Assessment of 
Structural Flood-Control 
Measures on Alluvial Fans 

October 1993 

Prepared By: 

Hydrologic Engineering Center 
US Army Corps of Engineers 
609 Second Street 
Davis, CA 95616-4687 

(916) 756-1104 
Prepared For: 

Federal Insurance Administration
Federal Emergency Management Agency
500 C Street, SW
Washington, D.C. 20472


1. INTRODUCTION
The Federal Emergency Management Agency (FEMA) prepares Flood 
Insurance Rate Maps (FIRMS) based on the results of Flood Insurance Studies 
(FIS). Those studies determine the areas with a i-percent annual chance of 
being inundated. The flood is called the Base (100-year) Ffood by FEMA. The 
FIS must evaluate the existing flood conveyance system, including installed 
flood-control measures. The determination of the inundated area may depend 
on whether flood-control measures protect part of the floodplain. With 
increased development in the arid western portions of the United States, more 
people are being exposed to the extreme flood hazards associated with flash 
floods, mud and debris flows, high flow velocity, channel avulsion, erosion and 
channel migration on alluvial fans. The dominant fluvial processes operating 
on alluvial fans are episodic in nature and are poorly understood. 
Nevertheless, one must consider whether flood-control measures perform well 
on alluvial fans, and if so, whether their performance can be evaluated. 

1.1 Geomorphology of Alluvial Fans 
Alluvial fans are depositional landforms, developed over a geologic time 
scale, located at the base of mountain ranges where ephemeral mountain 
streams emerge onto the lesser slopes of the valley floors (French, 1997). 
They are usually conical, or fan shaped in plan view. On topographic maps, 
alluvial fans appear as contour lines that are concentric about a canyon mouth 
(fan apex). Figures l-l and 1-2, respectively, are plan and profile views of an 
idealized alluvial fan. The figures also illustrate some of the terms used in this 
report. 

The broad use of the term alluvial fan in the geologic literature does not 
imply the existence of any specific hydraulic processes. Trends in deposition 
and erosion are episodic and locally variable on alluvial fans. 

There are fluvial systems which are not on the typical conical fan but 
maintain some of the characteristics of systems on alluvial fans, Arroyos are 
examples of distributary drainage networks with flow characteristics similar to 
those of a typical fan. 

Chapter 1. INTRODUCTION 


Plan 

Sheet 
Channelized , Braided , Flow 

Zone Zone  Zone 

The sediments deposited on alluvial fans are generally coarse grained, 
being composed of sand, gravel, and boulders. The depositional region is 
relatively close to the source region when compared with riverine situations. 
Alluvial fans are most common in arid or semi-arid areas where there is little 
vegetation; however, they also occur in polar and humid regions (Lecce, 1990 
and MacArthur, et al., 1992). There is considerable discussion in the 
geomorphologic literature (Lecce, 1990) regarding the formation of alluvial 
fans. 

Alluvial fans in the arid and semi-arid areas of the Southwestern United 
States are the focus of this study. In these areas, the important factors that 
contribute to the formation of alluvial fans are the hydrology, the geology and 
soil type, and the vegetative cover. Formation of an alluvial fan requires a 
source of sediment, a mechanism for moving that sediment, and an abrupt 
change in topographic slope and channel width that leads to shallow 
unbounded flow and sediment deposition. The relatively steep slopes often 
found on alluvial fans (3 to 20 percent) provide a high energy environment for 
the movement of water and sediment. The unbounded lateral dimension and 
rapid depositional nature of alluvial fans support frequent avulsions and flow 
spreading laterally on the fan surface. The hydrology of these areas is 
characterized by ephemeral (episodic) streams that only carry water and 
sediment during significant rainfall events. The predominant rainfall events that 

Chamer 1. INTRODUCTION 


Profile

Fan Surface 

Figure 1-2 Profile View of an Idealized Alluvial Fan 

form alluvial fans in these areas are localized thunderstorms. These storms 

may occur only once every several years over the watershed contributing to 
any particular fan (French, 1987). 

During the periods between large storms, unconsolidated sediment 
accumulates within the watershed and stream channels from minor storms, 
weathering, dry ravel erosion, earthquakes, etc., providing an abundant supply. 
This combination of steep slopes, abundant supply of sediment, and intense 
short-duration precipitation can produce mud and debris flows capable of 
moving large amounts of sediment onto the evolving fan. Debris flows can 
occasionally form sieve deposits on the fan (Rachocki, 1981). Sieve deposits 
occur when the water filters into the underlying coarse deposits, depositing the 
recently moved material. The stream channel system on alluvial fans is 
typically braided, which is both a characteristic of alluvial fans and an 
important factor in their formation (Rachocki, 1981). Beaty (1990) states that 
an average alluvial fan in the White Mountains of Southern California and 
Nevada could be formed in about 750,000 years by three average debris 
flows every 1,000 years, 

During a flood event, the flow may abandon the path lt has been taking 
and follow a new one. The occurrence, termed an avulsion, can result from 
floodwater overtopping a channel bank and creating a new channel. The 
overtopping may be caused by the sudden deposition of sediment and/or 
debris, or by the undercutting and subsequent failure of a channel bank. 
Because points below the avulsion may be in the path taken by the flood flow, 

Chapter I. INTRODUCTION 


either before or after the avulsion occurs, the probability of those points being 
inundated by the flood is greater than if the avulsion had not occurred. 
Through multiple avulsions over geologic time, the fan aggrades uniformly so 
that it tends to exhibit a concentric, semi-circular contour. 

Changes in flow and/or sediment supply to the fan can greatly affect the 
morphology of the apex and fan surface. If sediment supply from the 
upstream watershed to a fan is reduced due to changes in rainfall patterns or 
increased vegetation, incision of a channel will begin at the apex. When 
sediment transport capacity or competence of the flow exceeds the sediment 
load available at the apex, the flow will scour the fan surface and create an 
incised channel. As long as stream competence exceeds sediment supply, 
channel deepening and widening will continue. A return to higher sediment 
productivity in the watershed, that mayresult from forest fires or increased 
rainfall, will often cause the channel to backfill. In that case, erratic flood 
channels and sediment deposition will again dominate. If the reduction in 
sediment supply is permanent, an incised channel or entrenchment of the fan 
surface results. Normal depositional patterns on the fan are altered by the 
entrenchment in such a way that little deposition occurs near the apex, fan 
building commences downstream where the entrenchment ends, and flows 
radiate laterally over the fan surface. A new, secondary fan is established with 
its apex at the end of the entrenchment. 

Significant watershed and fan characteristics that influence flood behavior 
include: 

. watershed slope 
. watershed soil type, vegetation and land use 
. forest fire frequency 
. rainfall intensity and duration 
. longitudinal and lateral fan slope and shape 
. fan sediment type and vegetation 
. existence of an entrenched channel(s) 
. presence of rock outcrops or sills in the channel 
. location, density, geometry of development and/or structures (roads, 
fences, railroads, buildings) on the fan 

The influences and potential affects of these fan characteristics on the 
hydraulics and the sediment transport capability of flood waters must be 
considered when placing and designing structures on alluvial fans, 

Chapter I. INTRODUCTION 


1.2 Floods on Alluvial Fans 
Floods on alluvial fans, in the geographic regions discussed here, are 
generated by both localized convective storms and large scale cyclonic 
storms. Convective storms are the more common cause in Nevada, Arizona, 
Utah, and New Mexico; large cyclonic storms, generally, produce the flooding 
on alluvial fans in Southern California. Flooding on alluvial fans is infrequent, 
rapid, debris laden, and of high velocity. These floods can fill flood-control 
channels with debris, or erode elevated structures such as roadways, railroads 
and pipelines that cross the fan. Alluvial fans usually have multiple, braided 
channels that are subject to avulsion factors such as human activity modifying 
the landscape between floods, changes in vegetative growth, debris 
accumulation, eolian sediment deposits, etc. Brush fires in the contributing 
watershed can have a major impact on the availability and delivery of water 
and debris. Freshly burned watersheds can produce 10 to 100 times more 
sediment per unit runoff than unburned catchments (MacArthur, 1983). 
Precipitation and streamflow data for alluvial fan floods are sparse because 
alluvial fan flooding is typically caused by infrequent and intense storms. 
Long, dry periods between floods contribute to a rapidly declining public 
awareness of the flood hazard. In addition, most fans had not been 
developed; therefore, there are little or no long-term, historic flood records 
available for specific fans. 

FEMA (1989) has identified the following flood hazards that may be 
encountered on alluvial fans: 

. High velocity flow (15 to 30 ft/s) that can produce significant 
hydrodynamic forces on structures 

. Erosion/scour to depths of several feet 

. Deposition of sediment and debris to depths of 15-20 feet during a 
single event 

Debris flows and their associated impact forces and large sediment 
loads 

Mudflows and their associated deposits and need for removal 

Inundation, with its associated hydrostatic (buoyant) forces on 
structures (these forces are often affected by high sediment 
concentrations leading to modification of the effective fluid density 
and viscosity) 

Flashflooding, which means that there is little (ii any) warning time for 
evacuation and emergency actions to protect property 

Little or no long-term data; (event frequency and duration criteria 
must often be estimated from regional relationships based on limited 
data) 

Chapter 1. INTRODUCTION 


1.3 Purpose, Scope, and Study Limitations 
The FIA requires an assessment of the effectiveness of various structural 
approaches to flood control in alluvial fan Special Flood Hazard Areas 
(SFHAs). The purpose of this study is to document how installed flood-control 
measures have performed during major floods and to present current 
methodologies for assessing the performance and adequacy of the measures. 
Non-structural measures were not within the scope of this study, nor were 
methodologies for mapping alluvial fan hazards. This report is aimed at 
providing guidance to floodplain managers for use in assessing the adequacy 
of structural flood-control measures on alluvial fans (primarily improved 
channels, detention storage, diversions, and bypasses) for protection against 
the Base Flood. This report is not intended to be a design manual. 
Considerable study and testing, which are beyond the scope of this report, are 
required for proper design. 

