FEDERAL EMERGENCY MANAGEMENT AGENCY

TECHNICAL SERVICES DIVISION 
HAZARDS STUDY BRANCH 

RIVERINE EROSION 
HAZARD AREAS 
Mapping Feasibility Study 
September 1999 

FEDERAL EMERGENCY MANAGEMENT AGENCY

TECHNICAL SERVICES DIVISION 
HAZARDS STUDY BRANCH 

RIVERINE EROSION 
HAZARD AREAS 

Mapping Feasibility Study 

September 1999 


Cover: House hanging 18 feet over the Clark Fork River in Sanders County, Montana, after the river eroded its bank in May 
1997. Photograph by Michael Gallacher. 


Table of Contents 

Report Preparation........................................................................................xi

Acknowledgments.........................................................................................xii

Executive Summary......................................................................................xiv

1. Introduction........................................................................................1
1.1. Description of the Problem...........................................................................................................1
1.2. Legislative History........................................................................................................................1
1.2.1. National Flood Insurance Act (NFIA), 1968.......................................................................3
1.2.2. Flood Disaster Act of 1973...............................................................................................4
1.2.3. Upton-Jones Amendment, 1988........................................................................................4
1.2.4. National Flood Insurance Reform Act (NFIRA), 1994........................................................7
1.3. Extent of the Problem...................................................................................................................7
1.4. Purpose and Scope of the Study...............................................................................................10
1.4.1. Purpose.........................................................................................................................10
1.4.2. Scope............................................................................................................................11
1.5. Mapping of Riverine Erosion Hazard Areas..............................................................................12
2. Background of Riverine Erosion......................................................14
2.1. Fluvial Systems ...........................................................................................................................14
2.2. Factors Affecting Alluvial Channels...........................................................................................15
2.3. Channel Types ............................................................................................................................16
2.4. Channel Form and Processes....................................................................................................19
2.4.1. Definitions.......................................................................................................................19
2.4.2. Geomorphic Characteristics............................................................................................20
2.4.3. Planforms.......................................................................................................................22
2.4.4. Cross Section.................................................................................................................25
2.4.5. Slope.............................................................................................................................26
2.4.6. Roughness and Bed Configuration..................................................................................27
2.4.7. Mechanics of Sediment Transport...................................................................................29

2.5. Evaluation Methods for Channel Response to Imposed Changes.........................................30
2.5.1. Equilibrium Approaches...................................................................................................31
2.5.2. Fluvial Hydraulics.............................................................................................................32
2.5.3. Streambank Stability.......................................................................................................34
2.6. Computational Models................................................................................................................36
2.6.1. Modeling Approach.........................................................................................................37
2.6.2. Hydraulics......................................................................................................................38
2.6.3. Sediment Transport........................................................................................................39
2.6.4. Stability of Streambanks .................................................................................................39
2.6.5. Future Directions.............................................................................................................41
3. Literature Review..............................................................................43
3.1. Research Sources and Search..................................................................................................43
3.2. Literature Summary....................................................................................................................43
3.3. Selected Documents...................................................................................................................43
4. Analysis of Case Studies..................................................................44
4.1. AMAFCA Sediment and Erosion Design Guide........................................................................46
4.1.1. Overview........................................................................................................................46
4.1.2. Detailed Description........................................................................................................46
4.1.2.1. Level 1 -- Geomorphic and Other Qualitative Analysis..........................................47
4.1.2.2. Level 2  Quantitative Geomorphic and Basic Engineering Analysis.....................48
4.1.2.3. Level 3  Quantitative Analysis Using Numerical Models.......................................51
4.1.2.4. Prudent Line Analysis...........................................................................................51
4.1.3. Applicability....................................................................................................................52
4.1.4. Limitations......................................................................................................................53
4.1.5. Relevant References.......................................................................................................53
4.1.6. Mappability.....................................................................................................................54
4.1.7. Cost...............................................................................................................................54
4.1.8. Regulatory Potential........................................................................................................54
4.1.9. Summary........................................................................................................................55
4.2. Inventory and Analysis of Stream Meander Problems in Minnesota......................................56
4.2.1. Overview........................................................................................................................56
4.2.2. Detailed Description........................................................................................................56
4.2.2.1. Data Collection and Preparation...........................................................................57
4.2.2.2. Shift Measurement...............................................................................................57
4.2.2.3. Results Summary.................................................................................................58
4.2.2.4. Relationships Between Stream Parameters and Measured Results.....................58
4.2.3. Applicability....................................................................................................................58

4.2.4. Limitations......................................................................................................................58

4.2.5. Relevant References.......................................................................................................58

4.2.6. Mappability.....................................................................................................................59

4.2.7. Cost...............................................................................................................................59

4.2.8. Regulatory Potential........................................................................................................59

4.2.9. Summary........................................................................................................................59

4.3. Probabilistic Spatial Assessment of River Channel Instability.................................................60

4.3.1. Overview........................................................................................................................60

4.3.2. Detailed Description........................................................................................................60

4.3.2.1. Methodology........................................................................................................60

4.3.2.2. Case Study..........................................................................................................62

4.3.3. Applicability....................................................................................................................62

4.3.4. Limitations......................................................................................................................62

4.3.5. Relevant References.......................................................................................................63

4.3.6. Mappability.....................................................................................................................63

4.3.7. Cost...............................................................................................................................63

4.3.8. Regulatory Potential........................................................................................................63

4.3.9. Summary........................................................................................................................63

4.4. Geomorphology and Hydrology of the Santa Cruz River, Southeastern Arizona.................64

4.4.1. Overview........................................................................................................................64

4.4.2. Detailed Description........................................................................................................64

4.4.3. Applicability....................................................................................................................66

4.4.4. Limitations......................................................................................................................66

4.4.5. Relevant References.......................................................................................................66

4.4.6. Mappability.....................................................................................................................67

4.4.7. Cost...............................................................................................................................67

4.4.8. Regulatory Potential........................................................................................................67

4.4.9. Summary........................................................................................................................67

4.5. San Diego County Alluvial Studies.............................................................................................69

4.5.1. Overview........................................................................................................................69

4.5.2. Detailed Description........................................................................................................69

4.5.2.1. Computer Modeling..............................................................................................69

4.5.2.2. Definition of Hazard Areas....................................................................................70

4.5.3. Applicability....................................................................................................................70

4.5.4. Limitations......................................................................................................................71

4.5.5. Relevant References.......................................................................................................71

4.5.6. Mappability.....................................................................................................................71


4.5.7. Cost...............................................................................................................................72

4.5.8. Regulatory Potential........................................................................................................72

4.5.9. Summary........................................................................................................................72

4.6. City of Austin Technical Procedures for Watershed Erosion Assessments.........................73

4.6.1. Overview........................................................................................................................73

4.6.2. Detailed Description........................................................................................................73

4.6.2.1. Introductory Material.............................................................................................73

4.6.2.2. Impervious Cover Computation.............................................................................74

4.6.2.3. Watershed "Vital Statistics" ..................................................................................74

4.6.2.4. Urbanization Effects..............................................................................................75

4.6.2.5. Stream Inventory..................................................................................................75

4.6.2.6. Enlargement Potential...........................................................................................77

4.6.2.7. Nick Point and Meander Management..................................................................79

4.6.2.8. Prioritization of Erosion Problems.........................................................................79

4.6.2.9. Stream Management Recommendations.............................................................80

4.6.3. Applicability....................................................................................................................80

4.6.4. Limitations......................................................................................................................80

4.6.5. Relevant References.......................................................................................................80

4.6.6. Mappability.....................................................................................................................81

4.6.7. Cost...............................................................................................................................81

4.6.8. Regulatory Potential........................................................................................................81

4.6.9. Summary........................................................................................................................81

4.7. River Stability Studies for Virgin River, Santa Clara River, and Fort Pierce Wash, Utah......83 

4.7.1. Overview........................................................................................................................83

4.7.2. Detailed Description........................................................................................................83

4.7.2.1. Historical Evaluation, Virgin River..........................................................................83

4.7.2.2. Geomorphic Evaluation, Virgin River.....................................................................84

4.7.2.3. Conclusion............................................................................................................86

4.7.2.4. Santa Clara River.................................................................................................87

4.7.2.5. Fort Pierce Wash.................................................................................................87

4.7.2.6. Recommended Management Plans.....................................................................88

4.7.3. Applicability....................................................................................................................88

4.7.4. Limitations......................................................................................................................89

4.7.5. Relevant References.......................................................................................................89

4.7.6. Mappability.....................................................................................................................90

4.7.7. Cost...............................................................................................................................90

4.7.8. Regulatory Potential........................................................................................................90

4.7.9. Summary........................................................................................................................90

4.8. Hydrologic and Geomorphic Studies of the Platte River Basin, Nebraska............................91

4.8.1. Overview........................................................................................................................91

4.8.2. Detailed Description........................................................................................................91


4.8.3. Applicability .....................................................................................................................93 
4.8.4. Limitations......................................................................................................................93 
4.8.5. Relevant References.......................................................................................................93 
4.8.6. Mappability.....................................................................................................................93 
4.8.7. Cost...............................................................................................................................93 
4.8.8. Regulatory Potential........................................................................................................94 
4.8.9. Summary........................................................................................................................94 

4.9. Streambank Erosion Along Two Rivers in Iowa.......................................................................95
4.9.1. Overview........................................................................................................................95
4.9.2. Detailed Description........................................................................................................95
4.9.2.1. Migration Model....................................................................................................95
4.9.2.2. East Nishnabotna River........................................................................................96
4.9.2.3. Des Moines River.................................................................................................97
4.9.2.4. Conclusion............................................................................................................97
4.9.3. Applicability....................................................................................................................98
4.9.4. Limitations......................................................................................................................98
4.9.5. Relevant References.......................................................................................................98
4.9.6. Mappability.....................................................................................................................98
4.9.7. Cost...............................................................................................................................98
4.9.8. Regulatory Potential........................................................................................................99
4.9.9. Summary........................................................................................................................99
4.10. Channel Migration Studies in King County, Washington......................................................100
4.10.1. Overview.......................................................................................................................100
4.10.2. Detailed Description......................................................................................................100
4.10.2.1. Study Procedures...............................................................................................100
4.10.2.2. Study Area Characteristics.................................................................................101
4.10.2.3. Estimation of Channel Migration..........................................................................101
4.10.2.4. Estimation and Mapping of Future Channel Migration.........................................102
4.10.3. Applicability..................................................................................................................103
4.10.4. Limitations....................................................................................................................103
4.10.5. Relevant References.....................................................................................................103
4.10.6. Mappability....................................................................................................................104
4.10.7. Cost.............................................................................................................................104
4.10.8. Regulatory Potential......................................................................................................104
4.10.9. Summary......................................................................................................................104
4.11. Bank Erosion Field Survey Report of the Upper Mississippi River and Illinois 
Waterway..................................................................................................................................105 
4.11.1. Overview.......................................................................................................................105
4.11.2. Detailed Description......................................................................................................106

4.11.2.1. Data Collection and Parameter Classification.....................................................106

4.11.2.2. Characterization of Bank Erosion and Failure Mechanisms.................................106

4.11.2.3. The Illinois Waterway.........................................................................................106

4.11.2.4. The Upper Mississippi River...............................................................................107

4.11.3. Applicability..................................................................................................................107

4.11.4. Limitations....................................................................................................................108

4.11.5. Relevant References.....................................................................................................108

4.11.6. Mappability....................................................................................................................108

4.11.7. Cost.............................................................................................................................108

4.11.8. Regulatory Potential......................................................................................................108

4.11.9. Summary......................................................................................................................109

4.12. Arizona Standards for Lateral Migration and Channel Degradation.....................................110

4.12.1. Overview.......................................................................................................................110

4.12.2. Detailed Description......................................................................................................110

4.12.2.1. Guideline 1  Lateral Migration Setback Allowance for Riverine Floodplains 
in Arizona............................................................................................................110 

4.12.2.2. Guideline 2  Channel Degradation Estimation for Alluvial Channels in 
Arizona...............................................................................................................112 

4.12.3. Applicability..................................................................................................................113

4.12.4. Limitations....................................................................................................................113

4.12.5. Relevant References.....................................................................................................114

4.12.6. Mappability....................................................................................................................114

4.12.7. Cost.............................................................................................................................114

4.12.8. Regulatory Potential......................................................................................................114

4.12.9. Summary......................................................................................................................114

5. Assessment of Technical Feasibility .............................................115

5.1. Overview...................................................................................................................................115

5.2. Evaluation of Scientific Soundness.........................................................................................115

5.2.1. Field Investigations........................................................................................................115

5.2.2. Geomorphic Analysis....................................................................................................118

5.2.3. Engineering Analysis.....................................................................................................122

5.2.4. Mathematical Modeling.................................................................................................124

5.2.5. Conclusions...................................................................................................................125

5.3. The Sixty-Year Horizon.............................................................................................................125

5.4. Cost...........................................................................................................................................127

5.4.1. Data Source and Assumptions......................................................................................127

5.4.2. Affected Areas..............................................................................................................127

5.4.3. Applicability of REHA Study Approaches.......................................................................128


5.4.4. Costs of REHA Study and Mapping..............................................................................128
5.4.5. NFIP Costs...................................................................................................................130
5.5. Conclusions on Technical Feasibility......................................................................................130
6. Implementation...............................................................................131
6.1. Existing Regulations.................................................................................................................131
6.1.1. Federal Regulations......................................................................................................131
6.1.2. Local Regulations..........................................................................................................131
6.2. Options for Implementation......................................................................................................132
6.2.1. NFIP Expansion............................................................................................................132
6.2.2. Local Implementation....................................................................................................133
7. Conclusions....................................................................................134
8. References......................................................................................136
Appendix .....................................................................................................149


List of Tables 

Table 2.1. Variables affecting alluvial channels and their dependency (Schumm, 1971; Chang, 

1988).............................................................................................................................16
Table 2.2. Stream classification system based on USACE (1994)...................................................18
Table 2.3. Schumms summary of channel responses (adapted from Chang, 1988)........................22
Table 2.4. Planform patterns (USACE, 1994)..................................................................................24
Table 2.5. Sediment transport formulas and classifications..............................................................30
Table 2.6. Twelve computational models to predict channel geometry changes (ASCE, 1998b)......37
Table 2.7. Features of flow routing submodels of reviewed models (ASCE, 1998b)........................38
Table 2.8. Sediment transport characteristics of reviewed models (ASCE, 1998b)..........................40
Table 2.9. Streambank stability characteristics of the reviewed models (ASCE, 1998b)..................41
Table 4.1. Summary of case studies................................................................................................45
Table 4.2. Streams and location of study reaches............................................................................57
Table 5.1. Scope of field investigations for the case studies...........................................................116
Table 5.2. Data needs and procedures in geomorphic studies.......................................................119
Table 5.3. Data needs and procedures in basic engineering studies..............................................123
Table 5.4. Data needs and procedures in mathematical modeling studies......................................124


List of Figures 

Figure 2.1. Zones in a fluvial system..............................................................................................15
Figure 2.2. Relationships among variables for channel response (USACE, 1994)..........................17
Figure 2.3. Main mechanisms of cross sectional widening (adapted from ASCE, 1998a)..............25
Figure 2.4. Main mechanisms of cross sectional narrowing (adapted from ASCE, 1998a)............26
Figure 2.5. Processes affecting channel slope (USACE, 1994).....................................................27
Figure 2.6. Various bed forms and their progression under accelerating flow (USACE, 1994).......28
Figure 2.7. Mechanisms of bank failure.........................................................................................36
Figure 4.1. Location of case studies..............................................................................................44
Figure 4.2. "Prudent Line" definition (AMAFCA, 1994)...................................................................53


List of Photographs 

Photograph 1. Collapsed house on eroding streambank along the Cimarron River in Logan 
County, Oklahoma (March 1998). Photograph courtesy of Kathy Schmidt............2 

Photograph 2. A portion of a house has fallen into the Cimarron River in Logan County, 
Oklahoma (March 1998). Photograph courtesy of Kathy Schmidt.........................2 

Photograph 3. House hanging 18 feet over the Clark Fork River in Sanders County, Montana, 
after the river eroded its bank in May 1997. Photograph by Michael Gallacher......3 

Photograph 4. Duck Creek in Las Vegas, Nevada, after the July 8, 1999, storm. The stream 

eroded the railroad bridge foundation. Bank retreat reached the house behind 

the bridge. Photograph courtesy of Leslie Sakumoto.............................................5

Photograph 5. Duck Creek just upstream of the railroad bridge. The house foundation has 

been damaged although the bank has not fully collapsed. Photograph 

courtesy of Leslie Sakumoto..................................................................................5

Photograph 6. Mobile home destroyed after bank collapse in Flamingo Wash during the July 

8, 1999, flood in Las Vegas, Nevada. Photograph courtesy of Leslie 

Sakumoto...............................................................................................................6

Photograph 7. Trailer park destroyed due to erosion in Flamingo Wash during the July 8, 

1999, flood in Las Vegas, Nevada. Sedimentation substantially closed one of 

the spans of the bridge on the right. Photograph courtesy of Leslie Sakumoto......6 

Photograph 8. Satellite image of the Missouri River floodplain near Glasgow, Missouri. The 

green area shows the channel in 1879. The light yellow area corresponds to 

the present day channel. Photograph by USGS....................................................9

Photograph 9. Mass failure of streambank caused by minor flooding event.................................35


Report Preparation 

In reponse to the National Flood Insurance Reform Act of 1994, Section 577, 
this report was prepared by the Hazards Study Branch, Technical Services 
Division, Mitigation Directorate, FEMA. 


Acknowledgements

The project team expresses sincere thanks to the following persons who assisted in the review of 
this report: 

Project Working Group (PWG) 

Nani Bhowmik, Ph.D., Hydrology Division, Illinois State Water Survey, IDNR, Champaign, Illinois.
Dan Bunting, Regional Building Department, Colorado Springs, Colorado.
Howard Chang, Ph.D., San Diego State University, San Diego, California.
David Froehlich, Ph.D., Parsons Brinckerhoff, Morrisville, North Carolina.
Jonathan Fuller, P.E., P.H., JEFuller/Hydrology & Geomorphology, Inc. Tempe, Arizona.
Doug Hamilton, P.E., Exponent, Inc., Irvine, California.
Tom Hegemier, Raymond Chan and Associates Inc., Austin, Texas
Peter Lagasse, Ph.D., Ayres Associates, Fort Collins, Colorado.
Tim Morrison, Floodplain Management Section, Pima County Department of Transportation and

Flood Control District, Tucson, Arizona.
Robert Mussetter, Ph.D., Mussetter Engineering Inc., Fort Collins, Colorado.
John Parker, Ph.D., U.S. Geological Survey, Tucson, Arizona.
Mark Vian, New York City Department of Environmental Protection, Shokan, New York.
Alan Wald, Washington State Department of Ecology, Olympia, Washington.


Executive Summary

This Riverine Erosion Hazard Area (REHA) mapping feasibility study addresses requirements in 
the National Flood Insurance Reform Act (NFIRA) enacted in September 1994. Section 577 of 
NFIRA requires that FEMA submit a report to Congress that evaluates the technological 
feasibility of mapping REHAs and assesses the economic impact of erosion and erosion mapping 
on the National Flood Insurance Program (NFIP). The purpose of this study is to determine 
whether it is technologically feasible to map riverine erosion hazard areas. 

Section 577 of NFIRA has specifically defined an erosion hazard area as follows: 

Erosion hazard area means, based on erosion rate information and other 
historical data available, an area where erosion or avulsion is likely to result in 
damage to or loss of buildings and infrastructure within a 60-year period. 

In the context of this study, erosion is the removal of a volume of sediment from a stream reach. 
However, in riverine areas, a stream reach can be stable and still migrate back and forth. 
Channel instability occurs when natural or man-induced processes lead to excessive erosion or 
deposition. Therefore, when a stream migrates laterally but maintains its dimensions, pattern, 
and profile, stability is achieved even though the river is active and moves across the floodplain. 
For this study, a reach experiencing this type of lateral migration is considered to be eroding, 
and thus has an associated REHA. This is because stream migration can threaten buildings and 
infrastructure. 

Technological feasibility is defined as existence of: 

Methodologies that are scientifically sound and implementable under the NFIP. 
Scientific soundness means that the methodologies are based on physical or 
statistical principles and are supported by the scientific community. 
Implementable means that the approaches can be applied by FEMA as part 
of a nationwide program under the NFIP and for an acceptable cost. 

In the present study, the project team conducted a search of existing methodologies used to 
predict riverine erosion, with emphasis on case studies. In general, case studies were 
categorized as: 

1. Geomorphic methods - relying primarily on historic data and geomorphic investigations; 
2. Engineering methods - relying primarily on predictive equations based on engineering and 
geomorphic principles; and 
3. Mathematical modeling methods - relying primarily on computer modeling of fluvial 
processes. 
A Project Working Group (PWG) of experts in the field of riverine erosion was organized. Their 
functions were to provide guidance to FEMA on technological feasibility of mapping REHAs, to 
act as an information source to locate and select case studies, and to review and comment on 
reports prepared during the study. The PWG included a nationwide mix of individuals from 
academia; Federal, State, regional and local government; and the private sector. 

Based on the literature review, case study analysis, and input from the PWG, methodologies for 
analyzing and mapping REHAs were identified. A determination on technological feasibility was 
reached. 


Using cost data associated with existing case studies, the study team estimated the approximate 
unit cost (i.e., cost per river mile) of conducting riverine erosion hazard studies and adding the 
areas to existing Flood Insurance Rate Maps (FIRMs). The study team estimated the 
approximate overall costs for conducting studies and mapping the riverine erosion hazard areas 
nationwide. 

Riverine Erosion 

Fluvial systems respond to perturbations that may be the result of naturally occurring inputs, such 
as precipitation, or human intervention in the form of urban development, forestry, mining, flow 
diversions, flood regulation, navigation, and other activities. Complex physical processes whose 
mathematical characterization is still imperfect govern the response, although there is reasonable 
qualitative understanding of the nature of this response. The basic premise is that streams are 
constantly attempting to attain a state of balance involving their geometry (dimensions, pattern, 
profile), the properties of the bed and bank material, and the external inputs imposed. The 
process to achieve this state of equilibrium can span long periods and affect large areas. 

In the context of riverine erosion hazard areas, engineers are mostly concerned with migration of 
the channel alignment and various forms of erosion and deposition. These events can potentially 
occur in any stream environment but are often most dramatic in arid and semi-arid regions where 
the large sediment yields and the flashy character of floods can cause severe changes in channel 
configuration. 

Numerous factors affect the spatial and temporal response of a stream channel. These factors 
encompass various aspects of geomorphology and fluid mechanics and include fluid properties, 
sediment characteristics, discharge, sediment transport, channel geometry, and fluid velocities. 
The behavior of these variables depends on the time scale under consideration: short term, long 
term, and very long (geologic) term. For example, channel geometry can be considered relatively 
constant in the short term of a few weeks but highly variable in the geologic time frame. 

For most practical applications, engineers are interested in phenomena that take place in the 
short and long term; thus, certain variables can be considered independent. For instance, in the 
geologic time frame, valley slope is a function of geology and climate; however, short- and long-
term channel formation processes occur at a much faster rate, and valley slope can be 
considered independent in many instances. For short and long term analyses, it can be assumed 
that the discharge regime and sediment supply are the driving variables that act on channel 
boundaries and vegetation to produce changes in channel cross section, longitudinal profile, and 
alignment. 

Erosional and depositional processes in alluvial channels are defined as follows: 

Degradation: Lowering of the channel bed on a substantial reach length occurring 
over a relatively long period of time in response to disturbances that 
affect general watershed conditions, such as sediment supply, runoff 
volume, and artificial channel controls. 

Aggradation: Raising of the channel bed as a result of disturbances in watershed 
conditions that produce the opposite effect to those leading to 
degradation. 


General Scour: Lowering of the streambed in a general area as a consequence of a 
short-duration event such as the passage of a flood. Examples are 
the erosion zones near bridge abutments and those in the vicinity of 
gravel pits. 

Local Scour: Lowering of the bed due to localized phenomena such as vortex 
formation around bridge piers. 

Deposition: Raising of the streambed due to a specific episode. An example is 
the formation of a sand bar after a flood event. Deposition is used in 
this document as the counterpart to general scour. 

Lateral Migration: Shifting of the streambank alignment due to a combination of the 
above vertical erosional and depositional processes. The most 
common example is meander migration in the floodplain. Bank retreat 
due to mass failure is another example. 

Vertical variations in the streambed are additive in that the net change is the result of long- and 
short-term processes. For instance, a reach that is undergoing aggradation due to increased 
sediment yield from the watershed can also experience general and local scour as a 
consequence of flood events. 

Streams are constantly progressing towards a state of dynamic equilibrium involving water and 
sediment. The geometry of the stream undergoes adjustments so that the sediment transport 
capacity of the water is in balance with the sediment supply. Natural and artificial factors can 
upset this state of equilibrium. Earthquakes, large floods, climatic changes, urbanization, and 
construction of civil works in the waterway introduce changes in the sediment supply and amount 
of runoff reaching the stream. For example, development in the watershed typically increases the 
impervious area and hence the volume of runoff. Similarly, clear-cutting of forests increases the 
sediment yield to the stream. Dams trap sediment and have a regulating effect that increases 
low flows and reduces high flows. Channelization projects reduce channel length and therefore 
increase slopes. Diversions for irrigation or public water supply reduce the effective flows. 
Finally, an event such as a large flood can dramatically reshape the floodplain and increase 
channel width. 

Evaluation of Channel Changes 

Mathematical representation of fluvial fluid mechanics is difficult due to imperfect knowledge of the 
complex physical phenomena involved. The many attempts to modeling of fluvial processes have 
shortcomings largely due to the fact that sediment transport equations commonly overpredict or 
underpredict sediment loads by orders of magnitude of actual measured sediment transport 
rates. 

Some analysis methods are based on the hypothesis that the stream system tends toward a 
state of dynamic equilibrium in which the channel adjusts to changes in the water and sediment 
supply regimes. These methods include simple equations called "regime relationships," 
techniques based on mechanical stability conditions, and complex computer models. These 
equilibrium-based approaches have difficulties in accounting for ever-changing land use 
conditions. 

In addition to fluvial processes, numerous climatic, environmental and geotechnical factors are 
involved. Hydrodynamically induced erosion and deposition and the occurrence of mass failure of 
the streambanks drive channel cross sectional changes. Induced effects include changes in 


roughness, bed material composition, vegetation cover, and planform. Prediction of cross 
sectional adjustments can only be accomplished for site-specific conditions after the most 
significant geomorphological factors have been identified. Therefore, any prediction of channel 
geometry should be based on sound field observations. 

Literature Review 

Of several hundred pieces of literature, 108 articles and reports were evaluated to compile 
methods currently in use to predict channel changes. Of this set, the following 12 case studies 
were selected for detailed review: 

Case Study Title Location 
AMAFCA Sediment and Erosion Design Guide Albuquerque, New Mexico 
Inventory and Analysis of Stream Meander Problems in Minnesota 14 streams in Minnesota 
A Probabilistic Approach to the Special Assessment of River Channel Rillito Creek, Near Tucson, Arizona 

Instability 

Geomorphology and Hydrology of the Santa Cruz River, Southeastern Santa Cruz River basin, Arizona 
Arizona 
San Diego County Alluvial Studies San Diego County, California 
City of Austin Technical Procedures for Watershed Erosion Assessments Austin, Texas 
River Stability Study, Virgin River, Santa Clara River and Ft. Pierce Wash, Virgin River, Santa Clara River, and Ft. 

Vicinity of St. George, Utah Pierce Wash basins, Utah 
Hydrologic and Geomorphic Studies of the Platte River Basin, Nebraska Platte River basin, Nebraska 
Streambank Erosion Along Two Rivers In Iowa East Nishnabotna River and the Des Moines 

River, Iowa 
Channel Migration Studies in King County, Washington Snoqualmie, Tolt, Raging, and Green Rivers, 
King County, Washington 
Bank Erosion Field Survey Report of the Upper Mississippi River and Illinois Upper Mississippi River and Illinois 
Waterway Waterway, Minnesota, Wisconsin, Iowa, 
Illinois, and Missouri 
Arizona Standards for Lateral Migration and Channel Degradation Arizona (statewide) 

In assessing the technical feasibility of mapping REHAs, each case study was analyzed for 
applicability, limitations, potential for mapping riverine erosion, cost, and regulatory potential. 
These documents revealed that numerous techniques are currently in use covering geomorphic 
methods, basic engineering principles, and mathematical modeling. This diverse collection of 
techniques is necessary because of the uniqueness of each site and to address the objectives of 
the specific projects. 

Assessment of Technical Feasibility 

The case studies indicate that there are scientifically sound procedures for delineating riverine 
erosion hazard areas. Various geomorphic, engineering, and modeling procedures can be 
applied, depending on site-specific conditions. Specialized knowledge and experience are 
needed to draw conclusions that would lead to delineation of a hazard area. 

A time frame of 60 years has been specified in Section 577 of NFIRA as the interval of interest 
for delineation of riverine erosion hazard areas. Although it is feasible to use the specified 60


year time frame, the case studies and the opinions of the PWG indicate that existing techniques 
may be better suited for shorter time frames, e.g., 30 years with periodic revisions to the 
particular REHA study and delineation. This limitation arises from data inaccuracies, imperfect 
knowledge of sediment transport mechanics, and unknowns in future watershed development, 
hydrologic conditions, and magnitude and sequence of future flooding events. However, most 
structures have a useful life well over 30 years and predictions should somehow address a longer 
time span. 

Given a suitable time frame, future erosion could be estimated either extrapolating from historic 
data or through the use of mathematical models. In both cases, an estimate of the reliability of 
the prediction needs to be provided. 

Cost 

An approximate analysis was performed to estimate the total cost to the Federal government of 
mapping riverine erosion hazard areas. The sources of cost data include information provided by 
the PWG, costs reported in the case studies, FEMA reports and other literature, and cost data 
from previous studies performed by the project team members. The data are not sufficient to 
make reliable nationwide cost estimation; however, they can be used to perform an educated 
guess for total costs. 

Average study values are $2,000-$3,000 per mile for geomorphic methods, $6,000-$7,000 for 
engineering methods, and $10,000-$12,000 for mathematical modeling methods. If this effort 
were to be implemented as part of the NFIP, the cost to the Federal government would be 
between 200 and 300 million dollars. Section 577 of NFIRA specifies that, if REHA determination 
is found to be technically feasible, a cost-benefit study is to be conducted. The current study 
does not include these cost-benefit analyses. 

Implementation 

There are at least two potential options for implementation of a nationwide REHA delineation 
program: a federally run program and a locally run program. The federally run program would be 
integrated into the NFIP. The fundamental principle of this first option is to expand the current 
floodplain regulations to encompass riverine erosion. This option emphasizes authority from the 
Federal government. The existing framework can be modified to accommodate the new 
responsibilities of regulating erosion-prone areas. Disadvantages are the additional cost to the 
Federal government and the challenge of developing appropriate guidelines for REHA delineation 
in a field that requires flexibility and accessibility to a wide array of analytical options. 

The second option shifts the authority for regulating erosion-prone areas to the local jurisdictions. 
Implementation would be tailored to suit individual floodplain management needs. The Federal 
government would provide technical assistance, if required, and disseminate information. The 
main advantage is that the communities would have the flexibility to match their resources and 
needs with the complexity of the studies. 

Conclusions 

 It is technologically feasible to map riverine erosion hazard areas. Flexibility in the choice of 
analysis techniques is needed to address site-specific conditions. 
 REHA delineations for a period of 60 years are possible; however, better predictions may be 
achieved for a shorter time span, such as 30 years, with periodic revisions. 

 The analytical methods used should be able to provide an indication of the reliability of REHA 
delineations. 
 Average study values are $2,000-$3,000 per mile for geomorphic methods, $6,000-$7,000 
for engineering methods, and $10,000-$12,000 for mathematical modeling methods. 
 The cost of mapping REHAs nationwide ranges between approximately 200 and 300 million. 
This estimate is based on limited information. 
 Implementation of erosion regulations can be either done as an extension of the NFIP or 
delegated to local jurisdictions with support from the Federal government. 

1. Introduction 
1.1. Description of the Problem 
The National Flood Insurance Program (NFIP) administered by the Federal Emergency 
Management Agency (FEMA) covers damages caused by flooding and flood-related erosion. 
However, it does not cover damages caused by gradual (day to day) erosion. In fact, riverine 
erosion is not considered in insurance rates or shown on Flood Insurance Rate Maps (FIRMs). It 
was not until the Flood Disaster Protection Act of 1973 that Federal policy recognized the hazard 
of erosion by adding protection for flood-related erosion. 

This Riverine Erosion Hazard Area (REHA) mapping feasibility study was conducted in response 
to requirements in the National Flood Insurance Reform Act (NFIRA) enacted in September 
1994. Section 577 of NFIRA requires that FEMA submit a report to Congress that evaluates the 
technological feasibility of mapping REHAs and assesses the economic impact of erosion and 
erosion mapping on the NFIP. The purpose of this study is to determine whether it is 
technologically feasible to map riverine erosion hazard areas. 

Because riverine flood damage assessments generally consider inundation alone, the potential 
flood-related damages for rivers in arid and semi-arid regions may be significantly underestimated 
(Graf, 1984; NRC, 1999). Despite this observation, bank erosion along alluvial channels, caused 
by moderate or large non-flood (within channel) flows, is not recognized as a significant hazard in 
Federal floodplain management regulations. Several photographs below show various locations 
in the country where riverine erosion has caused substantial damage to buildings and 
infrastructure. In some of the cases, the areas were marked as not subject to flooding in FIRMs. 

The delineation of floodplains has generally been based on the application of established 
methodologies of hydrology and hydraulics for channels with fixed boundaries. In general, 
floodplain management and mapping have only relatively recently given more attention to the role 
of sediment and geomorphic and geologic controls on channel form and stability. 

1.2. Legislative History 
The NFIP legislative cornerstones and connections to erosion are: 

 National Flood Insurance Act, 1968 
 Flood Disaster Protection Act, 1973 
 Upton-Jones Amendment, 1988 
 National Flood Insurance Reform Act, 1994 
INTRODUCTION 


Photograph 1. Collapsed house on eroding streambank along the Cimarron River in Logan 
County, Oklahoma (March 1998). Photograph courtesy of Kathy Schmidt. 

Photograph 2. A portion of a house has fallen into the Cimarron River in Logan County, 
Oklahoma (March 1998). Photograph courtesy of Kathy Schmidt. 

INTRODUCTION 


1.2.1. National Flood Insurance Act (NFIA), 1968 
Federal involvement in non-structural means to decrease flood losses was relatively minor until 
the enactment of the National Flood Insurance Act (NFIA) of 1968. The NFIA created the NFIP 
to provide the availability of flood insurance, at actuarial premium rates, in communities which 
adopt and enforce floodplain management ordinances that meet minimum NFIP requirements 
(FEMA, 1991). However, the first few years after passage of the NFIA of 1968 saw community 
participation and sales of flood insurance policies disappointingly low (Mrazik and Kinberg, 1989). 
Then, in 1972, Tropical Storm Agnes caused Congress to re-examine existing Federal policies 
under the NFIP (Crowell, et al., 1999). 

Photograph 3. House hanging 18 feet over the Clark Fork River in Sanders 
County, Montana, after the river eroded its bank in May 1997. Photograph by 
Michael Gallacher. 

INTRODUCTION 


1.2.2. Flood Disaster Act of 1973 
The repercussions caused by Tropical Storm Agnes provided the impetus for the formulation and 
passage of the Flood Disaster Protection Act of 1973 (P.L. 93-234) (Miller, 1992). The Act 
strengthened the NFIA and expanded the NFIPs coverage by requiring the purchase of flood 
insurance as a condition of receiving Federal or federally backed financing for acquisition or 
construction of structures in flood hazard areas. 

1.2.3. Upton-Jones Amendment, 1988 
The Housing and Community Development Act of 1987 authorized FEMA to provide insurance 
payments under the NFIP for structures subject to imminent collapse due to erosion or 
undermining caused by waves or currents. This legislation is commonly referred to as the Upton-
Jones Amendment, and FEMA is responsible for determining whether a structure is subject to 
imminent collapse (FEMA, 1990). 

The Upton-Jones Amendment was originally designed for application in coastal areas; however, it 
does not exclude riverine areas, and claims have been made for structures along streams and 
rivers. In coastal areas, where average annual erosion rates are established, a structure that lies 
within 10 feet plus 5 times the annual erosion rate from an existing reference feature, such as a 
beach scarp or bluff crest, is defined as being within the zone of imminent collapse. An attempt is 
made to apply the same criterion to riverine areas; however, historic bank change information is 
often unavailable. 

In 1990, FEMA implemented a simple criterion for assessing stream bank and bluff stability in 
areas where no historical information was available. The purpose was to establish a procedure 
for determining if a structure lies within the zone of imminent collapse given limited information 
about the site. The procedure classifies a given slope as either stable or unstable, and 
determines the potential failure plane location if the bank were to fail. Given the height of the 
slope, angle of inclination, distance of the structure from the shoulder of the slope, soil strength, 
and the soils angle of internal friction, these two criteria can be determined (FEMA, 1990). In 
summary, if the slope is stable, the structure is outside the zone of imminent collapse. If the slope 
is unstable, and the structure lies within the potential failure plane, it is said to be within the zone of 
imminent collapse and eligible for benefits of the Upton-Jones program. 

The main purpose of the Upton-Jones Amendment was to provide funds for pre-flood mitigation 
activities to reduce future flood losses. However, review of the Upton-Jones claims history 
demonstrates that the program failed to generate interest; few insured eligible for Upton-Jones 
payments were applying for benefits. Moreover, although the intent of the Amendment was to 
encourage relocation and ultimately save money for the NFIP, most homeowners opted for 
demolition payments, thereby increasing expenditures to the NFIP (Davison, 1993). 

The Upton-Jones Amendment allowed claims for structures subject to imminent collapse due to 
bank erosion caused by waves and currents. Of the approved riverine Upton-Jones claims filed 
from May 1988 to December 1994, there was a total settlement of $3,200,000 and an average 
settlement of $43,000 (FEMA, 1998). Approximately 60 percent of these claims are from the 
southwest and northwest United States The Streambank Erosion Control Evaluation and 
Demonstration Act (SECEDA) of 1974 report (USACE, 1981) confirms that these areas are 
particularly vulnerable to riverine erosion and comprise approximately half of the estimated annual 
damages due to erosion. 

INTRODUCTION 


Photograph 4. Duck Creek in Las Vegas, Nevada, after the July 8, 1999, storm. The stream eroded the 
railroad bridge foundation. Bank retreat reached the house behind the bridge. Photograph courtesy of 
Leslie Sakumoto. 

Photograph 5. Duck Creek just upstream of the railroad bridge. The house foundation has been damaged 
although the bank has not fully collapsed. Photograph courtesy of Leslie Sakumoto. 

INTRODUCTION 


Photograph 6. Mobile home destroyed after bank collapse in Flamingo Wash during the July 8, 1999, 
flood in Las Vegas, Nevada. Photograph courtesy of Leslie Sakumoto. 

Photograph 7. Trailer park destroyed due to erosion in Flamingo Wash during the July 8, 1999, flood in Las 
Vegas, Nevada. Sedimentation substantially closed one of the spans of the bridge on the right. Photograph 
courtesy of Leslie Sakumoto. 

INTRODUCTION 


1.2.4. National Flood Insurance Reform Act (NFIRA), 1994 
When Congress enacted the Upton-Jones program into law, it was meant to be an interim 
program that would stay in effect until a more comprehensive erosion management program was 
legislated by Congress. As a result, between 1990 and 1994 a number of Congressional 
proposals were introduced that would have established erosion management authority under the 
NFIP and would have required FEMA to map erosion hazard areas for the entire Coastal and 
Great Lakes shorelines. Some proposals would have denied the availability of flood insurance for 
new or substantially improved structures located in 30-year erosion hazard areas; others would 
have made flood insurance available in erosion hazard areas, but at actuarial rates. These 
proposals were extensively deliberated, but ran into considerable opposition from real estate, 
developer, and property rights advocates who strongly opposed any form of erosion mapping or 
erosion-based insurance rate modifications that could potentially devalue or restrict use of 
property. As a result, none of the proposals were enacted (Davison, 1993). Ultimately, a 
compromise proposal was formulated between proponents and opponents of an FIA-
administered erosion management program. 

The compromise proposal required that FEMA study the issue of erosion mapping and erosion-
based insurance modification, rather than mandate immediate change to the NFIP. This proposal 
was included in the National Flood Insurance Reform Act, which was enacted into law on 
September 23, 1994. In this Act, Section 577, Evaluation of Erosion Hazards, authorizes that the 
FEMA Director may map a statistically valid and representative number of communities with 
erosion hazard areas throughout the United States, including coastal, Great Lakes, and , if 
technologically feasible, riverine areas. 

The major goals of Section 577 are to 1) list the communities likely to be identified as having 
erosion hazard areas, 2) estimate the number of flood insurance claims under the NFIP that are 
attributable to erosion, 3) determine the amount of flood insurance claims under the NFIP that are 
attributable to claims under the Upton-Jones program, 4) assess the full economic impact of 
erosion on the National Flood Insurance Fund, and 5) determine the costs and benefits of 
mapping erosion hazard areas. 

A distinction was made between open coast erosion and riverine erosion due to differences in 
the magnitude of these problems, and the physical processes involved. FEMA has been directed 
to map a statistically valid and representative number of communities with erosion hazard areas 
along the open coasts of the United States. An economic analysis has been conducted to 
determine the costs and benefits of mapping such areas based on actual and projected flood 
insurance claims attributable to coastal erosion. A similar economic analysis may be required for 
riverine sites, if mapping such areas is found to be technologically feasible. 

This project was conducted to determine the technological feasibility of mapping REHAs for 
communities within the United States. 

1.3. Extent of the Problem 
In recognition of the serious economic losses occurring throughout the United States due to 
stream bank erosion, Congress enacted The Streambank Erosion Control Evaluation and 
Demonstration Act (SECEDA) of 1974 (Section 32, Public Law 93-251), which authorized $50 
million for a national demonstration program. The U.S. Army Corps of Engineers (USACE) 
conducted this program. One of the purposes of this program was to evaluate the extent of 
streambank erosion in the United States SECEDA is one of the few initiatives that attempt to 

INTRODUCTION 


assess the erosion problem on a national level; therefore, the lessons learned from this Act are 
essential in determining if technologically feasible erosion prediction techniques are possible on 
this scale. However, the main focus of the SECEDA report, bank protection methods, differs 
from the purpose of the present study. Also, the SECEDA report (USACE, 1981) is 25 years 
old, and there have been significant advances in technology. In addition, urbanization in the last 
25 years has had an impact on the predicted number of bank miles affected. Due to 
urbanization, formerly stable channels become unstable as a result of increased flood peaks 
and/or flow volumes and reduced sediment loads (NRC, 1999). 