It must be recognized that the state-of-the-science and our understanding 
of mobile boundary hydrodynamic processes on alluvial fans are limited. 
These complex processes often evade theoretical attempts to characterize flow 
depth, location, orientation, velocity, sediment-carrying capacity, and event 
predictability with a high degree of accuracy. Present state-of-the-science 
methods depend on empiricism, experience, field observation, and the 
application of traditional clear-water assessment methods that have been 
modified to account for flow bulking and the unpredictable; and often episodic, 
nature of alluvial fan processes. 

This report documents flood experiences with flood-control structures on 
alluvial streams. While the emphasis is on alluvial fans, many of the potential 
flood hazards and performance problems for fans are the same as those on 
alluvial stream projects throughout the Southwest. It is intended that 
documented flood problems will lead to an awareness of potential causes for 
failure, and the required analyses to quantify important design parameters. 
The information and examples presented herein, are taken from field reviews of 
projects, interviews, and from various reports, papers, and publications listed in 
Chapter 4 of this report. The three basic approaches used in this study were 
to: 

1) Prepare an inventory of project experience; 

2) Conduct a library search for related studies and reports; and 

3) Summarize engineering assessment procedures for determining the 
adequacy of alluvial fan flood-control structures. 

6 Chapter 1. INTRODUCTION 


1.4 Definitions 
There are many terms used in this report and in the literature that are 
descriptive of alluvial fans and streams; sediment, debris, and mudflows; 
structural flood-control measures and their features; and the nature of the flood 
risk. Some of the terms are in general public usage, while others are technical 
and may not carry the same meaning among professionals. A glossary was 
developed for this project and it is included as Appendix A. The definitions are 
intended to be brief and informative; therefore, in some cases they may not be 
technically complete. The intent is to simply explain technical terms so the 
reader can better understand this report. 

1.5 Acknowledgements 
The Hydrologic Engineering Center (HEC) was authorized to perform the 
study for FEMA under Interagency Agreement ENW-90-E-3265, Project No. 
136669, undated. Subsequent project orders extended the contract duration 
to June 30, 1993. 

This study was performed under the management of Vern Bonner, Chief, 
Training Division, HEC. Mr. Darryl W. Davis was the Director of the Hydrologic 
Engineering Center during this investigation. The HEC study team included: 

D. Michael Gee, Project Engineer; Richard Hayes, field experience/case 
studies; Gary Brunner, engineering analysis; and Eric Butler, library research. 
Dr. Robert MacArthur, Resource Consultants and Engineers, Inc., prepared two 
chapters, expanded several others, and prepared a draft manuscript. All 
parties contributed to the study report. The final document editing was 
provided by FEMA. 
Dr. Frank Tsai was the FEMA project officer during the study. His support 
and guidance, along with the reviews from FEMA staff, were most helpful. 
Also, the support and information from the many Federal, State, and Regional 
offices is sincerely appreciated. 

Chapter 1. INTRODUCTION 


2. CASE STUDY REPORTS
The following case studies present reviews of flooding problems and, in 
some cases, failures of flood-control project features. Some of the examples 
are not specific to alluvial fans; however, they all represent the type of flooding 
problems and project failures that must be addressed in successful projects on 
alluvial fans. Some earlier flood experience has been incorporated into U.S. 
Army Corps of Engineers (USACE) and other agency standards and guidance 
documents. Most of the following information comes from office reports that 
are not readily available. Because there is limited access to these documents, 
major portions of the documents that meet the objective of this report have 
been paraphrased or copied in this chapter. Quotation marks and traditional 
reference marks have not been used because the information is taken from a 
single report. Full credit for the investigations and presented information 
belongs to the writers of the referenced reports. 

2.1 Los Angeles County 1938 Flood 
Location: Los Angeles County, California 
Reference: Report on Engineering Aspects -Flood of March 1938 
(USACE, 1938). 

While this is an old report, it documents flood-related problems concerning: 
channel bank protection; super-elevations at bends; transportation and 
deposition of debris in channels; action of debris basins; and dam operation 
and size of spillways. While the experience gained from studies of this type 
have been incorporated into the USACE Engineer Manuals, the summary of 
types and causes of failure are worth presenting here because they represent 
items that must be considered in the design and construction of flood-control 
measures. Excerpts on the subjects of bank protection, debris basins, and 
dam operation are presented here. 

Chapter 2. CASE STUDY REPORTS 


Flood Experience 

The series of storms that occurred in the Los Angeles County Drainage 
Area during the period of February 27 to March 3, 1938, caused the largest 
flood discharges on record. At many locations, the estimated peak discharge 
exceeded the previous record by two or three times. While a considerable 
amount of data were collected, there was a lack of direct measurement of 
discharge during the peak period. The flood was of short duration and the 
rapid change in stage made it difficult to collect field data. 

Bank Protection 

The following, are seven main classifications of bank protection: 

1) Pipe-and-wire fence
2) Riprap (dumped rock)
3) Rock paving (hand-placed)
4) Wire and rock mattress
5) Gunite slope paving
6) Reinforced concrete open channel
7) Reinforced concrete closed conduit

Pipe-and-Wire/Pile-and-Wire Fences. Fence-type protection was used 
extensively on the Rio Hondo and San Gabriel River and tributaries. It 
seemed to have played an important part in preventing bank erosion and 
confining the flood within channels in straight, or nearly straight, reaches. 
However, large sections failed on the outside of bends, or where some 
obstruction directed the main current against the fences, The weakness of 
this form of protection was its inability to deflect cross-flows sufficiently to 
prevent bank erosion on the back of the fence, and the ease with which 
pipe or piles were broken off or pulled out when encountering the full force 
of the current. In general, the stream alignment was assisted and the 
tendency to meander retarded in some cases, but this form of protection 
was ineffective at critical locations, 

Riprap Bank Protection. With few exceptions, riprap may be classed as 
derrick stone or dumped rock. Numerous failures of this type of levee 
occurred largely on the outside of bends or in the vicinity of an exceptional 
disturbance. The failures appear to have been started at the toe rock. The 
maximum velocity in the lower river was probably In excess of 18 ft/s. The 
irregularity of the pavement is believed to have set up scour along the toe, 
chopping the toe rock and cutting into the earth levee. 

Chamer 2. CASE STUDY REPORTS 


Rock Paving Bank Protection. The review of several creeks with this 
protection showed no damage with velocities up to 17 R/s and rather 
sinuous channel alignment. The upper Los Angeles River, wRh a design 
capacity of 40,000 ft3/s, had an estimated 60,000 ft% and flow velocity in 
excess of 20 ft/s. The failures there were attributed directly to poor channel 
alignment, disturbances created by side inflows, and ponding of local 
drainage behind levees. There was some indication that failure may have 
commenced immediately above the toe rock. Other failures seemed to 
have commenced about half-way up the slope. Once the stone was 
removed, water entered the fine sand of levee fill, and undercut the upper 
section. Failure progressed downstream until a breach of sufficient 
capacity to bypass the levee was made. 

Wire and Rock Mattress Protection. Two types of protection included one 
in which the toe of the slope is protected by a wire and rock mattress and 
the upper slope by wire laid on the slope, and the second in which the wire 
and rock mattress covers the entire levee face. These types of revetment 
withstood the flood as long as flowing water did not get behind them. 
Where this occurred, the rock mattress was rolled up and carried away. 

Gunite Slope Paving. Extensive failures of this class of protection occurred 
along the Los Angeles River. Failure seems to have been the result of 
water getting into the levee through cracks, causing settlement and 
breaking out a small section after this breach was made. The high velocity 
flows ripped off the thin gunite slabs. 

Reinforced Concrete Open Channel. No difficulties were experienced with 
this type of improvement, with the exception of several breaks in small 
channels and the Verdugo channel in the vicinity of a bridge, where a 
section of channel had been left unlined during the reconstruction. 

Reinforced Concrete Closed Conduit A great many small structures of this 
class were rendered useless by plugging with debris; however, only one 
important structural failure was noted. A double-barrelled conduit, each 18 
feet wide and 6 feet tall, became severely blocked with debris, causing the 
flow to blow off the cover slab. 

Debris Basins 

The 1933 fire and the disastrous New Years Day 1934 Flood hastened the 
adoption of the basin method of debris control. At the time of the March flood, 
there were 16 debris basins in the Los Angeles County area. During the years 
1933 to 1935, most of the areas involved were burned. As it takes 5 years or 
more to re-establish cover after a burn, the state of the cover in the drainage 
area was inadequate for the 1938 storm. 

Chapter 2. CASE STUDY REPORTS 


The debris basins functioned very satisfactorily within the limits of their 
capacity. In the areas wlth steep slopes, the material eroded was much larger 
and coarser than in the areas with flatter slopes. It was noted, as a typical 
case, that material accumulated to the crest of the outlet structure and was at a 
fairly uniform grade line to approximately the top of the inlet structure. When 
this condition was reached, a large amount of material passed over the outlet 
spillway during the peak inflow period. In one example, the debris slope 
backed up over the top of the inlet structure and piled up in the approach 
channel. In general, the action of all the debris basins was judged satisfactory. 

Outlet Channels. A common phenomenon was noted at a majority of the 

basins: a pulsating discharge from the basin to the outlet resulted in the 
formation of a standing wave, which proceeded down the channel at a high 
velocity. 

Dam Operation 

At the time, most of the reservoirs were small, local projects with small 
capacity and limited outlets, and so they had little effect on large flood peaks. 
The reservoirs serve mainly as debris basins and conservation structures. 
Although no appreciable damage to the structures resulted, the outlet and 
storage capacity of several structures were materially reduced by debris. 

2.2 Southern California 1969 Floods 
Location: Southern California 
Reference: Report of Engineering Aspects -Floods of January and 
February 1969 in Southern California (USACE, 1974). 

The floods that struck Southern California during January 18 to January 26, 
and February 20 to February 26, 1969, were the most damaging floods of 
record in six counties. Although past floods may have equaled, these floods 
were more damaging because of the intensive development that had taken 
place in recent years adjacent to unimproved or partly improved stream 
channels. More than 100 persons lost their lives in the floods, and property 
damage totaled more than $213 million. These were also the first great floods 
to occur since construction of the complex systems of flood-control projects by 
the USACE. This report presents information on each project sustaining flood 
damage or encountering problems not foreseen in design, on flood 
magnitudes, and on flood-damage data to permit future re-evaluation of 
practices and criteria. The report presents project-oriented information on 

Chapter 2. CASE STUDY REPORTS 


existing improvements sustaining flood damage, plus the performance of flood-

control improvements, special problems, and evaluation of existing design 

criteria. The following paragraphs summarize the performance review, special 

problems, and presents the evaluation of design criieria. 

Performance of Flood-Control Improvements 

Only improvements sustaining flood damage where the probable cause of 
damage could be determined are discussed. The information is presented by 
the following four project types: 

1) levees
2) channels
3) debris basins
4) reservoirs

Levees. The performance of USACE levees that sustained damage is 
discussed under the categories: single-levee projects and double-levee 
projects. All the levees were protected with stone revetments on the 
channel side. 

Sing/e-Levee Projects. The single-levee projects sustaining flood 
damage were Santa Maria Valley, the Santa Clara River levee, the Lytle 
Creek levee, the Mill Creek levee, and the Banning levee. Projects, both 
with and without groins, successfully withstood most of the damaging 
effects of the high-velocity floodflows. The major damages sustained by 
the levee revetments were probably caused by meandering flows that 
undermined the stone toe protection at isolated points. The erosion and 
subsequent slumping along the Santa Maria Valley levees, where the levee 
revetment was ungrouted, were clearly caused by undermining of the 
revetment toe as a result of impinging cross-stream flows. The severe 
scouring that occurred along the Mill Creek levees, where the levee 
revetment was grouted, was also caused by impinging cross-stream flows. 
Other failures were due to excessive streambed degradation and 
insufficient depth of the revetment toe. 

Investigation of effects of earlier floods revealed the ineffectiveness of 
the triangular-shaped toe protection on the grouted-stone revetment. This 
type of toe protection was used for the Lytle Creek levee and Muscoy 
Groin 4, as shown in Figure 2-l. Investigation of these two improvements 
after the 1969 floods, revealed the streamward displacement of the 
quarrystone toe protection that had originally been placed on the grouted-
stone revetment. The scattering of the displaced quarrystone away from 
the revetment and the absence of the bulk of the quarrystone in the eroded 

Chapter 2. CASE STUDY REPORTS 


Figure 2-1 Scour Adjacent to Muscoy Groin 4 (Photo 36 -Scour depth 
of about three feet below the foe of grouted-stone facing.) 

section at the toe indicated that this design is ineffective in providing 
additional protection against excessive scour along the toe. 

Double-Levee Projects. These projects consist of the San Gabriel River 
approach channel (which is also discussed under the section below titled 
Channels), the Riverside levees, and a 5-mile reach of the Santa Maria 
Valley levees. The San Gabriel approach and the Riverside levees 
performed well. Except for local scour near the stabilizers in the San 
Gabriel River approach, little degradation or aggradation occurred. The 
excellent performance of both projects reflected the efficiency of proper 
earth-bottom channel design. It was noted that this type of performance of 
channels with alluvial streambeds is possible where the upstream sediment 
supply is sufficiently large to replace the material transported out of the 
improved reach, and where the channels do not have excessive base 
widths that permit meandering. The reach of double levees in the Santa 
Maria Valley levees project is an example of a leveed channel with an 
excessive base width. The levees were damaged by impinging cross-
stream flows. 

Channels. Channels sustaining flood damage were the Los Angeles River, 
the San Gabriel River, and the Devil Creek diversion. Only the cause of 

damage on the San Gabriel River channel could be clearly defined. In 

general, little damage occurred in reaches of fully-lined concrete or 
grouted-stone channels. 

Chapter 2. CASE STUDY REPORTS 


The sloping drop structures in the San Gabriel River approach channel, 
upstream from the Santa Fe Reservoir, withstood the floodflows 
satisfactorily, except for isolated structural damage. The drop structures 
generally performed efficiently. The sloping grouted-stone aprons of the 
drop structures did not fail despite the abrasive actions of the moving rocks 
and gravel carried by the floods. The highly abrasive action of the 
floodflows was clearly evidenced by the sheared conditions of the stone 
projections on the surface of the sloping aprons. 

In the reach of the San Gabriel River channel from Santa Fe Dam to 
Whittier Narrows Reservoir, severe damage was sustained by revetments 
near the stilling basins of the drop structures. Grouting of the revetments 
would have prevented much of the damage that occurred. 

Debris Basins. A review of the debris basins indicated wide variations in 
the amount of debris and sediment trapped during the floods. The greatest 
amount of sediment and debris was produced from areas extensively 
burned in the fires of 1968. Generally, the USACE debris basins 
accumulation was well below their total capacity. Six basins, constructed 
by local interests, were filled with mud and debris during the floods. 
Aithough property downstream from the debris basins was severely 
damaged by mud and debris after the debris basins were filled, the 
damage would have been much greater if the basins had not trapped the 
bulk of the material. 

Reservoirs. Prado Reservoir was the only USACE reservoir that 
experienced serious problems in making planned releases; this was due to 
damage to downstream levees. Many non-Federal reservoir projects 
sustained damage during the floods. The damage included plugged-up 
outlet works, destroyed water-supply systems, damaged irrigation-outlet 
systems, undermined spillways, and eroded outlet channels. In addition, 
the capacities of the non-Federal reservoirs were severely reduced by 
sediment and debris deposition, 

Special Problems 

Sediment Transport. Sediment transported by the 1969 floods and 
deposited along channels and levees and in reservoirs and debris basins, 
caused serious damage to flood-control improvements. The collection of 
sediment data by the U.S. Geological Survey (USGS) and the data collected 
on sediment deposition during the 1969 floods, permitted an evaluation of the 
formulas used in design of USACE projects. The comparisons applied 
Du Boys formula, Einsteins bed-load method plus suspended load, Colbys 
curves, and the Toffaleti procedure. The results were plotted on a log-log 
display of flow in ft3/svs. sediment in tons/d. Generally, the methods 

Chapter 2. CASE STUDY REPORTS 


underestimated the sediment transport, compared with the USGS field data. 
The Einstein method gave the closest agreement to the measured data, even 
though the two differ by several hundred percent. The Toffaleti method gave 
the closest agreement for discharges, less than 1,000 ft3/s,but departs 
radically from the curve based on USGS data for greater discharges. 

Gravel Pi. Gravel pits and gravel-extraction operations in streambeds 
caused severe damage to USACE flood-control improvements during the 1969 
floods. The extensive gravel-pit operations in the streambed of the Santa Clara 
River and the stockpiling in the floodplain of materials from those operations 
accentuated the meandering qualities of the river. In reaches where complete 
failure of the levee occurred, the high-velocity floodflows of the meandering 
river were deflected toward the levee by a natural high bank on the opposite 
side of the stream. 

As a result of virtually uncontrolled mining of sand and gravel in the 
streambed of Lytle and Cajon Creeks, even insignificant flows caused serious 
degradation of the streambed. As mentioned above, gravel pits accentuated 
the meandering stream. When floods occurred, the headcutting action of 
floodflows moving into the pits scoured the streambed in an upward direction 
and attacked nearby flood-control improvements. The scouring action of the 
floodflows eventually caused failure of the levee toes. 

The experience gained during the 1969 floods emphasizes the need for the 
establishment of controls on gravel mining in the floodways of future projects. 
Such controls are usually administered by the flood-control district responsible 
for operation and maintenance of the project. Suggested requirements 
include: 

. No stockpiling of any kind and no other obstructions will be permitted 
in the floodway. 

. No excavation will be permitted within a strip extending 200 feet
streamward from the levee-control line.

No excavation will be permitted below a plane originating a distance 

200 feet horizontally and 10 feet vertically from the levee-control line, 

and extending to the channel side on a slope of 1 ft vertically, for every 

20 feet horizontally. 

. No excavation in the floodway will exceed a depth of 50 feet below the 
elevation of the corresponding point on the levee-control line. 

All extraction operations will be conducted in accordance with plans 
that have received prior official approval of the USACE, and which will 

Chapter 2. CASE STUDY REPORTS 


provide for continuous pits within the property of any one operation. 
Leapfrog operations will not be permitted, and the continuous pits will 
not be sinuous, with respect to either line or grade. 

Temporary excavation not conforming to the above rules will not be 
permitted. 

. Depth of new pits on the shore side of the levee shall not extend below 
a plane passing through the present ground surface at a point 60 feet 
south of the levee-control line, and dropping toward the south at a l-
on-l 0 slope. 

. Depth of new excavation in existing pits shall not extend below a plane 
starting at the point described above, and dropping toward the south at 
a l-on-5 slope. 

Evaluation of Existing Design Criteria 

Levees. The severe damage sustained by levees during the 1969 floods 
indicates that continued investigation and analyses should be made of design 
criteria pertaining to the depth of revetment toes. The insufficient depth of 
revetment toes was probably the major cause of levee damage to the Santa 
Maria Valley, the Santa Clara River and Mill Creek levees, and the Lytle and 
Cajon Creeks channel improvements. For levees where flow impingement is 
likely to occur during floods, the depth of the toe protection should not be less 
than the depth of the anticipated scour below the invert. When rigid 
revetments are used, the toe protection should be either an extension of the 
rigid lining to a depth below the estimated level of scour or a system of gabion 
mattress. The use of stone protection is discouraged because it was washed 
away during the 1969 floods. 

Channels. Current freeboard criteria was judged adequate. The 
conclusion was based on the absence of any evidence or report of spillage 
over the tops of the channel walls at any time during the floods. 

The riprap side-slope protection for earth-bottom channels functioned 
satisfactorily during the floods. Detailed analyses, made after the floods, of the 
thickness and grade of the riprap in place on three streams, indicated that the 
recommendation in Civil Works Engineering Bulletin 52-15 are reliable and 
conservative. 