Approximately one-third of the nations streams experience severe erosion problems, and 
landslides and mudslides are commonplace in some areas (NRC, 1999). As stated in the 
SECEDA report, there are approximately 7 million streambank miles in the United States, of 
which 8 percent or approximately 600,000 miles, are experiencing at least some degree of 
erosion (USACE, 1981). Of the 600,000 miles of eroding stream, approximately 142,000 bank 
miles (2 percent of the total bank miles) are experiencing extensive erosion. Due to rapid 
urbanization, especially for rivers with smaller watershed areas, the actual figure for bank erosion 
mileage could be significantly higher. The average damage per year of extensive erosion-related 
damage is approximately $450 million (in 1998 dollars), or approximately $3,000 per bank mile 
per year (USACE, 1981). Of this damage, over half is experienced in the southwest and 
northwest. It is important to note that the southwest is one of the most rapidly growing areas in 
the United States Cities such as Tucson, Phoenix, Las Vegas, and cities in southern California 
have documented problems associated with REHAs. 

One of the most important lessons learned from the SECEDA is quoted as follows (USACE, 
1981): 

Prediction of when, where, and extent of streambank erosion and/or bank 
instability remains clouded. The forces contributing to streambank instability 
are generally known and understood; however, application of these principles 
to the real world are complicated by the many processes acting 
simultaneously throughout a given river reach. Streams displaying very active 
tendencies to erode their banks often seem to reverse themselves and display 
periods of relative stability. 

Since the USACE report was published, there have been many advances in the field of river 
mechanics, some of which are compiled in publications such as the Arizona Design Manual for 
Engineering Analysis of Fluvial Systems (Simons, Li & Associates,1985) and the AMAFCA 
Sediment and Erosion Design Guide (Musseter, et al.,1994). 

Section 577 of NFIRA has specifically defined an erosion hazard area as follows: 

Erosion hazard area means, based on erosion rate information and other 
historic data available, an area where erosion or avulsion is likely to result in 
damage to or loss of buildings and infrastructure within a 60-year period. 

In the general context of this study, erosion is defined as the removal of a volume of sediment 
from a stream reach. However, in riverine areas, a stream reach can migrate back and forth 
without necessarily resulting in a sediment deficit for the reach. For example, a channel can have 
stable dimensions even though the stream is migrating laterally at a fairly constant annual rate. 
Thus, the amount of sediment entering the reach is about equal to the amount of sediment exiting 
it, with no net erosion occurring within the reach. Channel instability occurs when natural or man-
induced processes lead to excessive erosion or deposition. Therefore, when a stream migrates 
laterally but maintains its dimensions, pattern and profile, stability is achieved even though the river 
is active and moves across the floodplain. Photograph 8 shows a satellite image of the 

INTRODUCTION 


Missouri River floodplain near Glasgow, Missouri. The green area corresponds to the channel in 
1879. The light yellow area is the present day channel. The area in between the two channel 
positions experienced erosion at some point after 1879. The current channel has been stabilized 
with revetments that prevent further migration. 

Photograph 8. Satellite image of the Missouri River floodplain near Glasgow, Missouri. The 
green area shows the channel in 1879. The light yellow area corresponds to the present day 
channel. Photograph by USGS. 

For this study, a reach experiencing this type of lateral migration is considered to be eroding, 
and thus has an associated REHA, even though it is simply migrating back and forth. This is 
because such stream migration can significantly threaten buildings and infrastructure. 

INTRODUCTION 


Floodprone areas are determined according to national standards, which assume a stable 
channel. Computer programs used for floodplain studies, such as HEC-RAS, do not include the 
effect of moveable boundaries, varying erosiveness of bed and bank materials, and sediment 
transport, which is typical of alluvial streams. Therefore, additional interdisciplinary techniques 
based on the principals of fluvial geomorphology and hydraulic engineering must be considered 
for use in floodplain management studies aimed at REHA mapping. 

Changes in alluvial channels can be dramatic. The Santa Cruz River, an ephemeral river in 
southeastern Arizona, has a long history of channel instability. Since the late 19th century, lateral 
channel erosion has caused extensive property damage, particularly in Pima County. During the 
1983 flood, 13 people died and about $100 million of damage was caused in the Tucson area 
alone (Kressan, 1988). Most of the 1983 flood damage resulted from bank erosion on the Santa 
Cruz River and its tributaries, rather than inundation from over-bank flow. From 1982-84, the 
area of floodplain occupied by the channel along certain reaches has increased up to 137 percent 
(Hays, 1984). 

Alluvial channels in the Tucson basin have exhibited lateral bank migration, which caused land 
outside the designated 500-year floodplain to collapse into the channel (Kressan, 1988). 
Documented shifts in Rillito Creek of nearly 1,000 feet have occurred since the 1940s (Kressan, 
1988). Bank erosion can also be a significant hazard, even for moderate to large non-flood flows 
along the alluvial channels, such as those in the Tucson basin. 

Incised channels, such as the Santa Cruz River, present special problems in the interpretation of 
the flood hazard. Entrenchment occurs from entrainment and transport of alluvial materials by 
channel erosion. As a result, channels are frequently so deep that floods with recurrence 
intervals greater than 100 years are required for overflow. The geomorphic complexity of such 
alluvial streams creates difficulties for implementing Federal floodplain regulations that are based 
primarily on flooding. In fact, channel changes, such as meander migration and bank erosion, 
may constitute a greater hazard than overbank flow in some areas. 

In response to the 1983 flood, Pima County passed a revised floodplain management ordinance 
with a setback provision for structures on all property along unprotected channel banks for major 
river courses. In addition, recognizing riverine erosion as a statewide concern, the State of 
Arizona Department of Water Resources has adopted standards for identification of and 
development within erosion hazard areas (see Chapter 4). The state standards establish 
procedures for estimating setback distances for development along watercourses to allow for the 
lateral migration that may occur during future floods (Simons Li & Associates, 1984, 1985). 
Setbacks in naturalistic channels can provide protection to adjacent property similar to that 
provided by hard-lined channels. 

1.4. Purpose and Scope of the Study 
1.4.1. Purpose 
The purpose of this study is to determine the technological feasibility of mapping REHAs for 
communities within the United States. Technological feasibility is defined as existence of: 

Methodologies that are scientifically sound and implementable under the NFIP. 
Scientific soundness means that the methodologies are based on physical or 
statistical principles and are supported by the scientific community. 
Implementable means that the approaches can be applied by FEMA as part 
of a nationwide program under the NFIP and for an acceptable cost. 

INTRODUCTION 


1.4.2. Scope 
The study included the following tasks: 

Task 1 - Literature Search and Case Study Analysis 

The study team conducted an in-depth search of existing methodologies used to predict riverine 
erosion. The following groups were contacted: 

 Federal agencies 
 State authorities 
 Regional agencies 
 Research universities 
 Private firms 
 Professional organizations 
 Local government agencies 
The information was collected, categorized, and compared. Emphasis was placed on the critical 
review of existing case studies. In general, case studies were categorized as: 

1. Geomorphic methods - relying primarily on historic data and geomorphic investigations; 
2. Engineering methods - relying primarily on predictive equations based on engineering and 
geomorphic principles; and 
3. Mathematical modeling methods - relying primarily on computer models. 
Critical technical issues were identified related to frequency, statistics, definitions, data availability, 
data collection, and data accuracy and interpretation. 

Task 2 - Establish the Project Working Group 

A Project Working Group (PWG) of experts in the field of riverine erosion was organized. The 
responsibilities of the PWG were to provide guidance to FEMA on issues of technological 
feasibility of mapping REHAs, act as an information source to locate and select case studies, and 
review and comment on reports prepared during the study. The PWG included a mix of 
individuals who have similar expertise and come from throughout the United States representing 
academia; Federal, state, regional and, local government; and the private sector. The members 
of the PWG are cited in the acknowledgments. 

Task 3 - Input from the PWG 

Input from the PWG was solicited via the Internet and/or teleconference. Specific tasks 
conducted by the PWG were to: 

1. Review the preliminary results of the literature search. 
2. Identify other methodologies or new advancements and future research needs in riverine 
erosion studies. 
3. Discuss major technical issues identified in Task 1. 
4. Discuss the technologically feasibility of preparing REHA studies and identifying conditions 
and limitations for these studies. 
INTRODUCTION 


Task 4 - Selected Methodologies 

Based on the literature review, case study analysis, and input from the PWG, various 
methodologies were identified for analysis and mapping of REHAs. 

Task 5 - PWG Review 

A series of teleconferences were conducted to discuss the technological feasibility of mapping 
REHAs. Prior to the teleconferences, the PWG received reports for review and comment. The 
overall objective of determining the technological feasibility of mapping REHAs was addressed by 
the PWG at these times. 

Task 6 - Unit Costs of Riverine Erosion Hazards 

Using cost data associated with existing case studies, the study team estimated the approximate 
unit cost (i.e., cost per river mile) of conducting riverine erosion hazard studies and adding the 
areas to existing FIRMs. 

Task 7  Select Study Areas and Estimate Number of Communities Affected 

An approach for selecting study areas was recommended. A preliminary estimate of the number 
of studies and affected map panels was conducted for all participating communities covered by 
the NFIP. 

Task 8 - Overall Costs Within NFIP 

The approximate overall costs for conducting studies and mapping the riverine erosion hazard 
areas was estimated. 

Task 9 - Prepare Report with Recommendations 

This report was prepared. 

1.5. Mapping of Riverine Erosion Hazard Areas 
Mapping of riverine erosion hazards has a rather short history and has been done mostly at the 
local level and for specific projects. The purpose of mapping areas associated with natural 
hazards is to delineate zones and provide an assessment of the relative risk to which structures 
would be subject if they were located in such zones. In some cases, hazard-zone maps also 
include information that can be used for hazard forecasting and monitoring. A variety of hazard-
zone maps have been developed and published in the United States for various natural disasters. 
These hazard area maps include tornado maps, published by National Weather Service, 
earthquake fault zone maps developed and maintained by the State of California, and FEMAs 
FIRMs. 

Riverine erosion hazards have not been defined consistently except for a few cases. An example 
presented in Chapter 4 is the State of Arizona guidelines to define setbacks along erosion-prone 
streams. Another example also described in Chapter 4 is the "Prudent Line" approach used in 
Albuquerque, New Mexico, to define setbacks along arroyos. There have been other attempts at 
mapping that identify locations potentially subject to erosion and define the relative risk of erosion 
but do not specify the lateral extent of those risks. These relative measures of risk are of limited 
use for regulatory purposes. 

A viable methodology to characterize erosion hazards necessitates that the resulting maps allow 
users to determine whether a structure in or near the floodplain will have a high probability of 
being affected by erosion during its useful life. Therefore, the maps must show clearly defined 

INTRODUCTION 


areas in which, to the best information available, development must be regulated due to the 
dangers expected from erosion. In summary, the test for applicability of a procedure is its ability 
to determine REHAs and then be able to delineate them on a map so that those assets at risk 
can be identified. 

The main objective of this report is to determine if there are scientifically sound REHA delineation 
procedures. An additional consideration is whether a feasible framework can be proposed to 
develop erosion hazard maps on a nationwide scale. For this purpose, the report investigates the 
state-of-the-art in riverine erosion hazard characterization and summarizes several initiatives to 
develop erosion hazard maps and regulations. Chapter 2 presents background material on 
riverine erosion, Chapter 3 describes the literature search to arrive at the information used in this 
study, Chapter 4 describes 12 case studies in which various historical, engineering, geomorphic, 
and numerical modeling methods were used to define erosion hazard areas. Chapter 4 also 
analyzes the potential of such methods to develop hazard maps. Chapter 4 assesses the 
technological feasibility of mapping REHAs. Chapter 5 presents an approximate cost analysis of 
applying erosion characterization methodologies on a nationwide basis, and Chapter 6 discusses 
regulatory aspects of mapping REHAs. Chapter 7 summarizes conclusions and 
recommendations, and Chapter 8 lists the numerous references consulted in the development of 
this document. 

INTRODUCTION 


2. Background of Riverine Erosion 
Proper evaluation of erosion processes requires an understanding of the geomorphologic 
variables affecting a fluvial system. This section provides a concise overview of fluvial processes 
and their effects on channel formation, physical stream characteristics, response to watershed 
changes, and riverine erosion and deposition. This section also summarizes key research and 
references that have addressed the topic of characterization of riverine erosion problems. 

2.1. Fluvial Systems 
A watershed is a complex drainage system in which streams are the final receptors of water and 
sediment. In general, this drainage system has three zones (Schumm 1977; USACE, 1994): 

1. The erosional zone is the upper portion of the watershed, which is the source of most of the 
water and sediment. Streams in this zone are rather unstable and may present a braided 
pattern. 
2. The sediment transport zone is the middle part of a stream where a state of dynamic 
equilibrium tends to develop. Nevertheless, changes could be significant. 
3. The depositional zone is the lower portion of the stream where it is subject to the influence of 
the water body into which it empties. A delta typically forms in this zone. 
Figure 2.1 shows a schematic of the zones in a fluvial system. Specific features can create local 
depositional areas in the upper zone or local erosional areas in the lower zone. An example is an 
alluvial fan that creates a depositional zone. Nevertheless, in the very long term, the tendency of 
the stream is to flatten its channel slope by degradation in the upper reaches and aggradation in 
the lower portions. Engineered features placed in the channel can drastically hasten the naturally 
slow progression of processes. For example, a channelized portion of the stream increases the 
slope and may trigger erosion and formation of bends and deposition in unchannelized areas . 

Fluvial systems respond to perturbations that may be the result of naturally occurring inputs, such 
as precipitation, or human intervention in the form of urban development, forestry, mining, flow 
diversions, flood regulation, navigation, and other activities. Complex physical processes whose 
mathematical characterization is still imperfect govern the response, although there is reasonable 
qualitative understanding of the nature of this response. The basic premise is that streams are 
constantly attempting to attain a state of balance involving their geometry (dimensions, pattern, 
profile), the properties of the bed and bank material, and the external inputs imposed. The 
process to achieve this state of equilibrium can span long periods and affect large areas. 

In the context of riverine erosion hazard areas, engineers are mostly concerned with migration of 
the channel alignment and various forms of erosion and deposition. These events can potentially 
occur in any stream environment but are often most dramatic in arid and semi-arid regions where 
the large sediment yields and the flashy character of floods can cause severe changes in channel 
configuration. Similar behavior can occur in humid climate areas where rapid land use changes 
take place. 

BACKGROUND OF RIVERINE EROSION 


DEPOSITIONAL 
ZONE 
TRANSPORT 
ZONE 
EROSIONAL 
ZONE 
Delta 
Figure 2.1. Zones in a fluvial system. 

2.2. Factors Affecting Alluvial Channels 
Numerous factors affect the spatial and temporal response of a stream channel. These factors 
encompass various aspects of geomorphology and fluid mechanics and include fluid properties, 
sediment characteristics, discharge, sediment transport, channel geometry, and fluid velocities. 

The time scale is an important differentiator in the nature of fluvial changes. Schumm (1971) 
called short-term steady time and associated it with duration of the order of days. Long-term 
was labeled graded time spanning a few hundred years. Finally, very long-term was called 
geologic time, and it is measured in millions of years. Depending on the time scale under 
consideration, fluvial variables could be dependent or independent. For example, in the short-
term, sediment transport is a function of flow velocity, which in turn is a function of channel 
geometry and discharge; therefore, sediment transport and velocity can be considered 
dependent. However, in the long-term, both discharge and sediment transport are independent 
because they are the result of watershed processes to which the river must adjust itself (Chang, 
1988a). Table 2.1 shows a summary of these variables and their dependency according to time 
scale. 

BACKGROUND OF RIVERINE EROSION 


Table 2.1. Variables affecting alluvial channels and their dependency (Schumm, 1971; Chang, 
1988). I = Independent; D = Dependent; X = Indeterminate. 

Variable Status of Variable 
Short-Term Long-Term Very Long-Term 
(Steady Time) (Graded Time) (Geologic Time) 
Geology I I I 
Paleoclimate I I I 
Paleohydrology I I D 
Valley slope, width, and depth I I D 
Climate I I X 
Vegetation type and density I I X 
Mean water discharge I I X 
Mean sediment inflow I I X 
Channel morphology I D X 
Observed discharge and load D X X 
Hydraulics of flow D X X 

For most practical applications, engineers are interested in phenomena that take place in the 
short (steady) and long (graded) term; thus, certain variables can be considered independent. 
For instance, in the very long (geologic) term, valley slope is a function of geology and climate; 
however, short-and long-term channel formation processes occur at a much faster rate, and 
valley slope can be considered independent in many instances. For the time scales under 
consideration in this document, the most significant relationships are depicted in Figure 2.2, which 
indicates that channel geometry depends on discharge and sediment transport. 

2.3. Channel Types 
Various schemes have been developed over the years in an attempt to classify channels 
according to broad morphological aspects. According to Leopold et al. (1964), channel patterns 
were initially classified broadly as straight, braided or meandering. Despite its general 
usefulness, this classification has limitations in describing channel shapes. For instance, it is 
unlikely to find channels that are straight for more than a short distance; therefore, the term 
straight is relative and should refer to sinuous, irregular alignments. An exception is a channel 
controlled by fractured rock, which may actually show straight reaches along fractures and joints. 
Even in straight reaches, the flow pattern is such that the flow lines move from one bank to the 
opposite causing alternating point bars within the channel. Because of this natural tendency 
towards a sinuous channel, the term meandering should be applied to streams that have a 
relative periodicity in its planform; however, it is not easy to make this determination for all cases. 
Leopold et al. (1964) defined straight channels in terms of the sinuosity, which is the ratio of 
thalweg length to down-valley distance. Typical values of the sinuosity vary between 1 and 3. A 
channel with a sinuosity of 1.5 or less is said to be straight. 

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Figure 2.2. Relationships among variables for channel response (USACE, 1994). 

Other classification systems were developed as the study of river geomorphology evolved and 
more knowledge was acquired. Culvertson et al. (1967) created an alphanumeric system based 
on variability of channel width, braiding patterns, sinuosity, natural levees, floodplain width, 
vegetation, presence of oxbow lakes, meander type, and bank height. Brice (1983) introduced a 
classification system based on sinuosity, presence of point bars, braiding, and anabranching 
(Chang, 1988). Channels were classified as sinuous canaliform, sinuous with point bars, sinuous 
braided, and nonsinuous braided. Other classification schemes were proposed by Rosgen 
(1994, 1996) and Clark et al. (1995). The USACE (1994) developed the comprehensive system 
summarized in Table 2.2. 

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Table 2.2. Stream classification system based on USACE (1994). 

Stream Description 
Type 
Mountain 
Torrents 
Alluvial Fans 
Braided 
Rivers 
Arroyos 
Meandering 
Alluvial 
Rivers 
Modified 
Streams 
High velocity streams on steep slopes with a drop-and-chute structure often 
achieved by obstacles such as large boulders or debris. These streams are 
subject to scour and degradation caused by flood events. Very steep slopes 
can lead to debris flows that produce substantial movement of boulders and 
gravel. 
Occur usually in arid and semi-arid lands where a stream flowing through a 
stream valley enters a flat area. The coarse sediment carried by the stream 
deposits in a delta-like configuration characterized by multiple channels subject 
to shifting. The chief stability problem is caused by the unpredictability of the 
flow paths, which may cause erosion and deposition in unexpected places. 
Most recently, NRC (1996) defined alluvial fans as a sedimentary deposit 
located at a topographic break, such as the base of a mountain, escarpment, 
or valley side, that is composed of streamflow and/or debris flow sediments 
and that has the shape of a fan either fully or partially extended. 
The main characteristic of these streams is a series of interlaced channels 
defined by bars and islands. Braided streams often occur in upper and middle 
zones of the watershed and usually involve gravel and cobbles, although 
braiding may also occur in sands. Scour and deposition often cause shifting of 
the main channel. 
Present in arid and semi-arid lands, these are streams that remain dry most of 
the time and carry flow only during flood events. Discharge and sediment 
transport can be substantial during flow episodes. Incising channels, width 
enlargement, and deposition are typical problems associated with arroyos. 
These occur primarily in the middle and lower portion of the watershed. The 
planform of the stream is characterized by meanders that erode the 
streambank in the outer side of the bend and deposit material on the inner 
side. Meanders may migrate in the floodplain and can often become cut off 
periodically when two bends advance toward each other and curvature 
becomes severe. Cut-off meanders become isolated features called "oxbow 
lakes" that eventually fill with sediment. Traces of old meanders (scrolls) are 
easily distinguishable in aerial photographs. Measures that alter the supply of 
water or sediment have the potential to change cross sections, planforms, and 
gradients. 
This term generically encompasses those streams whose natural 
configuration has been modified by human intervention. These modifications 
include straightening, channelizing, enlargement, and base level changes 
caused by regulation of the receiving stream. Increased runoff from 
surrounding development also results in modifications. 

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Table 2.2. Stream classification system based on USACE (1994) (continued). 

Stream Description 
Type 
Regulated 
Streams 
Deltas 
Underfit 
Streams 
Cohesive 
Channels 
Regulation of tributaries by upstream reservoirs or diversions reduces flood 
flows and increases baseflow. These changes in the flow regime translate 
into altered morphological activity. If regulation facilitates sediment deposition 
in the channel and vegetation growth, the stream cross section will be 
reduced. However, if the stream carries substantial sediment loads that 
become trapped in the reservoir, the stream may cause erosion downstream 
of the dam. 
These features occur on flat slopes of the lower portion of the stream where it 
empties into relatively quiescent water such as the ocean or a lake. Sediment 
deposition due to reduced velocity forces the river to split into distributaries 
whose base level rises as the delta progresses into the water body. Deltas 
also exhibit the formation of natural levees along the distributaries. 
These are streams common in regions whose landscape formed as a result 
of glacial activity. Underfit streams occur in wide valleys formerly shaped and 
occupied by larger streams, usually the outlet to glacial lakes. Underfit 
streams are also found in abandoned riverbeds or channels downstream from 
reservoirs. Flat slopes, low velocities, and established vegetation make 
underfit streams generally stable. 
These are channels cut in cohesive materials such as marine clays, silted 
lakes, and glacial till plains. In marine deposits, these streams behave 
somewhat like meandering alluvial streams, although the meanders are flatter, 
wider, more uniform, and usually more stable. In glacial till the planform tends 
to be irregular. 

2.4. Channel Form and Processes 
Alluvial channels are characterized by movable bed and streambanks that change configuration 
according to erosion and deposition phenomena. Therefore, these phenomena have a direct 
effect on the geometry of the channel in that they can introduce changes in the cross section, 
planform, longitudinal profile, and bed topography of the stream. Modification of these channel 
features may occur at various rates depending on the intensity and variability of imposed 
conditions and on the time scale under consideration. This section describes the characteristics of 
these processes and their relationship to channel geometry. 

2.4.1. Definitions 
The determination of erosion hazard areas involves evaluation of several erosional and 
depositional processes that take place either gradually over long periods or episodically during 
isolated events. These processes are defined as follows (Simons, Li & Associates, 1985): 

Degradation: Lowering of the channel bed on a substantial reach length occurring 
over a relatively long period of time in response to disturbances that 
affect general watershed conditions, such as sediment supply, runoff 

BACKGROUND OF RIVERINE EROSION 


volume, and artificial channel controls. 

Aggradation: Raising of the channel bed as a result of disturbances in watershed 
conditions that produce the opposite effect to those leading to 
degradation. 

General Scour: Lowering of the streambed in a general area as a consequence of a 
short-duration event such as the passage of a flood. Examples are 
the erosion zones near bridge abutments and those in the vicinity of 
gravel pits. 

Local Scour: Lowering of the bed due to localized phenomena such as vortex 
formation around bridge piers. 

Deposition: Raising of the streambed due to a specific episode. An example is 
the formation of a sand bar after a flood event. Deposition is used in 
this document as the counterpart to general scour. 

Lateral Migration: Shifting of the streambank alignment due to a combination of the 
above vertical erosional and depositional processes. The most 
common example is meander migration in the floodplain. Bank retreat 
due to mass failure is another example. 

Vertical variations in the streambed are additive in that the net change is the result of long- and 
short-term processes. For instance, a reach that is undergoing aggradation due to increased 
sediment yield from the watershed can also experience general and local scour as a 
consequence of flood events. 

2.4.2. Geomorphic Characteristics 
Streams are constantly progressing towards a state of dynamic equilibrium involving water and 
sediment. The geometry of the stream undergoes adjustments so that the transport capacity of 
the water is in balance with the sediment supply. As sediment is deposited, the newly created 
terrain may be colonized by vegetation that contributes to the stability of the soil. 

In earlier stages of the field of geomorphology, a stream that exhibited the ability to adjust itself 
was called graded (Davis, 1902) to signify that there was a balance between erosion and 
deposition. Mackin (1948) defined the graded stream as one in which, over a period of years, 
slope is delicately adjusted to provide, with available discharge and with prevailing channel 
characteristics, just the velocity required for the transportation of the load supplied from the 
drainage basin. The graded stream is a system in equilibrium; its diagnostic characteristic is that 
any change in any of the controlling factors will cause a displacement of the equilibrium in a 
direction that will tend to absorb the effect of the change. These concepts have been 
superseded, as more knowledge became available. For instance Leopold (1964) points to the 
excessive emphasis on slope and the fact that lateral movement is also important but is 
neglected. Nevertheless, the graded stream is the classical concept that expresses the idea of 
equilibrium in rivers. Although difficult to define, equilibrium is a condition that implies the ability of 
the channel to adjust to changes in the independent variables (sediment load and discharge) and 
to attain a stable cross-sectional and longitudinal geometry. As mentioned earlier, the concept of 
equilibrium is relative to the time scale under consideration. 

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Natural and artificial factors can upset this state of equilibrium. Earthquakes, large floods, climatic 
changes, urbanization, and construction of civil works in the waterway introduce changes in the 
sediment supply and amount of runoff reaching the stream. For example, development in the 
watershed typically increases the impervious area and hence the volume of runoff. Similarly, 
clear-cutting of forests increases the sediment yield to the stream. Dams trap sediment and 
have a regulating effect that increases low flows and reduces high flows. Channelization projects 
reduce channel length and therefore increase slopes. Diversions for irrigation or public water 
supply reduce the effective flows. Finally, an event such as a large flood can dramatically 
reshape the floodplain and increase channel width. 

Engineers are interested in predicting changes in stream geometry that could ensue from 
modifications introduced elsewhere in the channel or the watershed. Thornes (1977) points to 
several difficulties in obtaining reliable predictions. One is the fact that there are many ways in 
which a channel can respond to change (indeterminacy). Another is that the same final 
configuration can be the result of different sequences of phenomena (equifinality). Finally, 
changes may induce streams to cross certain thresholds and completely alter their morphology. 

Nevertheless, adequate predictions can be made using the regime concept, which states that, 
for relatively constant discharges, channels reach a state of equilibrium where no net scouring or 
deposition occurs. Natural streams are far from experiencing relatively constant discharges; 
however, regime theory is still suitable for geomorphic analyses as long as the predictions are 
applied to adjustments to equilibrium and not to the overall transient behavior (Chang, 1988). 
Under this assumption, the discharge is assumed to be an independent variable affecting channel 
variables such as width, depth, meander wavelength, and velocity. 

An example of a tool for geomorphic analysis is the widely-used Lane Relationship (Lane, 1955), 
which describes the state of equilibrium as 

Qsd . QS (1.1) 

Where Qs is the sediment discharge, d the median sediment size, Q the water discharge, and S 
the channel slope. A net increase in the left side of the relationship will cause aggradation. 
Conversely, a net increase in the right-side product will lead to degradation. The state of 
equilibrium can be viewed in two ways. The first is one in which the geometry is in balance with a 
moving sediment discharge. The second is more applicable to cohesionless sand beds in which 
the sediment is at the threshold of movement. However, both views imply that there is a 
threshold that controls the channel geometry (Leopold, 1964). 

The Lane Relationship does not include channel geometric parameters but still allows qualitative 
analysis of channel response. For example, a reservoir raises the base level for the stream and 
hence decreases the upstream slope. In consequence, aggradation must take place to restore 
the original slope. Downstream from the dam, the sediment supply will be drastically reduced; 
therefore, degradation will occur. The channel will become more sinuous to decrease the slope, 
and the size of the sediment transported will increase (Chang, 1988). 

Schumm (1969) used Lanes and other regime relations to determine qualitative responses to 
various changes in channel processes. These relationships are summarized in Table 2.3. 
However, these are only general trends; channel response depends on numerous factors and 
varies with the scale of the phenomenon. 

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Table 2.3. Schumms summary of channel responses (adapted from Chang, 1988). 

Process B D F ....S P 
Qincreases alone (e.g.,downstream ofa treatmentplant) . . . . . . 
Qdecreases alone (e.g.,downstream a diversion) . . . . . . 
Qsincreases alone (e.g.,downstreamof mining operations) . . . . . . 
Qsdecreases alone (e.g.,afforestationor vegetationbuffering . . . . . . 
along streams) 
Qand Qsbothincrease (e.g., during urbanization) . .. . . .. . 
Qand Qsbothdecrease (e.g., downstream ofa reservoir) . .. . . .. . 
Qincreases and Qsdecreases (e.g.,climate change toward a .. . . .. . . 
more humid pattern) 
Qdecreases and Qsincreases (e.g.,irrigationdiversionplus .. .. . .. . . 
clearing for farming) 

B = width;D =depth;F = Width/Depthratio;. = Meanderwavelength; S = Slope; P = Sinuosity. 

. = Increase;. =Decrease;.. = Indeterminate 
Table 2.3 indicates that, in general, width increases with aggradation and decreases in a 
degrading channel. The trends shown in the table are generalizations that must be validated 
using site-specific data. The trends also depend on the time scale involved. Analytical 
determination of responses in width, depth, slope, and bank geometry must be conducted with 
physically based relationships. Some of these relations are well known; for instance, friction 
formulas such as Mannings formula can be used to determine flow depths. Other variables such 
as channel width and planform are not so readily defined. 

2.4.3. Planforms 
Stream planform can vary in numerous complex patterns (Table 2.4). A comprehensive set of 
possible configurations was presented by USACE (1994), developed after Mollard and Janes 
(1984). 

There have been many attempts to derive relationships between planform and other geometric 
stream characteristics. Examples are the various equations relating meander wavelength and 
amplitude to variables such as radius of curvature and stream bankfull width. In general, the 
bankfull width is the top width resulting from the channel-forming discharge. Some of these 
expressions were later shown invalid for numerous situations (Chang, 1988). For instance, 
Leopold and Wolman (1960) concluded that the radius of curvature of a meander rc was given by 

r= 2.4B (1.2)

c 

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where B is the bankfull width. However, although derived from data ranging from laboratory 
flumes to the Mississippi River, this expression appears to be valid only for developed meanders. 
As an improvement, Hey (1976) presented an alternate relationship that included the arc angle 
subtended by the meander curve. In another approach, Anderson (1967) used laboratory data 
to relate meander wavelength to cross sectional area, Froude number, and discharge. However, 
Edgar and Rao (1983) showed that the results were not applicable to field data. The conclusion 
is that these types of relationships cannot be generalized for generic cases and that their 
applicability must be closely examined before making any decisions based on their results. 

Nevertheless, research efforts have been valuable in providing numerous observations throughout 
the years that allowed drawing of useful qualitative relationships. For instance: 

 Braided rivers are usually shallow and wide and tend to exhibit a stable width of the braided 
area, 
 Meanders can migrate downvalley or undergo a periodic process of bed cut-off, 
 Extreme meandering patterns are associated with flat slopes and low width-to-depth ratios, 
 The total length of a natural channel tends to remain constant. For example, subsequent 
lengthening of other bends or creation of new meanders compensates bed cut-off. 
Research indicates that, beyond these qualitative relations, a general methodology is not 
available to predict planform changes in all physical settings. The most effective tools rely on 
developing site-specific information based on observations conducted at various points in time. 

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Table 2.4. Planform patterns (USACE, 1994). 

CHANNEL APPEARANCE 
CHANNEL 
TYPE 
TYPICAL 
ENVIRONMENT 
TYPICAL BED AND 
BANK MATERIALS 
(a) Regular 
serpentine meanders 
(b) Regular sinuous 
meanders 
Lacustrine plain Uniform cohesive 
materials 
Tortuous or contorted 
meanders, no cutoffs 
Misfit stream in glacial 
spillway channel 
Uniform cohesive 
materials 
Downstream 
progression 
Sand-filled meltwater 
channel 
Slightly cohesive top 
stratum over sands 
Unconfined meanders 
with oxbows, scrolled 
Sandy to silty deltas 
and alluvial floodplains 
Slightly cohesive top 
stratum over sands 
Confined meandering Cohesive top strata 
over sand substratum 
in steep-walled trench 
Slightly cohesive top 
stratum over sands 
Entrenched meanders Hard till or uniform rock Till, boulders, soft rock 
Meanders within 
meanders 
Underfit streams in 
large glacial stream 
spillways 
Cohesive materials 
Irregularly sinuous 
meanders 
Thin till over bedrock in 
plains 
Hard and softer 
materials 
Wandering Foothills and mountain 
valleys 
Cobble-veneered sand 
Anastomosing Foothills, plains, sand 
bed or gravel paved 
rivers 
Sand and gravel 
Classical braided Glacial outwash, 
foothills 
Sand and gravel 
Irregular channel 
splitting 
Large rivers in bedrock Alternate sand, gravel 
and rock 
Rectangular channel 
pattern 
Jointed rocks, mostly 
flat-lying sedimentary 
rocks 
Rock 
Lakes and rapids (R) Till-veneered shield 
terrain 
Till, cobbles, boulders, 
hard rock 

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2.4.4. Cross Section 
The shape of cross sections depends on runoff, sediment yield, bed and streambank materials, 
and vegetation. A stream in a state of dynamic equilibrium may exhibit relatively constant cross 
sections that are substantially altered only by flood events of large magnitude. After these 
infrequent episodes, the stream tends to recover its original configuration. Sustained erosion and 
deposition are usually the result of a fundamental change in the fluvial system; for instance, 
increased flows or sediment input. Figures 2.3 and 2.4 show the main mechanisms of cross 
sectional changes. 

The schemes in Figure 2.3 correspond to (a) widening without incision, (b) scour greater than 
deposition in meanders, (c) erosion due to flow deflected by a growing bar, (d) catastrophic bank 
failure due to instability caused by incision, and (e) erosion due to flow acceleration caused by 
aggradation that reduces the flow area. In Figure 2.4, the mechanisms depicted correspond to 

(a) formation of berms that are later colonized by vegetation, (b) deposition greater than scour in 
meanders, and (c) abandonment of a secondary channel or anabranch, or attachment of bars to 
the floodplain. 
Figure 2.3. Main mechanisms of cross sectional widening (adapted from ASCE, 1998a). 

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Figure 2.4. Main mechanisms of cross sectional narrowing (adapted from ASCE, 1998a). 

The spatial and temporal variability in the geometry of cross sections has a major influence in 
hydraulic modeling and hence in floodplain delineation. Cross sections may be considered 
relatively constant for low flows, but large differences may be present for greater flows. In 
addition, cross sections may change significantly in a short distance. For example, a meandering 
stream has a deep portion near the outer bend whereas the straight portions between meanders 
are more uniform and shallower. Cross sectional changes may also take place in engineered 
channels. For instance, aggradation may occur in a channel that was improperly designed with 
excessive width. 

2.4.5. Slope 
As discussed earlier, downvalley slope can be considered an independent variable for the time 
scales of interest in engineering applications. The channel slope is usually flatter than the valley 
slope and can be subject to modifications as a result of aggradation and degradation. The most 
common example is the reduction in the channel slope as a result of meandering. If a stream is in 
a state of dynamic equilibrium and the channel is straightened by cutting off one or more 
meanders, the resulting slope will be steeper than the initial condition and the stream will tend to 
lengthen its path through erosion and deposition. 

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In general, channel slopes have a somewhat concave profile, which is steeper in the upper 
reaches of the watershed and becomes flat toward the mouth. This shape is the result of 
degradation in the steeper portions and aggradation in the lower regions. Near the mouth, the 
aggradation process commonly leads to the formation of deltas. 

Local features can affect this natural tendency, sometimes severely. Geologic controls created 
by rock outcrops in the channel can slow down the progression of erosion. A reservoir in the 
stream causes settling of sediment and reduces its availability downstream. Aggradation takes 
place upstream and degradation downstream of the dam. Channelization can cause headcutting, 
which encourages degradation moving upstream and aggradation proceeding downstream. The 
point along the stream marking the progression of upstream degradation is known as the nick 
point. Figure 2.5 shows how these processes affect channel slope. 

2.4.6. Roughness and Bed Configuration 
The ability of flow to remove from and deposit material on the streambed induces the formation 
of features that change the bed configuration and affect the roughness properties of the channel. 
Form roughness includes the bed, streambanks, and alignment. The total resistance is given by 
the grain roughness plus the form roughness. 

Depending on the local flow conditions, bed forms may include ripples, dunes and antidunes, and 
bars. Other sources of form roughness are vegetation, bank protection measures, rock 
outcrops, and scour holes. Figure 2.6 shows the most commonly found features and their 
variation as flow accelerates. Increasing velocities induce crossing of a threshold at which the 
bed becomes flat. These changes trigger further adjustments in other river geomorphologic 
variables (Chang, 1988). 

Figure 2.5. Processes affecting channel slope (USACE, 1994). 

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Figure 2.6. Various bed forms and their progression under accelerating flow (USACE, 1994). 

Ripples are the smallest bed forms and have wavelengths of approximately one foot and heights 
of less than one inch. Their shape is smooth and almost symmetrical although the slope 
upstream could be milder than downstream. 

Dunes are larger than ripples and are usually accompanied by gravity water waves, which are 
out of phase with the dunes so that the water depth is greater over the troughs than over the 
crests. The shape is usually triangular with a mild upstream slope and a steeper downstream 
slope nearly equal to the angle of repose of the bed material. Ripples are sometimes found on 
the upstream slope. Eddies take place on the downstream side and lead to surface boils and 
turbulence. 

Antidunes are bed forms in phase with the gravity water waves and are sometimes called 
standing waves. Antidunes are associated with greater flow velocities. For high velocities, the 
water waves become unstable and break towards the upstream direction. The shape of 
antidunes varies from triangular to sinusoidal as the flow velocity increases. 

Bars are bed forms of dimensions greater than dunes and of the order of the channel width. 
Their vertical dimensions may be comparable to the flow depth. The term point bar usually 
refers to the depositional area near the inside of a bend. Point bars may change configuration but 
remain at the same location. Alternating bars refers to the periodic system of bars near 
alternate channel banks. These bars introduce sinuosity to the flow paths even if the channel is 
relatively straight. Alternating bars are much narrower than the channel width and tend to move 
slowly downstream. 

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Beds with coarse materials tend to develop a pool-and-riffle structure. The pools have milder 
slopes and finer material whereas the riffles are steeper and contain coarser material. Armoring 
is also common in these types of channels. 

The formation of bed forms explains the hysteresis effects noted in rating curves. A flood event 
can reshape a channel reach to the point that the roughness characteristics when the flood is 
receding are different from those during the initial stages. Therefore, for the same flow, the stage 
may be different depending on the hydrograph limb, and the rating curve may exhibit a loop. 

Three empirical methodologies are available to predict bed forms: Simons and Richardson 
(1961), Athaullah (1968), and van Rijn (1984). The methodologies are applied through diagrams 
that, for a given particle size and flow parameters such as Froude number, stream power, or 
shear stress, indicate the type of bed form likely to be present. Chang (1988) presents methods 
to estimate bed-form dimensions and include bed-form roughness in stage-discharge friction 
formulas. 

2.4.7. Mechanics of Sediment Transport 
The boundaries of the stream channel are usually soil material with a given resistance to erosion. 
Bed material can range from large boulders to very fine clay particles. In general terms, 
sediment can be cohesive, including clay, silt, and mixtures, or noncohesive, including sand, 
gravel, and larger particles. Transport of noncohesive materials is strongly dependent on particle 
size. The entire size distribution of the material is needed to ascertain its erodibility. The bond 
between particles in cohesive soil dictates its resistance to erosion and is far more important than 
size distribution. However, size becomes important once the material has been eroded and is 
transported by the flow (Simons, Li & Associates, 1985). 

An important sediment transport process is the development of an armor layer in beds containing 
gravel and cobbles. Water flowing over the mixture of sand and coarser material lifts the smaller 
grains and leaves an upper layer or armor of large particles. This armor protects the underlying 
sediment from further erosion and controls the subsequent behavior of sediment transport. A 
flood event of large magnitude can disturb the protective layer, and the armoring process will start 
again. 

Sediment transport exerts substantial control over morphology and channel geometric 
configuration. An indicator of this influence is the sediment load, which is the rate at which 
material moves in the stream as quantified in units of weight per unit time. The transport rate is 
closely dependent on the water discharge. The sediment load has several components. The 
bed load is that portion of the sediment that moves along the bottom by sliding, rolling or saltation. 
The suspended load is material carried in suspension and consists of particles that can be found 
in the bed, which become suspended due to turbulence. The sum of bed load and suspended 
load is known as bed-material load or total load. As will be seen later, there are equations to 
estimate bed loads, suspended loads, and bed-material loads. In addition, the stream carries the 
wash load, which is made of fine materials not found in the bed. The wash load does not depend 
on the carrying capacity of the stream but on the amount supplied by the watershed. There are 
no general expressions to predict the wash load, although site-specific regression equations may 
be available. 

Quantification of sediment transport is fraught with uncertainty because of the complexity of the 
phenomenon and its inherent spatial and temporal variability. Existing mathematical 
representations have relied heavily on experimental results. Chang (1988) groups the available 
sediment transport formulas according to the approach used to derive them. Three major 
approaches have been used: shear stress, power, and parametric. Formulas can also be 

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grouped according to the component of the total load they attempt to quantify: bed load, 
suspended load, or bed-material load. Table 2.5 summarizes the more commonly used 
formulas. 