Sediment transport through earth-bottom channels and a series of drop 
structures had been evaluated using Du Boys equation. While detailed 
analysis could not be made because information was not available on the 
quantity of sediment passing through Whittier Narrows Dam, an evaluation of a 

Chapter 2. CASE STUDY REPORTS 


reach with seven reinforced concrete-drop structures indicated that the method 
is adequate. Additionally, the method serves as a guide in determining the 
maximum degradation and aggradation of an invert during a design flood and, 
in turn, the design depth of toe protection and the heights of levees. 

The use of sloping bridge-pier extensions to reduce debris deposition was 
judged successful. No debris accumulated on the extensions provided on 
bridge piers in the Rio Hondo channel, which was the only channel with the 
extensions and the maximum discharge near the design discharge. 

Debris and sediment deposition in concrete channels was not a problem 
where debris basins were present. Extremely large amounts of sediment were 
deposited in the rectangular sections of the Devil Creek diversion channel, 
which did not have a debris basin. The design of concrete channels in 
drainage areas capable of producing large amounts of debris and sediment 
during floods, should include provisions for trapping such material at the head 
of the channel or moving the material through the channel. An alternative 
method for conveying all debris and sediment throughout the length of a fully 
lined channel is to design the channel with a steep grade and a trapezoidal 
configuration to maintain sufficient depth and velocity to move the material. 

Deb&Basin Capacities 

The criteria currently used by the USACE, Los Angeles District to determine 
design capacities was judged satisfactory. The criteria used are described in 
the paper titled A New Method of Estimating Debris-Storage Requirements for 
Debris Basins (Tatum, 1963). 

Reservoir Sedimentation 

Reservoir surveys made by the USACE, Los Angeles District after major 
storms, provide data on the rate of debris and sediment inflow to existing 
reservoirs. The data collected provide valuable information for use in 
estimating sediment allowances for future projects. Guides for estimating 
sediment yield are contained in the paper titled Factors Affecting Sediment 
Yield in the Pacific Southwest Area (PSIAC, 1968). 

Chapter 2. CASE STUDY REPORTS 


2.3 Riverside County 1980 Flood
Location: San Jacinto River Levee and Bautista Creek Channel, 
Riverside. California. 
Reference: Engineering Report, Report on Levee Failures and 
Distress San Jacinto River Levee and Bautista Creek 
Channel, Riverside County, Santa Ana River Basin, 
California (Engineer Team, 1980). 

This report reviews project design, construction, subsequent modifications, 
flood history, project performance, investigation of potential causes of failure, 
probable cause of failure, and remedial measures taken. Sections on project 
design, construction, and performance are presented here, along with the 
causes of levee failures and conclusions. The section on potential causes, lists 
the items that should be considered in levee design and construction. 

Background 

During February 1960, flooding caused the San Jacinto River flood-control 
project to undergo distress. Levees on both San Jacinto River and Bautista 
Creek reaches were breached, as evidenced in the aerial mosaics. Because of 
this occurrence and at the request of the USACE, Los Angeles District, an 
Engineer Team was formed and tasked with determining the probable cause or 
causes of failure; recommending remedial construction measures; and making 
recommendations as to the application of this experience to existing and future 
projects. The report sections describing the project design, construction, 
project performance, causes of levee failure, and conclusions are presented 
here. 

Project Description 

The San Jacinto River levee and the Bautista Creek channel improvements 
are located in Riverside County. They consist of a 3.7-mile levee on the left 
side of San Jacinto River, a 1.3-mile levee on the left side of Bautista Creek, 
and a 3.25mile concrete-lined channel on Bautista Creek upstream from State 
Highway 74. The federal cost of constructing this project was $3 million. The 
project units are designed to protect San Jacinto, Hemet, Valle Vista, and 
nearby agricultural areas. Since their completion in November 1961, the units 
have been maintained by the Riverside County Flood Control and Water 
Conservation District (RCFC&WCD). During the 1969 floods, they prevented 
damages estimated at $1.3 million. 

Chapter 2. CASE STUDY REPORTS 


Project Design 

The bases for design are included in the following reports prepared by the 
USACE, Los Angeles District: 

. Design Memorandum No. 1, Hydrology for San Jacinto River and 
Bautista Creek Improvements, July 1959; 

. Design Memorandum No. 2, General Design for Bautista Creek 
Channel, September 1959; and 

. Design Memorandum No. 3, General Design for San Jacinto River 
Levee, September 1960. 

Hydrology. The standard project flood (SPF) was used as the basis for 
design. The flood was developed in accordance with guidelines presented in 
Civil Works Engineer Bulletin No. 52-8, dated March 26, 1952. The standard 
project storm, general winter type, was employed for the drainage area 
tributary to the San Jacinto River levees. This storm is based on the assumed 
occurrence of a storm equivalent to that of January 1943, transposed and 
centered over the area tributary to the pertinent area. The standard project 
storm, local type, was used for the drainage tributary to the Bautista Creek 
improvement. This storm is based on the assumed occurrence of a storm 
equivalent in magnitude to that of March 1943, transposed and centered over 
the area. 

The resulting SPF peak discharges are 88,000 ft3/sfor the San Jacinto 
River improvement and 16,500 ft3l.sfor the Bautista Creek improvement. The 
SPF peak discharge for San Jacinto River is about 50 percent larger than the 
peak discharge that occurred during the flood record of February 1927. 

Hydraulics. The hydraulic design was based on the theoretical analyses 
and design practices previously approved for similar projects, The design 
conformed to the criteria, which applied at the time, published in chapters of 
the Civil Works Construction Engineer Manual and Civil Works Engineer 
Bulletin No. 52-l 5. 

Design Memorandum No. 3 describes the proposed plan of improvement 
and functional characteristics. The pre-project San Jacinto River channel 
flood-control levees, were constructed by local interests and were protected on 
the channel side with pipe-and-wire fencing. The estimated channel capacity 
was about 8,000 to 20,000 ft3/s,and the slope ranged from 0.00526 to 0.00935 
feet/ft 

Chapter 2. CASE STUDY REPORTS 


The levee along Bautista Creek was built in a reach where local interests 
had constructed sand levees and pilot channels. The channel sides were 
protected with pipe-and-wire fencing. The capacity of the pre-project Bautista 
Creek channel was about 75 percent of the design flood flow, and the slope of 
the channel ranged from 0.0100 to 0.0182 feet/ft. 

Water-surface computations were made by the reach method, using 
Mannings R. The computations were made on the basis of a design discharge 
of 86,000 ft3/sin San Jacinto River downstream from the confluence with 
Bautista Creek, and a design discharge of 16,500 ft3/sin Bautista Creek. The 
maximum water-surface computations to determine levee heights were based 
on an nvalue of 0.040. Depths ranging from 5.7 to 13 feet were computed for 
San Jacinto River; and from 3 to 6.6 feet, for Bautista Creek. The maximum 
mean velocities used to determine the slope and toe protection were based on 
an nvalue of 0.025. Velocities ranging from 7.3 to 15.5 ft/swere computed for 
San Jacinto River; and from 9.4 to 16.9 ft/s,for Bautista Creek. The water 
surface for San Jacinto River was computed based on the assumption that the 
existing left levee would be removed and the existing right levee would remain 
in place; however, for Bautista Creek, the water surface was computed based 
on the assumption that flow would be contained in an area bounded on the left 
by the levee, and on the right by high ground. 

A minimum freeboard of 3 feet above the computed water surface is 
provided along both streams. Superelevation was computed by the formula vT/gRc,where: V is the velocity of flow, T is the top width of flow, g is the 
gravitational constant, and Rc is the radius of the curve. The superelevation of 
the water surface ranged from 0.2 to 1 ft. 

Confluence computations were based on a flow of 74,000 ft%in San 
Jacinto River upstream from the confluence, and a flow of 12,000 ft3/sin 
Bautista Creek. This combination produces the maximum water-surface 
elevation in the confluence for the design discharge in San Jacinto River 
downstream from the confluence. 

Under the project document plan, the thickness of the revetment would 
range from 2 feet at the top of the levee, to 5 feet at the toe of the levee and 
the revetment would be underlain by a I-ftlayer of filter material. The adopted 
stone revetment, a 1.5-ftlayer of riprapover a 6-inchfitter blanket, is shown in 
Figure 2-2. (reprint of report Figure 2). The revised thicknesses were based on 
the then present-day criteria. 

Depth of toe was an item of considerable concern during the design of the 
project, as indicated by a review of District records. The adopted depths of toe 

Chapter 2. CASE STUDY REPORTS 


FLOODCONTROLlMPROfMENT

SANTA ANA RIVER BASIN. CALIF.

SAN JACINTORIVER LEVEE
AND BAUTISTACREEK CHANNEL

Chapter 2. CASE STUDY REPORTS 


for the Bautista Creek channel and the San Jacinto River levee are 5 and 10feet, respectively, below the low point of the streambed. 

Embankment and Foundation. The foundation materials are principally silty 
sands, sand-silty sands, and silts, with occasional gravel and cobbles. The 
upper 6 to 12 feet are loose to medium-dense. Groundwater was not found in 
any of the test holes that were drilled to a maximum depth of 35 feet along the 
project reach. The 1957 well records indicated that groundwater was about 10 
feet below the streambed at the downstream end of the project levees, and 60 
feet below the streambed at the upstream end. A typical embankment section 
is shown on the project map in Figure 2-2. Analysis of the slopes was based 
on drained strengths, Using the infinite slope method, the factor of safety for 
the end of the construction condition is 1.4. Steady seepage and drawdown 
conditions were not analyzed because the influence of seepage into the levee 
fills and foundations was considered to be negligible due to short-duration 
flows. 

Project Construction 

The dates for the completion of construction of the various reaches of the 
San Jacinto River levee and the Bautista Creek channel are presented in 
Figure 2-2. The Bautista Creek Channel Project is a concrete-lined trapezoidal 
channel with an energy dissipator at the downstream end. The portion of the 
Bautista Creek channel downstream of the concrete channel is a left-bank 
levee with a typical section similar to that shown for the San Jacinto River 
levee. It was constructed as part of the San Jacinto River Levee Project. 