Despite the intense efforts expended in the development of these formulas, evaluation against 
field data indicates that they commonly overpredict or underpredict sediment loads by orders of 
magnitude of actual measured sediment transport rates (Gomez and Church, 1989; Yang and 
Wan, 1991). This discrepancy is likely due to imperfect knowledge of the physics of sediment 
transport and also to the extensive variability and heterogeneity in hydrologic and geologic factors. 
For these reasons, no one formula is better than the others. Selection of a sediment transport 
formula must be dictated by how well the conditions of the problem at hand match the 
assumptions underlying the formula. If possible, applicability of the formula should be verified with 
site-specific field data. Collection of water samples for sediment analysis needs to be conducted 
carefully and using proper sampling protocols. When a water sample is collected, the sediment 
will include both the suspended and wash loads; therefore, it is necessary to separate the two 
components to verify sediment transport formulas. 

Table 2.5. Sediment transport formulas and classifications. 

Criteria Grouping 
Sediment Transport Formula 
DuBoys (1879)
Shields (1936)
Einstein Bed Load (1950)
Einstein Suspended Load (1950)
Meyer-Peter-Muller (1948)
Einstein-Brown (1950)
Parker et al. (1982)
Engelund-Hansen (1967)
Ackers-White (1973)
Yang (1972)
Colby (1964) 
Approach Shear Stress 
Power 
Parametric 
V V V V V V 
V V V 
V 
Load Component Bed Load 
Suspended Load 
Bed-Material Load 
V V V 
V 
V V V 
V V V V 

2.5. Evaluation Methods for Channel Response to Imposed Changes 
The previous discussion presented the generally accepted concept that streams tend to a state 
of dynamic equilibrium in which the geometry adjusts over time in accordance with discharge and 

BACKGROUND OF RIVERINE EROSION 


sediment influx patterns. Various geomorphologic variables were examined and qualitative 
relationships were discussed to predict channel geometry changes. This section presents a 
review of currently available methods to predict the magnitude and direction of these geometric 
changes. 

2.5.1. Equilibrium Approaches 
Several equilibrium-based techniques have been used to describe cross section geometry. 
These techniques range from regime relationships to complex computational models. This 
section presents a brief description of these techniques. 

The basic approaches used in the derivation of these techniques can be divided into three broad 
areas: regime theory, extremal hypothesis, and tractive force (ASCE, 1998a). The first two 
categories correspond to a state of dynamic equilibrium in which sediment moves but the system 
tends to a stable configuration. Tractive force methods refer to a state of static equilibrium in 
which sediment particles are at the threshold of motion. 

Regime relationships are based on empirical methods, and numerous equations have been 
published in the literature. A few of the most widely used equations for sand beds are those 
proposed by Lindley (1919), Lacey (1920), Simons and Albertson (1963), and Blench (1969). 
Hey and Thorne (1986) present a summary of gravel-bed formulas. Regime relationships have 
significant shortcomings. Examples of these limitations are: 

 The relations can be used reliably only in the geographic region where the basic data were 
collected. 
 Lateral migration is generally not included in the determination of hydraulic geometry. 
 The use of one representative discharge (e.g., the bankfull discharge, which is generally 
considered the channel-forming discharge) as one of the main independent variables may be 
insufficient to evaluate the effect of discharges in modifying the cross section. In addition, 
there is uncertainty in the determination of this representative discharge. 
 Vegetation, material resistance, sediment loads, groundwater levels and other variables 
have been sometimes included in the determination of geometry, but there is no consistency 
among researchers regarding their significance. 
 Sediment load is considered an independent input variable, but its magnitude is often 
unknown or uncertain at best. 
Extremal hypotheses are additional mathematical conditions that complement the fundamental 
fluid mechanics equations to allow computation of all variables in sediment transport. These 
conditions are usually expressed in terms of minimization or maximization of quantities such as 
stream power, energy dissipation, or sediment concentration (ASCE, 1998a). For example, 
Chang (1988) establishes that the state of dynamic equilibrium in a channel reach requires 
continuity of sediment, minimum stream power per unit channel length, and uniform streamwise 
power expenditure (energy slope). The computer program FLUVIAL-12 was developed on the 
basis of these assumptions (Section 3.5.2). Another application of extremal approaches can be 
found in Yang et al., (1981). 

Tractive force methods define a set of conditions that are conducive to mechanical stability of 
cross sections. These conditions arise from the momentum equation applied locally to particles 
on the boundary and requiring equilibrium among gravity, friction, and hydrodynamic forces. The 

BACKGROUND OF RIVERINE EROSION 


previously discussed Lanes Relationship (Lane, 1955) is one result from this approach. Tractive 
force methods imply a threshold channel geometry that can change in response to any variation in 
the force balance. Limitations associated with tractive force formulas are: 

 Channels must be straight. 
 Secondary flows are negligible. This condition limits applications in meandering channels. 
 Sediment must be noncohesive and uniform throughout the channel. 
 The theory does not allow sediment transport, which contradicts field observations. 
ASCE (1998a) reports developments that enhanced the applicability of tractive force methods. 
Parker (1978) extended the formulation to allow sediment transport. The resulting equations are 
complex and were solved only for special conditions. Ikeda et al. (1988) considered sediment 
heterogeneity, Ikeda and Izumi (1990) included the effect of vegetation, and Diplas and Vigilar 
(1992) applied numerical solutions of the differential equations to solve for the shape of the cross 
section. 

All three equilibrium approaches (regime theory, extremal hypothesis, and tractive force) have 
limitations when applied to channels of interest in engineering applications. For the purposes of 
this report, the two most important limitations are: 

 Watershed land-use changes impose ever-changing scenarios on the fluvial system to which 
the streams are constantly adjusting. 
 Streams in arid climates are subject to drastic changes in response to infrequent flow events. 
For example, some alluvial fans exhibit random behavior in the paths preferred by flow 
episodes. An equilibrium cross section cannot be defined for these situations. 
In addition, research indicates that changes occur over a wide range of time scales and that, in 
addition to fluvial processes, numerous climatic, environmental and geotechnical factors are 
involved. Induced effects include changes in roughness, bed material composition, vegetation 
cover, and planform (ASCE, 1998a). These observations suggest that prediction of cross 
sectional adjustments can only be accomplished for site-specific conditions after the most 
significant geomorphological factors have been identified. Therefore, any prediction of channel 
geometry should be based on sound field observations. 

2.5.2. Fluvial Hydraulics 
This section introduces the complexities and modeling difficulties in representing the mechanisms 
involved in fluvial hydraulics. A general review of the subject can be found in Knight and Shiono 
(1995). 

Fluvial processes are characterized by complex fluid mechanics phenomena. This complexity is 
the result of spatial and temporal variabilities, some of which have been discussed in previous 
sections. The following is a summary of these variabilities (ASCE, 1998a): 

Topography: Variability is due to planform configuration and the presence of bed forms. 
In addition, overflow into the floodplain during large-magnitude events can 
produce geomorphologic changes, which are very different from those 
stemming from in-channel flows. 

BACKGROUND OF RIVERINE EROSION 


Roughness: Varies according to the different materials along the banks and the bottom 
and the additional resistance of bed forms. 

Sediment: Materials in the bed and the banks are heterogeneous and may include 
both cohesive and noncohesive soils. Armoring produces a sediment 
distribution different from that of the bulk material. 

Water and These quantities are governed by unsteady phenomena. In addition, 

Sediment sediment supply is the limiting factor and does not depend solely on 

Transport: transport capacity. Therefore, floods of similar magnitude may carry 

different amounts of sediment. 

History of The current geometric configuration of channels is a cumulative result of 

Events: past events. 

Models have been developed to simulate some of the processes; however, the need to simulate 
variations in the cross section geometry implies that these models must be capable of some two-
dimensional representation. Three-dimensional models are required to simulate important 
mechanisms such as secondary currents in bends; however, these models are not cost-effective 
for most engineering applications. Therefore, two-dimensional models remain the most 
appropriate to make predictions on cross sectional changes. 

One of the most significant parameters associated with movement of sediment is the shear 
stress because its magnitude determines if bed and streambank materials will be eroded. In 
consequence, shear stress is a critical parameter in the definition of cross section geometry. 
ASCE (1998a) presents a summary of research efforts directed at determining the value of the 
shear stress. An important phenomenon is the redistribution of shear stresses under overbank 
flow conditions during large floods, which tends to affect the sediment transport rate. Depending 
on the direction of the effect, this stress redistribution can lead to erosion or deposition and thus 
to significant cross section adjustment. 

Variations in the longitudinal direction present computational challenges. For instance, many 
simulation models depend on the definition of cross sections representative of channel reaches. 
Therefore, some kind of averaging must be conducted to produce computational sections, which 
are not what exists in the channel in reality. The need for a similar procedure arises when 
selecting a value for the friction slope representative for the channel reach. This slope directly 
affects stream power, which is a fundamental quantity in movable-boundary models based on 
extremal hypotheses. As a consequence of these facts, any prediction in cross section changes 
should not be applied to a particular section but must be associated with the reach. In addition, 
streamwise averaging can be particularly complicated during overbank flow due to new flow 
patterns that may develop (ASCE, 1998a). For example, in meandering streams the sinuosity of 
the flow paths can vary depending on the elevation of the water surface. 

In principle, processes occurring at the banks are directly responsible for changes in the cross 
section geometry. Velocity, shear stress, secondary currents, and turbulence in the near-bank 
area control these processes. Knowledge of these processes is imperfect, as evidenced by the 
above-mentioned difficulties in ascertaining the physics of these phenomena. Tools based on 
empirical analyses are available to determine the magnitude and distribution of shear stresses 
(ASCE, 1998a). Within the computational limitations, determination of shear stresses is an 
important step. However, although the shear stress is the most important variable, it is not the 

BACKGROUND OF RIVERINE EROSION 


only one, and its role must be analyzed in conjunction with a general geomorphic context, the 
channel type, and the location in the watershed. 

2.5.3. Streambank Stability 
In addition to hydrodynamically induced erosion and deposition, another important mechanism in 
defining changes in cross section geometry is the occurrence of mass failure of the streambanks 
(Photograph 9). ASCE (1998a) discusses seven factors affecting the loss of material in the 
streambanks: 

Bank Erosion: This erosion is the result of hydrodynamic drag, which detaches particles 
and mixes them with the flow. Grain size and its distribution and the bonding 
mechanism among particles dictate resistance to erosion. Erodibility varies 
in space due to the various strata along the streambank and the presence 
of local heterogeneities. Noncohesive materials resist primarily by means of 
their submerged weight and, to some degree, from grain interlocking. 
Cohesive materials resist through particle electrochemical bonding. Critical 
shear stresses for cohesive materials are difficult to determine; however, 
research indicates that they are greater than for noncohesive particles. 
Consequently, cohesive banks tend to erode less than noncohesive banks. 

Weakening of Erodibility can be increased by weathering processes that tend to loosen 
Erosion particles. Some of these processes include, freeze-and-thaw cycles and 
Resistance: cracking caused by swelling and shrinkage. 

Mass Failure: Geotechnical stability of banks is similar to slope stability and depends on 
the height of the bank and cohesion and friction forces. Failure can occur as 
a plane slip or rotational slip of the soil wedge (Thorne, 1982) or toppling of 
a vertical slice triggered by a tension crack (Thorne and Tovey, 1981). 
Another possibility is the failure of a cantilever configuration associated with 
toe erosion (Thorne and Tovey, 1981). Mechanisms that trigger mass 
failure include incision of the channel that increases bank height or, as 
mentioned earlier, erosion of the bank toe. Figure 2.7 shows schematics of 
these failure mechanisms. Applications of these failure schemes can be 
found in Darby and Thorne (1996a), Thorne and Osman (1988), and 
Osman and Thorne (1988). 

Basal Endpoint This factor refers to the ability of fluvial erosion to remove the material 

Control: deposited by mass failure. If the flow can remove this material and continue 
eroding the bank, the section will be enlarged. If the flow is unable to 
remove all of the material, a berm will be formed and the section will be 
narrowed. 

Vegetation: Vegetation tends to increase the stability of material and hence reduce 
erosion rates. Vegetation may aid or prevent mass failure. Despite the 
evidence that vegetation influences soil erosivity and erodibility and mass 
failure, there is insufficient understanding about the complexity of this role. 

Seepage: Pore pressure increases can be triggered by rapid drawdown of water in 
the channel, leaving saturated banks. The water then seeps from the banks 
into the stream and may cause piping that leads to erosion of soil layers. 

BACKGROUND OF RIVERINE EROSION 


Bank Advance: This term encompasses all mechanisms that lead to narrowing of the 
section due to deposition. Morphological changes can cause aggradation 
and creation of berms. Colonization by vegetation improves the stability of 
these berms. 

An important limitation of the Osman-Thorne stability analysis procedures is the fact that pore-
water and hydrostatic confining pressures are not included. Another limitation is that the plane of 
failure is assumed to pass through the toe of the embankment. This prevents applicability to 
upper bank failures, which have been found to be common (ASCE, 1998b). Simon et al. (1991) 
solved some of these shortcomings. 

Photograph 9. Mass failure of streambank caused by minor flooding event. 

BACKGROUND OF RIVERINE EROSION 


COHESIONLESS 
COHESIVE 
COHESIVE 
Figure 2.7. Mechanisms of bank failure. (a) planar slip, (b) rotational slip, (c) toppling, (d) cantilever shear, 

(e) cantilever rotational, (f) cantilever tensile (ASCE, 1998a). 
2.6. Computational Models 
The foregoing sections evidence the fact that quantification of geometric changes in channel 
geometry is a difficult task and that prediction tools need to be improved to obtain reliable results. 
Several techniques have been introduced in previous sections. This section presents a summary 
of currently available computational models to predict geometry changes. The material in this 
section is a summary of a comprehensive review prepared by the American Society of Civil 
Engineers Task Committee on Hydraulics, Bank Mechanics, and Modeling of River Width 
Adjustment (ASCE, 1998b). 

Simulation of these complex temporal and spatial phenomena depends on the formulation of a 
conceptual model to represent the physical setting. Spatial conceptualization can be 
accomplished by selecting cross sections at given intervals to represent the channel configuration. 
For most practical applications, the simulation time spans years or decades. The soil material 
must be represented by a discrete selection of representative grain sizes. 

The next step is to select a computational model for the problem. Models can range from 
empirical equations based on field data to physically based numerical models. Numerical models 
are comprised of modules that simulate individual but coupled processes. Some of these 
modules are: 

BACKGROUND OF RIVERINE EROSION 


Hydraulic Module: Quantifies the fluid mechanics variables in the flow field, 
chiefly velocities and water-surface elevations. 

Bank Process Module: Defines streambank retreat or advance as a result of 
hydrodynamically induced erosion and deposition. 

Sediment Transport Module: Simulates the movement of sediments through the channel 
reach. 

ASCE (1998b) reviews 12 numerical models available to the engineering and scientific 
communities to simulate channel geometry changes. The following paragraphs describe the main 
features of these models. 

2.6.1. Modeling Approach 
The 12 models are based either on a geofluvial or extremal approach. The geofluvial approach 
couples flow and sediment transport modules with bank erosion and failure modules. As 
described in Section 2.5.1, the extremal approach is an equilibrium approach based on 
minimization of energy expenditure that tends to yield the final configuration of the channel. The 
models are listed in Table 2.6 along with the modeling approach they follow. 

Table 2.6. Twelve computational models to predict channel geometry changes (ASCE, 1998b). 

Model Approach Reference 
Darby-Thorne 
CCHEBank 
Kovacs-Parker 
Wiele 
RIPA 
Simon et al. 
Pizzuto 
STREAM2 
GSTARS 
FLUVIAL-12 
Alonso-Combs 
WIDTH 
Geofluvial, cohesive bank 
Geofluvial, noncohesive bank 
Geofluvial, noncohesive bank 
Geofluvial, noncohesive bank 
Geofluvial, cohesive bank 
Geofluvial, cohesive bank 
Geofluvial, noncohesive bank 
Geofluvial, cohesive bank 
Extremal hypothesis 
Extremal hypothesis 
Geofluvial, cohesive bank 
Geofluvial, cohesive bank 
Darby and Thorne (1996b) 
Li and Wang (1993) 
Kovacs and Parker (1994) 
Wiele (1992) 
Mosselman (1992) 
Simon et al. (1991) 
Pizzuto (1990) 
Borah and Bordoloi (1989) 
Yang et al. (1988) 
Chang (1988) 
Alonso and Combs (1986) 
Osman (1985) 

BACKGROUND OF RIVERINE EROSION 


2.6.2. Hydraulics 
Models can be one-, two-, or three-dimensional. A fourth category, quasi two-dimensional, 
represents the cross section with a series of one-dimensional stream tubes. 

Models can be steady or unsteady. An intermediate approach is to discretize the hydrographs 
into steps where flows are assumed steady. This approach is termed stepped hydrograph in 
ASCE (1998b). Other special enhancements to the hydraulic module include secondary flows, 
near-bank lateral shears, and spatial and temporal variability of friction factors. 

Friction losses can be modeled using a variety of flow resistance formulas. An important 
enhancement would be the inclusion of bed-form resistance, which is not implemented in any of 
the models investigated. 

Table 2.7 summarizes the main features of hydraulic modeling for the 12 models. 

Table 2.7. Features of flow routing submodels of reviewed models (ASCE, 1998b). 

Model Dimension Discharge Secondary Lateral Friction Flow 
Variation Flows Shear Factor Resistance 
With Time Formulasb 
Darby Thorne Quasi 2Da Stepped No Yes Time and Strickler 
hydrograph space 
variable 
CCHEBank 3D Unsteady flow Yes Yes Constant Keulegan 
Kovacs-
Parker 
2D Steady flow No Yes Constant Keulegan 
Wiele 2D Steady flow No Yes Constant Keulegan 
RIPA 2D Stepped 
hydrograph 
Yes No Constant Specified 
Simonet al. Quasi2Da Stepped No No Time and Strickler, 
hydrograph space Darcy, and 
variable Chezy 
Pizzuto 2D Steady flow No Yes Constant Einstein 
STREAM2 1D Stepped 
hydrograph 
No No Constant Specified 
GSTARS Quasi 2Da Stepped No No Time and Strickler, 
hydrograph space Darcy, and 
variable Chezy 
FLUVIAL-12 1D Unsteady flow Yes No Time and Strickler and 
space Brownlie 
variable 
Alonso-
Combs 
1D Stepped 
hydrograph 
No No Constant Specified 
WIDTH 1D Stepped No No Time and Strickler 
hydrograph space 
variable 

a

Quasi2D models refer to those models that simulate lateral variation of bed topography through use of multiple one-dimensional 
stream tubes. 

b

Strickler= Strickler (1923); Keulegan= Keulegan (1938); Einstein= Einstein (1950); Brownlie = Brownlie (1983). None of these 
formulas account for the effects of bed forms. 

BACKGROUND OF RIVERINE EROSION 


2.6.3. Sediment Transport 
Sediment transport is usually estimated by applying the velocity field to the bed material 
properties. The sediment continuity equation is used to determine if sediment will be eroded or 
deposited. 

The models reviewed use a variety of sediment transport methods (Table 2.4). Some models 
only allow the use of one formula, whereas others offer multiple choices to the user. As 
discussed in Section 2.4.7, sediment transport calculations should use formulas that were derived 
with materials that resemble the conditions of the problem to be solved. Even under these 
circumstances, large variations in the sediment transport rates should be expected. 

Changes in the bed topography are a result of the net accumulation or removal of material, which 
can be computed through the sediment continuity equation. Net flux across the channel reach 
should account for longitudinal and transversal variations of the sediment influx. Nevertheless, a 
simplified version of the continuity equation is routinely implemented along with assumptions to 
distribute the sediment loss or gain along the channel reach and across the flow section. These 
assumptions are more restrictive for one-dimensional models. Only three of the models reviewed 
do not allow variations of the sediment influx in the longitudinal direction. However, only six out of 
the twelve allow transversal sediment influx variations. 

All of the models perform bed load computations. Seven models allow computation of 
suspended loads. Sorting of sediments is related to armoring and is an important mechanism 
because it dictates the hydraulic resistance properties of the bed. Only six of the models allow 
for modeling of sorting. 

Table 2.8 summarizes the properties of sediment transport equations used in the models 
reviewed. 

2.6.4. Stability of Streambanks 
Table 2.9 compiles the key features of the reviewed models related to the handling of mechanical 
stability of the streambanks. 

As described in earlier sections, changes in streambank geometry are caused by gradual erosion 
and deposition and mass failure episodes distributed along the length of the river reach. Most 
models simulate bank retreat due to shear erosion, but only one of the models reviewed 
simulates channel narrowing due to deposition. Models based on extremal hypotheses 
(GSTARS and FLUVIAL-12) do not have modules to simulate bank stability processes. 

Simulation of fluvial entrainment of bank materials is crucial to determine the rate of bank 
movement as a result of both mass failure, gradual erosion, and deposition. Only one of the 
models reviewed does not simulate this process. None of the models include the effects of 
vegetation, wave action, seepage-induced piping, or other nonfluvial processes on bank advance 
or retreat. None of the models has the capability to account for layered or otherwise 
heterogeneous streambank materials. Erosion of cohesive materials is simulated by some of the 
models, but the empirical methods employed are not very reliable (ASCE, 1998b). Mass failure 
of cohesive streambanks is simulated in several of the models using the procedures developed 
by Osman (1985) and Osman and Thorne (1988). Some of the models can simulate planar and 
curved failure surfaces. 

BACKGROUND OF RIVERINE EROSION 


Table 2.8. Sediment transport characteristics of reviewed models (ASCE, 1998b). 

Model Transport Routing Bed Stream Trans-Verse Bed Susp. Sorting 
Equations Method Material Wise Flux Flux Load Load 
Difference Difference 
Darby-Thorne Engelund and 
Hansen (1967) 
Quasi 2D Sand Yes Yes Yes Yes Yes 
CCHEBank Meyer-Peter-
Muller (1948) 
2D Gravel Yes Yes Yes No No 
Kovacs-
Parker 
Kovacs and 
Parker (1994) 
2D Gravel No Yes Yes No No 
Wiele Parker (1979) 
and Meyer-
Peter-Muller 
2D Sand/ 
Gravel 
No Yes Yes No No 
(1948) 
RIPA Engelund and 
Hansen (1967) 
and Meyer-
Peter-Muller 
2D Sand/ 
Gravel 
Yes Yes Yes No No 
(1948) 
Simonet al. Yang (1973, 
1984), Ackers 
and White 
Quasi 2D Sand/ 
Gravel 
Yes No Yes Yes Yes 
(1973), and 
Engelund and 
Hansen (1976) 
Pizzuto Parker (1983) 2D Sand No Yes Yes No No 
STREAM2 Yang (1973), 
Graf (1971) and 
Meyer-Peter-
Muller (1948) 
1D Sand/ 
Gravel 
Yes No Yes Yes Yes 
GSTARS Yang (1973, 
1984), Ackers 
and White 
Quasi 2D Sand/ 
Gravel 
Yes No Yes Yes Yes 
(1973), and 
Engelund and 
Hansen (1976) 
FLUVIAL-12 Yang (1973), 
Parker et al. 
1D Sand/ 
Gravel 
Yes No Yes Yes Yes 
(1982), Ackers 
and White 
(1973), Engelund 
and Hansen 
(1967), and Graf 
(1971) 
Alonso-
Combs 
Alonso et al. 
(1981) 
1D Sand/ 
Gravel 
Yes No Yes Yes Yes 
WIDTH Engelund and 
Hansen (1967) 
1D Sand/ 
Gravel 
Yes No Yes Yes No 

BACKGROUND OF RIVERINE EROSION 


Table 2.9. Streambank stability characteristics of the reviewed models (ASCE, 1998b). 

Model Deposition Fluvial Type of Bank Longitudinal Cohesive Non-
Entrainment Failure Extent of Failure cohesiveb 
Included 
Darby-Thorne No Yes Planar/Curved Yes Yes No 
CCHEBank Yes Yes None No No Yes 
Kovacs-Parker No Yes None No No Yes 
Wiele No Yes None No No Yes 
RIPA No Yes Planar No No No 
Simonet al. No Yes Planar No No No 
Pizzuto No Yes None No No Yes 
STREAM2 No Yes Planar No No No 
GSTARS a a a a a a 
FLUVIAL-12 a a a a a a 
Alonso-Combs No No Planar No No No 
WIDTH No Yes Planar/Curved No No No 

a

 Models based on extremal hypotheses do not include bank mechanics modules 

b

 Assumed uniform in size 

For noncohesive banks, an important improvement was the introduction of the vectorial bed load 
equation by Kovacs and Parker (1994), which extended sediment transport computations for 
large angles. With this capability, the method can be applied to longitudinal and transversal 
slopes up to the angle of repose of the material. Nevertheless, the method could be further 
improved with the coupling of two- or three-dimensional flow models to describe the near-bank 
flow field. 

All but one model assume that mass failure occurs uniformly along the reach length. This 
assumption could overpredict mass waste material and was relaxed in the Darby-Thorne model 
with the introduction of a probability of failure. 

Models FLUVIAL-12 and GSTARS based on extremal hypothesis attempt to predict equilibrium 
channel morphology, and hence the result is total change in the cross sections instead of the rate 
at which this change occurs. A portion of the total change is assigned to the banks and the 
remainder to the bed. However, the user must specify the distribution of the streambank change 
between the two banks. According to ASCE (1998b), the procedures implemented in these 
models are plausible for noncohesive materials but inapplicable to cohesive sediments. 

2.6.5. Future Directions 
The ASCE (1998b) review of available models to simulate geometry changes concludes the 
following: 

BACKGROUND OF RIVERINE EROSION 

41 


 Some level of prediction is possible with the existing knowledge and tools. 
 Of the two major approaches, extremal and geofluvial models, the latter seems to be the 
most promising although at this time the models have not fully transitioned to the engineering 
community. 
 No one of the existing models seems to be universally applicable to all cases. 
 Few appropriate data sets exist for rigorous testing of available models. 
The ASCE (1998b) review lists a series of topics that are critical to improve current knowledge of 
fluvial geometry adjustment: 

 Improvement of currently available models through use of better submodels for hydraulics 
and bank mechanics. 
 Acquisition of comprehensive field and laboratory data for model testing and verification. 
These data must include cross sectional surveys, bed material size distribution and 
geotechnical characteristics, and discharge and sediment transport records. 
 Better characterization of the boundary shear stresses. 
 Improved bank mechanical stability modules to properly simulate erosion of cohesive 
material and distribution across the channel. 
 Definition of the longitudinal extent of mass failure. 
 Definition of the role of vegetation on flow and geotechnical stability. 
 Quantitative models for bank advance and retreat. 
 Improvement in the definition of shear stresses for entrainment of cohesive materials. 
 Better understanding of the dominant discharge. 
 Expansion of bank failure mechanisms to other commonly observed collapse scenarios. 
 Understanding of the role of overbank flows in cross sectional adjustment. 
BACKGROUND OF RIVERINE EROSION 


3. Literature Review 
3.1. Research Sources and Search 
The study team began with an in-depth search of existing methodologies in use to predict riverine 
erosion with emphasis on case studies. A search on the Internet led to identification of several 
databases that contained information on the subject. These databases included American 
Geophysical Union, American Society of Civil Engineers, U.S. Geological Survey, Uncover 
(commercial database) and university library databases. The publication types included manuals, 
journal articles, proceedings, case studies, reports, and textbooks. In addition, applicable literature 
was also found through the teams knowledge and experience in this subject. 

There were three levels of screening: an initial screening for relevance, a second screening for 
apparent applicability, and final screening to select key documents. The initial screening was 
performed using key words in database search queries. This initial screening yielded several 
hundred articles. Abstracts were read when available, and documents were selected for apparent 
applicability. 

This second screening yielded 108 pieces of literature (see Appendix). These pieces of literature 
were obtained and briefly reviewed. From this list of 108, 35 articles were selected and then 
further culled to12 that appeared to have the most promise. 

3.2. Literature Summary 
To document the results of the literature search, a database application was specifically developed 
using Microsoft Access. The documents found in the second level of screening (108) were 
entered into the database using two forms. The first form contains initial basic data elements 
including: article title, reviewer's initials, review date, where it appeared (journal, periodical, etc.), 
authors, date of article, number of pages, initial reaction to article, and an assessment as to 
whether or not the article requires further investigation. If the article required more in-depth 
investigation, the second, more detailed form was completed. 

The second form included the following elements: geographical location, state and flooding 
source, category of analysis (historic, geomorphic, mathematical), regulatory application or 
potential cost information, mapping feasibility, limitations or qualifications, short summary of the 
document, key words, and a list of relevant references mentioned in the article. 

3.3. Selected Documents 
From the final screening, 35 documents remained, and additional criteria further reduced this set 
of documents to 12. These criteria included geographic coverage, practical applicability, 
mappability, and potential to derive cost information. An attempt was also made to have 
representation among the three categories of analysis: historical, engineering and geomorphic 
methods, and mathematical models. Each of the 12 documents was studied in detail. A 
summary for each is presented in Chapter 4. 

ANALYSIS OF CASE STUDIES 


4. Analysis of Case Studies 
The twelve case studies selected covered portions of the Midwestern and Western United 
States. Figure 4.1 shows approximate locations of the study areas. The states involved are: 

 Arizona 
 California 
 Illinois 
 Iowa 
 Minnesota 
 Missouri 
 Nebraska 
 New Mexico 
 Texas 
 Utah 
 Washington 
 Wisconsin 

A summary of basic characteristics of the study sites is shown in Table 4.1. 

Albuquerque, NM 
Arroyo Studies 
Austin, TX Erosion 
Studies 
Utah River Stability 
Studies 
King Co., WA 
Channel Migration 
San Diego Co., CA 
Alluvial Studies 
Minnesota Stream 
Meander Study 
Rillito Creek, Tucson, AZ 
Probabilistic Study 
Santa Cruz River, AZ 
Geomorphologic Study 
Platte River, NE 
Geomorphic Study 
Iowa Streambank 
Erosion Study 
U. Mississippi/ Illinois 
Waterway Study 
Arizona Lateral 
Migration Standards 
Figure 4.1. Location of case studies. 

ANALYSIS OF CASE STUDIES 


Table 4.1. Summary of case studies 

Case Study Title Location Drainage Area (mi2) Stream Type Study Method Category 
AMAFCA Sediment and 
Erosion Design Guide 
Inventory and Analysis of 
Stream Meander 
Problems in Minnesota 
A Probabilistic Approach 
to the Special 
Assessment of River 
Channel Instability 
Geomorphology and 
Hydrology of the Santa 
Cruz River, Southeastern 
Arizona 
San Diego County Alluvial 
Studies 
City of Austin Technical 
Procedures for 
Watershed Erosion 
Assessments 
River Stability Study, 
Virgin River, Santa Clara 
River and Ft. Pierce 
Wash, Vicinity of St. 
George, Utah 
Hydrologic and 
Geomorphic Studies of 
the Platte River Basin, 
Nebraska 
Streambank Erosion 
Along Two Rivers In Iowa 
Channel Migration 
Studies in King County, 
Washington 
Bank Erosion Field 
Survey Report of the 
Upper Mississippi River 
and Illinois Waterway 
Arizona Standards for 
Lateral Migration and 
Channel Degradation 
Albuquerque, New 
Mexico 
14 streams in Minnesota 
Rillito Creek, near Tucson, 
Arizona 
Santa Cruz River basin, 
Arizona 
San Diego County, 
California 
Austin, Texas 
Virgin River , Santa Clara 
River, and Ft. Pierce 
Wash basins, Utah 
Platte River basin, 
Nebraska 
East Nishnabotna River 
and the Des Moines 
River, Iowa 
Snoqualmie , Tolt, Raging, 
and Green Rivers, King 
County, Washington 
Upper Mississippi River 
and Illinois Waterway, 
Minnesota, Wisconsin, 
Iowa, Illinois, and Missouri 
Arizona, statewide 
1 - 100 
Information not available 
920 
3,640 
40. (Model has been run 
for 1  105 mi2) 
1 30 
550 - 3,800 
Information not available 
960 - 1,450 
30 - 360 
28,900 mi2, for Illinois 
Waterway 
Information not available 
Arroyos 
Perennial 
Ephemeral 
Ephemeral and perennial 
Ephemeral and perennial 
Ephemeral and perennial 
Ephemeral and perennial 
Perennial 
Perennial 
Perennial 
Perennial 
Ephemeral and perennial 
Geomorphic and 
engineering analysis 
Geomorphic (using 
historic data) 
Mathematical (using 
historic data) 
Geomorphic (using 
historic data) 
Mathematical modeling 
Engineering and 
geomorphic analyses 
Geomorphic 
Geomorphic 
Engineering analysis with 
geomorphic analysis 
using historic data 
Geomorphic and 
engineering analysis 
(using historic and field 
data) 
Geomorphic and 
engineering analysis 
(using the field data) 
Geomorphic and 
engineering analysis 

ANALYSIS OF CASE STUDIES 


4.1. AMAFCA Sediment and Erosion Design Guide 
Document Title: Sediment and Erosion Design Guide (AMAFCA, 1994) 

Agency: Albuquerque Metropolitan Arroyo Flood Control Authority 

Authors: Robert Musseter, Peter Lagasse, and Michael Harvey 

Date: March 1994 

Study Method Geomorphic and engineering analysis
Category:

4.1.1. Overview 
The AMAFCA Sediment and Erosion Design Guide is a comprehensive design manual containing 
procedures to analyze arroyos and to design control projects. In this document, hydrologic, 
hydraulic, geomorphic, and sediment transport aspects are evaluated for the Albuquerque, New 
Mexico, area and specifically for the arroyo drainage system. The analysis procedures consist of 
three levels of analysis with increasing degree of complexity. 

Level 1 is a qualitative study based on planform analysis and bed material evaluations. This level 
of analysis is appropriate for cases with limited sources of information. Level 2 is an engineering 
analysis that provides a cumulative assessment of channel adjustment based on detailed studies 
of bed and bank materials, sediment transport properties, and quantification of channel changes 
and local scour. This level of analysis can be implemented using a moderate amount of 
resources. Level 3 is the most complex level and requires application of mathematical and 
physical models to simulate the effects of flooding on channel geometry. The Guide concentrates 
on Levels 1 and 2; a brief description of Level 3 is given. 

The AMAFCA Sediment and Erosion Design Guide defines hazard areas in the form of setbacks 
and discusses several erosion control and countermeasure criteria. This manual is an application 
and update of a similar document prepared by Simons, Li & Associates (1985). 

4.1.2. Detailed Description 
This manual provides specific relevant information on hydrologic analysis, the hydraulics of alluvial 
channels, and sediment transport analysis. The document also proposes guidelines for 
assessing the evolutionary stage of arroyos and forecasting the impact of development in the 
transformation of existing drainageways into arroyos. 

The information presented defines channel design criteria and areas subject to erosion. 
Setbacks, defined in terms of the concept of Prudent Line, are used to mark a zone of 
unacceptable erosion risk. The acceptable risk in the Prudent Line concept is defined in terms 
of the potential flooding effects of a 100-year event occurring at any time within a 30-year period. 
Also included is the cumulative erosion effect of all other storms weighted by their probability of 
occurrence. 

The AMAFCA Guide provides concise descriptions of geomorphology, watershed processes, 
and arroyo channel dynamics. Related topics include rainfall-runoff processes, sediment yield, 
channel incision and widening, alluvial channel hydraulics, sediment transport, channel adjustments 
and stability, and local scour. However, the most important component of the manual in 
connection with REHAs is the application of the Prudent Line concept. 

ANALYSIS OF CASE STUDIES 


The general approach is to analyze a given problem with progressively more complex techniques. 
Following the premise that accurate results can be obtained while minimizing costs, analyses can 
begin with a qualitative approach, continue with basic quantitative techniques, and, if necessary, 
use complex modeling procedures. The three levels of the approach are as follows: 

 Level 1  Geomorphic and Other Qualitative Analysis 
 Level 2  Basic Engineering Analysis (Hydrology, Hydraulics, and Sediment Transport) 
 Level 3  Mathematical and Physical Model Studies 

The purpose of this multilevel approach is twofold. First, it allows appropriate characterization 
techniques to be selected according to available data. Second, the sequence provides insight 
and direction for progression to the next more complex level based on information gathered at the 
previous level. As more data become available, iterations among the levels allow for more 
accurate solutions. In addition, the approach allows conclusions among the levels to be crosschecked. 

4.1.2.1. Level 1  Geomorphic and Other Qualitative Analysis 
The foundations of a Level 1 study lie primarily on fluvial geomorphology and channel mechanics. 
Successful application of these concepts requires sound knowledge and considerable practical 
experience. The goal of a Level 1 study is to develop an understanding of the fluvial systems 
response to watershed changes and thus be able to predict changes in bed geometry. In Level 
1, geomorphic principles that are applied to predict channel response require limited data but 
provide a qualitative assessment. 

Data for Level 1 must be able to characterize the conditions of the channel and demonstrate 
evidence of trends and changes in the fluvial system and the watershed. The following are 
examples of this information: 

 Area, vicinity, site, geologic, soils, and land use maps 
 Aerial photographs 
 Notes and photographs from field inspections 
 Historic channel profile data 
 Bridge as-built drawings and cross-section data 
 Existing and planned human activities (urbanization, clearing, channelization, sand and 

gravel mining, bend cutoffs, dam construction, reservoir operations) 
 Bed and bank material characteristics 
 Discharge regime 
 Data on morphological changes 

The steps involved in Level 1 investigations are described below. 

Step 1  Define Channel Characteristics 

Knowledge of the arroyo or drainageway characteristics provides a basic description of channel 
behavior in time and space. Channel incision, widening, and other features are indicators of past 
and present stability of channel alignment and banks. Bank stability is measured in terms of the 
geotechnical stability number, which is equal to the ratio of bank height to a critical bank height. 
Channel stability is assessed by means of the hydraulic stability factor, which is the ratio of the 
desired sediment supply to the actual supply. 

ANALYSIS OF CASE STUDIES 


Step 2  Evaluate Watershed Conditions 

This step seeks to evaluate sediment yield in response to watershed processes. Evaluation 
factors include land use, human activity, and vegetation. Changes in these variables are classified 
qualitatively; for instance, vegetation can be increasing, damaged, destroyed, or unchanged. The 
objective of this step is to correlate watershed changes with channel instability to understand the 
systems response. 

Step 3  Assess Overall Stream Stability 

The AMAFCA Guide includes charts to interpret planform characteristics as predictors of stability. 
Possible channel forms cover straight, meandering, and braided patterns. The charts allow 
overall stability to be evaluated based on the geotechnical stability number and the hydraulic 
stability factor. 

Step 4 Evaluate Lateral Stability 

Field inspections and material analysis can reveal bank configurations that may lead to failure. 
Alternatively, the bank stability analysis can be completed from data on bank position at different 
points in time as indicated in aerial photographs. This step should also seek to identify any 
potential avulsion effects. 

Step 5 Evaluate Vertical Stability 

Data records during several years are usually necessary to detect changes in bed elevation. 
Qualitative assessment of vertical stability problems can be obtained through the use of the Lane 
Relationship (Lane, 1955), sediment continuity, or channel dynamic equilibrium concepts. 

Step 6 Evaluate Channel Response to Change 

The previous steps provide information for understanding channel responses to changes or 
potential responses to proposed changes in the watershed. The Lane Relationship is an 
example of a tool to predicts overall channel changes. 

4.1.2.2. Level 2  Quantitative Geomorphic and Basic Engineering Analysis 
Level 2 improves the general information provided by Level 1 because it assigns quantitative 
measures to the general changes predicted. Level 2 relies mostly on basic engineering principles 
to quantify responses. The techniques used include evaluation of longitudinal slope changes, 
analysis of bed and sediment materials, sediment transport relationships, and frequency analysis 
for water and sediment. 

The data needed for this analysis includes the following types of hydrologic, hydraulic, and 
sediment transport analyses and information: 

 Watershed geometry 
 Channel geometry 
 Hydraulic channel data (roughness, cross sections, alignment) 
 Streamflow records 
 Land use, soils, and geologic data 
 Sediment discharge records 

ANALYSIS OF CASE STUDIES 


 Flood frequency records 
 Flood hydrographs and dominant discharge 
 Particle size gradations 
 Reservoir operation policies and sedimentation data 
 Hydrologic and hydraulic modeling 

The following are the steps to be conducted during Level 2 investigations. 

Step 1  Evaluation of Flood History 

Hydrologic characterization is critical in understanding the response and morphologic evolution of 
arid lands. For instance, arroyos in the Albuquerque area only exhibit flow in response to 
individual rainfall events. Analysis of changes in arroyo channels requires the development of a 
flood frequency curve for the 2-, 5-, 10-, 25-, 50-, and 100-year events. The AMAFCA Guide 
recommends making an analysis of hydrographs in arroyos using the Development Process 
Manual for the City of Albuquerque (City of Albuquerque, 1993). 

Step 2  Evaluation of Hydraulic Conditions 

Stability analysis requires knowledge of flow characteristics, including velocity, flow depth, top 
width, and other similar variables, to assess scour and sediment transport capacity. A hydraulic 
evaluation can be conducted using uniform flow equations and hydraulic-profile computer models 
(e.g., HEC-2, HEC-RAS). The AMAFCA Guide provides guidelines to evaluate the relationships 
between water surface and bed configuration in sand beds. Equations are provided to determine 
the amount of resistance opposed by sand-bed channels. Classic concepts of water surface 
superelevation at bends and supercritical flow are used to complement the analysis. 

Step 3  Bed and Bank Materials Analysis 

This step is essential to establish channel stability conditions and to conduct sediment transport 
analyses. Level 2 requires a detailed analysis of bed and bank materials to evaluate the 
erodibility and stability of the channel, which are functions of the grain size distribution. 
Techniques to measure grain sizes and determine distributions include sieving, photographic 
methods, and sedimentation methods. 

When the above methodologies are not applicable because of particle weight limitations or the 
presence of thin surficial layers, sampling techniques should be applied. The document provides 
a reference to areal, grid, and transect sampling (Kellerhalls and Bray, 1971) and discusses the 
application of grid sampling. 

Step 4  Estimation of Watershed Sediment 

Sediment production and transport in a watershed ultimately determine the amount of material 
reaching the stream network. Detention facilities are an important consideration. For instance, 
reservoir trapping of the sediment load may have serious consequences in the sediment transport 
regime downstream of the dam. The analysis must first obtain a qualitative evaluation of 
sediment sources and erosion processes. Nevertheless, sediment quantification tends to be 
imprecise. For this reason, assessments are better conducted by analyzing changes induced by 
disturbances in the watershed with respect to a baseline condition. The AMAFCA Guide 
recommends the use of the Pacific Southwest Interagency Committees rating methodology for 

ANALYSIS OF CASE STUDIES 


sediment yield (PSIAC, 1968) and the application of the Modified Universal Soil Loss Equation 
(MUSLE). 

Step 5  Evaluation of Bed Armoring Potential 

Incipient motion analysis can be used to assess relative channel stability. The critical point of 
balance between hydrodynamic and friction forces can be determined through the use of the 
Shields relation, which relates particle diameter to shear stress. Determination of the critical 
particle size for a range of discharges provides insight into the potential disruption to bed 
materials. In the sand beds typical of Albuquerque, even the smallest flows are capable of 
setting some of the bed material in motion. The formation of an armor layer provides a threshold 
discharge below which no degradation will occur. Conversely, degradation will occur for 
discharges exceeding this threshold. The potential for armoring can be evaluated using a simple 
relationship developed by the U.S. Bureau of Reclamation (USBR, 1984). 

Step 6  Evaluation of Degradation/Aggradation Potential 

Degradation or aggradation in a channel can be investigated through analysis of sediment 
continuity. Comparison between the estimated sediment inflow and the transport capacity of the 
channel indicates the change in the sediment stored in the reach. This analysis is usually 
conducted assuming a fixed bed but could be performed more accurately if the geometry of 
cross sections is allowed to vary in time according to sediment removal or deposition. However, 
the AMAFCA Guide states that this additional degree of complexity is not usually warranted for 
arroyos in Albuquerque. 

The procedure consists of subdividing the channel reach into subreaches and determining 
hydraulic characteristics using uniform flow calculations or a computer program such as HEC-2. 
The next step is to determine the sediment transport capacity of each subreach using a suitable 
method; for instance, the Meyer-Peter/Muller-Einstein Method (Simons, Li & Associates, 1985), 
Yangs Bed Material Equation, Colbys Method, or a power function such as the Zeller-Fullerton 
Relation. These methods are described in sufficient detail in the AMAFCA Guide. The continuity 
principle is applied assuming that the sediment transport capacity of an upstream subreach is 
equal to the sediment inflow to the next downstream subreach. 

Step 7  Evaluation of Lateral Erosion Potential 

Lateral erosion is best analyzed with a combination of Level 1 and Level 2 techniques. Field 
reconnaissance and comparison of channel patterns in aerial photographs can be used to 
estimate historic rates of lateral migration. Level 2 techniques include examining the stability of 
banks subject to undermining and tension fracturing. Several researchers have assumed various 
modes of wedge-shaped failure. The AMAFCA Guide suggests that those proposed by Ponce 
(1978) and Osman and Thorne (1988) are suitable for arroyos in Albuquerque. 

The method for assessing lateral migration can be chosen depending on the results of the 
sediment continuity analysis. If the reach is degrading, undercutting of the banks provides the 
supply of sediment and bank failure is the principal cause for lateral changes. If the channel is 
nearly in equilibrium and neither erosion nor deposition is expected, lateral migration is due to local 
erosion of bank material due to planform configuration. In this case, the best method for 
estimating lateral changes involves examining empirical or historic migration rates. In the absence 
of this information, the AMAFCA Guide presents a calculation method for sediment transport 
capacities based on the optimal shape of bends, bend shear stress, and sediment continuity. 

Step 8  Evaluation of Local Scour 

ANALYSIS OF CASE STUDIES 


This step analyses the local erosional phenomena caused by bridge piers and other similar 
obstacles in the flow. The Colorado State University equation is recommended for computation 
of pier scour (Richardson et al., 1991). Local scour also occurs downstream of check dams and 
other grade control structures. Estimation techniques include procedures in the Design Guide for 
Riprap-Lined Flood Control Channels (AMAFCA 1983) and the Veronese equation (Pemberton 
and Lara, 1984). The scour effects of revetments, spurs, and abutments can be quantified using 
the methods in the paper Evaluating Scour at Bridges, Hydraulic Engineering Circular No. 18, 
(Richardson et al., 1991). The AMAFCA Guide suggests procedures to evaluate scour induced 
by the presence of floodwalls. An equation presented in Hydraulic Design of Highway Culverts 
(FHWA, 1985) can be applied to quantify local scour at culvert outlets. 

4.1.2.3. Level 3  Quantitative Analysis Using Numerical Models 
This level involves the application of numerical models that implement the governing equations for 
physical processes in sediment transport and erosion. The Guide provides only a general 
description of the procedures involved. Various scenarios using these mathematical models need 
to be simulated on computers. Because Level 3 demands considerable resources, its application 
must be considered against project requirements, time, and budget constraints. 

Models are subject to the limitations in the theories that attempt to assign a mathematical 
representation to a physical phenomenon. The large degree of uncertainty in current 
understanding of sediment and erosion processes suggests that modeling results must be 
examined critically. In the end, the information from Levels 1 and 2 must be used to understand 
and validate the predictions of numerical modeling. 

There are numerous models available. The simplest ones are movable-bed models that simulate 
hydraulics while updating channel geometry to account for the effects of sedimentation and 
erosion. Examples of these models are the USACEs HEC-6 and BRI-STARS (Molinas, 1993). 

Other models, such as the FLO-2D or FLUVIAL-12, incorporate sediment transport calculations 
in hydraulic simulations. FLO-2D is a two-dimensional, finite-difference flood routing model that 
applies the full dynamic equations for channel flow and includes aggradation and degradation. 
The FLO-2D model offers the option to include any sediment transport equation. FLO 
Engineering, Inc., distributes FLO-2D. 

FLUVIAL-12, developed by Dr. Howard Chang, of San Diego State University (Chang, 1988), 
was developed to simulate water and sediment routing in channels. River change includes 
aggradation and degradation and allows the inclusion of physical constraints affecting channel 
erodibility. This model is one-dimensional, and the space domain is represented by cross 
sections along the channel. 

Physical models can be built in a hydraulics laboratory and often produce better results than 
numerical simulations due to the computational limitations in the mathematical models. Physical 
models are expensive, and their need must be justified by the project objectives. 

4.1.2.4. Prudent Line Analysis 
The definition of an erosion hazard by means of a Prudent Line is based on the concept of 
acceptable risk. The standard used in the AMAFCA Guide considers both the short-term 
flooding and erosion impact of the 100-year event and the long-term cumulative erosion effect of 
less severe events over the span of 30 years. 

These are the steps for determining Prudent Lines: 

ANALYSIS OF CASE STUDIES 


1. Conduct a Level 1 analysis to characterize vertical and lateral stability of the arroyo. 
2. Conduct a Level 2 analysis to determine the hydraulic behavior of the system. 
3. Conduct a Level 2 analysis to characterize sediment transport in the channel and assess the 
potential for lateral migration caused by both the 100-year event and the cumulative effect of 
less severe events. 
4. Use the results of the previous steps to delineate erosion hazard areas by plotting the 
estimated extent of both short- and long-term erosion at several cross sections. Figure 4.2 
shows three cases of hazard area delineation. 
The AMAFCA Guide refers to computer programs that have been developed to assist in the 
determination of Prudent Lines. 

The final portion of the guide is devoted to selection and implementation of erosion control 
measures. In addition to the Prudent Line, the document recommends the definition of a 
maintenance line beyond which protective measures must be implemented to prevent further 
erosion. 

4.1.3. Applicability 
This manual offers a systematic approach to characterization of riverine erosion hazards. It does 
not appear plausible that the results from Level 1 analyses alone will be sufficiently accurate. 
Level 3 analysis requires information and resources that may not be available to most 
communities. A combination of Level 1 and Level 2 appears to provide a adequate accuracy 
while imposing only moderate demands on the effort that must be undertaken. 

The AMAFCA Guide is directed specifically to arroyos in the Albuquerque area. However, the 
methodology can be readily expanded to other fluvial systems. 

ANALYSIS OF CASE STUDIES 


ANALYSIS OF CASE STUDIES 53 
Flow 
Flow 
Erosion on one bank only 
Hazard area limited by erosion 
boundary on one side and 
100-year floodplain on the other 
Erosion on both banks, 
100-year floodplain 
contained in erosion area 
Hazard area limited by erosion 
boundaries on both sides 
Erosion on both banks, 
erosion area contained 
in 100-year floodplain 
Hazard area coincides with 
100-year floodplain 
Short-term erosion 
(100-year flood) 
Long-term erosion 
(cumulative) 
100-year floodplain limit 
Erosion hazard area limit (Prudent Line) 
Figure 4.2. "Prudent Line" definition (AMAFCA, 1994). 
4.1.4. Limitations 
The limitations in the methodology stem mostly from those in the individual analysis techniques 
utilized. The main sources of uncertainty are the inherent inaccuracies in sediment transport 
computations. 
One important assumption is that bank-related sediment is provided by only one bank. This is 
usually the case for curved reaches but not for initially straight channels. In this case, the 
methodology adopts a conservative approach by extending the erosion area on both sides unless 
there is evidence that a geologic feature might exert some degree of erosion control. In addition, 
the Prudent Line concept may not be applicable in cohesive soils. 
The methods proposed can be applied in vertically stable or degradational reaches; however, 
behavior of aggradational reaches is more difficult to predict due to uncertainty in the 
development of flow paths. 
4.1.5. Relevant References 
Albuquerque Metropolitan Arroyo Flood Control Authority (1983), Design Guide for Riprap-Lined 
Flood Control Channels. 
Chang, H. H. (1988), FLUVIAL-12: Mathematical Model for Erodible Channels  Users Manual, 
San Diego, California. 
City of Albuquerque (1993), Development Process Manual, Section 22.2, "Hydrology," Volume 
2, Design Criteria, prepared by the DPM Drainage Design Criteria Committee.

Federal Highway Administration (1985), Hydraulic Design of Highway Culverts, U.S. Department 
of Transportation, Hydraulic Design Series No. 5. 

Kellerhalls, R. and D.I. Bray (1971), Sampling procedures for coarse fluvial sediments, ASCE 
Journal of the Hydraulics Division, V. 97, No. HY8, p. 1165-1179. 

Lane, E. W. (1955), Design of stable channels, Transactions of the American Society of Civil 
Engineers, 120, p.1234-1260. 

Molinas, A. (1993), Users Manual for BRI-STARS (Bridge Stream Tube Model for Alluvial 
River Simulation), National Cooperative Highway Research Program, Project No. HR15-11. 

Osman, A. M., and C. R. Thorne (1988), Riverbank Stability Analysis I: Theory, ASCE Journal 
of Hydraulic Engineering, vol. 114, HY2, pp.134-150. 

Pacific Southwest Interagency Committee (1968), Factors affecting sediment yield in the Pacific 
Southwest areas, Water Management Subcommittee, Sediment Task Force. 

Pemberton, E.L., and J. M. Lara (1984), Computing Degradation and Local Scour, Technical 
Guidelines for Bureau of Reclamation, Engineering Research Center, Denver, Colorado. 

Ponce, V.M. (1978), Generalized Stability Analysis of Channel Banks, American Society of Civil 
Engineers Journal of the Irrigation and Drainage Division, V. 104, No. 1R4, p. 343-350. 

Richardson, E.V., L.J. Harrison, and S. R. Davis (1991), Evaluating Scour at Bridges, Hydraulic 
Engineering Circular No. 18, U.S. Department of Transportation, Federal Highway 
Administration, Turner Fairbanks Highway Research Center, McLean, Virginia. 

Simons, Li & Associates, Inc. (1985), Design Manual for Engineering Analysis of Fluvial 
Systems, prepared for Arizona Department of Water Resources. 

U.S. Bureau of Reclamation (1984), Computing Degradation and Local Scour, Technical 
Guidelines for the Bureau of Reclamation. 
4.1.6. Mappability 
The methodology proposed is suited for mapping purposes and allows differentiation between 
short-and long-term erosion areas. A certain degree of engineering judgement must be applied 
during the actual delineation of erosion-prone areas when erosion appears to be controlled by 
localized features. 

4.1.7. Cost 
No information on cost is presented in the AMAFCA document. Dr. Peter Lagasse, member of 
the PWG, estimates $10,000 per stream mile, of which $5,000 are used for conventional 
baseline hydrology and hydraulic studies. 

4.1.8. Regulatory Potential 
The concept of the Prudent Line as a regulatory tool was implemented by AMAFCA following the 
example of other jurisdictions. These jurisdictions, for instance Fort Collins, Colorado (Grimm, 
1996), and Tucson, Arizona (Simons, Li & Associates, 1985, Kresan, 1988), have implemented 

ANALYSIS OF CASE STUDIES 


similar approaches based on three levels of analysis to delineate erosion hazard areas. The 
Prudent Line approach may not apply to cohesive soils. 

4.1.9. Summary 
The AMAFCA Guide presents a series of techniques and procedures to analyze lateral and 
vertical movement of large fluvial channels. The emphasis of the manual is to provide a technical 
basis for design of flood control projects. Hydrologic, hydraulic, geomorphic, and sediment 
transport aspects are evaluated. 

The general approach to sediment transport is subdivided into three levels with increasing 
degrees of complexity. Level 1 is based on planform analysis and basic bed material evaluations. 
Level 2 provides a cumulative assessment of channel adjustment based on detailed study of bed 
and bank materials, sediment transport properties, and quantification of channel changes and 
local scour. Level 3 involves application of mathematical models to simulate the effect of floods 
on channel geometry. 

The main outcome of the methodology is the development of a Prudent Line approach that can 
be used for delineation and regulation of erosion hazard areas. 

ANALYSIS OF CASE STUDIES 


4.2. Inventory and Analysis of Stream Meander Problems in Minnesota 
Document Title: Inventory and Analysis of Stream Meander Problems in Minnesota 
(MacDonald et al., 1991) 

Agency: St. Anthony Falls Hydraulic Laboratory 

Authors: Thomas E. MacDonald, Gary Parker, and Dave P. Leuthe 

Date: August 1991 

Study Method Geomorphic Equations (using historic data)
Category:

4.2.1. Overview 
The study was funded by the Legislative Commission on Minnesota Resources, State of 
Minnesota. The purpose of this quantitative geomorphic study is to identify meandering streams 
and rivers in the state of Minnesota experiencing bank erosion problems and to analyze the 
erosion problem both quantitatively and qualitatively. The report focuses on the analysis of 16 
stream reaches ranging from very small (Kanaranzi Creek, bankfull width 25 ft) to very large 
(Minnesota River, bankfull width 290 ft). The names and locations of stream reaches are listed in 
Table 4.2. 

The analysis was conducted using historic aerial photographs and topographic maps as basic 
data sources. Several parameters, including valley center line and stream center line, were 
measured for each reach from digitized photographs taken at two different time periods. The 
average annual shift of the stream was estimated using the computer program MEANDER. The 
study then attempted to establish a relationship between stream shift and the various stream 
parameters. Several relations were obtained. 

The study includes literature review, geographic, hydrologic and geomorphic information for each 
stream or river analyzed, description of aerial photographs and maps used for analysis, and 
results from the stream shift estimation program MEANDER. The last part of the document 
discusses plotting and multivariate linear regression analysis that were performed to seek 
relationships between stream parameters and results. The program MEANDER has been used 
in studies in other states, such as Illinois (Garcia, et al.,1996). 

4.2.2. Detailed Description 
This report consists of two parts. In the first part, stream reach descriptions, consisting of tables, 
ground and aerial photographs and graphics, provide information on each stream reach analyzed 
and present results for each reach. The second part explains how the data and measurements 
were gathered and how this information was used to obtain the results. 

ANALYSIS OF CASE STUDIES 


Table 4.2. Streams and location of study reaches. 

Stream Location 
Big Fork River 
Buffalo Creek 
Cottonwood River 
Hawk Creek 
Kanaranzi Creek 
Minnesota River A 
Minnesota River B 
Mississippi River A 
Mississippi River B 
Nemadji River 
Rice Creek 
Root River 
Rum River 
Wild Rice River A 
Yellow Medicine River 
Zumbro River 
Koochiching County 
McLeod County 
Brown County 
Renville County 
Rock County 
Nicollet and Big Earth Counties 
Scott County 
Aitkin County 
Aitkin County 
Carlton County 
Anoka County 
Houston County 
Isanti County 
Norman County 
Yellow Medicine County 
Wabasha County 

4.2.2.1. Data Collection and Preparation 
Selection of stream reaches was conducted through the Minnesota Department of Natural 
Resources. Channel shift was measured over a chosen time period to demonstrate that 
sufficient shift had occurred for meaningful measurement. For most reaches, this time period is 
approximately 20 years. 

A base map for each reach was created by photographically enlarging a portion of a USGS 7.5minute 
topographic map. The aerial photographic maps were then reproduced to the same 
scale. Stream and valley centerlines were digitized and smoothed to create coordinate data 
sets. Hydrologic and hydraulic information collected includes two-year storm discharge, bankfull 
width, stream depth, and channel slope. In addition, bed materials were analyzed to develop 
grain size distributions for each site. 

4.2.2.2. Shift Measurement 
The study a used a Lagrangian reference system to measure stream shift. With the Lagrangian 
reference, attention is focused on the area surrounding the stream centerline. Centerline 
movement is measured in the direction normal to the stream bank or, alternatively, the stream 
centerline. 

The first step in the shift measurement process was to equally space and smooth Cartesian x-y 
coordinates for the stream centerline at the beginning and end of the time period, as well as for 
the valley centerline. A piecewise spline function was fitted to each centerline. Then, the angular 
alignment of the stream centerline and the stream curvature were computed for each coordinate. 
The next step consisted of measuring the normal distance between each point in the initial 
centerline and the centerline at a later time. The normal shift at each coordinate was then broken 

ANALYSIS OF CASE STUDIES 


down into downvalley and crossvalley components. In the last step, each component was 
integrated over the entire reach to yield an average annual shift rate. 

4.2.2.3. Results Summary 
Many useful results are obtained through the shift measurement process. These include the 
stream sinuosity, average sinuosity change per year, average normal shift per year, average 
transverse shift per year, average longitudinal shift per year, and shift ratio. The average normal 
shift is the most basic and useful information because it indicates how much and how fast the 
stream banks were eroded during the period studied. Therefore, the normal shift could be used 
as a basis for predicting future shift. 

The floodplain area reworked by the stream per unit stream length per year was computed by 
integrating the area between the stream centerline at the beginning and end of the time period. 
This information allows floodplain managers to predict how much land surrounding the stream 
reach would be affected by meandering. 

4.2.2.4. Relationships Between Stream Parameters and Measured Results 
Relationships were sought among stream parameters (slope, sinuosity, 2-year discharge, 
bankfull width and depth) as well as between independent stream parameters and the dependent 
shift parameters. It was found that the average normal shift rate is strongly correlated to stream 
depth and discharge. In both cases, the correlation coefficient is higher than 0.73. Similar strong 
correlations were found between areas reworked by the stream, and depth and discharge. 

4.2.3. Applicability 
The report offers a systematic approach to characterizing stream shifts. The computer program 
MEANDER is available from St. Anthony Falls Hydraulic Laboratory. It is able to compute the 
average shift rate for a time period provided that stream reach data are available for the 
beginning and end of the study period. 

The relationships produced by regression analysis might be applicable to other stream reaches 
with similar hydrologic and geomorphic conditions. The accuracy of using such relationships to 
predict stream shift was not reported. 

4.2.4. Limitations 
The study focused on the shift of stream centerlines over time; however, bank movements were 
not studied in depth. The shift rules only apply to the reaches where they were developed. This 
observation is particularly important for smaller streams, which often change rapidly in space. 
Likewise, the given shift rates apply best for the period covered by the study, although they may 
be used for short-term predictions. Rapid changes in land use occurring in many watersheds 
may limit the applicability of the methodology. 

The study is based on descriptive information; it was not aimed at revealing causes of stream 
shift and erosion from either a geomorphic or an engineering point of view. 

4.2.5. Relevant References 
Garcia, M.H., L. Bittner and Y. Nio (1996), Mathematical Modeling of Meandering Streams in 
Illinois: A Tool for Stream Management and Engineering, 2nd edition, University of Illinois at 
Urbana-Champaign. 

ANALYSIS OF CASE STUDIES 


Hasegawa, K. (1989), Universal bank erosion coefficient for meandering rivers, ASCE Journal 
of Hydraulic Engineering, vol. 115, No. 6. 

Parker, G. (1982), Stability of the Channel of the Minnesota River near State Bridge No 93, 
Minnesota. St. Anthony Falls Hydraulic Laboratory, University of Minnesota 

Parker, G. (1983), Theory of meander bend deformation, River Meandering, Proceedings of 
the Conference Rivers 83, New Orleans, Louisiana. 

4.2.6. Mappability 
A stream centerline shift graph was prepared for each reach and presented in the report. It is 
easy to overlap it with a base map to produce a map to show the meander history and to predict 
stream shift in the near future. However, there is not sufficient information provided in the report 
to produce bank erosion maps. 

There are two possible directions to enhance the methodology for the purpose of mapping 
riverine erosion hazard areas. The model MEANDER could be used to delineate the stream 
banks instead of the centerline. The historic migration progress of the stream banks can be 
traced and the prediction can be made based on the historic trend. Another option is to use 
hydraulic models to simulate interactions between the stream bank and the flow (Hasegawa, 
1989) and use the results to estimate the width of the erosion band. This procedure may provide 
more accurate results, but significant amount of additional work is needed. 

4.2.7. Cost 
No cost information was reported. 

4.2.8. Regulatory Potential 
Although the study focused on the shifts of stream centerline, the method, with additional effort, 
can be further developed to meet the needs of measuring and estimating stream bank 
movements. 

The State of Minnesota is not regulating riverine erosion; therefore, the information obtained from 
the study has not been used for the regulation purposes. 

4.2.9. Summary 
Sixteen stream reaches in Minnesota were analyzed for stream shift and meander properties. 
Aerial photographs for a minimum of two different times, approximately 20 years apart, and 
topographic maps were used to produce x-y coordinate data files for valley centerlines and 
stream centerlines. The computer program MEANDER was used to measure various 
components of stream shift. The most useful parameters computed were the average normal 
shift and the rate of floodplain area rework by the stream reach. The results of the computer 
analysis were used in a regression analysis to determine relations between stream shift 
properties and stream parameters. The most useful relations include average normal shift versus 
depth, average normal shift versus discharge, rate of area rework versus depth, and rate of area 
rework versus discharge. 

The methodology may be further developed to meet regulatory needs. 

ANALYSIS OF CASE STUDIES 


4.3. Probabilistic Spatial Assessment of River Channel Instability 
Document Title: A Probabilistic Approach to the Special Assessment of River Channel 
Instability (Graf, 1984) 

Agency: Arizona State University 

Author: William L. Graf 

Date: 1984 

Study Method Mathematical method (using historic data)
Category:

4.3.1. Overview 
The paper reported a study funded by the Phoenix District of the U.S. Army Corps of Engineers 
(Graf, 1984). The purpose of the study was to provide an approach to predict the behavior of 
arid region rivers. The study was stimulated by the authors observation that economic analyses 
for flood mitigation in arid and semi-arid regions fail to take into account destructive channel 
migration and erosion. The approach presented in the paper combined geographic, geomorphic, 
hydrologic and statistical techniques to predict future channel locations. A case study of Rillito 
Creek near Tucson, Arizona, for a period from 1871 to 1978 provided a demonstration of the 
probabilistic method of erosion damage assessments and mapping. 

The study began with an analysis of past channel locations using historic aerial photographs, 
maps, and engineering surveys. Cell maps were developed from the above information, and the 
probability function of each cell for a given historic period was defined, allowing the erosion 
probability of a given cell to be calculated. This probability is directly proportional to the 
magnitude of annual floods during this historic period and inversely proportional to the upstream 
and lateral distances from the cell to the channel. The overall probability of erosion was derived 
from a series of historic probability parameters. These values were then mapped to create a 
spatial representation of the erosion hazard. The final product is a map showing long-term 
erosion probability along the stream. 

A practical planning implication of a probabilistic approach for prediction of channel migration and 
erosion is that it permits the economic assessment of potential losses under no project 
condition. In the case study, the method was used to run a simulation for a 50-year period. It is 
estimated that, given the land value in 1984, the erosion along the creek over the period 
represents an economic loss over $2 million, which is five times greater than the potential 
inundation losses. 

4.3.2. Detailed Description 
The paper consists of four major parts: methodology, case study, discussion of limitations, and 
conclusions. 

4.3.2.1. Methodology 
The following steps are involved in development of an erosion probability map. 

ANALYSIS OF CASE STUDIES 


Step 1. Develop a cellular map for each historic period. The base map was divided into square 
cells, 100 meters on the side, each designated as channel or nonchannel for any particular 
period. 

Step 2. Develop a probability function that relates the erosion probability for a cell to its location 
and hydrologic condition. The fluvial geomorphic literature suggested that location with respect 
to the active channel and the magnitude and frequencies of floods during the period in question 
are two most important factors in deciding the probability of a cell being eroded (Gregory, 1977). 
The following three-parameter erosion function was used 

log

S 
1k= 
. 
..
where , and are empirically derived coefficients, is the probability of erosion for abbbPa0123, , i,j,
given cell located at coordinates(),is the distance laterally across the floodplain to thei, j dl 
nearest active channel cell, isdistance upstream to the nearest active channel cell, is thedru 
return period of the peak annual flood that occurred in year , and is the number of years in theknUsing this function, the value of the probabilityoferosion for each cell in the period was 
The transition matrix, in which columns are based on theStep 3. Develop a transition matrix.
lateraldistancefroma cell to the channel and rows are based on the upstream distance along 
the floodplain from a cell to the channel,summarizesthe statisticalcharacteristicsofallnon-
channel cells for a particular time period. Each period of analysis, covering a few years, 
producesa transitionmatrix. Values of erosion probability for a selected cell vary from period to 
period. For example, assume that an element, , in the matrix represents all cells located 100a1 
metersdownstreamand 100 meters laterally from a channel at beginning of the period. If there 
are 59 of such cells and 30 where eventually eroded, 30/59 = 0.51 is the probability that cells with 
those distance characteristics would be ultimately eroded. If the sum of the returnintervalsofthe 
annual floods during the period in question is 132 for the period,the ordered setforcells100 
meters downstream and 100 meters laterally from the channel is (0.51, 100, 100, 132). 
Each period of analysis canStep 4. Estimate values of coefficient a, b, b and b of Eqn. 4.1.0123

period of interest. 

calculated. 

produce a transition matrix containing several ordered sets consisting of Pi,j, dl, du, and the 
summation of r values. The ordered sets from several periods provide the data to determine 
values for these parameters using regression methods. 

Step 5. Predict erosion. Assuming that the observed fraction of eroded cells is a reasonable 
indicator of the probability of future erosion for similarly situated cells, Eqn. 4.1 can be used for 
any initial conditions to predict erosion probability. Calculation of erosion probability can be made 
for a variety of time spans by manipulating the values for the sum of the annual flood return 
intervals. The function can generate a new transition matrix for future changes. The number of 
cells for each location type (e.g. 200 meters of lateral distance from the active channel) can be 
multiplied by the appropriate probability to predict the number of cells likely to be eroded over a 
given time period. The probabilities for the cells can also be mapped to present the spatial 
variability. 

.

n 

log10 

log 

log 

log 

(4.1)
+

+

+

Pij 

b 

d 

b 

d 

b

=

.
.

a0 

r

10 

1 

10 

2 

10 

3 

10

l 

u 

ANALYSIS OF CASE STUDIES 


4.3.2.2. Case Study 
The method was applied to Rillito Creek, located on the northern edge of Tucson, Arizona. Rillito 
Creek flows 18.4 kilometers through an alluvium-filled fault-block valley to its junction with the 
Santa Cruz River. At its mouth the Rillito drains an area of 2,378 square kilometers (918 square 
miles). The annual rainfall over most of the basin is 20 - 40 centimeters (8 - 16 inches). 

The locations of the channel were defined based on 25 historic documentary records during the 
period of 1871 to 1978. A review of these records shows that an unstable period extended from 
1871 to about 1937. Lower Rillito Creek has consistently been more unstable than the upper and 
middle reaches. All reaches have shown decreasing instability over the 107-year period. 

Transition matrices were developed which provided empirical values of the constants in Eqn. 4.1. 
Three matrices depicted changes for the periods of 1871-1912, 1912-1918, and 1918-1937. 
Seven matrices describe the more stable periods of 1937-1941, 1941-1949, 1949-1954, 19541960, 
1960-1967, 1967-1972, and 1972-1978. Prediction results indicate that the probability of 
erosion of all cells declines when distance from the channel increases. During the unstable period 
(1912-1937), this decline was less abrupt than in the more stable period (1937-1978). In the 
earlier period, distance from a cell laterally to a stream channel was as important as its distance 
upstream to a channel; in the later period, however, the importance of lateral distance declined 
more rapidly than the distance upstream to the nearest channel cell. 

As a test of the accuracy, Eqn. 4.1 with constants defined empirically for the 1937-1972 period 
was used to predict the 1978 map using the 1972 map as the starting condition. The resulting 
erosion probability map identified hazardous zones of instability and other more stable areas. 
During the test period, erosion was confined to cells that were mapped with the highest 
probability of erosion in the 1978 map except for six cells, which had been apparently influenced 
by gravel mining activities. 

4.3.3. Applicability 
The paper provides an approach to predict channel erosion by estimating probability of erosion 
for each cell. The approach depends on historic records; therefore, it is applicable to areas 
where sufficient historic records are available. 

When long-term predictions are needed, the probabilistic method is based on simulations utilizing 
random numbers as opposed to given probabilities to generate a series of flood events. For 
example, if the prediction is for a 50-year period, the calculation uses a single 50-year event, two 
25-year events, and so forth. The order of these events is randomly generated. Therefore, the 
exact course of the events in changing channel locations is not a primary product of the 
simulation. The most stable result is the number of cells lost rather than the specific location 
identity of those cells. 

4.3.4. Limitations 
Although the Rillito Creek example indicates that probabilistic approach to erosion damage 
assessment is possible, there are a number of concerns. 

First, the author indicated that the method proposed in this paper was experimental and it 
required further testing in other areas. However, no additional testing has been undertaken for 
other watersheds. Second, the method was designated to accommodate the continually eroding 
conditions along Rillito Creek. When a cell adjacent to the existing channel is eroded, no 
concomitant deposition is predicted for the opposite bank. Third, the length of the historic channel 

ANALYSIS OF CASE STUDIES 


record is critical in establishing a series of probabilities for erosion of cells. If the records are too 
short, the result will be dominated by short-term behavior of the system. The most important 
concern is that the method is based on established trends of the channel system and assumes 
the continuation of those trends. Although similar assumptions of system stationarity are common 
in engineering and hydrologic deterministic models, stationarity may not be assumed for erosion 
problems. 

4.3.5. Relevant References 
Gregory, K.J. (1977), River Channel Changes, John Wiley, New York. 

Graf, W. L. (1981), Channel instability in a braided, sand bed river, Water Resources 
Research, vol. 17, No. 4, p.1087-1094. 

Graf, W.L. (1983), Flood-related channel changes in an arid-region river, Earth Surface 
Process and Landforms, vol. 8, p.125-139. 

4.3.6. Mappability 
Several erosion probability maps of Rillito Creek, either for historic periods or for prediction, are 
included. Using different colors, the maps depict the erosion risk of each cell by its erosion 
probability. A probability map can be created for each channel studied. 

4.3.7. Cost 
Cost information was not reported. 

4.3.8. Regulatory Potential 
The probabilistic erosion maps could be used for regulatory purposes. For example, use of the 
land cells with higher erosion risk could be restricted. 

4.3.9. Summary 
The most important advantages of the cell map are that it can be developed by a Geographic 
Information System and that it can be used to reflect channel migration and erosion through 
empirical functions summarizing the past observed behavior of the channel. Geographic analysis 
of the likelihood that a given cell of land will experience erosion is based on the fundamental 
concept of distance decay away from a neighboring channel and the magnitude and frequency of 
flood events. Based on the assumption that channel systems possess stationarity, functions 
derived from historic records were used to extent past experience to predict probable future 
behavior. The probabilistic analysis that relies on grid cell maps of the near-channel landscape 
can be used to supplement more traditional engineering modeling approaches. 

ANALYSIS OF CASE STUDIES 


4.4. Geomorphology and Hydrology of the Santa Cruz River, 
Southeastern Arizona 
Document Title: Channel Change on the Santa Cruz River, Pima County, Arizona 193686 
(Parker, 1995) 

Agency: U.S. Geological Survey 

Author: John T. C. Parker 

Date: 1995 

Study Method Geomorphic analysis (using historic data)
Category:

4.4.1. Overview 
This study conducted by the U.S. Geological Survey (USGS) evaluated channel changes on the 
Santa Cruz River in Pima County, Arizona. The investigation covered geology, geomorphology, 
and hydrology of the river and was a project of the USGS in cooperation with the Pima County 
Department of Flood Control and Transportation District. 

4.4.2. Detailed Description 
This investigation focused on the major geomorphic and hydrologic processes affecting channel 
morphology in the Santa Cruz River. This investigation also examined the climate and human 
activities that have affected the location, magnitude, and timing of such changes. 

The methods used in this investigation include the following: 

 Documentation of channel changes; 
 Examination of factors that control the temporal and spatial patterns of the channel change, 
including the effect of human activities; and 
 Evaluation of available modeling techniques for channel changes. 
The primary methods used in this study were interpretation and analysis of aerial photographs 
supplemented by field observations and published and unpublished geomorphic, topographic, 
geotechnical, and historic data. The study area is the 70-mile reach of the Santa Cruz River 
through Pima County. At the downstream end of the study, the river basin has an area of 3,641 
square miles. The Santa Cruz River is ephemeral from the upstream end of the study area to the 
sewage treatment plant in northwest Tucson. Six study reaches along the 70-mile-long main 
stem of the river in Pima County were defined on the basis of morphology, historic stability, and 
channel changing processes. 

The coverage and quality of the aerial photographs used in this study varied from one location to 
another. A base map was developed from an aerial photograph taken in 1936 to document 
lateral channel change. The two reference systems used for longitudinal river position were axial 
distance, the distance along a straight line through the axis of a river reach, and river distance, the 

ANALYSIS OF CASE STUDIES 


distance along the meandering thalweg of the channel. The use of axial distance provides a fixed 
reference for measuring changes in channel width or position, whereas river distance changes 
over time as channels lengthen or shorten. 

The Santa Cruz River shows a tremendous amount of physical variation through the 70-mile study 
area. From 1936 to 1986, an increase in the top width of the channel and a decrease in river 
length characterized channel changes. Most of the channel straightening was the result of human 
intervention. The increase in channel width would have been greater if not for channelization and 
bank protection work. Most of the channel widening resulted from the large floods of 1977 and 
1983. In the Cortaro and Marana reaches, most of the channel changes involved lateral shifts in 
channel position. 

Lateral channel changes occurred by three basic mechanisms: meander migration, avulsion and 
meander cut off, and channel widening. Meander migration is the spatially continuous movement 
of the channel across its floodplain by the initiation of meanders and subsequent lateral extension. 
Meander migration tends to be the dominant mechanism of change during low to moderate 
discharge. Avulsion and meander cut off occur when overbank flow incises a flow path into the 
floodplain, causing large, abrupt shifts in channel position. In general, channel widening results 
from high flows that erode banks made of cohesionless soils. 

Vertical and lateral channel-change mechanisms operate in concert with bank-retreat 
mechanisms to produce widening of arroyos in incised reaches. Most of the reaches through 
Tucson qualify as incised reaches. Most of the arroyo widening occurs when the channel is 
deeply incised into poorly resistant sand and silt. The most rapid rates of arroyo widening have 
occurred in connection with the migration of confined meanders. Unlike channel widening, arroyo 
widening is not readily reversed. Nonetheless, this type of widening was reversed on the most 
deeply incised parts of the San Xavier reach of the Santa Cruz River. 

Hydrologic and climatic factors, such as the magnitude, duration, and intensity of precipitation 
events, and the frequency of flood events generally control the timing and magnitude of channel 
changes on the river. Changes in channel geometry caused by successive floods or changes in 
roughness caused by vegetation growth also contribute to the temporal variability of channel 
changes. Spatial variability of channel change in the Santa Cruz River, such as the location of 
channel change and its magnitude in response to a given discharge, is largely controlled by 
topographic, geologic and human-introduced factors. These factors include sediment sources, 
bank materials, vegetation density, pre-existing topography, control size and quantity of bed load, 
resistance to erosion, valley and channel slope, and channel geometry. 

The flood history of the Santa Cruz River in this century shows three distinct periods: 1915 to 
1929, 1930 to 1959, and 1960 to 1986. The large floods in 1915 to 1929 and 1960 to 1986 
caused substantial channel changes throughout the study area. Some reaches, however, were 
characterized by considerable lateral instability throughout the study period, including 1930 to 
1959 when annual floods were generally moderate. 

The floods of 1977 and 1983 were the two largest floods of record on the Santa Cruz River. The 
1983 flood was the single largest episode of channel change to occur on the Santa Cruz River 
since at least 1915. That flood produced an enormous magnitude of channel and arroyo 
widening and lateral shifts in channel position throughout the study area. 

Available models for prediction of channel change generally do not address lateral changes, and 
those that do limit the types of channel change mechanism that can be modeled. The application 
of probabilistic models of channel changes is not appropriate on the Santa Cruz River because of 
the change in resistance to erosion with time. 

ANALYSIS OF CASE STUDIES 


In conclusion, the general stability of various reaches can be evaluated by recognition of major 
channel-changing mechanisms operating in a reach and identification of the local topographic, 
geologic, and cultural controls on channel changes. 

4.4.3. Applicability 
The methods used in this study have identified mechanisms of channel change on the Santa Cruz 
River, timing and magnitude of the channel change with respect to floods, and physical conditions 
associated with channel instability. A relation exists between hydrologic regimen and the rate and 
magnitude of channel change. This relationship is modified by resisting forces and hydraulic 
conditions such as channel morphology. The author suggests that further analysis of the historic 
database would increase understanding of the river system. However, it would not produce the 
quantitative information necessary to predict channel change in response to floods. 

4.4.4. Limitations 
There are a few limitations with the method used in this study related to the coverage, quality and 
resolution of photographs and the time interval between photograph series. Unless there is 
evidence that such photographs have been taken before and after flooding, it is difficult to 
determine the extent of erosion and channel change due to a given magnitude and volume of 
discharge. Also, the author suggests that additional research needed includes analysis and 
evaluation of the following factors: the nature of bank material, in particular their cohesive 
properties; mechanical processes, such as cracking and piping; stream bed material 
composition; interactions between streamflow and soilhydrologic processes in channel banks; 
and formation of armored channels and point bars in rapidly varying flow. 

4.4.5. Relevant References 
Baker, V.R. (1977), Stream-channel response to floods, with examples from central Texas, 
Geological Society of American Bulletin, v. 88, p. 1057-1071. 

Fan, S. S., ed. (1988), Twelve Selected Computer Stream Sedimentation Models Developed in 
the United States, Washington D.C., Federal Energy Regulatory Commission report. 

Guber, A. L. (1988), Channel Changes of the Xavier Reach of the Santa Cruz River, Tucson, 
Arizona, 1971-1988, unpublished Masters thesis, University of Arizona, Tucson, Arizona. 

Hays, M. E. (1984), Analysis of Historic Channel Change as a Method for Evaluating Flood 
Hazard in the Semiarid Southwest, Masters thesis, University of Arizona, Tucson. 

Klimek, K. (1974), The retreat of alluvial river banks in the Wisloka Valley (South Poland), 
Geographia Polonica, v. 28, p. 59-75. 

Kresan, P.L. (1988), The Tucson, Arizona, Flood of October 1983-Implications for Land 
Management Along Alluvial River Channels, in Baker, V.R., R.C. Kochel, and P.C. Patton, 
eds., Flood Geomorphology, John Wiley & Sons, New York, New York. 

Parker, J. T. C. (1995) Channel Change on the Santa Cruz River, Pima County, Arizona, 193686, U.S. Geological Survey Water-Supply Paper 2429. 

ANALYSIS OF CASE STUDIES 


Sabol, G.V., C.F. Nordin, and E.V. Richardson (1989), Scour at Bridge Structures and Channel 
Aggradation and Degradation Field Measurements, Arizona Department of Transportation 
Research Project. 

Slezak-Pearthree, M., and V.R. Baker (1987), Channel Change Along the Rillito Creek System 
of Southeastern, Arizona, 1941 through 1983, Arizona Bureau of Geology and Mineral 
Technology, Geological Survey Branch, Special Paper 6. 

Webb, R.H., and J.I. Betancourt (1992), Climatic Variability and Flood Frequency of the Santa 
Cruz River, Pima County, Arizona, U.S. Geological Survey, Water supply Paper 2379. 

4.4.6. Mappability 
The rates of lateral channel migration through bank erosion, meander propagation, and channel 
avulsion can be measured from sequential aerial photographs. Limitations include the quality and 
resolution of the photographs and the length of time between photographic series. Unless 
photographs have been taken immediately before and after a major flood, it is difficult to know 
just how much erosion and channel migration is due to a given magnitude and volume of 
discharge. It can also be difficult to distinguish the changes that have occurred from natural 
processes from those related to human activity. The methods used in this study were relatively 
coarse and, according to the author, accurate to no more than 10 meters on the average, 
although this accuracy varied considerably depending on the location, photograph quality, and 
availability of registration points. Greater accuracy can be achieved, but at greater cost. The 
most important information that can be obtained from mapping channel movements is the extent 
of geological controls on channel positions and relative rates of erosion. By overlaying channel 
position through time with surficial geology and geomorphology, the effect of different quaternary 
units and landforms (e.g. terraces, paleochannels) can be clearly seen, and the units most 
susceptible to erosion can be determined. The author emphasized the use of geologic-
geomorphic mapping here because he thinks that, especially in the arid and semi-arid regions, it 
is the most significant control on the location of channel change. The rate at which such change 
takes place is governed by the magnitude and frequency of floods, which in dry climates is 
extremely difficult to forecast; however, by understanding geological controls, at least the location 
of greatest hazards can be delineated. 

4.4.7. Cost 
No cost information was provided in the document, since the author was a student employee with 
limited experience when he did this project. An experienced geologist or hydrologist may take a 
few months to map channel change in a reach of about 100 miles. 

4.4.8. Regulatory Potential 
If the limits of significant channel erosion and migration can be determined on the basis of 
geologic controls, such controls can be used to delineate areas of most significant hazard. 

4.4.9. Summary 
The primary methods used by the author in this study were the interpretation and analysis of 
aerial photographs supplemented by interpretation of field observations and geomorphic, 
topographic, and historic data. Although an appropriate model for predicting channel change on 
the Santa Cruz River has not been identified, the stability of reaches relative to one another and in 
time can be evaluated through recognition of the local controls and the major channel-changing 

ANALYSIS OF CASE STUDIES 


mechanisms operating in a reach. Much of the channel change that has occurred during the 
study period has been human-induced. 