The Bautista Creek levee has a maximum height of 10 feet and the stone 
revetment toe is 8 to 9 feet below the line of backfill at the face of the levee. 
This distance corresponds to 5 feet below the low point of the streambed. The 
levee section was built with streambed materials and borrow from an existing 
levee. These materials were placed in 12-inch layers, compacted with four 
passes of a 50-tonrubber-tired roller. 

The borrow for the San Jacinto River levee was obtained by removing 
about four miles of existing levee between Cedar Avenue and the downstream 
end of the project. The remainder of the levee fill came from streambed 
materials similar to the foundation materials previously described. 
Construction of the levee was the same as for Bautista Creek. The 
construction-control data show that the densities varied from 98 to 106 percent 
of the standard American Association of State Highway and Transportation 
Officials (AASHTO) maximum density. 

Chapter 2.CASE STUDY REPORTS 


Riprap. Stone for the project was obtained from the Bernasconi Pass 
Quarry and the Juaro Quarry. The stone tested had a bulk-specific gravity of 

2.71 to 2.76 and an apparent specific gravity of 2.73 to 2.78. The construction 
control riprap gradations, taken at the plant located at the quarry, are not 
representative of the stone gradation on the levee, in part, because of 
segregation that results from handling and placement. It has been verified that 
a jaw crusher was used to control the maximum size of stone, but it is not 
known whether a screen was used to remove the finer stone throughout the 
production. The stone was transported to the levee crown in end dump trucks 
and then was dumped into a skip that was crane-operated. The skip was 
used to place the stone and drag the slope. 
Project Performance 

Before the February 1980 Flood Since the completion of the project, high 
flows have occurred in 1965, 1966, 1969, and 1978. In November 1965, a 
multiple (10) corrugated metal pipe and dip crossing with concrete overflow at 
Main Street were washed out. During the February 1969 storms, the Bautista 
Creek channel was degraded. Afterwards, the seven stabilizers previously 
mentioned were constructed. Five of the stabilizers were damaged during the 
1978 storm and were repaired in 1978 by a RCFC&WCD contract. The 
RCFC&WCD has kept a record of degradation and aggradation in Bautista 
Creek and has furnished a drawing showing streambed profiles at various 
times. Severe degradation of the streambed (about 10 feet), was noted before 
the floods of 1969. The RCFC&WCD has noted that the energy dissipator 
derrick stone has been repaired since the original construction. 

A review of the aerial mosaics presented in Design Memorandum No. 3
and post-construction aerial photographs, indicates that topographic features
have directed flows into the San Jacinto River levee in the general vicinity of
the February 1980 breach. A long-time resident of the area commented after
the break that it was the third time that the water broke through the same
reach. The first two breaks occurred in locally constructed levees before the
construction of the USACE levee.

February 1980 Flood Rainfall occurred over the watershed for nine 
consecutive days, from February 13 to February 21, 1980. Mean seasonal 
precipitation ranged from about 14 inches at San Jacinto to about 45 inches at 
San Jacinto Peak, averaging about 20 inches over the total area. 

The peak discharge of February 21, 1980, in San Jacinto River above 
Bautista Creek is 17,300 ft3/s, about a 30-year flood. The estimate of a 6,000 
ft3/s discharge on Bautista Creek represents about a 70-year flood. Based on 
these two discharges, the peak discharge, which occurred at the San Jacinto 

Chapter 2. CASE STUDY REPORTS 


levee, is estimated to be about 25,000 ft3/s, representing a flood recurrence 
interval of about 25 years, 

During the February 1980 Flood. On February 21, 1960, the Bautista Creek 
and San Jacinto River levees were breached. The breach in the Bautista 
Creek levee extended from approximately sta. 61+00 to sta. 59+00. The 
breach in the San Jacinto River levee extended from approximately 
sta. 169+00 to sta. 154+00, before flood fighting operations controlled the 
erosion. At several other locations erosion occurred, generally below the line 
of backfill. 

The RCFC&WCD has provided eyewitness accounts of the San Jacinto 
River levee breach. Excerpts from these eyewitness reports state: 

Water Master for the Hem&San Jacinto Area of Eastern
Municipal Water District...was on Mountain Avenue at
approximately 7:00 a.m. and observed a 204 wide breach in the
levee at that time and reported to their headquarters.

Other eyewitness accounts following the initial breach give an account of the 
progress of the failure. An eyewitness account of observations at 7:45 a.m. 
reports: 

Levee disintegrating on the upstream side of breach rapidly.
Flood through breach surging in river in waves 5 to 10 ft high.
.+ I330a.m.Breach +700 ft wide at this time At the location
of breach, the main direction of the river flow was 225to the
downstream tangent, as observed.

Eyewitness estimates of the flow through the breach ranged from 75 to 95 
percent of the river flow. 

Causes of Levee Failures 

The Engineer Team considered the following six as possible causes of 
levee failures, and their application to the subject project: 

1) Overtopping
2) Internal erosion (piping)
3) Slides within the levee embankment and/or foundation
4) Surface erosion
5) Undermining of bank protection (scour)
6) Channel configuration

Overtopping. Based on high-water marks, probable maximum height of 
ride-up, speculative height of waves, and their influence on probable 

Chapter 2. CASE STUDY REPORTS 


maximum water levels, overtopping did not occur and, therefore, was not a 
cause of failure. 

Internal Erosion (piping). There was no evidence to suggest the 
occurrence of piping, even though the characteristics of embankment and 
foundation materials make them susceptible to internal erosion. Observed 

rodent activity is not considered to be significant. The small differential 

head does not produce sufficient hydraulic gradient in levee sections to 
develop piping. Thus, internal erosion (piping) was not a cause of levee 
failure. 

Slides Within the Levee Embankment and/or Foundation. Levee design 
exploration and stability analyses indicated levee embankment and 
foundations to be stable. Minor erosion at the landside toe of the levee 
upstream of the San Jacinto River levee breach is not considered to be 
significant. The levee has a conservative cross section, embankment and 
foundation materials have high strengths; and no evidence of through or 
underseepage exists. Consequently, Wis concluded that, because slides 
did not occur within the levee embankment or foundation, they were not a 
cause of levee failure. 

Surface Erosion. Levee failures can be caused by surface erosion of riprap 
bank protection because of action from excessive stream currents and/or 
waves. Surface erosion will then occur when the tractive force produced 
by flow velocity exceeds the critical tractive force for stone stability. Waves, 
caused by unstable streambed formations near the bank or flow 
impingement on the bank (both conditions occurred in San Jacinto River), 
produce uplift pressures on bank protection stone that, in combination with 
stream velocity, can cause surface erosion when tractive forces are smaller 
than critical. Consequently, when riprapbank protection is designed for 
flow velocity alone and significant waves occur along the bank, surface 
erosion may occur for flows substantially smaller than the design 
discharge. 

To determine whether surface erosion was a cause of levee failure on 
San Jacinto River, observations of in-place stone were made and four in-
place gradations were taken, as previously noted. Based on visual 
observations, there was no evidence that significant surface erosion had 
occurred, although some localized areas of stone were judged to be fine 
and others to be coarse. The gradations indicate one sample to be 
undersized with respect to project specifications; however, the original 
design appears to be following the criteria used at the time of construction, 
namely, gradation control at the quarry only. Therefore, the areas of 
undersized stone may be due, in part, to segregation that occurred during 
handling and placement, 

Chapter 2. CASE STUDY REPORTS 


Observations and sampling of in-place riprapindicate that, because 
removal of the bedding layer from beneath the ripraphad not occurred, it 
is an unlikely cause of surface erosion leading to levee failure. Although 
two of the in-place gradations show the bedding layer to be finer than 
specified, this condition could have resulted from silting by flow sediments 
and/or contamination from sampling procedures, since demarcation 
between bedding and embankment materials probably was not distinct. In 
any event, itis believed that the finer gradation of the bedding material was 
not a significant factor in levee failure. 

In one trench, where scour depths were near the bottom of the riprapprotection, some riprapwas located at the scour level riverward of the 
ripraptoe. This stone was either removed from the ripraplayer by surface 
erosion or undermined in the breach area and transported downstream 
along the scoured streambed. The latter case appears to be the most 
likely reason for finding displaced riprap.

Based on present criteria (ETL 1110-2-120)a significantly thicker layer 
or heavier stone would be required to withstand flood velocities. Although 
no evidence was found that surface erosion was a significant factor in levee 
failure, the undersized riprapprotection compared with present criteria 
would probably be subject to failure by surface erosion during larger floods 
up to design-flood magnitude. 

Undermining of Bank Protection (scour). Inspection of Bautista Creek 
upstream of the levee suggests that construction of the concrete channel 
caused sediments, naturally carried by the creek, to be deposited upstream 
of the channel inlet. The resultant delivery of relatively sediment-free water 
to the leveed reach along with the steep slope of this reach (greater than 1 
percent) caused general streambed degradation downstream of the 
concrete channel. The subsequent nearly complete filling from the valley 
immediately upstream from the concrete channel inlet with deposited 
sediment and the construction of channel stabilizers by the RCFC&WCD 
have reduced, and in the upstream part of the reach have reversed, the 
general tendency of the streambed to degrade. 

The RCFC&WCD had documented the general degradation of Bautista 
Creek through most of the leveed reach. The level of backfill (still evident 
along much of the levee) provides a reference plane for evaluating the 
approximate depth of scour and/or channel degradation. Comparing the 
design depth of riprap toe with the depth of the existing streambed below 
the backfill reference level indicates that the streambed below the backfill 
reference level is at about the same level as the riprap toe along much of 
the levee. Visual inspection of exposed riprap at the streambed tends to 
confirm that the riprap toe is exposed and damaged in some locations. 