ANALYSIS OF CASE STUDIES 


4.5. San Diego County Alluvial Studies 
Document Title: San Diego County Alluvial Studies 

Agency: San Diego County 

Author: Howard Chang, San Diego State University 

Date: 1984 

Study Method Mathematical modeling
Category:

4.5.1. Overview 
The County of San Diego, California, was the subject of a number of studies aimed at 
investigating the effects of erosion in floodplain management of alluvial channels. A substantial 
portion of this work was accomplished through the work of Dr. Howard Chang, of San Diego 
State University, using several versions of his model, FLUVIAL. In its latest version, this finite-
difference model computes hydraulic profiles, sediment transport, and streambed geometry 
changes for a given flooding event. Streams that were analyzed include the San Luis Rey, San 
Diego, and San Dieguito Rivers and Moosa Canyon Creek. 

The results of hydraulic modeling were used as a starting point to estimate potential channel 
changes as a result of the 100-year event. The results were complemented with geomorphic 
and engineering information to arrive at definition of erosion hazard areas. The County developed 
floodplain management procedures that incorporate effects of erosion and sedimentation in the 
form of setback restrictions and a Resource Protection Ordinance. 

4.5.2. Detailed Description 
4.5.2.1. Computer Modeling 
The FLUVIAL model was initially developed in 1972 and has undergone several enhancements 
through the years. The latest version is called FLUVIAL-12 and is capable of simulating water 
and sediment routing through natural and artificial channels of general configuration for a given 
flow period. The model has been tested and calibrated with several streams in semi-arid and 
humid environments. Although intended for perennial streams, with additional effort, the model 
can simulate ephemeral streams (Chang 1988a,b). 

FLUVIAL-12 simulates changes in channel geometry resulting from aggradation and degradation 
processes. These processes cause changes in width and alignment as well as variations in the 
bed topography. Limiting conditions can be formulated to model the effect of physical constraints 
such as bedrock outcrops, grade control structures, and bank resistance. Typical applications 
include general erosion and deposition studies, bridge scour evaluations, sediment transport 
analyses, gravel mining impact assessment, and design of bank protection and grade control 
structures. 

The foundation for modeling river changes lies on the tendency of natural streams to reach a 
state of dynamic equilibrium involving, water, sediment, and channel resistance and geometry. 
The equilibrium conditions are characterized by uniform sediment discharge along the channel and 
uniform stream power expenditure .QS, where . is the unit weight of the water-sediment mixture, 

ANALYSIS OF CASE STUDIES 


Q the discharge, and S the energy gradient. The stream adjusts the power expenditure by 
scouring the bed and reducing the width or filling the bed and increasing the width. FLUVIAL-12 
computes changes in bed geometry by implementation of these principles. The main 
components of FLUVIAL-12 are: 

Water Routing: Uses the continuity and momentum equations to determine temporal and spatial 
variations of stage, discharge, energy, and other hydraulic variables. This module includes 
evaluation of both longitudinal and transverse flows. 

Sediment Routing: This module computes sediment transport capacity and actual sediment 
discharge and solves the sediment continuity equation. Six formulas are provided as options for 
sediment capacity computation: Engelund-Hansen, Yang's unit stream power, Graf, Ackers-
White, Meyer-Peter & Muller, and Parkers formula for gravel. 

Channel Geometry Changes: Uses the erosion or deposition data computed by the sediment 
routing module to predict how the section geometry is affected. From the energy gradient, the 
module computes the direction of width adjustment. The rate of width adjustment is computed 
based on sediment rate, bank stability properties, and bank erodibility. After the width is 
adjusted, the remainder of the total correction is applied to the channel bed and distributed 
according to the tractive force. 

Channel Migration: Curvature-induced scour and deposition are computed from the flow 
curvature along the stream. Transverse sediment transport is used to predict changes in the bed 
topography at a given cross section. 

FLUVIAL-12 can simulate a river system for an indefinite period, as long as a hydrograph is 
available. The computation depends on the assumption of a stable portion of the stream that can 
provide suitable boundary conditions. 

4.5.2.2. Definition of Hazard Areas 
The County of San Diego used the modeling results of FLUVIAL-12 to define erosion and 
sedimentation hazard areas. The following are the general steps to accomplish this objective: 

1. Gather information to develop an input data set for FLUVIAL-12. Obtain the hydrograph for 
the 100-year event and model the channel reach starting with the existing configuration. 
Define the 100-year floodplain limits for the final configuration of the channel. 
2. Use photographs, maps, history of past channel changes, and geomorphic and engineering 
investigations to evaluate the results of hydraulic modeling and to identify erosion and 
sedimentation hazards. Use engineering judgement to develop a "safety factor" for these 
hazard areas (Chang, 1998). 
3. Delineate hazard areas on floodplain maps and establish setback requirements. 
4.5.3. Applicability 
The procedure adopted by San Diego County is a fully developed example of erosion-hazard 
area delineation. In principle, the methodology can be transferred to other regions. A major 
advantage is the incorporation of physically based models of most of the major factors influencing 
channel change. 

ANALYSIS OF CASE STUDIES 


4.5.4. Limitations 
The methodology calls for analysis of erosion and sedimentation produced by the 100-year 
event. Even though the 100-year event is likely responsible for substantial modification of the 
channel geometry, the approach does not include the potential effects of other floods which may 
change the channel and invalidate the starting condition used to model the 100-year event. Most 
likely, a study revision should be undertaken every time the channel suffers major alterations 
(Chang, 1998). 

The methodology does not incorporate procedures to analyze the rate of erosion. FLUVIAL-12 
does not limit the simulation time; however, it does need to have a defined flow record for the 
entire duration of the simulation. 

FLUVIAL-12 is a complex movable-bed model that requires specialized skills to run and interpret 
its results. Careful attention is needed to ensure that field conditions match the assumptions in 
the model. Modeling of alluvial channels using FLUVIAL-12 can be expensive. These skills and 
resources may not be available to some jurisdictions. 

Specific procedures need to be defined to evaluate the results of modeling and use them to 
delineate hazard areas. 

4.5.5. Relevant References 
Chang, H. H. (1982), "Mathematical model for erodible channels," Journal of the Hydraulics 
Division, American Society of Civil Engineers, 108(HY5), p. 678-689. 

Chang, H. H. (1984), "Modeling of river channel changes," ASCE Journal of Hydraulic 
Engineering, vol. 110, No. 2, p.157-172. 

Chang, H. H. (1984), "Water and sediment routing through curved channels," Journal of the 
Hydraulics Division, American Society of Civil Engineers, 101(HY4), p. 644-658. 

Chang, H. H., and D. Stow (1991), "Computer simulation of river channel changes induced by 
sand mining," Proceedings of the International Conference on Computer Applications in 
Water Resources, July 3-6, Taipei, Taiwan, vol.1, p. 226-234. 

Chang, H. H., and J. C. Hill (1977), "Minimum stream power for rivers and deltas," Journal of the 
Hydraulics Division, American Society of Civil Engineers, 103(HY12), p. 1375-1389. 

Cooper Engineering Associates (1987), Flood Plain Management Study for Moosa Canyon 
Creek, prepared for County of San Diego Department of Public Works, July. 

National Research Council (1983), An Evaluation of Flood-Level Prediction Using Alluvial River 
Models, National Academy Press, Washington, D.C. 

4.5.6. Mappability 
San Diego County has produced floodplain maps that show erosion hazard areas. The 
procedure to actually define the extent of hazard areas is mostly based on engineering judgement 
used to complement the results of hydraulic modeling. The methodology can be used to prepare 
erosion hazard maps provided that a consistent set of procedures and guidelines is available to 
derive the extent of these areas. 

ANALYSIS OF CASE STUDIES 


4.5.7. Cost 
No information on cost is presented. Although there are large cost differences according to the 
physical setting, Dr. Chang estimates that the cost is in the order of several thousand dollars per 
mile. The cost per mile decreases for large studies. 

4.5.8. Regulatory Potential 
San Diego County developed procedures for management of alluvial streams. The County also 
developed a floodplain mapping procedure and a Resource Protection Ordinance. There are 
three management options: 

1. Based on past events and existing information, the stream is identified as a "special erosion-
sedimentation" area. 
2. An erosion/sedimentation study is conducted including alluvial hydraulic modeling of the 100year 
flood and additional geomorphic and engineering investigations. Flood elevations for the 
100-year event are computed for the final configuration of the channel. Setbacks are defined 
based on the results of this study. 
3. In cases of sand and gravel mining where there are substantial bed and width changes, an 
equilibrium invert is used for hydraulic modeling. 
The Resource Protection Ordinance mandates that all proposed development be located beyond 
the setbacks for the erosion-sedimentation hazard area as shown in County floodplain maps. 
Development is only allowed if documentation is presented to demonstrate that adequate 
protection can be provided in a manner compatible with the natural characteristics of the stream. 

4.5.9. Summary 
San Diego County has developed floodplain management procedures for alluvial channels. The 
procedures are based on hydraulic modeling using FLUVIAL-12 complemented with geomorphic 
and engineering studies to define erosion and sedimentation hazards. The methodology 
evaluates the effects of the 100-year event on the channel geometry. Development in hazard 
areas is restricted by setbacks that become part of floodplain maps. 

ANALYSIS OF CASE STUDIES 


4.6. City of Austin Technical Procedures for Watershed Erosion 
Assessments 
Document Title: Technical Procedures for Watershed Erosion Assessments (City of 
Austin, 1997) 

Agency: Drainage Utility Department, Austin, Texas 

Author: Raymond Chan & Associates, Austin, Texas 

Date: 1997 

Study Method Engineering and geomorphic analyses
Category:

4.6.1. Overview 
This document was prepared by Raymond Chan & Associates for the Drainage Utility 
Department of the City of Austin, Texas. The report summarizes the procedures to conduct 17 
watershed erosion assessments within the City's jurisdiction. An individual report was prepared 
for each watershed study. The studies were performed in 1997 and included only watersheds 
greater than 640 acres. 

The procedures seek to characterize the changes in channel geometry, identify potential erosion 
problems, and estimate sediment yield as a result of channel erosion. Major tasks to accomplish 
these objectives include: 

 Stream inventory, 
 Erosion problem identification, 
 Characterization of urbanization effects, 
 Prioritization of property protection and channel restoration, 
 Development of an Erosion Hazard Indicator, 
 Nick point identification and management strategies, 
 Identification of meander migration, and 
 Estimation of channel enlargement and sediment yield. 
4.6.2. Detailed Description 
4.6.2.1. Introductory Material 
An initial discussion on stream equilibrium concepts describes land use changes and the response 
they generate in channels and watersheds. For example, aggradation can be the result of 
deforestation in favor of agriculture. Afforestation as replacement of meadowlands often results 
in channel erosion and enlargement due to both suppression of thick grass turf and supply of 
debris that can cause log jams. In general, development introduces an initial increase in sediment 
loading that translates into aggradation due to reduced channel capacity. After development, the 
sediment supply decreases but flows increase in rate and volume due to larger impervious 
surfaces. Although the most noticeable effect is enlargement of the channel, most of the studies 
cited in the document evidence the wide range of morphological responses depending on 
watershed and stream characteristics. The most important factors are: 

ANALYSIS OF CASE STUDIES 


 Basin area, 
 Temporal variability, rate, and volume of discharge, 
 Quantity, particle size, and transport rate of the sediment, 
 Resistance of bed and bank materials, 
 Type and spatial distribution of riparian vegetation, 
 Longitudinal valley gradient, 
 Magnitude, timing, and distribution of urbanization influence in the watershed, and 
 Scale of features in the fluvial system. 
A typical feature is the formation of an inset channel within the enlarged channel. The inset 
channel conveys the low flows. The Technical Procedures document uses numerous references 
to examine fluvial responses to urbanization and stream channel evolution. A large portion is 
devoted to the concept of channel equilibrium and its four possible states: 

 Neutral: no channel form alterations under small disturbances, 
 Stable: temporary alteration followed by return to original state upon cessation of small 
disturbance, 
 Unstable: permanent alteration following a small disturbance without achieving a new stable 
behavior, and 
 Metastable: no alteration following a small disturbance but general system alteration towards 
a new equilibrium state as a consequence of a major disturbance. 
A fluvial system may move from one state of equilibrium to another either smoothly or 
catastrophically. The transition and new state depend on the characteristics of the stream and 
the spatial and temporal distribution of the disturbance. Most engineering analyses use "regime 
theory," which assumes that the system is in a state of metastable equilibrium. That is, the river 
reacts to major flooding events, but its form is dictated mostly by events with a return period 
between 1.5 and 2 years (bankfull discharge). The document cites literature documenting the 
shift of dominant discharges to the 2- to 3-month event (mid-bankfull discharge) when the 
watershed undergoes urbanization. 

The Technical Procedures document ends its introductory section with a description of 
watersheds in the City of Austin and the studies that were conducted in them, mostly using the 
Rapid Geomorphic Assessment (RGA) protocol. The document concludes that the stream 
channel morphology within the City of Austin appears to be consistent with regime theory. 

4.6.2.2. Impervious Cover Computation 
The University of Texas performed this computation using a grid layout to determine drainage 
area and impervious cover. The calculation was done for each reach at geomorphic survey 
sections. Little detail is provided on the GIS-based model that was applied to accomplish this 
task. Impervious cover is estimated for both existing and future conditions. 

4.6.2.3. Watershed "Vital Statistics" 
Basic characteristics are gathered to provide a general assessment of the watershed. Sources 
of information include USGS topographic maps, the Travis County soil survey, development data 
from the Bureau of Economic Geology, maps to identify parks, and drainage improvement plans 
from the City's Drainage Utility. 

The results are presented in a watershed report that details drainage area, existing and future 
impervious cover, summary of watershed development, stream length and slope, major 

ANALYSIS OF CASE STUDIES 


tributaries, soil description, list of parks, unique watershed features, and existing and upcoming 
infrastructure projects. 

4.6.2.4. Urbanization Effects 
The regional regression equations developed by the USGS are used to estimate existing and 
future peak discharges for the 2- and 10-year events. These equations require using the 
impervious fractions previously computed. The flows are computed for every reach and every 
geomorphic survey cross section. The percent increase in the peak flows can be used as an 
indicator of management needs for the watershed. 

4.6.2.5. Stream Inventory 
Erosion Priority Rating 

The purpose of this task is to identify and prioritize erosion problems. A field survey of the stream 
is conducted to select reaches, measure geomorphic parameters, assess erosion problems, 
determine probable causes, assign priorities, and formulate mitigation options. Streams are 
investigated if the drainage area is at least one square mile. A stream inventory form was 
developed for this purpose. 

Items to be noted include buildings, parking lots, bridges and public facilities, retaining walls, trees, 
utility poles and utility crossings, fences, steep banks within parks, and significant loss of land. 
Priorities were assigned as: 

Priority 1 - A primary structure, road, or public facility currently threatened and requiring 
immediate attention. 

Priority 2 - Other items such as retaining walls, fences, trees and woodlands currently 
threatened. This also includes areas that are facing substantial land loss due to erosion. 

Priority 3 - Items not currently threatened but that may be in danger in the future due to erosion. 

A detailed geotechnical investigation is performed later on Priority 1 areas. The information 
gathered includes assessment of vegetation, substrate material, bank soil strata, cross section 
measurements, indicators of erosion potential, slope stability, and structure stability. 

Identification of Erosion Causes 

Photographs are used to document the conditions of the creek. The objective is to collect this 
information at various points in time to observe changes in bottom width, bank slopes, vegetation, 
baseflow conditions, and channel substrate. This information can be used to assess 
maintenance needs in the watershed. Causes of erosion include: 

 Effects of development (widening, downcutting, aggradation, slope failure, planform change, 
channel straightening due to increased velocity) 
 Pipelines along and crossing the stream 
 Manholes 
 Storm sewer outfalls (pipes and channels) 
 Rock berms causing scour holes, bank widening, and upstream aggradation 
 Severe bends 
 Erodible soils 
 Past channelization projects 
 Culverts without energy dissipators 
ANALYSIS OF CASE STUDIES 


 In-channel disturbances 
Inset Channel Geometry 

Periodic measurements of inset channel width and depth are taken at the bankfull level in riffle 
sections. These measurements are used in geomorphic analyses. Bankfull indicators include 
exposed roots, moss lines, exposed alluvium, and vegetation type and condition. 

Reach Classification 

The stream is subdivided into reaches defined according to the following criteria: 

 In straight channels, length is at least 20 bankfull widths with the bankfull width given by the 
1.2-year event. In meandering channels, the length is greater than two meander 
wavelengths, 
 A 10 percent increase either in the flow rate of the basin imperviousness, or 
 Effective drainage area greater than one square mile. 
These reaches are also termed response segments and are used to examine ongoing 
geomorphic processes. 

The document stresses the point that morphologic classifications are not appropriate in cases 
where development is causing channel adjustment because the form-process relationship is being 
disrupted. No classification system has been devised for urban streams. Therefore, it is more 
appropriate to classify channels according to the erosion susceptibility of boundary materials. For 
the City of Austin, it was determined that the channels respond in one of three basic patterns: 

 Valley formation: banks and bed are both susceptible to scour, 
 Channel widening: banks are more susceptible to scour, and 
 Channel downcutting: bed is more susceptible to scour. 
The susceptibility to scour is computed as the sum of stickiness, plasticity, and firmness indexes 
as produced by standard soil consistency tests. 

Reaches are classified according to three standard types: alluvial channel, rock-bed channel, and 
rock-controlled channel. Structural channels are viewed as a fourth type. These designations are 
assigned based on a combination of examination of maps and photographs and the field survey. 
Reaches can be classified further using a system developed by Knighton (1987) which describes 
the boundary material composition of substratum, substrate, and least resistant bank. These 
materials can be rock, undisturbed overburden, alluvium, or armor. This classification system is 
used in determining the enlargement ratio. 

Geomorphic Assessment of Reaches 

The document cites three adjustment scenarios: 

 Initial phase: Roughness and depth adjust rapidly to changing conditions. Affected features 
are microforms with relaxation times of the order of 100 year. 
 Second phase: Width is modified until depth and roughness are adjusted. Affected features 
are mesoforms with relaxation periods between of the order of 101 to 102 years. 
 Third phase: Longitudinal slope is adjusted and reaches equilibrium with width, depth, and 
roughness. This is a macroform feature with relaxation time of the order of 102 to 103 years. 
ANALYSIS OF CASE STUDIES 


The Technical Procedures document indicates that true representation of any fluvial system 
should take into account micro, meso, and macroscale features. However, the fine spatial and 
temporal variability of microforms makes measurement and monitoring impractical. On the other 
extreme, macroform dimensions and time scales are too large to be of practical management 
value. Consequently, geomorphic assessments are to be based on mesoforms with a general 
understanding of the other scales involved. 

Relaxation periods for disturbances applied on mesoforms are defined as 10 to 55 years for 
alluvial and rock-bed channels; however, no guidance is provided to select a value. No relaxation 
period was defined for rock-controlled channels, but 55 years is suggested for use in 
computations. 

Each reach is analyzed through Rapid Geomorphic Assessment (RGA) to determine its stability 
conditions. A RGA form is used to check for the presence of geomorphic processes indicative of 
aggradation, degradation, widening, and planform adjustment. Scores for these processes are 
used to arrive at a Stability Index (SI) that describes the reach as stable, transitional, or in 
adjustment. The results of these analyses can be summarized in a color-coded map showing the 
stability classification for each reach. However, the SI does not indicate the intensity of 
geomorphic activity, whether the processes are still active, or the sequence in which they 
occurred. These issues need to be resolved with additional historic or anecdotal data. 

In addition to the RGA form, the fluvial geomorphology field reconnaissance form records a large 
amount of information: 

 General site data, 
 Photographic records, 
 Bank material composition for several stratigraphic units (soil consistency, particle size, soil 
class, and plasticity index), 
 Environmental integrity (channel alteration, sediment deposition, embeddedness of coarse 
material in fine material, bank condition, riparian vegetation), 
 Sketch of planform geometry, 
 Field erosion process classification (shear dominated, falling-stage dominated, and/or 
controlled by other nonfluvial processes like expansion-contraction, chemical weathering, 
rainfall impact, and animal trampling and burrowing), 
 Rosgen (1994, 1996) classification (entrenchment ratio, width/depth ratio, sinuosity, 
substrate, substratum, slope), and 
 Manning's rating curve. 
4.6.2.6. Enlargement Potential 
The theory to estimate channel enlargement was proposed by Morisawa and LaFlure (1979) and 
modified to produced the Austin Enlargement Curve. The basic assumptions are: 

 Change in impervious land cover in the watershed is a suitable surrogate for instream 
erosion potential changes, 
 Scour adequately represents instream erosion potential, 
ANALYSIS OF CASE STUDIES 


 Channel enlargement varies inversely with drainage area and channel boundary resistance, 
and 
 For the purposes of assessing response to changes, channels can be classified in three 
categories: rock controlled, rock bed controlled, and alluvial. 
Morisawa and LaFlure (1979) developed a curve in which enlargement is a function of area with 
imperviousness greater than 5 percent. This curve was modified by providing different curves for 
each of the three channel categories as observed for the City of Austin. This approach does not 
include the effect of past channel maintenance activities. Another curve termed "channel 
relaxation curve" is used in the computation, but the document does not mention its origin. The 
procedure to estimate channel enlargement is the following: 

1. Using regional regression equations, compute the bankfull discharge for existing conditions. 
2. Determine bankfull depth as an average of field-observed values and those computed using 
Manning's formula. Calculate flow area ABFL EXT. 
3. Using land use maps and other information, determine the average age of development 
impacting the stream (ti). 
4. Define the relaxation period (tr) corresponding to the channel type. Compute the ratio tI / tr. 
5. Compute existing and future watershed impervious cover. Review the Stability Index SI and 
study any existing photographs to check whether the development pattern and impervious 
cover conclusions are sensible. 
6. Determine the ultimate enlargement ratio for existing conditions RE EXT ULT from the 
Morisawa-LaFlure curves modified for the City of Austin. Use tI /tr in the "channel relaxation 
curve" to determine RE EXT / RE EXT ULT, where RE EXT is the existing enlargement ratio. 
Compute RE EXT. 
7. Compute the pre-development bankfull channel area as ABFL PRE = ABFL EXT / RE EXT. 
8. Obtain RE FUT ULT from the modified Morisawa-LaFlure curves using the future impervious 
cover. Compute future bankfull channel area from ABFL FUT ULT = ABFL PRE  RE FUT ULT . 
9. Compute the difference between ABFL FUT ULT and ABFL EXT. This difference will provide an 
estimate of sediment yield. 
10. Determine the appropriate enlargement process. The channel may be widening, 
downcutting, or both. Downcutting may be constrained by grade-control structures such as 
culverts. Widening may be restricted by resistant channel boundaries. 
11. Analyze the possibility of further bank failure due to enlargement and the ensuing increase in 
top width. For example, near vertical or unvegetated slopes are unstable, and widening may 
cause catastrophic failure and increase sediment yield. The increase in the top width can be 
viewed as an erosion hazard indicator. 
12. Compute sediment volume yield as channel length times the total bank loss. This estimate 
will be useful only as a planning tool due to variability along the channel reach. Use a suitable 
density (e.g., 120 pounds per cubic foot) to compute the weight of sediment yield. 
ANALYSIS OF CASE STUDIES 


13. Determine the new cross section. This requires assessment of bank failure mechanisms 
and estimation of final bank slopes. Another consideration is the distribution of widening 
between the two banks according to the relative resistance of bank materials. 
14. Formulate management options depending on all of the observed conditions. For example, if 
the watershed is near ultimate development and the creek has nearly achieved its final 
configuration, only local erosion problems may need to be addressed. 
4.6.2.7. Nick Point and Meander Management 
Nick points and meander issues will be identified in the stream inventory phase to complement the 
stream stability analysis. Nick point migration indicates downcutting of the stream bed that may 
result in bank instability, which in turn causes channel widening and increased sediment load. 
Similarly, meander migration involves erosion and may affect structures located on the banks. 

Nick points can occur naturally or be the result of structures such as pipeline crossings and 
bridges. Migration of nick points may range from a few inches to hundreds of feet per year, 
depending on the resistance properties of the bed material. The migration rate is usually an 
indicator of the associated problem because it indicates the rate at which the channel is 
reconfiguring itself in response to outside impacts. However, the severity of any migration rate is 
often dictated by the structures in danger. For example, a few inches of annual migration may 
have serious consequences on the geotechnical stability of the stream banks. 

Characterization of nick points involves field documentation of location, height, substrate and 
substratum description, features affecting nick point movement, and distance to grade control 
structures that may stop migration. Mitigation measures for nick point problems usually involve 
the introduction of a control. The two major options are burial with material to avoid exposure of 
the bed and introduction of hard outcrops (rock or concrete) to interrupt bedding planes of soft-
hard materials. 

Meander severity is treated according to the methods in Hickin and Nanson (1984). Erosion 
proceeds in the outer bend of meanders whereas deposition takes place in the inner bend. Most 
of this process takes place in the downstream third of the meander length. Therefore, meanders 
move both laterally normal to the valley slope and down slope parallel to the valley. Hickin and 
Nanson (1984) determined that the rate of channel migration is directly proportional to stream 
power and channel width and inversely proportional to bend radius, bed material shear 
resistance, and bank height. Further, the migration rate is maximum when the ratio of bend 
radius to channel width r/WBFL is between 2 and 3. This conclusion was derived from streams in 
prairielands but was extrapolated to the Austin area. 

The procedure in the document consists of identifying channels with a sinuosity of 1.2 or greater 
and then determining the r/WBFL ratio. If the ratio falls within the fast-migration range, this factor is 
taken into consideration when developing management options. 

4.6.2.8. Prioritization of Erosion Problems 
The Technical Procedures document proposes a prioritization scheme based on scores assigned 
to the threatened resources and the potential problems. Resource values vary from a maximum 
of 100 for a major road to 35 for a yard. Little information is provided on how the scores are 
actually estimated; however, the process is systematic and leads to ranking of reaches based on 
the significance of erosion and affected structures. The procedure is implemented by means of 
spreadsheets. 

ANALYSIS OF CASE STUDIES 


4.6.2.9. Stream Management Recommendations 
The Technical Procedures document devotes one chapter to the development of restoration 
measures at the channel and watershed levels. These measures can involve floodplain 
management, channel geometry modifications, vegetation restoration, flow modification, and 
substrate manipulation. The Technical Procedures document outlines an 11-step procedure to 
identify and design appropriate restoration measures. 

4.6.3. Applicability 
Most of the techniques described in the Technical Procedures document are directly applicable to 
the riverine erosion investigations. The document offers a systematic procedure based on 
geomorphic and engineering principles to analyze erosion problems. A major contribution of the 
methodology is its emphasis in examining the temporal variations of channel evolution as a 
function of continuing development. Although mapping the extent of hazard areas is not one of its 
objectives, the Technical Procedures document suggests that channel enlargement computations 
combined with geotechnical stability considerations can be used to delineate erosion hazard 
areas. 

The methodology was developed specifically for streams in the Austin area but, in principle, can 
be modified for other regions. 

4.6.4. Limitations 
The methodology depends on the development of site-specific channel enlargement curves, 
which may be costly to obtain. However, a carefully designed research program may provide 
sufficient information to typify stream systems into regionally valid categories. Such an approach 
would lessen the burden when conducting studies at a national scale. The limitations of such 
regional studies must be taken into account when conducting site-specific analyses. 

Delineation of hazard areas requires development of new procedures for geotechnical bank 
stability analysis. The Technical Procedures document alludes to the need for these procedures, 
but a national methodology requires explicit definition of methods. These can be easily defined 
from a variety of sources. 

The methodology only computes total channel enlargement and does not distribute it among the 
two banks and the stream bed. This limitation may be alleviated in cases where erosion takes 
place either at the banks or at the bottom. Nonetheless, a procedure needs to be devised to 
handle all situations. 

4.6.5. Relevant References 
Hickin, E.J., and G.C. Nanson (1984), "Lateral migration rates of river bends," ASCE Journal of 
Hydraulic Engineering, vol. 110, No. 11, p.1557-1567. 

Knighton, A. D. (1987), Downstream River Channel Adjustments, in River Channels: 
Environment and Process, Institute of British Geographers special publication, K. Richards 
(ed.), Basil Blackwell, Inc., New York. 

Morisawa, M., and LaFlure (1979), "Hydraulic Geometry, Stream Equilibrium and Urbanization," 
in Adjustments of the Fluvial System, D. D. Rhodes and G.P. Williams (eds.), Proceedings 
of the 10th annual Geomorphology Symposium Series, Binghamton, New York, Kendall Hunt 
Dubuque, Iowa. 

ANALYSIS OF CASE STUDIES 


Renwick, W.H. (1992), "Equilibrium, disequilibrium, and nonequilibrium landforms in the 
landscape," in Geomorphic Systems, J.D Phillips and W. H. Renwick (eds.), Proceedings of 
the 23rd annual Geomorphology Symposium Series, Binghamton, New York, Elsevier, 
Amsterdam. 

Rhoads, B. L. (1994), "Fluvial Geomorphology," Progress in Physical Geography, 18(4), p. 588

608. 
Rosgen, D. L. (1994), "A classification of natural rivers," Catena, 22, pp. 169-199. 

Rosgen, D. L. (1996), Applied River Morphology, Wildland Hydrology, Pagosa Springs, 
Colorado. 

Simons, Li & Associates, Inc. (1985), Design Manual for Engineering Analysis of Fluvial 
Systems, prepared for Arizona Department of Water Resources. 

Thorne, C.R., and M.K. Tovey (1981), "Stability of composite river banks," Earth Surface 
Processes and Landforms, 6, pp. 469-484. 

Thorne, C.R., and M.R. Welford (1994), "The equilibrium concept in geomorphology," Annals of 
the Association of American Geographers, v. 84, No. 4, p. 666-696. 

Thorne, C.R., and J. Lewin (1982), "Bank processes, bed material movement, and planform 
development," in Adjustments of the Fluvial System, D. D. Rhodes and G.P. Williams (eds.), 
Proceedings of the 10th annual Geomorphology Symposium Series, Binghamton, New 
York, Kendall Hunt Dubuque, Iowa. 

4.6.6. Mappability 
The results of this methodology produce maps for indicators of erosion and localized erosion 
features such as nick points. The color-coded maps show channels in adjustment, transition, or 
stable conditions. This kind of information is useful for watershed and stream management but 
not for delineation of riverine erosion hazard areas. The maps need to be enhanced to show the 
result of channel enlargement computations. 

4.6.7. Cost 
No information on cost is presented in the guideline document. Mr. Tom Hegemier of Raymond 
Chan & Associates estimates an average cost of $2,000 per mile. 

4.6.8. Regulatory Potential 
The methodology could potentially be enhanced to allow regulation based on delineation of 
hazard areas. 

4.6.9. Summary 
The Technical Procedures document proposes a systematic methodology aimed mainly at 
characterization of erosive state of a stream. Geomorphic and engineering methods are outlined 
to classify channels and determine if they are adjusting through high geomorphic activity, in 
transition to a new equilibrium form, or stable. The basic tool is a set of curves to estimate 

ANALYSIS OF CASE STUDIES 


channel enlargement. The document presents techniques to analyze localized features such as 
nick points and meanders. 

While in its present form the methodology is extremely useful as a watershed and stream 
management tool, refinements are needed to allow delineation of hazard areas. 

ANALYSIS OF CASE STUDIES 


4.7. River Stability Studies for Virgin River, Santa Clara River, and 
Fort Pierce Wash, Utah 
Document Title: River Stability Study, Virgin River, Santa Clara River and Ft. Pierce 
Wash, Vicinity of St. George, Utah (CH2M Hill and J.E. Fuller/Hydrology 
& Geomorphology, 1996) 

Agency: City of St. George, Utah 

Authors: CH2MHill and JE Fuller/Hydrology & Geomorphology, Inc. 

Date: 1996 

Study Method Geomorphic analysis
Category:

4.7.1. Overview 
This report describes the results of an investigation of river stability for three major watercourses: 
the Virgin River, the Santa Clara River and the Fort Pierce Wash, in the vicinity of St. George, 
Utah. The objective of the study was to develop a floodplain management plan to help the City of 
St. George better manage flood and erosion hazards associated with development in floodplains 
located within the city limits. As a result of the study, erosion hazard boundary corridors along 
each of three rivers have been established. The report describes the methodologies and 
assumptions used to develop the erosion hazard boundaries. The erosion hazard boundary 
corridors are presented on aerial maps. 

The basic approach used in the study is geomorphic analysis, entailing the following tasks: data 
collection, field reconnaissance, historic evaluation, and geomorphic evaluation. Hydraulic and 
sediment transport analysis and modeling were not included. 

4.7.2. Detailed Description 
Historic evaluation and geomorphic evaluation are essential parts of the study. Evaluation of 
historic river channel changes and understanding of the nature of past river behavior provide a 
basis for predicting the type of channel changes that could occur in the future. Geomorphic 
evaluation allows predicting where future aggradation, degradation, and lateral channel migration 
are likely to occur. Similar methodologies for historic and geomorphic evaluation were applied to 
all three streams. In the following sections, the methodologies are described in detail for the 
Virgin River and in brief summaries for the other two streams. 

4.7.2.1. Historical Evaluation, Virgin River 
The Virgin River is one of the major tributaries of the lower Colorado River. The drainage basin of 
the Virgin River covers about 3,800 square miles of southern Utah and northern Arizona. Its 
perennial flow and fertile floodplain have supported agricultural activities since prehistoric times. 

ANALYSIS OF CASE STUDIES 


The historical evaluation of the Virgin River was based on following types of data: 

 Historic descriptions from pioneer diaries (1853 to 1902) 
 Historic aerial maps and ground photographs (1902 to 1995) 
 Topographic maps and cadastral survey maps (1870 to 1992) 
 Hydrological and climatic data 
 Published engineering reports 
Review of historic description shows that the appearance and character of the Virgin River have 
substantially changed. The river observed by the pioneers was narrower, had grassy banks and 
was lined by tall trees and swampy grassland. The river seen in the earliest photographs (1902) 
is wide and braided with a barren active floodplain and vertical cut banks. 

A total of nine aerial photographs, dating back to 1938, and a cadastral survey map of 1870 
were compared to identify the type and extent of changes on the active river channel and 
floodplain. Map overlays were prepared from each set of historic aerial photographs to compare 
the locations of the active channel and floodplain. Measurements made from the overlays 
indicate that the active channel of the Virgin River decreased in width by about 62 percent from 
1938 to 1993 (684 to 257 feet), and the maximum active floodplain width decreased from 5,100 
to 2,200 feet. The largest measured lateral migration distance due to bank erosion was more 
than 900 feet. 

Hydrologic and climatic information was analyzed. There were 12 floods with discharges greater 
than 10,000 cubic feet per second (approximately a 5-year flood) recorded for the Virgin River at 
Virgin, Utah, with the largest recorded natural flood in December 1966. Comparison of the 1960 
and 1966 aerial photographs indicated that the active channel of the Virgin River shifted 
significantly within the floodplain and that bank erosion reached up to about 800 feet in some 
places. Another large flood, the January 1989 Quail Creek flood, occurred due to a reservoir 
failure. Channel movement was not as significant as other historic natural floods due to the 
flood's short duration. The paleoflood hydrologic study revealed that there were at least 12 
floods larger than the 1966 event and that there have been at least two floods larger than the 
Quail Creek Flood since about 900 AD. 

From the historical evaluations, the following important conclusions were obtained: 

 The Virgin River has been subject to large, erosive floods for at least the past 1,000 years. 
These erosive floods have caused the active channel to frequently shift locations within the 
geologic floodplain. 
 Channel movement and/or bank erosion of 800 to 2,000 feet was not uncommon during the 
7 to 14 year period between dates of aerial photographs. 
 During the past 35 years, the Virgin River has become narrower and deeper, and the active 
floodplain has become densely vegetated. 
4.7.2.2. Geomorphic Evaluation, Virgin River 
The geomorphic evaluation estimates where future aggradation, degradation, and lateral channel 
migration are likely to occur. Several river stability assessment methodologies were used 
including river-classification based techniques, interpretation of historic river response, USACE 
allowable velocity and hydraulic geometry procedures, Federal Highway Administration guidance, 
and sediment continuity relationships. As empirically based methodologies, they are more 
appropriate for predicting the direction of expected channel change than the exact magnitude of 
future channel changes. The authors believe that methodologies that allow precise determination 

ANALYSIS OF CASE STUDIES 


of future (long-term) river behavior simply do not exist. However, the authors also believe that 
by application of the geomorphic principles and methodologies, combined with the interpretation 
of the historic information, a reasonable assessment of the potential for future erosion can be 
made. The erosion corridor was defined based on past river behavior and the existing river 
channel conditions. Engineering judgement was used in developing approximate future erosion 
hazard corridor limits. 

The study summarized variables that affect river behavior into eight categories: hydrology, flow, 
flood characteristics, riverbed and bank sediment, climate, time scale, channel vegetation and 
watershed characteristics. Sources for data for these categories used for the geomorphic 
evaluation include: 

 Historic records on channel change, 
 Aerial photographs and maps, 
 HEC-2 output from the effective FEMA Flood Insurance Study, and 
 Engineering and scientific reports. 
A visual survey was conducted, and observations on above variables were recorded. The 
longitudinal profile and channel bed elevation data were analyzed. Results indicate that long-term 
scour of up to 12 feet has occurred since the 1960s and that net long-term scour has probably 
been occurring in the study reach since at least 1862. Channel degradation on this scale tends to 
reduce bank stability and increase the potential for lateral erosion. 

Stability of channel reaches was evaluated using two classification systems for the Virgin River 
study reach: the Federal Highway Administration (FHWA, 1991; Brice, 1983) system and the 
Rosgen system, both described in detail below. 

Federal Highway Administration classification system: The method (FHWA, 1991) was 
developed primarily for the evaluation of river stability near bridge structures and uses readily 
identified river characteristics to predict stability. The system assigns a relative erosion potential 
classification based on the 12 river characteristics below: 

 River Size: The potential for and scale of lateral erosion increase with river size. 
 Flow Habit: Perennial rivers tend to be more erosive than ephemeral rivers, except in arid 
regions where flash floods and unstable bank materials cause significant lateral erosion. 
 Bed Materials: The Virgin River has a sandy gravel bed and stratified sand and gravel bank 
materials susceptible to scour. 
 Valley Setting: Channel reaches with low relief (less than 100 feet) typically have more 
erosion-prone banks. 
 Floodplains: Channels with wide floodplains (more than 10 times channel width) typically 
experience more lateral channel movement than channel with narrow floodplains (2 to 10 
times channel width). 
 Natural Levees: Rivers with natural levees tend to have low rates of lateral migration. 
 Apparent Incision: Rivers with vertical cut banks are generally unstable. 
 Channel Boundaries and Vegetation: Alluvial rivers are more susceptible to lateral erosion 
than non-alluvial rivers. The Virgin River is an alluvial river except for a small portion. Banks 
with less than 30 percent woody vegetation and banks with cohesive soils are generally less 
susceptible to scour. The Virgin River does not have these characteristics. 
 Sinuosity: The Virgin River has a sinuosity less than 1.5 and probably is not subject to 
meander geometry relationships. 
 Degree of Braiding: Braided rivers are laterally unstable. The Virgin River study reach was 
generally braided form 1938 to 1967 and retains some characteristics of a braided river at 
high flows today. 
 Degree of Anabranching: The study reach is not anabranched, as defined by FHWA. 
ANALYSIS OF CASE STUDIES 


 Variability of Width and Development of Bars: Rivers with relatively uniform width and 
narrow, regular point bars tend to have slow lateral migration rates. The study reach has 
irregular channel widths and wide irregular bars; therefore, it is subject to rapid migration. 
Under this classification system, the Virgin River study reach scores high in all of the categories 
that indicate lateral instability. It indicates that the river reach has high potential for rapid erosion 
and lateral channel movement. 

Rosgen Classification System. The Rosgen system (Rosgen, 1985, 1994, 1996) was 
developed from data from the intermountain region of the western United States and has many 
adherents among state and Federal agencies in Utah. The system classifies river reaches into 
different types based on its slope, channel type, bed materials, entrenchment, erodibility and type 
of sediment. 

The Rosgen bank erosion matrix (Rosgen, 1996) was used to evaluate bank erosion potential. 
The bank erosion matrix assigns measurable parameters describing bank conditions. These 
parameters include bank height/bankfull height, root depth/bank height, root density, bank angle in 
degree, and surface protection. Points are assigned to each parameter, which are then are 
summed and adjusted for bank material composition and stratification. A high total score 
indicates high erosion potential. The banks of the Virgin River in the study reach scored in the 
range of very high to extreme categories of bank erosion potential. 

The study evaluated the potential for bed erosion by comparing the permissible velocity, 
established by USACE (1995) for design of non-scouring flood control channels, with the average 
channel velocities determined from Flood Insurance Studies. The results showed that floods with 
10-year or greater return periods would exceed the permissible velocities. The USACE criteria 
relating flow depth and velocity to the movement of bed materials and erosion of cohesive bank 
materials (USACE, 1990) indicate that the Virgin River will erode its bed and banks at flow rates 
substantially less than the 10-year flood velocities and depths. The criteria developed by Thorne 
and Osman (1988) as well as by Rosgen (1996) were used to evaluate bank stability. Results 
indicated that the vertical-cut banks along the Virgin River are unstable. Finally, the channel 
migration rates during the period of record were determined by comparing channel locations in 
sequential aerial photographs. The results show the potential for very rapid bank migration within 
the study reach. 

Sediment data were obtained from several engineering reports (SCS, 1990; Biowest, 1995) and 
USGS measurements at the Hurricane, Utah, station. These sediment data tend to support the 
theory that much of the geomorphic work on the Virgin River is accomplished during floods, 
instead of during bankfull discharges and low flows. 

Based on conclusions of historic and geomorphic studies and engineering judgement, an erosion 
hazard corridor was defined. 

4.7.2.3. Conclusion 
From the above analysis, the study concluded that the Virgin River is subject to extreme bank 
erosion, lateral channel movement, and channel changes in bed elevation. Bank erosion with 
accompanying lateral migration of up to 2,000 feet over the past 120 years was identified using 
historic maps and aerial photographs. Human modifications in the floodplain of the Virgin River 
and attempts to control erosion have generally been unsuccessful during the 140-year period of 
settlement. No trend toward increase of the river stability was identified. For all these reasons, a 
wide erosion hazard corridor was established. 