Chapter 2. CASE STUDY REPORTS 


Examination of the scour gage data indicates that scour along the levee 
was approximately to the rock toe, except in the breach area where scour 
was several feet below the rock toe. These data indicate that scour was 4 
to 5 feet below the levee toe at sta. 54+58, upstream and downstream of 
the breach. Based on observed conditions and scour gage information, it 
is quite evident that undermining of the bank protection caused the levee 
failure at Bautista Creek. 

During the initial field inspection and preparation of the preliminary 
report, there was no readily apparent or obtainable information upon which 
to determine the cause or causes of levee failure at the main breach in the 
San Jacinto River levee, other than the evidence that most of the river flow 
impinged on and then flowed along the levee in the areas where the 
breach subsequently occurred. This evidence suggested the possibility 
that deep scour occurred along the levee in the area of flow impingement, 
which undermined the levee toe and caused failure of the levee. 

Subsequent excavation and inspection of trenches provided positive 
evidence of scour depths. A trench, located a short distance downstream 
of the breach, revealed that the depth of scour was approximately to the 
bottom of the rock toe. A second trench, located within the breach area 
and approximately 50 feet riverward of the original levee rock toe, indicated 
the depth of scour to be approximately at the same level as the bottom of 
the original rock toe. Considering the magnitude of the 1980 flood 
compared with other floods that occurred subsequent to completion of the 
project, it is reasonable to conclude that the maximum depth of 
postconstruction scour occurred during the 1980 flood. This evidence 
suggests that the maximum depth of scour at the rock toe resulting from 
impingement of flow on the levee face during the February 1980 flood was 
at or below the bottom of the rock toe at the time of the levee breach. 
Consequently, undermining of the bank protection by scour appears to be 
the principal cause of the San Jacinto levee failure. 

Below the Main Street crossing, the similar evidence of impingement 
and flow along the levee face suggests that the levee distress there was 
caused in the same manner as it was for the main breach. 

Channel Configuration. The channel configuration appears to have been a 
significant factor contributing to levee failure, inasmuch as the resulting flow 
impingement on the levee causes deeper scour at the toe of rock 
protection. Flow impingement was particularly significant on the left levee 
of San Jacinto River between sta. 164+00 and sta. 169+00. Upstream 
from this location, the abrupt junction of Bautista Creek with San Jacinto 
River and the protection wall upstream of the water-well area resulted in 
impingement of flows at the upstream end of the right Indian levee with 

Chapter 2. CASE STUDY REPORTS 


some distress at that point. The upstream end of the Indian levee 
deflected flows across San Jacinto River to impinge at an angle of 
approximately 25 degrees on the left levee at the above-referenced failure 
location. This angle of impingement contributed to 75 to 95 percent of the 
flow that passed through the levee break. Similar, but less noticeable, 
irregularities in channel bank alignment farther downstream on San Jacinto 
River and on Bautista Creek resuited in flow impingement at several 
locations where levee distress occurred. Therefore, it is evident that 
channel configuration contributed to levee failures by producing flow 
impingement on levees that, in turn, produced deeper scour and 
undermining of the levees. 

Conclusions 

Based on the information available, the Engineer Team has reached the 
following conclusions regarding the causes of levee failures: 

. Failure of the levees, in whole or in part, was caused by undermining of 
the levee toe, influenced by flow impingement due to adverse channel 
configuration, 

. There is no evidence that inadequate or improper maintenance
contributed to the failure.

. Considering the customary practices and procedures at the time of 
construction, the project was constructed substantially according to 
plans and specifications, These procedures did result, however, in 
riprap levee slope protection that was, at some locations, somewhat 
smaller than called for in the design. 

. The riprap protection was designed based on the criteria in effect at the 
time. Present criteria would call for a thicker layer of heavier and more 
uniformly graded riprap. 

. The depth of scour was properly recognized in the original design of 
the levee slope protection as an important design consideration; 
however, the effect of flow impingement on producing greater depths of 
scour in certain locations was not recognized, as riprap toe protection 
was not taken to greater depths in those locations. 

Two factors contributed to the failure of the Bautista Creek levee: (1) 
inability to provide sufl icient depth of riprap protection to accommodate 
the increased streambed degradation caused by reduction in sediment 
load due to the presence of the upstream concrete channel and inlet; 
and (2) the excessively steep streambed slope in the levee reach. 

Chapter 2. CASE STUDY REPORTS 


2.4 Las Vegas 1975 Flash Flood 
Reference: A Brief Hydrologic Appraisal of the July 3-4, 1975, Flash 
Flood in Las Vegas Valley, Nevada, (Katzer, T.L.; Glancy, 
P.A.; and Harmsen, L., 1976). 

This report focuses on the storm, flood characteristics, sediment transport, 
and resulting damage of the 1975 Las Vegas flash flood. There were no major 
flood-control projects in place at this time. (Subsequently C/ark County 
completed flood-control facilities in 1987 (Reel & Bond, 1988) and the USACE, 
Los Angeles District, is proceeding with a Las Vegas Wash & Tributaries project 
that incorporates the earlier County project.) Portions of the report sections on 
the flood characteristics and sediment transport are presented here because 
they provide insight on the consequence of thunderstorm events on alluvial 
fans and the resulting sediment and floodflow damage. Automobiles, roads, 
and utilities were severely damaged from this event. 

Background 

Heavy thunderstorm precipitation on the afternoon of July 3, 1975, between 
metropolitan Las Vegas and the mountains to the south, west, and north, 
caused flashflooding in the city area. Total storm precipitation equaled or 
exceeded 3 inches in some areas. Peak flows of Tropicana Wash, Flamingo 
Wash, Las Vegas Creek, and Las Vegas Wash were the highest ever 
determined. 

Flood Characteristics 

Source Area. The alluvial fan system southwest, west, and north of 
metropolitan Las Vegas received the greatest amount of precipitation and, 
therefore, contributed most of the runoff. The complex drainage patterns 
superimposed on the alluvial surfaces indicate that this type of storm runoff 
has occurred many times in the past. Much of the alluvial surface area was 
inundated by shallow sheet flow. The vegetation on the alluvium is sparse to 
moderate, consisting of desert shrubs and grasses, and is not very effective in 
retarding flows and promoting infiltration, Thus, as sheet flow moves 
downslope it tends to become channelized. As flow capacities of major 
channels are sometimes exceeded, aerially widespread flooding occurs during 
particularly large runoff events. 

Chapter 2. CASE STUDY REPORTS 


Peak Flows. Hydrologically, the July 3, 1975, flood may have been the 
greatest flood in Las Vegas history. Peak flows in most major drainages 
exceeded those previously measured or estimated. However, quantitative 
records are completely lacking on some earlier floods; therefore, the 1975 
floodflows may have been exceeded in the past, at least at some sites along 
some tributaries, 

Peak flows at gages were determined and flood hydrographs were 
developed for four recording stations. Flood peaks generally diminish in a 
downstream direction in the absence of additional tributary inflow. This 
reduction in peak flow is at least, in part, the resuit of some of the flow being 
temporarily stored or retarded on the floodplain because of localized flooding. 
Some of this localized flooding is frequently caused by flood debris clogging 
bridge and culvert openings, thereby, reducing channel capacities and forcing 
some flow out of the main channels, 

The peak flow rates per unit area of contributing drainage area, are not 
particularly great when compared to other flash floods in Nevada; in other 
floods, peaks as high as 7,000 to 8,000 (f@)/mi*from small drainages have 
been determined by USGS investigations (data in files of the USGS, Carson 
City, Nevada). 

Las Vegas Creek probably peaked sometime about 4 p.m., P.D.T, and was 
the first known tributary to peak on July 3, followed by Flamingo and 
Tropicana Washes. The first flows reached the Flamingo Wash gaging station 
at Maryland Parkway at 5:00 p.m., about 5 hours after the storm started, with 
the peak occurring at 6:30 p.m. and lasting just a few minutes. By 7:30 p.m., 
the flood crest had dropped about 3 feet and was decreasing rapidly. This 
was the only gaging station that operated throughout the peak-flow period; 
however, the gage became inoperative later during the flow recession. No 
known data fix the time of peak flow on Duck Creek. 

Flow Velocities. Mean velocities of peak flows at the indirect-measurement 
sites are calculated to have ranged from about 2 ft/s on Tropicana Wash near 
Interstate Highway 15, to as high as 15 ft/s on Las Vegas Wash near North Las 
Vegas. Maximum point velocities within the cross sections at these sites are 
unknown, but they are inherently somewhat greater than the average velocity. 

One current-meter flow measurement was made during the flood in a 
channel reach characterized by heavy salt cedar growth at the Las Vegas 
Wash near Henderson gaging station. The measured stream discharge was 
3,500 ft3/s. Velocities ranging up to 3.4 ft/s in individual vertical sections were 
noted, and the mean velocity for the entire cross section was 1.38 ft/s. This 
measurement was made about 3 hours after the peak had passed. 

Chapter 2. CASE STUDY REPORTS 


The approximate 4-hour time lag between the start of sheet flow on the 
alluvial fans (about noon) and the beginning of flooding in the metropolitan 
area (about 6.5 mi maximum distance), gives a general suggestion of the 
average integrated flow velocities from points throughout the drainage. The 
time of travel of the storm runoff, however, is the product of a complex mixture 
of many factors and is, primarily, affected by storm and land surface 
characteristics, 

Sediment Transport 

The intense rainfall and heavy runoff caused a substantial amount of 
erosion, sediment transport, and sediment deposition. The field-
reconnaissance nature of this investigation did not allow any quantitative 
measurements of erosion or sediment deposition. Also, an unknown fraction 
of the total sediment transported by the storm runoff was deposited in Lake 
Mead near the mouth of Las Vegas Wash and, therefore, is not readily 
accessible to quantitative assessment. This report addresses only some of the 
more obvious qualitative aspects of sediment erosion, movement, and 
deposition by the flood. 