ANALYSIS OF CASE STUDIES 


4.7.2.4. Santa Clara River 
Santa Clara River is a tributary of the Virgin River. Its drainage basin covers about 545 square 
miles of mountainous terrain north of the City of St. George, Utah. The river has perennial flow; 
its floodplain has been exploited for agriculture. Although floods and flood-related channel 
changes have occurred during the last 140 years, the lateral position of the channel within its 
floodplain has been very stable. Only in recent years has flood-related channel instability caused 
problems for landowners. 

The same historic and geomorphic analyses used on the Virgin River were applied to the Santa 
Clara River. Evaluation of the historic channel changes of the Santa Clara River reaches 
demonstrated the following conclusions: 

 The Santa Clara River experienced a period of channel bed degradation during the late 
1800s that caused the main channel to become incised. However, the channel pattern 
apparently did not change significantly. The channel was relatively stable from 1938 to 
1994. During this period the watershed experienced an extended drought, the record flood, 
and many other large floods. 
 Recent channel instability is concurrent with a period of human disturbances of the floodplain 
and channel. Undisturbed reaches have been stable since 1938. Recent channel instability 
is due to relatively small-scale bank failures, but generally the bank has not eroded beyond 
the limits of the incised channel. 
 Geomorphic evaluation consisted of field visual survey, hydraulic data analysis (from FEMA 
Flood Insurance Studies), channel longitudinal profile development and analysis, and river 
stability evaluation using the FHWA and Rosgen classification systems. 
 The study reach has been relatively stable, except where bank vegetation has been 
disturbed. Historic channel degradation has over-steepened banks in portions of the study 
reach. 
 Hazard of bank erosion and lateral channel movement is high where bank vegetation has 
been removed and is moderate elsewhere. 
 Long-term scour has lowered the channel bed elevation by as much as 12 feet since 1850 
and has increased the potential for bank erosion and lateral channel movement. 
 Overall, the study reach is subject to lateral erosion and degradation where bank vegetation 
is disturbed. 
Based on these conclusions, the results of the historic analysis and engineering judgement, an 
erosion hazard corridor for the Santa Clara River was defined. 

4.7.2.5. Fort Pierce Wash 
The Fort Pierce Wash drainage basin covers about 1,600 square miles of southern Utah and 
northern Arizona. Its watershed consists primarily of poorly vegetated, arid, high desert areas 
with dry washes in hilly, bedrock terrain. The study reach has ephemeral flow; recently, near-
perennial discharge has begun to flow in the wash. The discharge comes from irrigation return 
flow, mining tailwater and other sources related with urbanization of the St. George area. 

ANALYSIS OF CASE STUDIES 


Historical evaluation and geomorphic analysis were applied to evaluate stability of the study 
reach. Channel changes that occurred along Fort Pierce Wash during the historic period are 
summarized as follows: 

 The wash has undergone significant modifications of its channel and floodplain in the past 30 
years. The active channel in the disturbed reach has changed from a wide, braided river to a 
narrow, confined channel choked by dense growth. The disturbed reach narrowed by more 
than one order of magnitude. Lateral channel movement has not been a significant hazard 
along Fort Pierce Wash since 1938, partly because that none of the floods recorded in the 
past 90 years for the study reach exceeded the 20-year recurrence interval. 
 Human activities on the floodplain, mainly sand and gravel mining and urban development, 
have strong impacts on channel changes. Mining has moved material from channels and 
floodplain, and two mining pits have intercepted the main channel in two places. 
Development has encroached on the floodplain. 
 The geomorphic evaluation indicated that human impacts have had the greatest effect on 
channel stability in the study reach. Lateral channel stability has not been a problem in the 
past and is not expected to cause significant problems in the future, except for areas near 
the sand and gravel mining reach. 
An erosion hazard corridor was developed along the study reach. 

4.7.2.6. Recommended Management Plans 
Based the on above analysis, the study recommended floodplain management plans for each 
river. The general recommendations for management of all reaches include the following 
elements: 

 Adopt the erosion hazard corridor. 
 Amend the existing flood control ordinances and policies to include river management 
policies. These policies would support preservation of the natural river systems, promote 
land uses that are compatible with a natural river system, and limit construction of structural 
improvements within the erosion hazard corridor. Exceptions would be given to measures 
that protect existing structures needed for public safety or where the channel threatens to 
move outside of the established erosion hazard corridor. 
 Regulate all new developments within the corridor by requiring a special use permit. 
 Establish a no-build zone close to the banks. Habitable structures should be set back a 
minimum distance from the top of the bank. 
Specific recommendations are made for each of the studied rivers. 

4.7.3. Applicability 
The report described a typical river stability study in arid areas. The study analyzed historic 
records to interpret river behavior in the past and used geomorphic analysis based on field 
observation and historic data to predict the erosion potentials for the future. Two river stability 
classification systems, one developed for nationwide use and another based on regional features, 
were used to evaluate the stability of study reaches. This methodology is applicable to other 
rivers. 

ANALYSIS OF CASE STUDIES 


Based on the analysis, erosion hazard corridors were defined for all studied river reaches. The 
corridors were developed based on the following factors: 

# Judgement 
 Historic bank movement distances measures on maps and photographs 
 Meander geometry relationships 
 Regime/hydraulic geometry relationships to identify unstable/stable reaches 
 Geometric mapping of soil units 
 Presence of permanent improvements, such as bank protection, bridges, and pipeline 
crossings, that would be protected and repaired. 

Although the above elements do not generate a specific distance to use as setback from banks, 
they provide a sound base to support proper engineering judgement to delineate erosion 
corridors. The maps in the report illustrate these corridors. 

The study recommended management plans for each river studied. The common elements of 
the management plans are applicable to other rivers with similar natural and human-introduced 
characteristics. 

4.7.4. Limitations 
It is clear that using the procedures described in the report to determine erosion corridors 
requires specialized knowledge and that the quality of the work would largely rely on the project 
engineers experience and judgement. 

4.7.5. Relevant References 
Biowest, Inc. (1995), Virgin River Geomorphic and Hydrologic Studies Related to Channel 
Forming Flows, Reported prepared for Utah Division of Wildlife Resources, Salt Lake City, 
Utah. 

Federal Highway Administration (1991), HEC-20: Stream Stability at Highway Structures, 
Washington, D.C. 

Rosgen, D. (1985), A Stream Classification System, in Riparian Ecosystems and Their 
Management  Reconciling Conflicting Uses, Proceedings of the First North American 
Riparian Conference, April 16-18, 1985. Tucson, Arizona. 

Rosgen, D. L. (1996), Applied River Morphology, Wildland Hydrology, Pagosa Springs, 
Colorado. 

Soil Conservation Service (1990), Virgin River - Utah, Cooperative Study, prepared in 
cooperation with Utah Division of Natural Resources. 

Thorne, C. and A. Osman (1988), Riverbank Stability Analysis. II: Applications, ASCE Journal of 
Hydraulic Engineering, vol. 114, No. 2, p. 151-172. 

U.S. Army Corps of Engineers. (1990), Stability Assessment for Flood Control Channels (Draft). 
U.S. Army Corps of Engineers (1995), Hydrologic Design of Flood Control Channels, EM11102-
1601. 
ANALYSIS OF CASE STUDIES 


4.7.6. Mappability 
The study mapped historic river channel locations for the Virgin River from 1874 to 1993, and 
erosion hazard corridor boundaries along the studied reach for each of three rivers. 

4.7.7. Cost 
No cost information is reported. Mr. Jon Fuller, a member of the PWG, estimates that the cost is 
approximately $1,000 per mile for the geomorphic analysis. 

4.7.8. Regulatory Potential 
The results of the study can be used for floodplain management as well as regulation. The study 
recommended necessary revisions to ordinances of the City of St. George to incorporate 
floodplain management policies that would reduce human-induced damages to the river systems. 
Specifically, the study recommended requiring a special use permit to regulate all new 
development within the erosion hazard corridor. To obtain a permit, the developers within a 
corridor must meet following requirements: 

 Meet NFIP requirements for development within a floodplain. 
 Provide an engineering and geomorphic study prepared by a professional engineer licensed 
to practice in the State of Utah and certifying that the proposed development will not be 
affected by erosion over a 100-year planning period. 
 Demonstrate that proposed bank stabilization, if any, will not deleteriously affect reaches or 
development upstream or downstream. 
 Demonstrate the stability of proposed bank stabilization, if any. Local scour, long-term 
degradation, channel movement, and bank erosion must be explicitly addressed in the bank 
protection design. 
 Hold the City harmless from any and all claims resulting from erosion or any other flood-
related damage to development within the erosion hazard corridor. 
 Provide for perpetual maintenance of the bank stabilization at no cost to the City or any other 
public agency. 
 Provide a maintenance and access easement adjacent to any bank stabilization project. 
 Obtain necessary floodplain, wetlands (404), and water quality (401) permits or approvals 
for any construction activities at no cost to the City. 
4.7.9. Summary 
The report describes historic and recent channel changes on the Virgin River, Santa Clara River, 
and Fort Pierce Wash in the City of St. George, Utah. The document describes methodologies 
used to predict future river behavior and erosion. These methodologies rely heavily on 
interpretation of historic channel behavior but also include consideration of engineering and 
geomorphic techniques for evaluating channel stability. The historic, geomorphic, and engineering 
analyses were used to support flood control and erosion management for the City of St. George. 

ANALYSIS OF CASE STUDIES 


4.8. Hydrologic and Geomorphic Studies of the Platte River Basin, 
Nebraska 
Document Title: Relation of Channel-Width Maintenance to Sediment Transport 
and River Morphology: Platte River, South-Central Nebraska 
(Karlinger et al., 1983) 

Agency: U.S. Geological Survey 

Authors: Michael R. Karlinger, Thomas R. Eschner, Richard F. Hadley, 
and James E. Kircher 

Date: 1983 

Study Method Geomorphic analysis 

Category: 

4.8.1. Overview 
This study was undertaken as a result of the need to maintain a specific channel width along the 
Platte River, south-central Nebraska, for migratory-bird habitat. The study reach extends from 
Ashland, Nebraska, to just upstream of the confluence with the North Platte River, near Keystone, 
Nebraska. Three different methods were developed to estimate the water discharge necessary 
to maintain the current channel width. The methods include an empirical estimation based on 
combining a sediment-water discharge relation with a steady state flow duration curve, 
investigation of macroforms and channel geometry as an aid in predicting the current channel 
width, and use of Parkers equations (Parker, 1977). 

The report reviews, in detail, the channel geometry, sediment characteristics, and vegetation 
effects before applying the three methods. 

4.8.2. Detailed Description 
The history of channel change was investigated through observations of changes in the channel 
pattern, width, depth, slope, and bed forms. Information was collected from aerial photographs, 
USGS maps, topographic surveys, and field inspections. The data were analyzed and plotted to 
represent changing conditions over time. 

The channels sediment characteristics were also analyzed. Both bed sediment and bank 
sediment samples were collected and analyzed to determine trends in sediment size. The 
stream distances versus the median sediment size were plotted to obtain a representation of the 
sediment characteristics for the entire river section. The effect of changes in vegetation on 
channel width and sediment transport was also considered. 

Once the channel conditions were investigated, three methods were applied to obtain the 
discharge necessary to maintain the channel width. 

Method One 

The first method involves determining a predominant discharge for which a specific amount of 
sediment will be transported, thus ensuring that the channel width remains the same. The first 
step is to determine a relationship between sediment transport rate and water discharge, and a 

ANALYSIS OF CASE STUDIES 


flow-duration curve for the site of interest. Sediment transport is computed using the Modified 
Einstein Method, and a power regression equation is determined to relate sediment transport 
rate to water discharge. This relationship can be used with the flow duration curve to assign 
probabilities of exceedence to sediment transport. This sediment duration curve is then used to 
compute the expected sediment transport rate. The water discharge associated with the 
maximum contribution to the expected sediment transport rate is assumed to be the "effective 
discharge" responsible for carrying most of the sediment load. This discharge must be 
maintained if the channel shape is to remain unaltered. This discharge transports most of the 
sediment by virtue of both its magnitude and the frequency of occurrence. Therefore, the 
"effective discharge" is that for which the product of discharge times probability of occurrence is 
maximum. 

Method Two 

The second method involves determining a relationship between discharge and macroform 
characteristics. The premise is that macroforms can be colonized by vegetation and alter the 
flow section; therefore, the flow regime should be able to transport these macroforms 
downstream if the channel is to remain unaltered. 

Macroforms are large bed forms that are submerged only during high flows and are proportional 
to the channels dimensions. In this study, the following equation was used to determine the flow 
necessary to move a macroform downstream. 

1/ n

.

.

hs Lf 

2at

.

L 

sin

a

(4.2)
Q

=

. 
..

. 
..

where hs is the depth of scour, Lf the macroform length, a the angle formed by the intersection of 
the channel bankline with the crestline of the macroform, and a and n are regression coefficients. 
The time required to move a unit width of macroform a distance .L is termed t.L 
. 

This discharge has a lower limit equal to the flow that causes a minimum water depth over the 
macroforms necessary for incipient sediment motion. In the Platte River, this depth was 
estimated as 20 cm. 

Method Three 

The third method involves the application of Parkers equations (Parker, 1977), which specify 
conditions for balance of sediment transport and deposition to attain equilibrium. There are three 
regime equations. The first one relates grain size to particle fall velocity and channel slope. The 
second and third equations relate water and sediment discharge to variables describing particle 
size and channel features including grain size, channel slope, and channel width. 

Parkers equations assume that 1) channel reaches are straight; 2) bed and bank materials are 
noncohesive; and 3) sediment grain sizes are uniform throughout the reach. Because the third 
condition does not hold for the Platte River, average sediment size was inferred from field 
measurements. Several parameters (channel width, discharge, depth, and slope) were 
measured at five locations along the river reach. The data were then inserted into the equations 
to determine the sediment size. The observed width versus sediment size was plotted for the five 
locations, thereby enabling the user to apply Parkers equations for any site within the reach. 

ANALYSIS OF CASE STUDIES 


4.8.3. Applicability 
The first and third methods are applicable for the purposes of the REHA study. The concept of a 
predominant discharge, described in the first method, could be used by communities as a 
monitoring technique. A change in the resulting function could indicate changes in channel 
morphology. In addition, the calculation of widths based on a given discharge from the first and 
third methods could be approximately mapped to illustrate erosion hazard areas associated with 
the various discharges. 

The second method is not applicable for the study of REHAs beyond usage as an indicator of 
channel stability. 

4.8.4. Limitations 
There are several limitations associated with each method. None of the three methods consider 
channel meandering. The methods are generally only applicable to straight channel reaches. 

Although not an insurmountable limitation, it should be noted that to use the first method data must 
be available to establish the sediment versus water-discharge relation and the flow-duration curve 
for each site of interest. Sediment transport equations yield greatly varying results depending on 
the parameter values used and the equation selected. In consequence, large variations can be 
expected in any derived quantities. 

In addition to the fact that the second method has minimal application to the REHA study, the 
report indicates that the derivation of Eqn. 4.2. may suffer from some statistical inconsistencies. 

There are several limitations associated with the use of the third method. In addition to being 
applicable only to straight channel reaches, Parkers equations are limited by the assumption that 
the bed and bank materials are non-cohesive. Parker also indicates that appropriate particle 
sizes are among the smaller sizes. The report indicates that the dependence of the design 
sediment size on the channel slope is assumed negligible for the calibration curve and the curve is 
based on maximum widths due to constraining vegetation. Special consideration needs to be 
made when using the calibration curve for other sites. 

4.8.5. Relevant References 
Parker, G. (1977), Self-Formed Straight Rivers with Stable Banks and Mobile Bed in Non-
Cohesive Alluvium. Part I: The Sand-Silt River, Department of Civil Engineering, University 
of Alberta, Alberta, Canada. 

Andrews, E. D. (1980), Effective and Bankfull Discharges of Streams in the Yampa River Basin, 
Colorado and Wyoming, Journal of Hydrology, v. 46. 

4.8.6. Mappability 
The first and third methods presented in this study could serve as bases for assessing the effects 
of channel changes and can be used to approximately map channel changes to illustrate erosion 
hazard areas. 

4.8.7. Cost 
No cost information was provided. 

ANALYSIS OF CASE STUDIES 


4.8.8. Regulatory Potential 
The methodology, in particular methods 1 and 3, could potentially be enhanced to be used as a 
tool for regulatory purposes. 

4.8.9. Summary 
This study of the physical characteristics of the Platte River channel provides a basis for 
understanding the channel-forming processes in the stream. These characteristics and 
understanding are used to estimate discharge and channel width relationships. This study 
consists of a qualitative assessment of geomorphic and channel conditions on the Platte River 
with some quantitative assessments involving measurements and analysis of the bed and bank 
materials. The three methods described can be used to determine the discharge necessary to 
maintain a specific channel width. The first and third methods may be applicable to the REHA 
study in estimating channel widths for given discharges. However, the methods have several 
limitations and their applicability to other sites needs to be further investigated. 

ANALYSIS OF CASE STUDIES 


4.9. Streambank Erosion Along Two Rivers in Iowa 
Document Title: Streambank Erosion Along Two Rivers in Iowa (Odgaard, 1987) 

Agency: Iowa Institute of Hydraulic Research, University of Iowa 

Author: A. Jacob Odgaard 

Date: 1987 

Study Method Historic analysis and geomorphic equations
Category:

4.9.1. Overview 
This research paper is one of the early studies conducted in the United States to relate the rate of 
bank erosion to river characteristics. The U.S. Geological Survey supported the research. The 
objective of the study is twofold: to evaluate the contribution of bank erosion to the sediment load 
and, more importantly, to identify and quantify specific channel characteristics correlated to bank 
erosion. 

The study was based on Ikedas theory of river meanders, which assumes that the rate of bank 
retreat is proportional to the difference between the near-bank depth-averaged mean velocity 
and the reach-averaged mean velocity at bankfull discharge (Ikeda et al. 1981). The analytical 
framework for this study is an algebraic relation between the migration rate and channel 
characteristics based on Ikedas theory and a meander flow model developed by the author. 

The study first evaluated the rates and the processes of bank erosion for reaches of two streams 
in Iowa: the East Nishnabotna River and the Des Moines River. The streams are alluvial with 
point bars and their widths vary from relatively constant along the length (equiwidth) to wide 
bends. The evaluation was made using historic records (aerial photographs, maps, and stream 
records), field measurements, and soil analyses. The erosion rates were then correlated to 
channel characteristics such as width, depth, curvature, arc angle of channel centerline, channel 
slope, friction factor, and bank vegetation. The correlation was studied using the migration 
model, which simulates the velocity distribution in the bends and the location of the erosion 
occurrence. The final step of the study was to estimate the total sediment influx to the river from 
cutbank erosion. 

4.9.2. Detailed Description 
The paper consists of three major parts: the migration model, two case studies of the East 
Nishnabotna and Des Moines Rivers, and conclusions. 

4.9.2.1. Migration Model 
A meander flow model describes the meander bends as constant-radius curves whose migration 
is a combination of downvalley translation and lateral expansion. Two basic assumptions behind 
this model are the following: 

 The rate of bank retreat is proportional to the difference between the near-bank, depth-
averaged mean velocity and the reach-averaged mean velocity at bankfull discharge, and 
ANALYSIS OF CASE STUDIES 


 The velocity distribution in bends, or lag in velocitys response to curvature change, 
determines the distance from the crossover point to the first erosion occurrence on the outer 
bank. 
The migration rate and the bank erosion were related using the following relationship: 

v/u = e ub /u (4.3) 

in which v is the linear rate of bank retreat, u is the reach-average mean velocity, e is an erosion 
constant, and ub is the difference between the near-bank depth-averaged mean velocity and the 
reach-averaged mean velocity u at bankfull discharge. An equation for the near-bank depth-
averaged mean velocity was obtained by solving the continuity and momentum equations and an 
equation for lateral stability of the streambed (Odgaard, 1986). The solution is for steady, 
subcritical, turbulent flow in constant radius segments. The coefficient e is the equivalent of the 
scour factor (Parker, 1983). 

The migration model predicts the zero point for v, which is the point of first outer bank erosion 
occurrence. The location of this point along the bend depends on the radius-width ratio, bend 
angle, and upstream channel geometry. The model predicts that sine-generated bends with 
large radius-width ratio migrate by having both downstream translation and lateral expansion; and 
the same prediction applies to bends with smaller radius-width ratios if the bend angle is large. 
For sine-generated bends with relative small radius-width ratios and small bend angles, the 
migration is driven by downstream translation only. 

The average rate of erosion along the eroding part of the bank was assumed to be 
approximately half of the maximum linear rate of bank retreat. The areal rate of erosion per bend 
was obtained by multiplying the average linear rate of bank retreat by the bend angle and the 
radius of curvature referring to the channel centerline. The volumetric rate was obtained by 
multiplying the areal rate and the height of the cutbank. Summation over all bends along a river 
yields the total volumetric rate of cutbank erosion. 

The method was applied to the East Nishnabotna River and the Des Moines River. 

4.9.2.2. East Nishnabotna River 
The East Nishnabotna River is located in southwestern Iowa where it drains about 960 square 
miles of the Missouri River basin. The East Nishnabotna basin consists of loess deposit with 
about 30 percent clay content and a thickness of about 18 feet. The erosion study covers a 
40.6-mile reach of the river between Atlantic and Red Oak. Flow and sediment data for channel 
sections were obtained from the USGS Water Resources data for Iowa (1961-1986), and 
photographs for channel segments were obtained from the Agricultural Stabilization and 
Conservation Service of the U.S. Department of Agriculture. Two field investigations, one made at 
near bankfull stage and another at extreme low flow, were conducted to survey the cross 
sections. Soil samples were collected during the field investigations. 

The 40.6-mile reach was subdivided into 136 straight and constant radius segments; from them, 
72 were selected as study bends. All 72 bends experienced bank erosion during the period of 
analysis. Historic bank retreat was determined by superimposing sequential photographs using 
the technique described by Brice (1982). The data analysis also included defining the location of 
erosion occurrence relative to the crossover points and determining the channels bankfull width, 
cutback height, curvature, bend angle and bed level variations over the period of record. The 
channel characteristics were then used as an input to the migration model to predict bank erosion 
retreat, erosion velocity, and location of the first outer bank erosion point. 

ANALYSIS OF CASE STUDIES 


The study compared erosion velocities measured in the East Nishnabotna River along the 
cutbanks with the erosion velocities predicted by the migration model. The results showed the 
observations of erosion velocity followed the same general trend of model predictions. However, 
there was significant scatter between the observed first outer bank erosion point and the one that 
the model predicted. 

The study then estimated the average bend retreat rate, v. While the actual value of v varies 
from bend to bend, the average bend was migrating at a rate of 25.6 feet per year. The areal 
loss from outside banks along the lower half of the study reach was estimated, based on historic 
measurements, to be 13.7 acres per year, or about 0.6 acre per mile per year. Along the upper 
half of the reach, the loss was 5.0 acres per year, or about 0.28 acre per mile per year. 

4.9.2.3. Des Moines River 
The Des Moines River flows to the southeast across central Iowa to its confluence with the 
Mississippi River in the southeastern corner of the state. The study covered the lower 143 miles 
of the river, downstream from the Red Rock reservoir. The valley length of this reach is 
approximately 115 miles and adjacent drainage area is 2,200 square miles. The floodplain 
consists of alluvial deposits with silt and clay on top of a mixture of sand and gravel. A total of 45 
bends were identified comprising 45 percent of the 143-mile reach. The study reach was divided 
into three subreaches with about equal number of bends in each. Erosion data were obtained 
from a study conducted by the U.S. Army of Corps of Engineers (USACE, 1979), in which areal 
losses were determined from historic aerial photographs taken between 1938 to 1976. 

The analysis of the historic data showed that over the 37-year period, a total of 2,996 acres were 
lost along cutbanks. The annual loss rate, 0.56 acre per mile, is almost the same as the East 
Nishnabotna reach (0.60 acre per mile). The bank retreat for average bends in three reaches 
varied from 7.8 to 12.1 ft per year. The amount of volume loss and the amount of sediment influx 
to the water were also estimated. 

The meander flow model was used to predict the erosion retreat using the measured channel 
characteristics as the input. The values for three reaches were then plotted to compare with the 
measurements from the historic data. The plot showed a good correlation between the 
measured and predicted values. 

4.9.2.4. Conclusion 
The above analyses showed that there were several similarities between the study reaches of 
the two rivers, although they are different in channel size and discharge patterns. The East 
Nishnabotna River has natural flow, and the Des Moines River has controlled releases from the 
Red Rock reservoir. Both reaches were experiencing cutbank erosion in bends at a rate of 13 to 
26 feet per year. The erosion constant, which is a measure of the erodibility of the bank 
materials, is about the same for these two reaches. The study also found that the erosion 
velocities were generally higher by a factor of about 2 or more in banks flanked by matures trees. 

The analyses indicated that the theoretically developed relation between erosion velocity and 
channel characteristics, as described in the meander flow model, is valid. 

The author concluded that the similarity between the two rivers in terms of erosion rates and 
pattern suggested that the findings were typical for the region, including Iowa and possibly 
neighboring states, because of a notable uniformity in the regions floodplain features. 

ANALYSIS OF CASE STUDIES 


4.9.3. Applicability 
The study provided a relatively simple approach to predict channel erosion from channel 
characteristics by application of a meander flow model. The model is applicable to other river 
reaches in the region to estimate annual erosion rate; however, additional procedures must be 
developed to develop riverine erosion maps. 

4.9.4. Limitations 
Although the two examples of the East Nishnabotna and Des Moines Rivers indicated that 
applying the meander flow model to the erosion damage assessments is possible, there are two 
main concerns. 

First, the model was only tested in two rivers. From figures included in the paper, it appears that 
the model was able to predict the erosion velocity with reasonable accuracy; however, the 
prediction of the first erosion point was not satisfactory. The paper did not present more 
comparisons of observed or measured data with the prediction. 

Second, the study did not generate any maps to identify the erosion hazard area, although the 
estimated first occurrence of erosion could be used as an indicator of high erosion hazard areas 
on maps. The applicability of using the modeling results to develop erosion maps needs to be 
further explored. 

4.9.5. Relevant References 
Brice, J. C. (1982), Stream Channel Stability Assessment, Report FHWA/RD-82-021. Federal 
Highway Administration, Washington, D.C. 

Ikeda, S.G., G. Parker, and K. Sawai (1981), Bend theory of river meanders; 1: Linear 
Development, Journal of Fluid Mechanics, v. 112, p. 363-377. 

Odgaard, A. J. (1984), Bank Erosion Contribution to Stream Sediment Load, Iowa Institute of 
Hydraulic Research Report 280, University of Iowa, Iowa City. 

Odgaard, A. J. (1986), Meander-flow Model, 1, Development, ASCE Journal of Hydraulic 
Engineering, vol. 112 , p. 1117-1136. 

Parker, G. (1983), Theory of Meander Bend Deformation, in River Meandering: Proceedings 
of the Conference Rivers 83, American Society of Civil Engineers, New York. 

U.S. Army Corps of Engineers, Rock Island District (1979), Des Moines River Bank Erosion 
Study, Iowa and Missouri Stage 2, Final Feasibility Report. 
4.9.6. Mappability 
Procedures must be developed to convert model predictions into the boundaries of erosion 
hazard zones. 

4.9.7. Cost 
Cost information was not reported. 

ANALYSIS OF CASE STUDIES 


4.9.8. Regulatory Potential 
The approach, including the meander flow model, procedures to collect and analyze historically 
recorded data, and comparison of the measured/recorded and predicted bank erosion to test the 
model, has a potential to provide technical support for regulating riverine erosion hazard areas. 

4.9.9. Summary 
The processes of bank erosion and retreat rates were evaluated for reaches of two alluvial 
streams in Iowa. The evaluation was made using historic records (aerial photographs, maps and 
stream flow records), field measurements, and soil analyses. The model simulates the erosion 
velocity in the river bends and the location of erosion occurrence. The total sediment influx was 
also estimated for the studied channel reaches. 

The migration model and the methodology to use the model to predict erosion location has the 
potential to be used as a tool to identify the erosion hazard areas. However, the model needs to 
be further tested and mapping procedures must be developed before the methodology can be 
applied for the purposes of riverine erosion hazard mapping. 

ANALYSIS OF CASE STUDIES 


4.10. Channel Migration Studies in King County, Washington 
Document Title: Tolt and Raging Rivers Channel Migration Study, King County, 
Washington (Shannon & Wilson, 1991) 
Green River Channel Migration Study (King County Surface Water 
Management Division, 1993) 
Channel Migration in the Three-Forks Area of the Snoqualmie River 
(King County Surface Water Management Division, 1996) 

Agency: King County Department of Natural Resources, Surface Water 
Management Division 

Authors: Shannon & Wilson, Inc. 
Susan Perkins, King County Surface Water Management Division. 

Date: See dates next to document titles 

Study Method Geomorphic and engineering analysis (using historic and field observed 
Category: data) 

4.10.1. Overview 
These three studies were performed for flooding sources in King County, Washington, with 
known erosion hazards. The basic premise of the studies is that future river migration rates and 
types of channel migration in each river will, on average, be similar to past behavior under the 
same water and sediment discharge regime. The methods used in the study are primarily 
geomorphic and engineering analysis based on historic data, detailed study of bed and bank 
materials, and quantification of channel changes analysis. The studies provide detailed analysis 
and mapping of erosion hazard areas. 

4.10.2. Detailed Description 
The methodology consists of compiling historic aerial photographs, surveys, and available maps 
for the period of study. The periods of study include 19421993, 18981992, and 19361991 
for the Snoqualmie, Green River, and Tolt and Raging Rivers studies, respectively. 

4.10.2.1. Study Procedures 
Each of the studies was conducted individually. In general, the studies used the following 
procedures: 

 Investigating channel characteristics of study areas. The investigation consists of field 
surveys and review of historic maps, aerial orthophotographs, and aerial photographs. 
Features examined during the field survey included geologic materials, river bank height and 
composition, levees and revetments, vegetation type and age, presence of eroding banks, 
abandoned channels, deposition zones and descriptions of river and floodplain morphology. 
Sediment sampling was also conducted. Erosion zones were identified as a result of the 
investigation. 
 Determining historic channel migration rates. The average historic channel migration rate 
for each reach was determined by first comparing the channel locations on maps or aerial 
photographs at different time periods, then calculating the average migration rate for each 
channel reach where erosion had occurred. 
ANALYSIS OF CASE STUDIES 


 Estimating potential for erosion and enlargement of floodplain channels. The enlargement 
potential was estimated by calculating the boundary shear stress that flowing water would 
exert on each channel during a 100-year flood. Factors affecting channel migration rates 
were analyzed. 
 Determining hazard area from future channel migration. The prediction of the probable limit 
of future migration was determined based on an assumption that future rates and types of 
channel migration will, on average, be similar to the past behavior under that same water and 
sediment discharge regime. The basic elements of the prediction are as follows. The 
appropriate historic period to use for prediction was determined based upon whether 
hydrologic changes had significantly influenced channel migration. Next, the probable outer 
limits of future channel migration were determined based on historic meander belt width and 
measured bend amplitudes for each reach, ignoring existing or potential revetments and 
levees. 
 Developing mitigated channel migration hazard map. The outer limits of future migration 
were modified to account for constraints. Major roads and subdivisions were recognized as 
likely to be protected from erosion hazards; constraints were added into the natural limits of 
the channel migration to produce a Mitigated Channel Migration Hazard Map. The mitigated 
hazard area was then subdivided into areas of severe and moderate hazard, using 
calculated historic channel migration rates. 
The above general procedures were followed in the all three studies; however, each study was 
somewhat different, depending upon study area characteristics. 

4.10.2.2. Study Area Characteristics 
The Snoqualmie study investigates 12 miles of the upper reach of the river, including its three 
forks, with a contributing drainage area of 358 square miles. The Tolt and Raging Rivers are 
tributaries to the Snoqualmie River; the study encompasses a 5.9-mile reach of the Tolt River 
with a drainage area of 101 square miles and an 8.1-mile reach of the Raging River with a 
drainage area of 33 square miles. The Green River study investigates 20 miles of river, including 
the Howard Hanson dam. All of the studied rivers are experiencing rapid migration. Human 
activities, such as gravel mining and logging, are prevalent in many of the study areas. Some of 
the areas also have flood control structures, levees, and/or bank protection. The studies provide 
documentation on the hydrologic and geologic conditions of the areas, such as the largest floods 
of record, bank composition, and sediment size. 

4.10.2.3. Estimation of Channel Migration 
For each study, the rivers were divided into distinct reaches based on different rates and types of 
channel migration and corresponding differences in river morphology. The report provides a 
description of the morphology of each reach. 

Average historic channel migration rates were calculated for each reach. The procedure for 
calculating migration rates involved dividing the reach into 100 or 200 feet intervals (depending on 
the size of the reach). For each station where erosion occurred, the distance between the 
channel edges was measured from successive maps. The results are tabulated as migration 
rates occurring for specified time periods. Some of the periods are selected based on changes 
in the flow regime or bank protection. For example in the Green River study, migration rates are 
calculated for the pre-dam era (circa 1900-1940) and the post-dam era (1960-1992). For the 
Snoqualmie study, calculations included 1942-1961 (the pre-bank armoring period) compared to 
1961-1993 (the post-bank armoring period). Other comparative calculations were made to 
illustrate the effects of major channel-altering events such as large floods or channel avulsions. 

ANALYSIS OF CASE STUDIES 


The reports also discuss the factors affecting channel migration rates. Patterns of shear stress 
and channel stability, changes in gradient, bed elevation, bank materials, vegetation, flood control 
structures, bank armoring, levees, and channel straightening were all examined to determine the 
factors affecting the channel migration rates over time. The Snoqualmie study also considered 
changes in channel sinuosity and bend geometry. 

4.10.2.4. Estimation and Mapping of Future Channel Migration 
Tolt and Raging Rivers: Both short-term (10 years) and long-term (100 years) erosion hazards 
were mapped for the Tolt River. To map the 10-year hazard, the study assumed that bank 
erosion would occur only in the following locations: currently eroding areas, outer banks of bends, 
more probable avulsion sites, and sites likely to erode as a result of upstream channel shifting. 
The calculated average migration rate over each reach was selected from periods of record at 
least 10 years in length, which include one or more severe floods. In this study, two records met 
this qualification, and the higher of the two migration rates was selected. The selected migration 
rate was then multiplied by the time period, and the resulting width was mapped. The widths 
were mapped from the existing channel location assuming the channel would migrate laterally. 
The 100-year hazard was divided into areas of high and moderate hazards. The high-hazard 
zones were calculated by multiplying the calculated average migration rates for the entire period 
of record by 50 years. The resulting widths were mapped assuming the channel would migrate 
in both directions from the existing channel. The moderate hazard zones were calculated by 
multiplying the calculated average migration rates for the entire period of record by 100 years. 
The resulting widths were mapped assuming the channel would migrate in either direction from 
the existing channel. 

Avulsions were also considered in the Tolt and Raging River study. The likelihood of an avulsion 
occurrence is categorized as either more probable or less probable. More probable is 
defined as an existing creek or side channel that diverges from the main channel of the river in a 
downstream direction. Less probable is assigned when one or more conditions would have to 
change for an avulsion to occur. Avulsions were mapped for the Tolt River based on either rates 
of growth of avulsion for reaches where avulsion data were available or the highest shifting and 
widening rate. An avulsion on the Tolt River was assumed to start as a 30-foot wide channel, 
erode at the higher avulsion rate for 10 years, and then drop back to the normal channel 
migration rate for the remainder of the prediction period. For the Raging River, where avulsions 
were less significant, an avulsion was assumed to start as a 30-foot wide channel, then erode at 
the normal migration rate for the remainder of the prediction period. 

Green and Snoqualmie Rivers: Several additional items were considered in the mapping for 
these two rivers. First, based on hydrologic changes, the appropriate historic period was 
selected. For example, in the Green River study, there was evidence to suggest that some of 
the reaches had changed significantly as a result of the diversion of the White River and 
construction of Howard Hanson Dam. Therefore, when making migration predictions, more 
recent migration behavior was used instead of the entire period of record. Second, two 
geomorphological channel characteristics were used to estimate the outer boundary of the 
erosion hazard area: the meander belt width and the bend amplitude. The meander belt width is 
used to estimate the maximum valley width occupied by all the mapped channels for the relevant 
time period. The bend amplitude is used to map the median amplitude of the bends that grew in 
each reach during the relevant time period. The hazard zone was then extended from the existing 
channel the greater of the two distances (the meander belt width or the bend amplitude). If the 
extension reached a valley wall or high terrace, the hazard area was extended in the opposite 
direction. As indicated on the maps provided with the report, the resulting hazard area 
encompassed all former river channels in both studies. Third, another map was produced, which 
scaled back the erosion hazard area to reflect the effects of existing roads, levees, and bank 
protection. 

ANALYSIS OF CASE STUDIES 


An underlying assumption for the Green River and Snoqualmie studies was that the river would 
shift into all high avulsion potential channels and then migrate laterally according to the migration 
rates calculated for the reach. High avulsion potential is defined as creeks and well-defined 
former channels that are flooded deeply and frequently (at least once every two or three years), 
diverge from the main channel in a downstream direction, and are (or could become) directly 
connected to the river. 

Lastly, the erosion hazard area was further divided into severe and moderate hazard areas by 
using a procedure similar to the one used in the Tolt and Raging River study. For the severe 
hazard area, it was assumed that during a 100-year period the river would migrate for 50 years 
in each direction from its present position. The calculated migration rate multiplied by 50 years 
results in the widths that were mapped. The moderate hazard area is defined as the land 
between the outer boundary of the severe hazard area and the outer boundary of the mitigated 
channel migration hazard area. 

4.10.3. Applicability 
These studies are applicable to the REHA study. The final products of the studies are maps 
delineating erosion hazard zones. The maps provide a general erosion hazard area and further 
divide the area into zones of different degrees of hazards. Although the studies are all in the 
same county, they encompass a variety of basin sizes, flood control structures, and land-use 
conditions. 

4.10.4. Limitations 
Many assumptions and selections seem subjective. For example, studies do not provide 
justification for the time periods and methodology used to define the moderate and severe hazard 
area zones that would be necessary for the purposes of the REHA study. The selection of 
barriers to migrating channels is somewhat subjective. No stability analysis was done to 
determine how effective these structures would be in preventing channel migration. 

The channel migration estimates are based on historic data; therefore, years of photographs and 
documentation of the migrating channel must be available to perform the type of analysis used in 
the studies. 

There are some errors associated with the mapping. For example, if the area used to calculate 
the migration rates is too small, the depicted widths may be inaccurate. 

The studies do not consider the effects of landslides due to undercutting of valley walls. 

The basis of using historic trends for a prediction is the assumption of stationarity. Although the 
assumption may be applicable to study areas that are not significantly impacted by the human 
activities, this is not always valid for most cases. 

4.10.5. Relevant References 
Dunne, T., W.E. Dietrich, and D. Nimick (1976), The Yakima River Between the Selah and Union 
Gaps, in Jones and Jones, 1976, Hydrology: Seattle, Report to the City of Yakima 
Department of Parks and Recreation, Appendix A. 

Dunne, T., and W.E. Dietrich (1978), A River of Green, in Jones & Jones, 1978, Hydrology, 
Sedimentation, Channel Migration, and Flood Diking Along the Green River: Seattle, 

Report to the City of Yakima Department of Parks and Recreation, Appendix A. 

ANALYSIS OF CASE STUDIES 


4.10.6. Mappability 
The studies provide numerous maps. The maps depict historic channel locations, locations of 
geologic and man-made channel constraints, unconstrained channel migration hazards, and 
mitigated channel migration hazards. 

4.10.7. Cost 
No cost information was included in the report. King County and Ms. Susan Perkins, a consultant 
to the County, estimated that it cost between $3,000 to $6,000 per river mile to complete the 
channel migration hazard studies. The cost per mile would be less for large projects. The 
estimate includes digitizing maps into GIS format. 

4.10.8. Regulatory Potential 
Currently, a formal regulatory policy is in place adopted by the County Council as part of the King 
County Flood Hazard Reduction Plan in November 1993. The King County Surface Water 
Management Division recommended the adoption of the policy to prevent future development in 
channel migration hazard areas through land use regulation. The hazard maps produced by the 
studies were transmitted to the King County Department of Development and Environmental 
Services to use in regulating development under the Sensitive Areas Code. King County has also 
implemented a 100-foot buffer zone for class 1 streams in accordance with the King County 
Sensitive Areas Ordinance. 

4.10.9. Summary 
The three studies performed for King County are based on engineering and geomorphic analysis. 
The methodology applied includes compiling historic data, conducting field investigations, 
researching and analyzing channel characteristics, calculating channel mitigation rates, and 
mapping erosion hazard areas. 

The studies are appropriate for the purposes of the REHA study. However, some of the 
limitations such as defining and providing justification for the severe and moderate erosion hazard 
designations need to be resolved. 

The main result of the studies is the production of maps depicting erosion hazard areas, which 
are being used for regulatory purposes. 

ANALYSIS OF CASE STUDIES 


4.11. Bank Erosion Field Survey Report of the Upper Mississippi River 
and Illinois Waterway 
Document Title: Bank Erosion Field Survey Report of the Upper Mississippi River and 
Illinois Waterway -- Interim Report for the Upper Mississippi River  
Illinois Waterway System Navigation Study (Illinois State Water 
Survey,1997) 

Agency: USACE, Illinois State Water Survey (ISWS), and Iowa Institute of 
Hydraulic Research (IIHR) 

Authors: Nani Bhowmik and David Soong, ISWS; Tatsuaki Nakato, IIHR; Mike 
Spoor, USACE, Huntington District, Jeff Anderson, Anderson 
Environmental Services, and Dan Johnson, USACE, Rock Island District 

Date: November 1997 

Study Method Geomorphic and engineering analysis based on historic data 
Category: 

4.11.1. Overview 
This report summarizes findings from several phases of the Upper Mississippi River/ Illinois 
Waterway (UMR/IWW) bank erosion study, which is a part of the UMR/IWW system navigation 
study. The USACE Districts of Rock Island, St. Louis, and St. Paul led the study under the 
authority of Section 216 of the Flood Control Act of 1970. The objective of the study is to 
examine the feasibility of navigation improvements for a 50-year planning horizon from 2000 
through 2050. 

Tasks completed to the date of publication of the report include a literature review, an aerial 
reconnaissance survey, and field surveys to qualitatively assess the relative significance of 
commercial navigation on bank erosion. The research team consists of the ISWS, the University 
of Iowa's IIHR, and the USACEs Rock Island, St. Paul, St. Louis, and Huntington Districts. The 
principal authors for this report are ISWS and IIHR. 

Field surveys were conducted in the Fall of 1995. The surveys covered 854 miles in the UMR 
and the IWW. The study team selected 72 erosion sites (29 sites on the IWW and 43 sites on 
the UMR) for detailed study. 

The report summarized approaches and findings of the field survey. The surveys evidenced that 
approximately 115 bank miles on the IWW and 240 bank miles on the UMR are severely eroded. 
These figures amount to approximately 20 percent of the total bank length of the IWW and 14 
percent of the UMR that are actively eroding. The severely eroded reaches are marked on 
navigation charts. 

The next phase of the study was to develop a model to assess the risk of bank erosion based on 
site-specific field data for existing and future conditions in the UMR/IWW. 

ANALYSIS OF CASE STUDIES 


4.11.2. Detailed Description 
The objective of the field survey is twofold: 

 Documenting bank conditions along both sides of the surveyed rivers on their navigation 
charts and identifying major erosion sites; and 
 Selecting representative sites, collecting data on each site, and developing explanations as to 
the causes and failure mechanisms of erosion at each site. 
4.11.2.1. Data Collection and Parameter Classification 
The study developed a systematic approach to selecting the study sites. First, the sites were 
initially selected based on aerial oblique videotape, photographs, and information from operation 
and maintenance personnel of the USACE. The final selection of the sites was done during field 
reconnaissance. The sites were classified as detailed study and observation sites. The 
location of each site was selected from maps and was located using a Global Positioning System 
(GPS) during the field trip. Each site was then mapped on the navigation chart. Standard 
procedures were used to collect bank data. The field data were then entered into a database for 
each river. 

Field parameters were classified according to site location, site attributes, and erosion attributes. 
The study adapted a near-shore bank failure model in bank assessment. The model 
characterizes a typical bank section using three features: scarp, berm, and bench. Soils were 
classified based on the Unified Soil Classification System. 

4.11.2.2. Characterization of Bank Erosion and Failure Mechanisms 
The report described, in general terms, the characteristics of the riverbank and near-bank 
benches: soil, slopes, depositional features, failure and erosion mechanisms. Stage histograms 
were developed for selected sites for relating erosion patterns, bank slopes and other features to 
hydrologic conditions described by stage-duration data. 

4.11.2.3. The Illinois Waterway 
The field survey was conducted by the ISWS, the IIHR, and the USACE. 

Detailed field data were collected at a total of 29 study sites and 3 observation sites. Data 
collected at all sites include bank sections, core samples of bank materials, and measurements 
of at least one cross section. A total of 80 bank sections from 29 eroded sites were measured. 
All sites were grouped into six bank types based on their physical features (slope and height of 
scarp, slope and shape of bench, soil type, vegetation cover, near-bank and underwater 
materials, and ordinary high water level) and erosion potential, which is defined as the most likely 
erosion processes. However, the grouping is somewhat subjective; the report did not develop 
quantitative criteria for all physical features. Some qualitative descriptions, such as gently 
sloping, were used for the classification. Under this system, types 1 and 2 indicate high potential 
for erosion, types 3 and 4 indicate moderate potential for erosion, and types 5 and 6 indicate 
active but less severe erosion. Hydrologic conditions prior to the field survey were provided to 
compare with the conditions prior to two previous surveys in 1984-1985 (Warren, 1987) and 
1988 (Hagerty, 1988). 

The report described each site in detail; a site map showing the locations of bank erosion was 
prepared for each study site. 

ANALYSIS OF CASE STUDIES 


During the field data collection, the survey team identified the probable causes of erosion at all of 
the bank sections investigated. The study found that 27 percent of bank sections showed 
erosion occurring only at high stages, while 63 percent of the bank sections had erosion within the 
normal range of stage fluctuation  between the ordinary high water marks and normal pool 
stage. This observation indicates that erosion cannot be completely attributed to large floods; the 
rework and transport processes, as caused by waves and currents, are significant at stages 
between the normal pool and the high water marks. 

The study identified major erosion causes. About 74 percent of the bank sections showed 
evidence of seepage effects, 28 percent showed evidence of waves and seepage impacts, and 
24 percent showed impacts of traffic-induced disturbance. 

Analysis of the erosion mechanisms at all measured bank sections indicated that about 10 
percent of locations are in types 1 and 2 (high erosion potential), 21 percent in types 3 and 4 
(moderate erosion potential), and 17 percent in types 5 and 6 (low erosion potential). The 
remaining bank sections showed some deviation from these defined types or a combination of 
different types. 

4.11.2.4. The Upper Mississippi River 
The field survey was conducted by the ISWS, IIHR, and the USACE. 

A total of 43 sites distributed along the river through Minnesota, Wisconsin, Iowa, Illinois, and 
Missouri were selected as major study sites. The sites were classified into six types according to 
observed bank erosion features: soil type and deposition, slope of scarp, formation of terraces, 
and bank failure features. The classification method and criteria for the classification are different 
from those used in the IWW survey. 

A detailed site description and site map showing the location of erosion bank were prepared for 
each site. Based on individual geomorphologic and hydraulic characteristics, traffic-wave induced 
erosion potential was estimated for each study site. 

The report provided a descriptive summary of findings of the field survey. Much of the summary 
is descriptive. The study did not estimate bank retreat due to erosion; however, it described the 
field observations of impacts of the Great Flood of 1993 at most of the study sites. The study 
concluded that flood effects appear to be more significant than other erosion mechanisms. Stage 
recession after floods is greater in the upper ends of pools, and gradients and exposed bank 
heights for emergent seepage are larger in upper ends of pools. 

On the basis of the field study, approximately 14 percent of the Mississippi River banks were 
estimated to be actively eroding as of 1995. 

4.11.3. Applicability 
The document reported the findings of field surveys of the UMR/IWW System Navigation Study. 
It did not present or discuss any systematically defined methodology; in fact, the two parts of the 
study (IWW and UMR) are different in many aspects, most significantly, in the selection of site 
parameters and the classification of erosion mechanism. Both methods may be applicable to 
some locations, but it is difficult to make a general evaluation for overall applicability to other 
locations. 

ANALYSIS OF CASE STUDIES 


4.11.4. Limitations 
The report provided detailed bank erosion information of UMP and IWW in 1995; these results 
and findings are site-specific and not transferable. 

4.11.5. Relevant References 
Bhowmik, N.G. and R. J. Schicht. Bank Erosion of the Illinois River, Report of Investigation 92, 
Illinois State Water Survey, 1980 

Bhowmik, N.G., M. Demissie, and C.-Y. Guo. Waves Generated by River Traffic and Wind on 
the Illinois and Mississippi Rivers. Illinois State Water Survey Contract Report 293, 1982 

Hagerty, D. J. Illinois Waterway Bank Evaluation. Unpublished report, submitted to the U.S. 
Army Corps of Engineers, Rock Island District. 1989 

Spoor, M.F. and D.J. Hagerty. Bank Failure and Erosion on the Illinois Waterway. Proceedings, 
International Symposium on Sediment Transport and Modeling, ASCE, New Your, 600-605, 
1989 

Warren, R.E. The Impact of Bank Erosion on Prehistoric Culture Resources in the Lower Illinois 
River Valley. Technical Report 87-211-10, Illinois State Museum Society, Springfield, IL, 
1987 

4.11.6. Mappability 
There are two types of maps developed from the field surveys. The site map developed for 
each study site shows erosional banks identified during the site investigation. Overall results of 
the field survey are presented in Charts of the Illinois Waterway and Upper Mississippi River 
Navigation Charts. In these navigation charts, banks are classified into eight categories and 
color-coded accordingly. The categories are the following: scarp approximately 4 feet or higher, 
scarp approximately less than 4 feet, moderate to minor erosion with occasional vegetation, 
mostly sand bench with vegetation growing on upper bank, alternate erosion and disposal, 
dredge material disposal, stable, and riprap/rock outcrop/bank protection/seawall. 

Only erosion sections along the riverbanks were mapped; the lateral span of erosion at a location 
was not shown on either site maps or navigation charts. 

4.11.7. Cost 
No cost information was reported. Additional information provided by the ISWS indicated that the 
total costs for the IWW survey were about $190,000; the average cost per mile was about $660. 

4.11.8. Regulatory Potential 
The study is designed to investigate impacts of navigation on bank erosion; the report mainly 
contains descriptions of the field surveys and findings. Although some information may be used 
for riverine erosion evaluation, its overall usefulness for regulation purposes is limited. 

ANALYSIS OF CASE STUDIES 


4.11.9. Summary 
As a part of the UMR/IWW System Navigation Study, a field investigation of the current state of 
bank erosion on the UMR and the IWW was conducted. The purpose of the study was to 
estimate the relative significance of navigation impacts in the context of riverbank erosion 
processes. 

For the selected sites on the IWW, the research team observed multiple erosion processes at 
most of the selected bank sections. The most frequently identified erosion mechanisms are 
seepage, stage fluctuations, flood flows, navigation traffic, wave action and eddies, and disturbed 
flow. 

Bank failure and erosion conditions on the UMR also showed significant flood impacts. Analyses 
of surficial soil samples showed that the banks were mantled by primarily sand and gravel in the 
upper reach of the river, silt and sand in the middle reach, and clay and silt in the lower reach. 
Most of the bank failure and erosion sites showed that flood damage is the dominant erosion 
cause. 

Based on results of field survey, the study team recommended developing correlations between 
apparent navigation-induced erosion and physical parameters, including proximity to narrow 
channel reaches and locks, mooring and fleeting activities, soil and sediment characteristics, and 
land use. The risk assessment study will develop models to assess the risk of bank erosion, 
which is identified as directly related to the increase in commercial navigation and recreation traffic 
in the rivers. 

ANALYSIS OF CASE STUDIES 


4.12. Arizona Standards for Lateral Migration and Channel Degradation 
Document Title: State Standards for Watercourse System Sediment Balance (Arizona 
Department of Water Resources, Flood Warning, and Dam Safety 
Section, 1996) 
Agency: Arizona Department of Water Resources, Flood Warning, and Dam 
Safety Section 
Authors: Simons, Li & Associates 
Date: September 1996 
Study Method Geomorphic and engineering analysis based on historic data 

Category: 

4.12.1. Overview 
The Standards document presents the following three guidelines to characterize erosion hazard 
areas: 

Guideline 1: Lateral migration setback allowance for riverine floodplains in Arizona, 

Guideline 2: Channel degradation estimation for alluvial channels in Arizona, and 

Guideline 3: Evaluation of river stability impacts associated with sand and gravel mining. 

Guidelines 1 and 2 are the most relevant for the purposes of evaluating riverine erosion hazard 
areas. Guideline 1 provides procedures for estimating the setback distance along streams within 
which erosion is expected. Guideline 2 presents procedures applicable to estimate degradation 
in natural channels. The Guidelines present three levels of methodologies with increasing degree 
of detail in the analyses. Level 1 offers a preliminary assessment, Level 2 requires additional 
effort and provides refined estimates, and Level 3 allows for detailed analyses generally needed 
only for special circumstances. 

4.12.2. Detailed Description 
4.12.2.1. Guideline 1  Lateral Migration Setback Allowance for Riverine 
Floodplains in Arizona 
This Guideline provides techniques to estimate the setback where development must be 
restricted along a stream to minimize the potential damages caused by erosion of the 
streambanks. Three levels of analysis are available with increasing degrees of complexity. 

Level 1 

This level requires as basic data the drainage area to the point of interest and the peak 100-year 
discharge. The method is applicable to basins with an area less than 30 square miles. The 
setbacks are given by 

Setback = (Q100)0.5 for straight or mildly-curved channels (rc > 5T) (4.4) 

ANALYSIS OF CASE STUDIES 


Setback = 2.5 (Q100)0.5 for channels with curvature such that rc = 5T (4.5) 

where Q100 is the 100-year peak discharge in cubic feet per second, rc is the radius of curvature 
measured at the centerline of the stream, and T is the top width of the channel. The minimum 
setback is 20 feet for a straight channel and 50 feet for a curved channel. The setback is 
measured in feet outward from the 100-year floodway or the top of the channel bank, whichever 
is greater. In channel bends, the setback is applied to the outer bend. 

Level 2 

The Standards suggest that this level can be used to demonstrate stability of the banks during the 
100-year flood and to justify a smaller setback than that produced by Level 1 computations. 
Three methods are provided: Allowable Velocity Analysis, Tractive Stress Analysis, and Tractive 
Power Analysis. 

In the Allowable Velocity Analysis the velocity of the 100-year peak flow within the stream is 
compared to a maximum velocity below which erosion does not occur. The source of this 
maximum allowable velocity is a chart developed by the Soil Conservation Service (now the 
Natural Resources Conservation Service). Two curves in the chart show the variation of the 
maximum permissible velocity as a function of grain size as indicated by the diameter for which 75 
percent of the material is finer (D75). One curve applies for sediment-laden water (more than 
20,000 parts per million) and the other for clear water (less than 1,000 parts per million). The 
conditions in Arizona require the curve for sediment-laden water. The velocity thus found is then 
multiplied by correction factors to account for channel alignment, bank slope, and flow depth. 

The Tractive Stress Analysis is subdivided into two cases: Case 1 applies when the D75 of the 
sediment is between 0.25 and 5.0 inches. Case 2 applies when D75 is less than or equal to 0.25 
inches. For Case 1, a formula is provided to compute the tractive stress for an infinitely wide 
channel as a function of D75, Mannings n, the energy slope, and the flow depth. This basic 
stress is multiplied by a correction factor to obtain the value for trapezoidal channels of finite 
width. The value obtained is compared with the maximum allowable tractive stress that 
measures the grains propensity to be eroded by the flowing water. The allowable stress is a 
function of the channel side slopes, the angle of repose of the material, and D75. For Case 2, the 
procedure is similar, but a graphical solution, instead of formulas, is used to arrive at the values 
for the tractive and allowable stresses. 

Tractive power is defined as the product of the mean flow velocity times the tractive stress. The 
Tractive Power Analysis can account for processes like cementation that reduce the erodibility of 
the soil. A soil sample is collected to conduct an unconfined compression test. The resulting 
unconfined compression strength is reduced by a factor of safety equal to or greater than 2. The 
procedure provides a chart zoned according to combinations of the compression strength and the 
tractive stress. A line marks the boundary between erosive and nonerosive conditions. Using the 
compression strength from the test and the computed tractive power, the flow conditions can be 
classified as erosive or nonerosive. 

Level 3 

This is the most complex level of analysis and involves the use of mathematical models to 
simulate hydraulics and sediment transport and the resulting erosion or deposition. The analysis 
involves an evaluation of historic data, field data collection and geomorphic evaluations, and 
hydraulic and sediment transport modeling. 

ANALYSIS OF CASE STUDIES 


4.12.2.2. Guideline 2  Channel Degradation Estimation for Alluvial 
Channels in Arizona 
This Guideline describes procedures that may be used to predict channel degradation in streams 
and unlined channels. The total degradation impacts the depth to which structures must be 
buried. As with Guideline 1, three levels of analysis are presented. 

Level 1 

This level requires knowledge of the 100-year peak discharge. The total scour depth ds is 
computed as the sum of general degradation dgs and long-term degradation dlts. General 
degradation dgs is given by 

dgs = 0.157 (Q100)0.4 for straight channels (4.6) 

dgs = 0.219 (Q100)0.4 for channels with curvature (4.7) 

The Guideline states that Eqn. 4.7 should not be used unless significant curvature is evident in 
the channel reach; however no information is provided to decide whether the channel has a 
curvature. 

Long-term degradation dlts is computed as 

dlts = 0.219 (Q100)0.4 (4.8) 

This equation should only be used in the absence of downstream channel controls. The minimum 
total scour depth is 3 feet. The total scour is applied to the lowest point in the cross section. 

Level 2 

The Guideline indicates that this level can be used to demonstrate the ability of the channel to 
resist degradation and to justify lesser degradation depths, which lead to less burial requirements. 
Four alternatives are available to conduct Level 2 analyses: Erodibility Evaluations, Armoring 
Potential Evaluation, Channel Profile History Comparison, and Grade Stabilization Measures 
Adequacy Analysis. 

Erodibility Evaluations can be conducted using any of the three methods provided under Level 2 
of Guideline 1: Allowable Velocity, Tractive Force, and Tractive Power. 

Armoring Potential Evaluations and incipient motion analyses can be performed to evaluate 
channel stability. Incipient motion occurs when the hydrodynamic forces equal the gravity and 
friction forces and movement of the grain is imminent. If a particle reaches the incipient motion 
threshold, all other finer particles are assumed to have been lifted. The Guideline uses the 
Shields diagram to evaluate the state of incipient motion. For the turbulent flow range, the critical 
particle diameter Dc is given by 

t 

Dc = p (4.9) 
0.047(. s -. ) 

where tp is the boundary shear stress acting on the grain, and .s and . are the specific weights of 
sediment and water respectively. The boundary shear stress is 

ANALYSIS OF CASE STUDIES 


t p = 
1 
f.V 2 (4.10) 

8 

where f is a friction factor (a function of Mannings n), . is the density of water, and V is the flow 
velocity. A chart is provided to correct this shear stress for channel curvature. 

The armoring potential is estimated by comparing the critical diameter in Eqn. 4.9 with the 
diameter for which 90 percent of the material is finer (D90). Armoring can occur if Dc < D90. After 
determination of the fraction of the material with sizes equal to or larger than Dc, the depth of 
scour necessary to establish an armor layer .Za is given by (USBR, 1984) 

. 1 .

.Za = ya .-1. (4.11) 
. Pc . 

where Pc is the fraction of the material coarser than Dc, and ya is the thickness of the armor layer, 
usually taken as a multiple of Dc. Field observations indicate that at least two layers of armoring 
particles are needed for a stable armor. 

If sufficient data are available, a Channel Profile History Comparison can be conducted to 
identify the condition of various areas in the channel. The approach should be used to 
demonstrate stability or aggradation rather than degradation. The analysis should include an 
evaluation of expected future trends. 

Grade Stabilization Measures Adequacy Analysis should be conducted for measures 
implemented to reduce potential degradation. The Guideline states that local procedures may be 
in place for various jurisdictions in Arizona and suggests two documents for all other cases. 

Level 3 

The nature and scope of this level is the same as for Level 3 in Guideline 1. 

4.12.3. Applicability 
The Guidelines provide standardized procedures to analyze channel stability and define erosion 
setbacks. 

4.12.4. Limitations 
The Guidelines allow for great latitude in the techniques that may be used to define erosion 
setbacks. The limitations of the approach are those of the analytical tools selected for the 
analysis. The information provided is insufficient to assess the shortcomings of Level 1 
techniques. Therefore, a potential user cannot evaluate the reliability of the predictions. No 
procedures are available to determine aggradation. 

A limitation is the emphasis on the 100-year flood event, which is assumed to be the controlling 
event. Events of lesser magnitude can also introduce substantial changes in the channel 
geometry. Except for Level 3 methods, the techniques do not take into account some important 
geotechnical variables in streambank analysis; for instance, slope stability. 

There is indication that the procedure can be extended to other regions; however, the procedures 
used to derive Level 1 tools are not disclosed. 

ANALYSIS OF CASE STUDIES 


4.12.5. Relevant References 
Simons, Li & Associates, Inc. (1985), Design Manual for Engineering Analysis of Fluvial 
Systems, prepared for Arizona Department of Water Resources, Tucson, Arizona. 

City of Tucson Department of Transportation (1989), Standards Manual for Drainage Design 
and Floodplain Management in Tucson, Engineering Division, Tucson, Arizona. 

4.12.6. Mappability 
The procedure to map erosion hazard areas through a setback is straightforward and can be 
accomplished easily. 

4.12.7. Cost 
No cost information was reported. 

4.12.8. Regulatory Potential 
The procedure is already implemented as a regulatory instrument. 

4.12.9. Summary 
The Standards document presents two guidelines to characterize erosion hazard areas: 

Guideline 1: Lateral migration setback allowance for riverine floodplains in Arizona. 

Guideline 2: Channel degradation estimation for alluvial channels in Arizona. 

Guideline 1 provides procedures for estimating the setback distance along streams within which 
erosion is expected. The resulting setback can be used for mapping of hazard areas. Guideline 
2 presents procedures applicable to estimate degradation in natural channels. This guideline is 
intended to set minimum burial depths. 

The Guidelines are structured in three levels of methodologies with increasing degree of 
complexity. Level 1 offers a preliminary assessment using simple equations, Level 2 requires 
standard engineering calculations, and Level 3 calls for numerical modeling using specialized 
computer programs. 

The Guidelines are already used for regulatory purposes in Arizona. 

ANALYSIS OF CASE STUDIES 


5. Assessment of Technical Feasibility 
5.1. Overview 
Section 577 of the National Flood Insurance Reform Act of 1994 discusses mapping erosion 
hazard areas for riverine areas, if technologically feasible. The purpose of this study is to 
determine whether it is technologically feasible to map REHAs. 

Technologically feasible was defined earlier in the report as: 

Methodologies exist that are scientifically sound and implementable under the 
NFIP. Scientifically sound means that the methodologies are based on 
physical or statistical principles and are supported by the scientific community. 
Implementable means that the approaches can be applied by FEMA as part 
of a nationwide program under the NFIP and for an acceptable cost. 

Scientific soundness and cost acceptability will be covered in this chapter. Applicability as a 
nationwide program will be covered in Chapter 6. 

This chapter is organized into four main topics: Evaluation of Scientific Soundness, the 60-Year 
Horizon, Cost, and Conclusions on Technological Feasibility. These topics are developed as 
follows: 

 The Evaluation of Scientific Soundness contains summaries of material covered in Chapter 4. 
 Section 577 defines hazard areas as those where erosion is likely to result in damage to 
buildings and infrastructure within a 60-year period. Discussion of the options to address the 
60-year period is presented in this chapter. 
 Total cost to the NFIP is determined using unit costs developed in the study and applying 
them to counties prone to riverine erosion. 
 The conclusions on technological feasibility are based on the analysis of the 12 selected 
documents and suggestions and recommendations from the Project Working Group (PWG). 
5.2. Evaluation of Scientific Soundness 
The case studies in the previous chapter indicate that it is possible to conduct riverine erosion 
studies and establish conclusions regarding the likelihood of future erosion. The case studies also 
illustrate the fact that erosion studies can follow various methodologies depending on data 
availability, funding and resource limitations, stream characteristics, and the needs that motivated 
the study. This observation implies that there is significant flexibility in conducting these studies, 
although not all methodologies allow delineation of erosion setbacks along streams. In general, 
the studies seem to fall under three categories: 1) geomorphic analysis; 2) engineering analysis; 
and 3) mathematical modeling. These categories will be used as a framework to analyze 
scientific soundness. 

5.2.1. Field Investigations 
The majority of the case studies required substantial field investigations of watersheds and 
stream channels. Table 5.1 summarizes the type and scope of the field investigations for each of 
the studies. 

ASSESSMENT OF TECHNICAL FEASIBILITY 


Table 5.1. Scope of field investigations for the case studies. 

Case Study Title Field Investigations 
AMAFCA Sediment and 
Erosion Design Guide 
Level 1: Geomorphic and other qualitative analyses 
 Survey of local stability problems 
 Observations on flow patterns 
 Identification of aggradation and degradation zones 
 Sampling of bank and bed materials 
 Inventory of activities affecting the stream (e.g., urbanization, channelization, dams, and other human 
activities) 
Level 2: Basic engineering analysis 
 Field verification of hydrologic parameters 
 Cross section and channel profile topographic surveys 
 Survey of bridges, culverts, and other hydraulic structures 
 Estimation of roughness coefficients 
 Sedimentsampling 
 Sediment transport rate measurements 
 Sampling of bed and bank materials 
Level 3: Mathematical modeling 
 Same as in Level 2 but in greater detail 
Inventory and Analysis of 
Stream Meander Problems 
in Minnesota 
 Sedimentsampling 
A Probabilistic Approach to 
the Special Assessment of 
River Channel Instability 
 No field investigation needed 
Geomorphology And 
Hydrology of the Santa Cruz 
River, Southeastern Arizona 
 Periodic fluvial geologic surveys 
 Periodic topographic, geomorphic, and geotechnical surveys 
 Periodic aerial photographic surveys 
 Topographic survey of stream profile and cross sections 
 Stream reconnaissance surveys 
San Diego County Alluvial 
Studies 
 Field verification of hydrologic parameters 
 Measurements of hydrographs 
 Measurements of stage and discharge at cross sections 
 Cross section and channel profile topographic surveys 
 Survey of bridges, culverts, and other hydraulic structures 
 Estimation of roughness coefficients 
 Sedimentsampling 
 Sediment transport rate measurements 
 Sampling of bed and bank materials 

ASSESSMENT OF TECHNICAL FEASIBILITY 


Table 5.1. Scope of field investigations for the case studies (continued). 

Case Study Title Field Investigations 
City of Austin Technical 
Procedures for Watershed 
Erosion Assessments 

 
 
 
 
 
 
 
 
Sedimentsampling 
Sampling of bed and bank material 
Survey of riparian vegetation 
Watershed surveyto verifywatershed boundaries, geology, urbanization (existing or future projects), and 
structures at risk 
Topographic survey of stream profile and cross sections 
Periodic stream reconnaissance surveys to assess aggradation/degradation, widening, planform 
changes, slope failure, severe bends, nick points, erodibility of soils, bankfull width and depth, channel 
type, channelization projects, hydraulic structures, presence of utilities, and other disturbances 
Geotechnical survey to evaluate slope stability 
Survey or roughness coefficients 
Discharge measurements 
River Stability Study, Virgin 
River, Santa Clara River and 
Fort Pierce Wash, Vicinity of 
St. George, Utah 
 
 
 
Visual survey to assess relative river stability; physical characteristics observed are banks and cutting, 
vegetation on banks, floodplain, bars, bank and bed sediments, bank protection, bridge scour, and 
tributaries 
Suspended sediment measurements 
Floodplain land use 
Hydrologic and Geomorphic 
Studies of the Platte River 
Basin, Nebraska 

 
 
 
 
 
 
Sedimentsampling 
Sampling of bed and bank material 
Survey of riparian vegetation 
Topographic survey of stream profile and cross sections 
Stream reconnaissance surveys to assess channel patterns, scour, and bed forms 
Water samples for suspended sediment analysis 
Discharge measurements 
Streambank Erosion Along 
Two Rivers In Iowa 
 
 

 
Cross section surveys 
Sampling of bed sediment 
Streamflowobservations 
Bank vegetation survey for type and distribution 
Channel Migration Studies in 
King County, Washington 
 
 
 

 
 
 
 
Sampling of bed and bank material 
Survey of bank heights and bank erosion 
Survey of riparian vegetation type and age 
Geologic survey 
Watershed survey to inventory urbanization, roads, and other infrastructure at risk 
Topographic survey of stream profile and cross sections 
Stream surveys to assess aggradation/degradation and floodplain morphology 
Survey of levees, revetments, dams, and other hydraulic structures 

ASSESSMENT OF TECHNICAL FEASIBILITY 


Table 5.1. Scope of field investigations for the case studies (continued). 

Case Study Title Field Investigations 
Bank Erosion Field Survey 
Report of the Upper 
Mississippi River and Illinois 
Waterway 
 Field verification of hydrologic parameters 
 Cross section and channel profile topographic surveys 
 Survey of bridges, culverts, and other hydraulic structures 
 Estimation of roughness coefficients 
 Sedimentsampling 
 Sediment transport rate measurements 
 Sampling of bed and bank 
Arizona Standards for Lateral 
Migration and Channel 
Degradation 
Level 1 
 No field investigations needed 
Level 2 
 Soil samples for grain analysis 
 Soil samples for unconfined compression test 
 Water samples for suspended sediment analysis 
Level 3 
 Same as Level 3 in AMAFCA Sediment and Erosion Design Guide 

5.2.2. Geomorphic Analysis 
This study category involves comprehensive evaluation of stream morphology, watershed 
characteristics and historic data. The objective is to assemble all of these pieces of information to 
assess channel stability and predict changes in depth and width. 

Of the twelve case studies, nine of them report application of geomorphic methods to some 
extent. Some of the case studies involve joint application of geomorphic and engineering 
principles but are included in this section because the approach is predominantly geomorphic. 
Table 5.2 summarizes the data needs and analysis procedures employed in each study. 

The geomorphic procedures in Table 5.2 are all useful in providing a picture of the erosion 
hazards, although not all of the methodologies had the delineation of erosion setbacks as ultimate 
goal. Nevertheless, the literature indicates that geomorphic methods can be used to define 
erosion hazard areas to some degree. One difficulty is the wide variation of time scales 
associated with the data ranging from geologic to recent events. Information from all of these 
time scales must be assembled to be able to make predictions a few decades into the future. 
Another challenge is that past phenomena are the result of varying watershed conditions due both 
to natural events and to human intervention; for example, urban development, channel 
modification, and hydraulic structures. The analyst is often confronted with drawing conclusions 
for the short- to mid term by extrapolating to future conditions that will also be characterized by 
varying watershed characteristics. Geomorphic analysis requires professionals with a specialized 
set of skills and experience that may be in short supply. 

ASSESSMENT OF TECHNICAL FEASIBILITY 


Table 5.2. Data needs and procedures in geomorphic studies. 

Case Study Title Data Needs Analysis Procedures 
AMAFCA Sediment and 
Erosion Design Guide 
(Level 1) 
 
 
 

 

 




 
 
 
 
 
Topographic maps (USGS quads, 
local mapping, control points, etc.) 
Historic stream plans and profiles 
Field notes and photographs on bank 
geometry, stability problems, flow 
patterns 
Planimetric maps 
Aerial and satellite photographs 
Transportationmaps 
Bridge and culvert as-built plans 
Geologic maps 
Soils maps 
Land use 
Rainfall records 
Streamflow data (flooding events) 
Sedimentation data (bed and bank 
sediment size, transport rates, 
aggradation and degradation areas) 
Water quality data (sediment 
concentrations) 
Water use data (irrigation, power 
generation, diversions, etc.) 
Watershed development plans 
(urbanization, channel modifications, 
dams, and streambed mining) 
 
 
 
 
 
 
 
 
 
 
 
 
Use topographic, soils, land use, and geologic maps to 
identify instability problems 
Use maps to locate braids, bars, tributaries, and channel 
controls such as rock outcrops 
Use soil and land use maps to evaluate vegetative cover and 
other factors affecting sediment supply 
Analyze aerial photographs to assess watershed 
development and changes in stream patterns: bend 
migration, bankline failure, and erosion progress 
Determine stability of streambanks using photographs and 
field notes 
Use as-built plans and observations to evaluate scour 
Analyze impact on instability of existing and planned human 
activities (streambed mining, channel modifications, 
urbanization, dam construction) 
Identify changes in hydrology and hydraulics using rainfall and 
streamflow records 
Consolidate the analyses above into an overall assessment 
of stream stability 
Combine stream bank field notes and bankline failure 
observations from aerial photographs to evaluate lateral 
stability and estimate lateral erosion rates 
Evaluate vertical stability from information on bank failure, 
historic streambed profiles, and long-term trends in stage-
discharge relationships 
Use simple predictive geomorphic tools (e.g., the Lane 
Relationship) to evaluate channel response mechanisms 
Inventory and Analysis of 
Stream Meander 
Problems in Minnesota 
 
 

 
 
Grain size distribution 
Historic aerial photographs 
Topographic maps 
2-year flood discharge as the bankfull 
discharge 
Stream parameters (slope, bankfull 
width, and depth) 
 
 
 
Process photographs and maps to obtain digitized valley 
centerlines and stream centerlines 
Clean and smooth lines and run computer program 
MEANDER to determine the stream's transversal and 
longitudinal shift, sinuosity change rate, and reworked 
floodplain area 
Develop relationships between the shift parameters from 
MEANDER and stream parameters: width, depth, flow rate, 
and slope 
Geomorphology and 
Hydrology of the Santa 
Cruz River, Southeastern 
Arizona 
 

 


 
 
Geologic maps and reports 
Topographic maps 
Historic stream and plans and profiles 
Hydrologic data 
Climatic data 
Seasonal sediment concentration 
Aerial and satellite photographs 
 
 
 
Use geologic data to define tectonics, depositional patterns, 
floodplain landforms and sedimentation 
Use hydrologic data to determine runoff volumes and history, 
frequency, spatial and temporal characteristics of flood 
events 
Use climatic data to define meteorological characteristics of 
major storms 

ASSESSMENT OF TECHNICAL FEASIBILITY 


Table 5.2. Data needs and procedures in geomorphic studies (continued). 

Case Study Title Data Needs Analysis Procedures 
Geomorphology and 
Hydrology of the Santa 
Cruz River, Southeastern 
Arizona (continued) 
 
 
Field observations on 
aggradation/degradation, channel 
widening, planform changes, bank 
structural condition, and mass wasting 
Watershed plans (urbanization, 
channel modifications, dams, effluent 
discharge, and streambed mining). 
 
 
Analyze field data and maps to summarize channel 
changes: meander migration, avulsion, meander cutoff, 
channel widening, mass wasting, slope degradation, vertical 
channel change, and human-induced changes 
Identify effects of geology, topography, hydrology, climate, 
and human intervention in controlling channel change 
City of Austin Technical 
Procedures for 
Watershed Erosion 
Assessments 
 
 
 
 

 


 
 
 
 
 
 


 
Watershed boundaries and surface 
area 
Sediment size distribution and 
transport rate 
Soil consistency of bed and bank 
materials and geotechnical data on 
slope stability 
Type and spatial distribution of 
riparian vegetation 
Watershed boundaries 
USGS regression equations 
Geologic maps 
Soils maps 
Magnitude, timing and distribution of 
urbanization influence in the 
watershed 
Inventory of structures at risk 
Longitudinal stream valley gradient 
Cross section measurements 
Field observations on 
aggradation/degradation, widening, 
planform changes, slope failure, 
severe bends, nick points, and 
erodibility of soils, channel type, 
channelization projects, hydraulic 
structures, presence of utilities, and 
other disturbances 
Measurements of bankfull width and 
depth 
Roughness coefficients 
Discharge measurements 
Scale of features in the streams 
 
 
 
 
 
 
 
 
 
Use watershed data to estimate impervious areas 
Use watershed development information and maps to 
prepare summary of drainage, development patterns, 
stream geometry and topology, soils, and inventory of 
parklands and other watershed features 
Use USGS regression equations to estimate 2- and 10-year 
peak flow events for all reaches 
Use field data to prioritize erosion problems and the assets 
at risk. Use geotechnical data on highest priority sites to 
assess slope stability 
Identify causes of erosion 
Classify channels according to 1) erosion susceptibility of 
bed, banks, or both; 2) bed material (alluvial rock-bed 
controlled, rock controlled, and structural); 3) the Knighton 
(1987) system 
Use Rapid Geomorphic Assessment (RGA) to compute the 
Stability Index (SI) describing the channel as stable, 
transitional, or in adjustment 
Determine "channel enlargement curve" after Morisawa and 
LaFlure (1979) 
Use channel enlargement curve to estimate ultimate channel 
geometry and approximate sediment yield (see Section 
4.6.2.6 for a detailed description) 
River Stability Study, 
Virgin River, Santa Clara 
River and Fort Pierce 
Wash, Vicinity of St. 
George, Utah 
River Stability Study, 
Virgin River, Santa Clara 
River and Fort Pierce 
Wash, Vicinity of St. 
 

 

 

 
 

Historic aerial photographs 
Topographic maps 
Geologic maps and reports 
Archeologicaldata 
Historic accounts and diaries 
Floodplainmaps 
Available hydraulic models 
Hydrologic and paleohydrology data 
Climatic data 
 
 
 
 
Use historic information to determine stream alignment and 
characteristics in the past 
Produce map overlays from configurations at various points 
in time 
Quantify geometric changes in channel and floodplain 
geometry to estimate lateral migration 
Correlate flood occurrence to channel changes caused by 
erosion 

ASSESSMENT OF TECHNICAL FEASIBILITY 


Table 5.2. Data needs and procedures in geomorphic studies (continued). 

Case Study Title Data Needs Analysis Procedures 
George, Utah (continued)  Suspended sediment data 
 Flood photographs 
 Inventory of human activities in the 
watershed (ditches, diversions, 
reservoirs, grazing activities) 
 Bridge construction plans and 
maintenance records 
 Use geomorphic data to classify the stream according to 
Rosgen (1994) and FHWA's system (FHWA, 1991; Brice, 
1983) 
 Evaluate bank erosion using Rosgen's bank erosion matrix 
(Rosgen, 1996) and the approach in Thorne and Osman 
(1988) 
 Evaluate erosion potential using the maximum permissible 
velocity 
 Consolidate all information to define an erosion corridor 
Hydrologic and 
Geomorphic Studies of 
the Platte River Basin, 
Nebraska (Methods 1 
and 3) 
Method 1 
 Flow duration curve 
 Regression equation relating 
sediment transport rate to water 
discharge 
Method 3 
 Channel slope, depth, and width 
 Sediment size distribution and 
specific gravity 
Method 1 
 Use flow duration curve and regression equation to compute 
duration curve for sediment transport 
 Determine "effective discharge" as water discharge 
associated with maximum contribution to expected 
sediment transport rate 
Method 3 
 Compute particle fall velocity 
 Use channel slope and particle fall velocity to compute 
relative smoothness from the first Parker (1997) equation 
 Use relative smoothness and channel geometric 
parameters to compute water discharge and sediment 
transport rate using the remaining two Parker (1977) 
equations 
Channel Migration 
Studies in King County, 
Washington 
Channel Migration 
Studies in King County, 
Washington (continued) 
 Properties of bed and bank material 
 Bank heights and erosion problems 
 Type, age, and distribution of riparian 
vegetation 
 Geologic maps 
 Watershed development maps 
 Aerialphotographs 
 Stream profile and cross sections 
 Flood magnitudes 
 Data on aggradation/degradation 
 Floodplain morphologic data 
 Inventory of levees, revetments, dams, 
and other hydraulic structures 
 Review historic maps and photographs and determine 
migration rates 
 Divide streams into geomorphologically uniform reaches 
 Compute shear stress caused by the 100-year flood and 
analyze potential channel enlargement 
 Evaluate effect of hydrologic changes in channel migration 
potential. Select those periods consistent with current 
conditions 
 Use historic data on bend amplitude and meander belt width 
to estimate outer limits of future channel migration 
 Adjust the limits using information on likely erosion 
constraints (e.g., roads, levees, bank protection, and 
development). Produce a Mitigated Migration Hazard Map 
 Adjust the map using evidence of frequent avulsion 
 Use historic data to classify erosion hazard as moderate or 
severe 
Bank Erosion Field 
Survey Report of the 
Upper Mississippi River 
 Aerial oblique videotape 
 Site photographs 
 Topographic maps 
 Use soils data to classify soils 
 For all study sites, use bank characteristics and hydrologic 
data to assess bank erosion and failure mechanisms 

ASSESSMENT OF TECHNICAL FEASIBILITY 


Table 5.2. Data needs and procedures in geomorphic studies (continued). 

Case Study Title Data Needs Analysis Procedures 
and Illinois Waterway 
 
 


  
Navigationcharts 
Geometry of banks 
Properties of bed and bank materials 
Cross sections 
Vegetationcover 
Historic flood data and water level 
 
 
Compare erosion with stage records to define correlation 
with flood events, traffic waves, and currents 
Prepare color-coded erosion location map 
Arizona Standards for 
Lateral Migration and 
Channel Degradation 
(Level 1) 
 
 

 

100-year peak discharge 
Ratio of channel curvature to top width 
100-year floodway 
Top of channel bank location 
Drainage area 
 
 
Use 100-year discharge in setback regression equation 
appropriate for curvature-top width ratio 
Use 100-year discharge in total scour regression equation 
appropriate for channel curvature 

5.2.3. Engineering Analysis 
This study category involves application of basic engineering principles and is often 
complemented with geomorphic methods. Some of the tools employed include friction formulas 
for hydraulic conveyance, flood frequency procedures, hydrologic and hydraulic models, sediment 
yield, sediment transport, geotechnical analysis of slope stability, and soil property testing. The 
objective is to analyze flow regimes and their effect on structural stability to predict changes in 
channel geometry. 

Several case studies involve joint application of geomorphic and engineering principles. However, 
only three are included in this section because the approach is predominantly based on 
engineering principles. It should be noted that the most effective use of engineering tools is 
accomplished when preceded or combined with geomorphic analyses; therefore, separation of 
the two approaches is not always possible or desired. Table 5.3 summarizes the data needs 
and analysis procedures employed in each of the three case studies. 

Application of basic engineering methods affords a series of tools for quantification of erosion 
rates. The analyst must compile the information available from both geomorphic and engineering 
tools and use considerable judgement in delineating an erosion hazard area. Of special concern 
is the inability of many of these simple tools to consider past erosion history. The analyst must 
carry out this temporal integration by consolidating all of the information available. 

ASSESSMENT OF TECHNICAL FEASIBILITY 


Table 5.3. Data needs and procedures in basic engineering studies. 