Erosion. In spite of the reported intense nature of precipitation at many 
localities from time to time during the storm, subsequent observations did not 
generally disclose extensive rill erosion of the general landscape; however, 
many striking examples of ditch, gutter, and gully erosion were seen 
throughout areas subjected to intensive runoff. Major stream channels also 
exhibited numerous striking examples of lateral channel cutting and bank 
caving; however, obvious vertical downcutting along reaches of major channels 
was not common in and near the metropolitan area, possibly because the 
major channels are extensively underlain by deposits of caliche (calcitecemented 
alluvium) that effectively armor the streambeds against vertical 
erosion. Vertical scour damage occurred locally at the downstream ends of 
culverts and similar drainage structures. Some concrete protective aprons or 
wingwalls were undercut and seriously damaged by the highly turbulent flow. 
A particularly dramatic example of this type of damage occurred near the 
mouth of Las Vegas Wash, where concrete box culverts through the high fill of 
Northshore Road were progressively undermined after turbulence and vertical 
channel downcutting of flood flow destroyed the effectiveness of the protective 
riprap armor lining the channel, and mantling the downstream fill slope. 
Damage at this site continued even long after peak flows had subsided, and 
the highway fill section required extensive reconstruction to prevent complete 
failure. 

There was severe but typical examples of eroded roads at diverse locations 
in the Las Vegas metropolitan area. In most situations, roads that were 
overtopped by heavy flows failed from progressive headward channel cutting 

Chapter 2. CASE STUDY REPORTS 


through the roadbed. In other places, road-surfacing was laterally displaced in 
masse by streamflow. A particularly severe example of eroded roadway 
occurred where Lamb Boulevard was cut by Las Vegas Wash a short distance 
south of the intersection of Lamb Boulevard and Owens Avenue. 

Probably the most pronounced example of vertical and lateral erosion 
along a major stream channel occurred in the lower reaches of Las Vegas 
Wash. The site is near the former location of a Geological Survey streamflow 
gage that was lost when the stream bank eroded during the flood. Recent 
drastic channel erosion in lower Las Vegas Wash had occurred prior to the 
July 3-4 flood, but the floodflows greatly accelerated the erosion and were 
largely responsible for the chaotic results. 

The suspended-solids content of Las Vegas Wash at Nothshore Road still 
showed pronounced effects of the flood 11 days after the peak flow, and had 
not recovered to background levels more than 4 months after the flood. 

Lateral channel cutting by overbank floodflows also affected constructed 
features other than road surfaces. The overbank flow undercut masonry block 
walls, sidewalks, street curbing, sewer lines, and street signs. 

A minor erosion problem, having the potential for serious consequences, 
was the exposure of a natural-gas line by erosion. The line was constructed 
on top of the land surface and covered only with a relatively thin blanket of 
alluvium. The path of the pipeline lies across numerous shallow gullies that 
drain surface flow down the alluvial fan, creating the potential for exhumation 
by moderate to heavy surface runoff. An exposed pipeline would be 
vulnerable to further flood damage and vandalism that could trigger more 
serious problems. 

Sediment Deposits. Sediment deposits created many problems and may 
actually have caused greater overall economic damage than that damage 
caused by erosion. One of the most obvious sediment deposits that received 
early cleanup attention was in Flamingo Wash at the Caesars Palace parking 
lot. Although the deposit covered only a few acres at most, cleanup probably 
involved removal of several acre-feet of sediment, 

Another obvious problem area of sediment deposition was at Winterwood 
Golf Course near the junction of Flamingo Wash, and Las Vegas Wash in 
southeast Las Vegas. The deposits covered many acres, but the depths of 
most of the deposits are uncertain. Total volume of the deposits was at least 
several acre-feet. 

Sediment was also profusely deposited on numerous streets, highways, 
lawns, and in homes, businesses, and other buildings. Cleanup of much of 

Chapter 2. CASE STUDY REPORTS 


this sediment probably accounted for a large part of the cost of the flood 

damage. Sediment deposition at the delta of Las Vegas Wash in Lake Mead 

was probably great. The effects of this sediment transport on lake and stream 

biota are unknown, but may have been significant. 

Patti&-Size Distribution of the Transported Sediment, The sediment loads 
transported by floodwaters consisted of three basic components: 

1) Man-made objects
2) Natural organic debris (mostly trees and brush)
3) Natural inorganic particles (mineral and rock material)

Man-made Objects. This component was probably the smallest volume of 
material transported, but involve the greatest economic impact because of 
the high financial losses associated with displacement and damage of 
automobiles and other expensive articles. 

Natural Organic Debris. Organic debris probably makes up a minor 
fraction of the total weight and volume of all sediment transported, but was 
important because the debris and man-made objects together effectively 
blocked and clogged culverts. The clogged drainage ways ponded and 
diverted floodflow, which caused increased flooding and damage. The 
bulky character of much of the organic debris and the man-made objects, 
as well as their generally floatable nature, contributes to the clogging 
problems. Fine-grained organic debris and small man-made objects 
probably had only minor effects on the floodflow movement. 

Natural Inorganic Particles. The nonorganic mineral and rock material 
made up the majority of the weight and volume of sediment transported 
and deposited by the flood. Almost all observed sediment deposits, both 
overbank and in-channel, were dominated by fine-grained sediments (sand, 
silt, and clay). Undoubtedly, some coarse material moved, but cursory 
visual inspection suggests that gravel and boulders were only a minor part 
of the total weight and volume of transported sediment. The main-channel 
flow commonly displayed the competence to move automobiles, concrete 
drainage pipe, and other large heavy objects over considerable distances; 
therefore, if gravel and boulder transport did not occur, it was probably 
because that size of material was unavailable for transport in most major 
channels. The particle-size distribution of the sediment apparently moved 
by the flood was, therefore, controlled more by availability than by the 
competence of flows required to move it. 

Chapter 2. CASE STUDY REPORTS 


Damage 

Heavy damage occurred along Flamingo Wash in the vicinity of Caesars 
Palace, where automobiles were parked in the floodplain, despite several signs 
warning of flash floods. Several hundred cars were damaged by submersion 
and collisions when they were moved by the floodwaters. Many of the vehicles 
were piled up at the entrance to drainage structures under Las Vegas 
Boulevard South, commonly referred to as The Strip. The obstructions 
caused increased backwater, and more cars and a larger area were inundated. 

Many automobiles in various parts of the flooded city suffered similar 
consequences. Several autos were lost when they were driven onto flooded 
sections of streets and the flows swept the vehicles off the roadways. 

Overbank flooding of major creeks caused great damage to buildings that 
were invaded by the turbid water. Many utility poles tilted to non-vertical 
positions during the flood. Streets were inundated and later, left coated with 
sediment, as were lawns and other improved real estate features. Curbs and 
drainage structures were undermined and pipelines were exhumed and 
commonly damaged. Sewage plants were inundated and deactivated by mud 
and water. 

2.5 Saddleback Diversion Harquahala Valley 
Watershed 
Location: Maricopa County, Arizona 
Reference: Engineering Report, Saddleback Diversion Harquahala 
Valley Watershed (SCS, 1987). 

Project Description 

Saddleback Floodwater Diversion Channel is a 4.73-mile-long channel that 
takes the principal spillway outflow from Saddleback Flood Retention Structure 
(FRS). Approximately 1,900 feet of the channel from the FRS to the 
Courthouse (McDowell Road) bridge is lined with grouted rock riprap, shown in 
Figure 2-3. Downstream from the bridge, the unlined channel intercepts 

,drainage from an 8.6-square-mile area across an alluvial fan. There are four 
grouted rock drop structures to maintain grade and to reduce velocity within 
the channel. The diversion channel outflow is a natural alluvial wash in an 
undeveloped area. The diversion protects agricultural development adjacent 

Chapter 2. CASE STUDY REPORTS 


LFi gure 2-3 Lined Diversion Channel, Looking Upstream Toward FRS 
to the diversion. Figure 2-4 is a photocopy of the project alignment on a site 
photograph, flown in March 1976, (north direction is the bottom of the page). 

Runoff from the fan area flows into a collector channel running parallel to 
the diversion channel. A collector dike separates the collector channel from 
the diversion channel. The dike has a 12-ft top width and a 3-to-l-side slope 
toward the collector channel. The collector channels were formed as lateral 
swales to direct the overland flow into side inlets located along the length of 
the diversion channel. The collector channels were a maximum of 2 feet deep, 
with approximately, a 4-to-l-side slope toward the natural fan, and bottom 
widths ranging from 10 to 30 feet. Figure 2-5 shows a plan view of the 
collector channel (labeled: lateral swale) and side weir inlet, plus a cross-
section view (B) of the collector channel and dike. 

The original project had 18 side inlets at locations where the diversion dike 
intercepts natural washes that drain the west slopes of Saddleback Mountain. 
The side inlets are trapezoidal-shaped weirs, 2 feet deep, with 10 to 60 feet 
bottom widths, and 3-to-l-side slopes. (See Figure 2-5. As-Built Drawing.) 
The widths were sized based on the estimated contributing area for each inlet. 
All but one inlet are protected with grouted rock 2 feet thick. The diversion, 
side inlets, and collector channels were designed for a 50-year, 24hour storm. 

Chapter 2. CASE STUDY REPORTS 


Chapter 2. CASE STUDY REPORTS 


22-d 
P

PCANMS
igure 2-5 As-Built Drawing: Collector Channel and Side Weir 

Chapter 2. CASE STUDY REPORTS 


Flood Experience 

On September 2, 1984, a storm passed through the watershed with an 
approximate duration of 4 hours. The storm produced an estimated outflow of 
739 ft3/sat the Saddleback FRS outlet and approximately 12,355 ft3/sat the 
diversion outlet, The design discharges are 1,I20 ft3/sand 6,060 ft3/s,
respectively. The diversion performed well during and after the storm, with a 
maximum water level within 1 ftof the top to the diversion dike. There was 
little erosion damage to the diversion channel. 

The runoff from the alluvial fan caused sediment bar development in the 
collector channels, which caused the collector dike to be overtopped by 
floodwater; 15 of the side inlets were overtopped or flanked by erosion through 
the abutments. The damage occurred even in locations where the inflow did 
not exceed design discharge. Figure 2-6 shows severe erosion to the levee 
due to overtopping (photoNo.9 from referenced report). The grouted side 
inlet is on the right edge of the photo and a small gully through the levee is at 
the left of the grouted inlet. 