Case Study Title Data Needs Analysis Procedures 
AMAFCA Sediment and 
Erosion Design Guide 
(Level 2) 
 Flood frequency curves 
 Flood hydrographs 
 Rainfall data 
 Bankfulldischarge 
 Stream profile, alignment, and cross 
sections 
 Roughness coefficients 
 Geometric data on bridges and 
culverts 
 Land use maps 
 Soils maps 
 Geologic maps 
 Sediment size distribution in 
watershed and channel 
 Mechanical properties of bed and 
bank materials: size, shape, fall 
velocity, cohesion, density, and angle 
of repose 
 Measurements of sediment transport 
rates 
 Reservoir operation and 
sedimentation data 
 Use watershed data and rainfall records to conduct 
hydrologic modeling (e.g., HEC-HMS) and develop flood 
frequency curves 
 Evaluate flood history to identify wet-dry periods 
 Use hydraulic modeling (e.g., HEC-RAS or Manning's 
formula) to define velocity, flow depth, top width, and other 
flow properties of the main channel and overbanks 
 Analyze characteristics of bed and bank materials: size, fall 
velocity, and mechanical properties 
 Use maps of land use and soils, and watershed plans to 
evaluate sediment yield from the watershed for existing and 
future conditions. The RUSLE methodology can be applied 
for this purpose 
 Use bed and material size distribution to perform incipient 
motion analyses and analyze bed armoring 
 Compute or estimate sediment transport and apply 
sediment continuity to determine aggradation/degradation 
potential. Various sediment transport formulas can be used 
(Table 2.4) 
 Evaluate lateral erosion potential by analyzing slope stability 
and bank failure mechanisms (e.g., Osman and Thorne, 
1988) 
 Evaluate local scour using formulas in AMAFCA (1994) 
Streambank Erosion 
Along Two Rivers In Iowa 
 Topographic maps 
 Aerialphotographs 
 Bed and bank material properties 
 Type and distribution of vegetation 
along banks 
 Streamflowrecords 
 Data on areal loss from banks 
 Roughness coefficients 
 History of channel planform, width, 
depth, curvature, slope, and bed 
forms 
 Develop a conceptual model of the stream in which the 
meanders are approximated as constant-radius curves 
 Determine the near-bank, depth-averaged mean velocity 
using equations in Odgaard (1986) 
 Determine the lateral migration rate using the scour factor in 
Parker (1983) 
 Use equations in Odgaard (1987) to compute maximum 
rate of bank retreat, average areal and volumetric erosion 
rates, and downvalley migration rates 
Arizona Standards for 
Lateral Migration and 
Channel Degradation 
(Level 2) 
 100-year peak discharge 
 Suspended sediment concentration 
 Channel curvature, bank slope, width, 
and flow depth 
 Grain size distribution 
 Energyslope 
 Manning's roughness coefficient 
 Unconfined compression strength 
 Use grain size to determine maximum allowable velocity and 
compare with 100-year discharge velocity 
 Use grain size distribution, flow depth, energy slope, and 
roughness coefficient to determine tractive stress. Correct 
for channel geometry and compare with maximum allowable 
tractive stress 
 Compute tractive power and use unconfined compression 
strength to derive maximum allowable tractive power 

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5.2.4. Mathematical Modeling 
This approach is the most complex level of analysis and requires discretization of the fluvial 
system into small units where computational models are applied to determine variations of the 
channel geometry in time. Variable material properties can be assigned to each discretized unit 
to account for localized conditions. The computations are usually complex and could be statistical 
or physically based in nature. 

Two case studies involve mathematical modeling. The first case study is based on probabilistic 
analysis; the second is based on simulation of the basic equations for hydraulics and sediment 
transport. Table 5.4 summarizes the data needs and procedures for the two case studies. 

Table 5.4. Data needs and procedures in mathematical modeling studies. 

Case Study Title Data Needs Analysis Procedures 
A Probabilistic Approach 
to the Special 
Assessment of River 
Channel Instability 
 Aerial photographs for several time 
periods. 
 Divide all of the aerial photographs into uniform square cells. 
Designate those cells occupied by the channel. 
 Assign "coordinates" to each cell based on lateral and 
upstream distances to the channel. 
 For each period of analysis, count the cells that were eroded 
for each pair of "coordinates" between the photographs 
defining the beginning and end of the period. Compute the 
erosion probability as the number of eroded cells divided by 
the total number of cells for those coordinates. 
 Repeat the procedure for other time periods. 
 Use regression techniques to define the erosion probability 
as a function of lateral and upstream distance to the channel 
and of the return period of flooding events. 
 Use the equation to predict future erosion extent. 
 Develop erosion probability map to identify potential hazards. 
San Diego County Alluvial 
Studies 
 Initial profile and cross sections of the 
channel 
 Channelroughness 
 Sedimentcharacteristics 
 Hydrographs 
 Downstream stage-discharge rating 
curve 
 Physical constraints (e.g., rock 
outcrops, check dams, and 
nonerodible banks) 
 History of floods and induced channel 
change 
 Enter initial conditions of channel geometry 
 Enter hydrograph and simulation time step 
 Run FLUVIAL-12 and obtain bed profiles at all sections. 
 Calibrate model to known conditions 
 Verify with another set of known conditions 

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Table 5.4 illustrates two very different approaches to mathematical modeling. They both 
discretize the area of interest and apply equations to each of the discrete elements; however, the 
results are quite different. The statistical model predicts the probability of erosion occurrence at 
various locations along river reaches whereas the physically based model provides the channel 
geometry and alignment at various points in time. Therefore, the statistical model needs to be 
interpreted further to define erosion hazard boundaries whereas these hazard areas can be 
directly obtained from the output of the physically based model. 

Data needs, computational algorithms, and output are specific for each model (see Table 2.6) 
and cannot be generalized. However, mathematical models are usually designed to simulate the 
evolution of channel morphology under a variety of scenarios. As such, these models are 
naturally suited for hazard area delineation. On the other hand, the mathematical representation 
of sediment transport and erosion processes is still imperfect and prone to large errors. In 
addition to being calibrated and verified, models should be analyzed critically in light of their 
limitations to understand the nature and magnitude of potential errors. 

5.2.5. Conclusions 
The case studies indicate that delineation of riverine erosion hazard areas is feasible. There are 
various geomorphic, engineering, and modeling procedures that can be applied, although the 
accuracy of such methods could be quite variable. Considerable judgement and experience are 
needed to draw conclusions that would lead to delineation of a hazard area. 

The basic data used by geomorphic and engineering methods are nonstationary in the sense that 
the response of the fluvial system is the result of time-variable watershed conditions. This 
characteristic implies that extrapolation directly from historic data may need to correct for past 
and future variability. 

In addition to issues of spatial accuracy, hazard area delineation has a temporal component. This 
issue is analyzed in the following section. 

5.3. The 60-Year Horizon 
To be useful, the REHA delineated should have an associated time span within which erosion is 
expected to occur within the delineation. Evidence of erosion in geologic times could be strongly 
supported with field observations but may be of limited application for floodplain management due 
to the long time scale involved. Therefore, planners and decision-makers need to know the time 
frame for which a particular hazard area is valid. For management purposes this time frame 
should be of the order of magnitude of decades, which is consistent with the useful life of 
structures in the built environment. For example, a structure's useful life usually extends well 
beyond the 30 years of a typical mortgage, and there are many structures in the United States 
older than 100 years. 

A time frame of 60 years has been specified in Section 577 of NFIRA as the interval of interest 
for delineation of riverine erosion hazard areas. This designation likely stems from a similar 
requirement for assessment of coastal erosion, but there is no apparent scientific basis to choose 
60 years. However, the case studies and the opinions of the PWG indicate existing techniques 
are better suited for shorter time frames; in consequence, larger uncertainties will likely affect 
erosion predictions for 60-year periods. This difficulty arises from limitations in data accuracy, 
errors in sediment transport models, and unknowns in future watershed development, hydrologic 
conditions, and magnitude and sequence of future flooding events. 

From the review of the case studies, two approaches emerge to address the erosion time frame: 

ASSESSMENT OF TECHNICAL FEASIBILITY 


1. Extrapolation from site data: This is the approach applied by those case studies that yield 
an erosion-prone area as part of the results. As a general procedure, the configuration of 
the stream at two or more points in time is used to determine average migration rates that 
can be translated into erosion and deposition rates. Ideally, the rates would be adjusted to 
account for variable watershed conditions in the historic period. These rates can then be 
used to predict the stream's location at some time in the future. In the simplest case, 
migration can be assumed linear in time by selecting a migration rate variable in space but 
constant for the time period under consideration. A more suitable approach uses a 
"frequency weighted" erosion rate (i.e., the expected annual erosion rate), which includes the 
erosion caused by the possible range of flows weighted by their frequency of occurrence. 
The predicted alignment can be modified to account for localized conditions such as identified 
geologic controls and existing hydraulic structures. The prediction should also incorporate 
expected future watershed development. 
2. Mathematical modeling up to a specified time: Continuous simulation models (e.g., 
FLUVIAL-12) make use of streamflow hydrographs and time series for watershed 
characteristics to simulate temporal variations in stream geometry. In theory, the simulation 
can proceed indefinitely for as long as detailed temporal resolution of input data is available. 
To predict future conditions, the model requires estimated future time series of the input 
data, which can be statistically generated or inferred from historic data and expected future 
trends. In practice, forecasts for input data may be reliable only for a short period in the 
future. The basic procedure involves using available data to generate time series up to a 
specified target date and running the model to simulate changes in the stream. 
Application of either approach raises the issue of uncertainty surrounding both the time frame and 
the spatial location of the REHA limits. For the floodplain manager, assessment of erosion risk 
must include an estimation of the confidence interval of erosion predictions both in time and 
space. The need for this information is especially critical when restrictions are placed on 
development and other activities due to erosion hazards. 

An analyst experienced in the application of geomorphic and engineering methods can use 
professional judgement in assigning a confidence interval to a particular REHA delineation. 
However, when judgement is involved, there is a distinct possibility that two analysts may arrive at 
different conclusions due to the subjective nature of the analysis process. In addition, there is an 
acknowledged lack of professionals with the necessary training and expertise in this field to 
conduct these assessments. 

Continuous simulation applying mathematical models offers a more systematic approach that 
also lends itself to replicability. The historic data can be used to derive statistical distributions for 
streamflow and for other input parameters that can be considered random variables. Using 
Monte Carlo simulation techniques, the distributions are sampled to generate several hypothetical 
future scenarios for which the model is run. With a sufficiently large number of scenarios, the 
resulting set of erosion areas can be used to derive REHA statistics and confidence intervals 
(Froehlich, 1997). In addition to the skills needed to run the fluvial model, this approach requires 
knowledge of stochastic processes and Monte Carlo simulation. For instance, expertise is also 
required to assign probability distributions to model parameters. After running all of the 
simulations, considerable effort is needed to process the results from the individual runs and 
derive output statistics. 

Implementation of the approach is expensive because it involves numerous runs, but the 
procedure is straightforward and lends itself to drawing objective conclusions. Uncertainty is built 
into the approach as the various probability distributions are sampled to generate run scenarios; 
therefore, the confidence intervals produced by the Monte Carlo approach can be used for risk-
based analysis. 

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Avulsion presents an additional challenge because of the catastrophic nature of the phenomenon 
and its unpredictability. It is possible to identify past and potential avulsion sites during stream 
reconnaissance; however, it is doubtful that the occurrence of avulsion can be predicted reliably 
even for the near future. The cumulative effect of long-term avulsion is unknown. 

5.4. Cost 
5.4.1. Data Source and Assumptions 
A detailed estimation of total REHA analysis and mapping costs is beyond the scope of this 
study. However, an approximate cost of implementation has been estimated. The purpose of 
the cost estimation of this study is threefold: 

 identifying elements that should be included in the cost estimation; 
 establishing a conceptual cost estimation framework; and 
 providing approximate NFIP cost for REHA analysis and mapping. 
The sources of cost data for this report include information provided by the PWG, costs reported 
in the case studies (Chapter 4), FEMA reports and other literature, and cost data from previous 
studies performed by the project team members. The data are not sufficient to make a reliable 
nationwide cost estimation; however, they can be used to perform an educated guess for total 
costs. 

Several assumptions were made to estimate costs of nationwide REHA mapping. First, the 
REHA mapping would be based on the existing FIRMs of a community. The hydrologic and 
hydraulic studies used to develop FIRMs could be used as the base for riverine erosion studies, 
and the same base map would be used to map REHAs. The costs of the basic hydrologic and 
hydraulic studies to develop FIRMs and the cost for preparing base maps were excluded from 
the REHA cost estimation. Only the costs that are solely designated for performing riverine 
erosion studies and mapping the erosion boundaries are included. 

Second, cost data, especially data from case studies, are from different time periods. For the 
estimate presented herein, all costs are assumed in 1997 dollars; inflation rates were not 
considered. 

Third, although study costs vary with amount of stream miles, there is not enough information to 
quantify economies of scale. For simplicity, the average unit cost per map panel was used as the 
base rate. Under this assumption, the average unit cost was first estimated and then multiplied 
by the number of panels to obtain the total costs. 

Procedures of cost estimation are presented in following sections. 

5.4.2. Affected Areas 
To determine the extent of areas prone to riverine erosion, the Project Team designated levels of 
erosion as high, intermediate, or low for each county in the entire United States. These initial 
designations were supplemented with input from the PWG and the literature that was reviewed 
(listed in the Appendix). Quantitatively, a county's erosion potential was designated as "high" if 
more than 60 percent of stream miles in the county were subject to riverine erosion. An average 
percentage of erosion-prone stream miles of 80 percent was used in the cost estimation. 
"Intermediate" was assigned to counties with erosion prone stream miles from 30 to 60 percent, 
and an average of 45 percent was assumed. "Low" was assigned to counties with less than 30 
percent erosion-prone stream miles, and 15 percent was assumed as an average. 

ASSESSMENT OF TECHNICAL FEASIBILITY 


5.4.3. Applicability of REHA Study Approaches 
Not all erosion prone streams would be studied using technical approaches that belong to the 
same study category. This is because counties with erosion-prone streams in limited areas 
would be less motivated to develop REHA maps; while counties that have many erosion-prone 
streams widely distributed would have more urgent needs and interest in mapping REHAs. In 
addition, even in counties with severe erosion problems, less populated areas, such as areas 
mapped as Zone A in FIRMs, are unlikely to be studied and mapped by using detailed, advanced 
and costly technical approaches such as mathematical modeling. More likely, they would be 
mapped using a rather simplified, less costly approach. 

From these considerations, a fraction of stream miles was designated to be studied by each 
selected study category: geomorphic, engineering, and mathematical modeling, as defined in 
Section 1.4.2, for each level of erosion. The same percentage was applied to all counties, 
regardless of their location. 

Using limited available information and considerable judgement, the Project Team estimated the 
approximate ranges for percentage of stream miles shown in Table 5.5. For example, it was 
assumed that an area with intermediate levels of erosion could be adequately studied with 
geomorphic methods applied to between 50 and 80 percent of the stream miles, engineering 
methods used in 20 to 40 percent of the stream miles, and mathematical modeling needed only 
for a maximum of 10 percent of the stream miles. 

Table 5.5. Assumed ranges for percentage of stream miles by study category for each level of 
erosion. 

Level of Erosion 
Study Category 
High Intermediate Low 
Geomorphic 30%50% 50%-80% 70%-100% 
Engineering 45%-60% 20%-40% 0%-30% 
Mathematical Modeling 5%-10% 0%-10% 0% 
5.4.4. Costs of REHA Study and Mapping 
REHA average unit cost is defined as the sum of the study cost, the mapping cost, and the cost 
of publication and distribution. These components are explained below. 

 Study cost. Costs for performing analysis to determine riverine erosion boundaries ranged 
from $2,000-$12,000 per stream mile, depending upon the study category. Average values 
are $2,000-$3,000 per mile for a study using geomorphic methods, $6,000-$7,000 for using 
engineering methods and $10,000-$12,000 for using mathematical modeling methods. 
Those data are from limited case studies as well as from the PWG and the project team; 
actual study costs per mile may vary in a wider range. The study cost per map panel was 
computed by multiplying the cost per mile and the average miles per panel. The nationwide 

ASSESSMENT OF TECHNICAL FEASIBILITY 


average is 2.2 stream miles per panel and is based on analysis of Q3 digital flood data 
conducted for the cost estimate for the FEMA map modernization report (FEMA, 1997). 

 Mapping cost. The cost for mapping erosion boundaries, including reviewing study results 
and transferring them into a digital map, was estimated using data for preparing FIRMs. In a 
digital mapping format, REHA boundaries would be stored as an additional layer in the 
FIRMs. The cost of preparing this layer was estimated as $1,000 per panel, one-third of the 
current mapping cost for a FIRM panel. 

 Publication and distribution cost. REHA maps would be published and distributed together 
with FIRMs. Additional costs for publication and distribution are limited and were estimated 
as $100 per panel. 

The average unit cost is estimated using following procedures for a given county: 

 Compute the erosion-prone stream miles per panel. Different percentages are used for 
counties with different levels of erosion, as discussed in Section 5.4.2. 

 Estimate the REHA study cost. The fraction of stream miles studied by each study category 
and the cost per mile for each study category are used to compute the study cost for each 
study category, using information presented in Section 5.4.3 (Table 5.5) and this section. 
The study cost per panel is obtained by adding the cost for each study category, then 
multiplied by the miles per panel. 

 The average unit cost is the sum of the study cost, the mapping cost, and the publication and 
distribution cost. 

The average unit cost for mapping REHAs varies with the level of erosion. Table 5.6 shows a 
summary of costs per panel by level of erosion. 

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Table 5.6. Average unit cost for each level of erosion 

Cost per Panel, $ Level of Erosion 
Study Mapping Publication/ 
Distribution 
Total 
High 7,400-11,000 1,000 100 8,500-12,100 
Intermediate 2,800-5,400 1,000 100 3,900-6,500 
Low 700-1,400 1,000 100 1,800-2,500 
5.4.5. NFIP Costs 
The cost per county for mapping REHAs was determined by applying the cost per panel to the 
number of panels in a county. The nationwide cost was obtained by summing all of the county 
estimates. The approximate cost to the NFIP is in the range of 200 to 300 million dollars. 

Section 577 of NFIRA specifies that, if REHA determination is found to be technically feasible, a 
cost-benefit study is to be conducted. The current study does not include these cost-benefit 
analyses. 

5.5. Conclusions on Technical Feasibility 
There is evidence that a time frame can be associated with a particular REHA delineation using 
one or a combination of the three analysis methodologies. However, specialized knowledge and 
expertise are needed to accomplish this task and evaluate the reliability of the results. Due to 
inherent inaccuracies in data collection and to the limitations of current models in attempting to 
represent the complex phenomena associated with fluvial processes, it is likely that various levels 
of reliability for the results will occur for a 60-year horizon. This difficulty is compounded by the 
unpredictability of avulsion. 

A more reasonable option would be to use a time frame that is long enough so that the stream 
may have time to change appreciably but short enough so that the uncertainty in forecasts can be 
reduced. It appears that a shorter time frame, such as 30 years, with periodic revisions meets 
these conditions. However, as pointed out earlier, the useful life of structures may extend well 
beyond 30 years. Some progressive communities may prefer to use the "geologic floodplain" for 
management purposes. 

Another option consistent with individual floodplain management needs is to assign a suitable time 
frame depending on local conditions and data availability. For example, more frequent 
assessments would be conducted for areas with severe erosion problems. Longer periods can 
be analyzed as better data become available. 

ASSESSMENT OF TECHNICAL FEASIBILITY 


6. Implementation 
Previous chapters addressed the technical aspects of delineating riverine erosion hazard areas 
and assessing the time frame for erosion to occur in those areas. The material in those chapters 
summarizes scientifically sound methodologies for REHA delineation. The approximate cost of 
conducting these activities was also examined. 

This chapter completes the analysis of technical feasibility by providing a description of 
implementation options for a nationwide program of REHA delineation. Information is also 
provided on regulations already in place in some jurisdictions that incorporate erosion into 
floodplain management activities. 

6.1. Existing Regulations 
6.1.1. Federal Regulations 
The NFIP regulations contain provisions to handle flood-related erosion to some extent. Section 

60.5 of the regulations states that FEMA is to furnish information so that communities with 
erosion-prone areas can implement floodplain management plans. In cases where they have 
been identified, erosion hazard areas are designated as Zone E on FIRMs, although there are 
currently no flood insurance rates for Zone E. The FEMA-supplied information sets forth the 
minimum requirements of floodplain regulations, but the communities can acquire additional data 
to complement this information. 
If in the process of applying for flood insurance coverage a community has identified erosion-
prone areas, the Federal regulations require issuance of construction permits in these areas and 
a review to establish the safety of proposed projects and the risk of causing additional hazards 
elsewhere. If these negative aspects are identified, the community must request modification of 
the project or deployment of mitigation measures. If a Zone E has been designated, in addition 
to the above requirements, the community is to require setbacks for new development to create 
a safety buffer consistent with the useful life of the structures and physical setting: geology, 
hydrology, topography, climate, and other site characteristics. No permanent structures can be 
placed in the buffer, but the zone can be used for open space purposes. 

The regulations state that communities must include erosion hazards in their planning to avoid 
development in hazard areas, promote open space utilization, and implementation of preventive 
measures in E zones (setbacks, shore protection, relocation and property acquisition of erosion-
prone areas). The planning process should include coordination with other jurisdictions and the 
State to ensure consistency. 

6.1.2. Local Regulations 
The Federal regulations are flexible to allow communities freedom to implement additional 
requirements. One example is the Floodplain and Erosion Hazard Management Ordinance for 
Pima County, Arizona. The ordinance requires the issuance of floodplain use permits that include 
review for impacts on riparian vegetation and for building setbacks. The ordinance states 
different setback criteria for major watercourses and minor washes in the county. 

As defined in the Pima County ordinance, major watercourses constitute those streams where 
the 100-year flood discharge is greater than 2,000 cubic feet per second. In the absence of 
"unusual conditions," the setback is 100 feet for streams with 100-year peak discharges between 
2,000 and 10,000 cubic feet per second and 250 feet when the discharge is greater than 10,000 

IMPLEMENTATION 131 


cubic feet per second. The setback is 500 feet for a few critical sections of certain county 
streams; for example, the Santa Cruz River and Rillito Creek. These setbacks can be reduced 
provided that additional analysis is performed to the satisfaction of the County Engineer. 
Examples of "unusual conditions" include historic meandering of the watercourse, large excavation 
pits, poorly consolidated banks, natural channel armoring, proximity to bridges or rock outcrops, 
and changes in flow amount, direction, and velocity in the watercourse. If "unusual conditions" 
prevail, the setback will be established on an individual case basis and approved by the County 
Engineer. The analysis must consider potential danger to life and property. 

Minor watercourses are streams where the 100-year peak flow is less than 2,000 cubic feet per 
second. Where no "unusual conditions" exist, the setback for these streams is 50 feet, unless 
approved bank protection measures have been deployed or additional analysis prove that the 
setback can be safely reduced. As with major watercourses, "unusual conditions" dictate the 
need for individual assessment of site-specific conditions. 

There are other communities in the United States that manage riverine erosion through adopted 
regulations or guidance through the local floodplain management program. In addition, under the 
Community Rating System (CRS), communities can receive credit for regulating erosion hazards. 
For example, up to 10 points can be awarded for disclosure of hazards other than flooding, 
including erosion; up to 50 points can be earned if the community maps areas with special flood-
related hazards; and up to 100 points can be assigned for regulations keyed to special flood-
related hazards. 

6.2. Options for Implementation 
The foregoing section suggests two potential options for implementation of a nationwide REHA 
delineation program. The first is a federally run program similar and integral to the NFIP. The 
second is a locally run program with support from the Federal government. The following 
subsections describe the general vision and features of these options as well as some of their 
advantages and disadvantages. 

6.2.1. NFIP Expansion 
The fundamental principle of the first option is to expand the current floodplain regulations to 
encompass riverine erosion. This option emphasizes authority from the Federal government. 

The process would parallel the current determination of floodplains. Qualified study contractors 
would conduct riverine erosion studies according to approved guidelines. The studies would 
undergo technical review before being approved by FEMA for inclusion in FIRMs. Actuarial 
studies would be conducted to account for erosion risks, and E zones would be used to 
determine insurance rates. Communities would be required to comply with Federal requirements 
to be eligible for insurance coverage. Additional legislation may need to be passed to regulate 
activities within REHAs. Similar to flooding studies, the regulations would require that REHAs be 
restudied periodically, which would lend itself to implementing the appropriate time frame with 
periodic revisions as discussed previously in Section 5.5. 

The main advantage of this option is the fact that there is already a regulatory structure in place to 
regulate flood-prone areas. The existing framework can be modified to accommodate the new 
responsibilities of regulating erosion-prone areas. One obvious disadvantage is the additional 
cost to the Federal government to implement the program. Another disadvantage is the risk of 
imposing rigid guidelines for REHA delineation in a field that requires flexibility and accessibility to 
a wide array of analytical options. A related disadvantage is the potential for developing 
adversarial situations among the Federal government, local jurisdictions, and citizens due to the 

IMPLEMENTATION 132 


inherent uncertainty in REHA delineation. One last disadvantage is the apparent shortage of 
qualified professionals to conduct and review REHA determination studies. 

6.2.2. Local Implementation 
The second option shifts the authority for regulating erosion-prone areas to the local jurisdictions. 
Implementation would be tailored to suit individual floodplain management needs. The Federal 
government would provide technical assistance, if required, and disseminate information. 

Under this option, the communities would determine the need to undertake REHA delineation 
studies that would be paid with local funds, although Federal funding may be available in some 
cases. The sole responsibility of selecting a consultant and approving the work would rest with 
the community's engineering staff. The Federal government could provide some review functions. 
The community would use the results of the studies to develop ordinances to manage erosion-
prone areas. The main responsibility of the Federal government is to publish erosion hazard 
areas in FIRMs for informational purposes. For insurance purposes, the Zone E designation 
could have the same rates as Zone X. 

An advantage of this option is that if follows the philosophy of current FEMA initiatives to support 
progressive communities, such as Cooperating Technical Communities and Project Impact. The 
delineated REHAs would be distributed digitally as additional layers in the Digital Flood Insurance 
Rate Maps (DFIRMs) as a tool for communities to use in evaluating exposure to multiple hazards. 
Another advantage is that the communities would have the flexibility to match their resources and 
needs with the complexity of the studies. The community would be responsible for defending its 
local regulations in potentially controversial situations. 

Similar to the first option, local implementation may be limited by a lack of skilled professionals to 
delineate REHAs. This shortcoming presents an additional disadvantage to the communities 
because they may not have the expertise or resources to evaluate potential consultants. Another 
disadvantage is that communities may not be politically or financially capable of revising REHA 
studies as often as required to maximize validity. 

Yet another disadvantage is the potential risk of inconsistencies and conflicts across jurisdictional 
boundaries. One community's lax erosion area regulations may be the cause of increased 
hazards to neighboring communities. These types of situations currently arise in connection with 
water quantity, but the authority of the Federal government serves to resolve these conflicts. In 
the case of erosion, however, the absence of a strong Federal arbiter compounded by the 
uncertainty associated with erosion hazard characterization could make these interjurisdictional 
differences more common. 

IMPLEMENTATION 133 


7. Conclusions 
Riverine erosion is a complex physical process that involves interaction of numerous factors: fluvial 
hydraulics, geotechnical stability, sediment transport, and watershed characteristics, including 
hydrology and sediment yield, past and future land use, and vegetation, among others. The 
study of riverine erosion is multidisciplinary in nature and requires experienced geomorphologists, 
hydrologists, hydraulic engineers, geotechnical engineers, photointerpreters, planners, and 
mapping specialists. Some of these professions require advanced degrees in their specialties. 
Valuable input is also needed from local floodplain managers. 

Despite decades of research into the physical processes associated with riverine erosion, 
knowledge of the subject is still imperfect, and much work remains to be done. Accurate 
mathematical representation of these processes has not been achieved yet, and available tools 
produce results surrounded by varying degrees of uncertainty. Nevertheless, there are analytical 
procedures that can be used to characterize riverine erosion and that, depending on the 
application, can yield reliable results. For example, because of limitations in data availability and 
model capabilities, it is extremely difficult to reproduce detailed time variation of stream 
movement; however, it is entirely feasible to analyze channel history and infer trends in the stream 
alignment and average migration rates. 

The analysis approaches currently in use by professionals in the field can be broadly classified 
into three categories: geomorphic analysis, basic engineering analysis, and mathematical 
modeling. Data needs and complexity are increasingly more rigorous from geomorphic analysis 
to mathematical modeling. Field investigations are a major component of all three approaches. 
Geomorphic analysis is characterized by evaluation of readily available information to infer 
relationships among sediment, flow regime, and channel geometry. Basic engineering analysis 
complements geomorphic analysis and introduces equations to quantify relationships among 
factors affecting channel configuration. Mathematical modeling couples analytical representations 
for processes in fluid mechanics and sediment transport to simulate the interaction of several 
physical phenomena and their impact on channel geometry. Modeling is the most complex 
approach, and its implementation requires considerable expertise and resources. 

The 12 case studies investigated for this report indicate that there are scientifically sound 
approaches to analyze riverine erosion. Some of the methods are suitable for qualitative 
assessments of erosion hazards; others establish conditions to be maintained if erosion is to be 
prevented. A few of the methods in the case studies can be applied to delineate erosion hazard 
areas, although the accuracy is highly variable. 

Two issues impact the significance and usefulness of a REHA: the degree of uncertainty in the 
spatial location and the time frame for which the REHA is applicable, that is, the time for which it 
is estimated that erosion will take place within the REHA. These two issues are related in that 
uncertainty is greater for long time frames. On the other hand, a very short time frame for which 
uncertainty is much reduced may be useless for floodplain management because of the minimal 
amount of erosion expected to occur. Section 577 of NFIRA specified a period of 60 years for 
REHA delineation; however, it appears that a shorter time frame, such as 30 years with periodic 
revisions, presents a more practical alternative. 

A preliminary cost analysis was performed based on limited information obtained as part of the 
current study and from personal experience within the Project Team and the PWG. Average 
study values are $2,000-$3,000 per mile for geomorphic methods, $6,000-$7,000 for 
engineering methods, and $10,000-$12,000 for mathematical modeling methods. If this effort 
were to be implemented as part of the NFIP, the cost to the Federal government would be 
between 200 and 300 million dollars. Section 577 of NFIRA specifies that, if REHA determination 

CONCLUSIONS 134 


is found to be technically feasible, a cost-benefit study is to be conducted. The current study 
does not include these cost-benefit analyses. 

A program for nationwide delineation of REHAs can be implemented under two scenarios. The 
first option is an expansion of the current NFIP regulatory scheme to include erosion. This option 
emphasizes authority at the Federal government level, and its foremost advantage is that there is 
already a framework in place for implementation. The main disadvantage is the additional 
regulatory burden and cost to the Federal government. The second option shifts responsibility to 
the local jurisdictions to conduct REHA delineation voluntarily. The Federal government would 
publish REHAs for informational purposes. The main advantage is that the delineations would be 
conducted according to the individual needs of the community. A disadvantage is the risk of 
adversarial situations that a community may face from land use restrictions. 

CONCLUSIONS 135 


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REFERENCES 146 


Appendix

APPENDIX 147 


List of Publications Investigated 

Ahmed, A. F. and M. M. Gasser (1992), "Bank erosion study of the nile river at bani mazar," in 
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Alaska Division of Emergency Services (1992), Mantanuska River Erosion Task Force Interim 
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Albuquerque Metropolitan Arroyo Flood Control Authority (1994), Sediment and Erosion Design 
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Andrews, E. D. (1979), Scour and Fill in a Stream Channel, East Fork River, Western 
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Annandale, G. W. and D. L. Parkhill (1995), "Stream bank erosion: application of the erodibility 
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Arizona Department of Water Resources, Flood Warning and Dam Safety (1996), State 
Standard for Watercourse System Sediment Balance, Phoenix, Arizona. 

Arizona Geological Survey (1992), Geomorphic Assessment of Flood-Prone Areas on the 
Southern Piedmont of the Tortolita Mountain, Pima County, Arizona. 

Ayres Associates (1983), Cottonwood Creek Drainage Basin Planning Study. 

Baker, V.R., R.C. Kochel, and P.C. Patton, eds. (1988), Flood Geomorphology, p. 465-489, 
John Wiley & Sons, New York, New York. 

Biedenharn, D. S., P. G. Combs, G. J. Hill, C. F. Pinkard, Jr., and C. B. Pinkston (1989), 
"relationship between channel migration and radius of curvature on the red river," in Sediment 
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Binnter, L., Y. Nio, and M. Garca (1994), "Mathematical models to assess stream dynamics," 
in Hydraulic Engineering'94, p. 391. 

Bond, L. A. (1997), Community Rating System Credit for Mapping and Management of Areas 
with Uncertain Flow Paths, Engineering Report to Community Task Force. 

Borah, D. K. and M. S. Dashputre (1994), "Field evaluation of the sediment transport model 
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Borah, D. K. and P. K. Bordoloi (1989), "Stream bank erosion and bed evolution model," in 
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Brabets, T. P. (1994), Scour Assessment at Bridges from Flag Point to Million Dollar Bridge, 
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Bryan, B. A. (1989), Channel Evolution of the Hatchie River near the U. S. Highway 51 
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89-598. 

APPENDIX 148 


Burkham, D. E. (1972), Channel Changes of the Gila River in Safford Valley, Arizona 18461970, USGS Professional Paper 655-G. 

Chang, H. H. (1984), "Modeling of river channel changes," ASCE Journal of Hydraulic 
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CH2M Hill, and R. H. French (1992), Alluvial Fan Data Collection and Monitoring Study for the 
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CH2M Hill and J. E. Fuller/Hydrology & Geomorphology, Inc. (1996), River Stability Study, Virgin 
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prepared for City of St. George, Utah. 

City of Austin (1997), Technical Procedures for the Watershed Erosion Assessments, prepared 
by Raymond Chan & Associates, Austin, Texas. 

Comes, B. M. (1990), "Identification techniques for bank erosion and failure processes," in 
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Copeland, R, R., and W. A. Thomas (1996), Sediment Impact Assessments Using the 
Hydraulic Design Package-SAM, USACE Waterway Experiment Station, Proceedings of 
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Daniel, J. F. (1971), Channel Movement of Meandering Indiana Streams, USGS Professional 
Paper 732-A. 

Darby, S. E. and C. R. Thorne (1994), "A physically based model of channel widening," Hydraulic 
Engineering '94, p. 944. 

Darby, S. E. and C. R. Thorne (1994), "Prediction of tension crack location and riverbank erosion 
hazards along destabilized channels," Earth Surface Processes and Landforms, vol. 19, p. 

233. 
Darby, S. E. and C. R. Thorne (1996), "Numerical simulation of widening and bed deformation of 
straight sand-bed rivers. I: Model development," ASCE Journal of Hydraulic Engineering, 
vol. 122, No.4. 

Darby, S. E., C. R. Thorne, and A. Simon (1996), "numerical simulation of widening and bed 
deformation of straight sand-bed rivers. II: Model evaluation," ASCE Journal of Hydraulic 
Engineering, vol. 122, No. 4, p. 194. 

Darby, S. E. and C. R. Thorne (1996), "development and testing of riverbank-stability analysis," 
ASCE Journal of Hydraulic Engineering, vol. 122, No. 8, p. 443. 

Davis, R. J. and K. J. Gregory (1994), "A new distinct mechanism of river bank erosion in a 
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DeLong, L. L. (1986), "Extension of the unsteady one-dimensional open-channel flow equations 
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Hydrologic Sciences. 

APPENDIX 149 


Eschner, T. R., Hadley, R. F., and Crowley, K. D. (1983), Hydrologic and Morphologic Changes 

in Channels of the Platte River Basin in Colorado, Wyoming and Nebraska: A Historical 

Perspective, Geologic Survey Professional Paper 1277-A. 

Faye, R.E., W.P. Carey, J.K. Stamer, and R.L. Kleckner (1980), Erosion, Sediment Discharge, 

and Channel Morphology in the Upper Chattahoochee River Basin, Georgia, USGS 

Professional Paper 1107. 

Federal Emergency Management Agency (1990), Analysis of Stream Bank and Bluff Stability 

for Use in the Evaluation of Flood Insurance Claim Eligibility Under The Upton/Jones 

Amendment, Washington, D.C. 

Federal Interagency Stream Restoration Working Group (1998), Stream Corridor Restoration 
Handbook. 

Graf, W. L. (1981), "Channel instability in a braided, sand bed river," Water Resources 
Research, vol. 17, No. 4, p. 1087. 

Graf, W. L. (1983), "Flood-related channel change in an arid-region river," Earth Surface Process 
and Landforms, vol. 8, p. 125. 

Graf, W. L. (1984), "A probabilistic approach to the spatial assessment of river channel 
instability," Water Resources Research, vol. 20, p. 953. 

Grimm, M. (1996), "Applied geomorphology and floodplain management," Land and Water, 
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Grissinger, E. H., and J. B. Murphey,. (1985), Channel cross-section changes in Mississippi's 
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Hagerty, D. J., and M. F. Spoor (1989), "Alluvial Stream Bank Erosion -- A 10-Year Study," in 
Sediment Transport Modeling, p. 594. 

Hagerty, D. J., and M. N. French (1988), "An Investigation into Causes of Bank Caving," in 
Critical Water Issues and Computer Applications, p. 254. 

Hasegawa, K. (1989), "Universal bank erosion coefficient for meandering rivers," ASCE Journal 
of Hydraulic Engineering, vol. 155, No. 6, p. 744. 

Hey, R. D. (1990), "Environmental river engineering," Journal of the Institute of Water and 
Environmental Management, vol. 4, p. 358. 

Hjalmarson, H. W., and S. P. Kemna (1991), Flood Hazards of Distributary-Flow Areas in 
Southwestern Arizona, USGS Water-Resources Investigations Report 91-4171. 

Holtorff, G. (1989), "The Formation of Meandering and Braided Channels," in Sediment 
Transport Modeling, p. 70. 

Jacobson, R. B., and K. Oberg (1997), Geomorphic Changes on the Mississippi River Flood 
Plain at Miller City, Illinois as a result of the flood of 1993, USGS Circular 1120-J. 

APPENDIX 150 


Jay, D. A., and C. A. Simenstad (1996), "Downstream effects of water withdrawal in a small, 
high-gradient basin: Erosion and deposition on the Skokomish River delta," Estuaries, vol. 
19, No. 3, p. 501. 

Johannesson, H, and G. Parker (1985), Computer Simulated Migration of Meandering Rivers 
in Minnesota, Project Report 242, St. Anthony Falls Hydraulic Laboratory, University of 
Minnesota. 

Jones, J. A. A. (1989), "Bank erosion: A review of British research," Hydraulic Engineering'89, p. 

283. 
Karlinger, M. R., T. R. Eschner, R. F. Hadley, and J.E. Kircher (1983), "Relation of channel-width 
maintenance to sediment transport and river morphology: Platte River, South-Central 
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King County Surface Water Management Division (1993), Green River Channel Migration 
Study. 

King County Surface Water Management Division (1996), Channel Migration in the Three Forks 
Area of the Snoqualmie River. 

Kircher, J. E. (1983), Interpretation of Sediment Data for the South Platte River in Colorado and 
Nebraska, and the North Platte and Platte Rivers in Nebraska, Geologic Survey 
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Kovacs, A. and G. Parker (1994), Erosion of Steep River Banks and Time Evolution Towards 
Equilibrium Channel Shape, in Engineering Mechanics, p. 896. 

Kresan, P.L. (1988), The Tucson, Arizona, flood of October 1983 - Implications for land 
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Lagasse, P. F., J. D. Schall, and M. Peterson (1985), "Erosion risk analysis for a southwestern 
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Lawler, D.M. (1993), "The measurement of river bank erosion and lateral channel change: A 
review," Earth Surface Processes, vol. 18, p. 777. 

Liou, J. (n.d.), Evaluation of Existing Mathematical Fluvial Models for National Flood Insurance 
Studies, 329 p. 

Lohnes, R. A. (1991), "A method for estimating land loss associated with stream channel 
degradation," Engineering Geology, vol. 31, p. 115. 

MacDonald, T. E., G. Parker, and D. P. Leuthe (1991), Inventory and Analysis of Stream 
Meander Problems in Minnesota, St. Anthony Falls Hydraulic Laboratory, University of 
Minnesota. 

MacMurray, H. L., and M.N.R. Jaeggi (1990), "Modeling erosion of sand and silt bed river," 
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APPENDIX 151 


Madej, M. A., W. E. Weaver, and D. K. Hagans (1994), "Analysis of bank erosion on the Merced 
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March, D. E., S. R. Abt, and C. R. Thorne (1993), "Bank stability analyses versus field 
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McCaffrey, W. F., J. C. Blodgett, and J. L.Thornton (1988), Channel Morphology of Cottonwood 
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Odgaard, A. J. (1987), "Streambank erosion along two rivers in Iowa," Water Resources 
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Odgaard, A. J., S. C. Jain, and D. J. Lutzbetak (1989), "Hydraulic mechanisms of riverbank 
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Osman, A. M., and C. R. Thorne (1988), Riverbank stability analysis I: Theory, ASCE Journal 
of Hydraulic Engineering, vol. 114, No. 2, pp.134-150. 

Osterkamp, W.R., L.J. Lane, and G. R. Foster (1983), An Analytical Treatment of Channel 
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Parsons Brinkerhoff, Quade, & Douglas (1997), Methodology for Predicting Channel Migration, 
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Parker, J. T. C. (1995), Channel Change and Sediment Transport in Two Desert Streams in 
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Parker, J. T. C. (1995), Channel Change on the Santa Cruz River, Pima County, Arizona, 
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Pemberton, M., J. Bradbury, P. Cullen, and G. Dixon (1995), "Monitoring and management of 
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APPENDIX 152 


Romero-Daz, M. A., F. Cabezas and F. Lpez-Bermdez (1992), "Erosion and fluvial 
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Rosgen, D. L. (1994), "A classification of natural rivers," Catena, vol. 22, No. 3, p. 169. 

Sevon, W.D. (1989), "Erosion in the Juniata River drainage basin, Pennsylvania," 
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Shan S. F., ed. (1988), Twelve Selected Computer Stream Sedimentation Models Developed in 
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Shannon and Wilson, Inc (1991), Tolt and Raging Rivers Channel Migration Study, King County, 
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Shields, Jr., F. D., S. S. Knight and C. M. Cooper (1995), "Rehabilitation of watersheds with 
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Simon, A. (1989), "The discharge of sediment in channelized alluvial streams, Water Resources 
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Simon, A. (1994), Gradation Processes and Channel Evolution in Modified Tennessee 
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Simon, A. and C. R. Hupp (1986), Channel Widening Characteristics and Bank Slope 
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Simon, A., and C. R. Hupp (1992), Geomorphic and Vegetative Recovery Processes Along 
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Simon, A., M. Rinaldi, and G. Hadish (1996), "Channel evolution in the loess area of the 
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Simons, Li & Associates, Inc. (1985), Design Manual for Engineering Analysis of Fluvial 
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Spoor, M. F. (1989), "International research on mechanics of river bank erosion -- A summary 
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APPENDIX 153 


Thorne, C. R., and A. M. Osman (1988), Riverbank stability analysis II: Applications, ASCE 
Journal of Hydraulic Engineering, vol. 114, No. 2, p. 151-172. 

Thorne, C.R., and V.R. Baghirathan (1994), "Blueprint for morphologic studies," in The Variability 
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Thorne, C. R., R.G. Allen, and A. Simon (1996), "Geomorphological river channel reconnaissance 
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Turnipseed, D. P. (1993), Lateral Movement and Stability of Channel Banks near Two Highway 
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Turnipseed, D. P. and K. Van Wilson, Jr. (1992), Channel and Bank Stability of Standing Pine 
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APPENDIX 154