Evaluation 

An engineering evaluation was performed by the Soil Conservation Service 
(SCS) and reported in the referenced Engineering Report. This project 
summary paraphrases sections of that study report. The project has been 

Figure 2-6 Severe Erosion on North Side from Overtopping (photo 

No. 9 - Station 80+12 /n/et #7) 

Chapter 2. CASE STUDY REPORTS 


completely repaired and was in excellent condition when the site was visited 
on November 3,1990. 

While the storm event generally exceeded the design capacity at some 
locations in the project, the accumulation of sediment across the collector 
channels caused overtopping and breaching of the collector dikes. The storm 
caused flow braiding on the alluvial fan, erosion, and deposition of sediment in 
the collector channels. The collectors were not designed with sedimentation 
considerations, and there was no freeboard added to the dike. The 
accumulation of sediment deposits in the collector channel reduced the 
capacity and effectiveness of the collector to distribute the fan runoff to the 
side inlets. 

There were locations on the fan where new drainage channels were formed 
during the event. Beside the contribution of sediment, these shiis in runoff 
changed the contributing area to some of the inlets; therefore, some inlets 
received runoff from more area than expected, while others had a decrease. 

The SCS Engineering Report identiiied the following design criteria that 
need to be re-examined: 

. Freeboard requirements for side inlets 

. Freeboard requirements for collector channels 

. Sediment accumulation in collector channels 

. Over-designing collector channels to account for shifting, braided flow 
on alluvial fans 

The project has been repaired with the following treatments: 

. Side inlets were repaired and some enlarged or additional inlets were 
added to increase capacity. 

. Collector channels were enlarged to provide for sediment. 

. Additional side inlets were placed in locations where new major washes 
developed. 

While the overflow of the collector dike required considerable repair, all the 
overflowing water was carried by the diversion channel, which performed well 
during and after the storm. 

Chapter 2. CASE STUDY REPORTS 


No probability was assigned to the 1984 storm, but it did exceed the 50year 
design discharges at the lower end of the project. With additional 
capacity, this project should be able to offer flood protection at the I-percentannual 
chance level. 

2.6 Lowell Creek Diversion 
Location: Seward, Alaska 

Reference: Office Reports and Project Files, USACE, Alaska District, 
Anchorage, Alaska. 

Project Description 

The project consists of a dam and emergency spillway that diverts Lowell 
Creek at the apex of the fan into a tunnel such that flows are completely 
removed from the fan, as shown in Figure 2-7. The drainage area upstream of 
the project is 4.02 mi2. The diversion dam is about 400 feet long with a 
maximum height of 25 feet, The uncontrolled spillway is about 60 feet long, ,. 
with a crest elevation 4 feet below the top of the dam. Flow overtopping the 
spillway would follow the former course of Lowell Creek through the City of 
Seward. The diversion tunnel is a 10-ft horseshoe about 2,000 feet long, on a 
gradient of 4.3 percent. A sharp drop at the tunnel entrance accelerates the 
water to a velocity of about 40 ft/s. This high velocity is necessary to ensure 
that all debris will pass into and through the tunnel. The tunnel is concrete 
lined throughout and the floor is armored with 40-lb railroad rails welded to 
channel cross-ties embedded in the floor, The design drawings indicate a 
minimum concrete thickness of 8 inches, with 8 inches of concrete below the 
floor rails. The space between the rails is filled with abrasion-resistant 
concrete. This project was constructed in the early 1940s. 

Basis of Design 

Hydrology. The project was constructed to replace a previous project, built 
in 1929, that consisted of a timber flume to carry water and debris through 
Seward to Resurrection Bay. That flume required heavy maintenance and had 
deteriorated to such an extent by 1937 that a replacement flood control project 
was urgently needed to protect Seward. The diversion/tunnel project was 
probably designed based upon observations of floodflows during the period in 
which the flume was in place. Hydraulic calculations indicate that the tunnel 
capacity is 3,200 fts/s and the spillway capacity is 1,600 ft3/s,giving a total 
system capacity of 4,800 ft3/s.Contemporary frequency computations indicate 

Chapter 2. CASE STUDY REPORTS 


1Seward, AK 1 
1 
Figure 2-7 Lowell Creek Diversion 

that the project is capable of conveying one-half of the PMF (2,200 ft3/s)
without flow over the spillway, provided that the tunnel is not blocked by 
debris, landslide, or ice accumulation. Note that the USACE, Alaska District 
uses a procedure to modify frequency curves to reflect the possible occurrence 
of surge-release flood events. These events occur when the channel is 
temporarily blocked by a landslide, which is subsequently overtopped and 
breached during the storm. 

Hydraulics. No records of the design criteria or methodology were found. It 
appears that classical hydraulic computations were performed based upon 
observations of floods carried by the flume. It is not known if the tunnel was 
designed to flow full at its design flow. 

Sediment/Debris Historical information from the period during which the flume 
was in place was apparently used. The significance of the debris load was 
recognized by the designers, although little quantitative information was 
available. It was reported, for example, that 10,000 yd3 of material were 
deposited in the flume in 11 hours during a flood in 1935. 

Chapter 2. CASE STUDY REPORTS 


Flood Experience 

The flood of October 1986 is believed to be the largest known flood since 
Seward was established in 1900. The peak discharge was estimated to be 
1,900 ft3/s,which is approximately the l-percent chance exceedance flood. 
During this flood, the water surface came to within 6 inches of the spillway 
crest, due to a landslide above the tunnel entrance that temporarily blocked 
the tunnel. The City of Seward had an observer at the dam during this event. 
He reported the conditions and the potential flood area downstream of the 
spillway was evacuated in 30 to 45 minutes. The tunnel was also partially 
blocked by a similar landslide in August 1966, when the water level came 
within 2 feet of the spillway crest. Major damage to the tunnel lining occurred 
in 1986 and continues to be a problem. 

Over the years, the concrete lining of the tunnel has been abraded; 
reinforcing rails have broken loose: and some of the natural rock has eroded 
due to the high-velocity movement of large material through the tunnel. After 
both the 1966 and 1988 floods, large rocks (4 to IOtons) were found in the 
tunnel. No major repairs were made to the tunnel until 1968, when loose rails 
in the center of the invert were removed and replaced. By 1974, much of the 
concrete replaced in 1968 had eroded and the rails were again coming out. 
(The rails fail by working loose at the upstream ends, bending upwards, and 
trapping more debris creating the possibility of a tunnel blockage.) Erosion of 
a large hole through the tunnel lining in the inlet transition began around 1977. 
With the loss of the tunnel lining, the tunnel is no longer structurally sound and 
may not resist the overburden compressive forces, leading to the possibillty of 
a collapse. In 1981, the State of Alaska appropriated $8million for tunnel 
repairs that were performed in 1984. During these repairs, failed rails in the 
middle third of the tunnel were removed and not replaced. By 1987, the 
middle third of the tunnel floor had eroded to bedrock at 18 locations. It was 
noted at that time that the tunnel walls had not sustained significant damage 
over the 40+years of operation. By January 1988, an estimated 30 percent of 
the middle third of the tunnel floor had eroded to bedrock at 44 separate 
locations, and much of the bedrock had also been scoured. Emergency 
repairs were made in early 1988 by the USACE.These repairs were made 
only to damage from the 1986 flood and did not restore the entire tunnel to its 
original design, Part of the tunnel floor was unrepaired and left covered by a 
thin (2-inch or less) layer of concrete. 

The tunnel outlet, which consists of an open concrete flume 10feet wide 
by 100 feet long, has also experienced damage from erosion, including loss of 
reinforcing rails. The ground level downstream of the outfall to Resurrection 
Bay has been raised due to the accumulation of material transported through 
the tunnel. In September of 1982, the material transported through the tunnel 
during a flood, blocked a road at the end of the tunnel (not shown in Figure 

Chapter 2. CASE STUDY REPORTS 


2-7); apparently this event has occurred several times, The diversion of 
sediment may have had negative impacts in the fan area as well. In 1964, for 
example, Seward experienced catastrophic landsliding along the distal edge of 
the fan delta during The Good Friday earthquake. It has been speculated that 
this landsliding would not have been so severe, or nonexistent, had Lowell 
Creek continued to deliver sediment to the distal edge of the fan delta. 

Several design deficiencies have been identified that need to be addressed 
in the future. 

. No provision was made for flood passage through the area downstream 
of the spillway. Since construction of the project, this area has become 
heavily developed to include single-family homes, senior citizens 
apartments, and a hospital. The former creek bed is now occupied by 
a major residential and business street (Jefferson Street). 

. There was no emergency plan developed for action to be taken should 
the tunnel be blocked or long-term impoundment behind the dam 
occur. (The dam was not designed for long-term impoundment, and 
the possibility of seepage and subsequent failure exists). 

. There was no provision for a monitoring and warning system. Lowell 
creek is subject to flashflooding, and there is extreme hazard to life and 
property should the tunnel diversion system fail. 

2.7 Fourth of July Creek Levee System 
Location: Seward, Alaska 

Reference: Office Reports and Project Files, USACE,Alaska District, 
Anchorage, Alaska. 

Project Description 

The project is located on an alluvial fan directly across Resurrection Bay 
from the City of Seward. It consists of a levee system that was constructed to 
protect land on the fan for development. The levees confine the stream along 
the southeast margin of the fan delta. This project has reduced the active 
surface of the fan by about 70 percent. The project drainage area is about 2.5 
mi2.Channel slopes are about 900 ft/miin the canyons, 200 ft/minear the 
apex of the fan, and 50 ft/mijust above the levee system. Fourth of July Creek 
is a typical glacial-fed stream with low flows occurring in late fall, winter, and 
early spring, and high flows occurring in summer and early fall. High summer 

Chapter 2. CASE STUDY REPORTS 


flows carry a large amount of suspended sediment derived from glacial 
outwash.Low winter flows are mainly derived from groundwater seepage, and 
carry little sediment. The project was constructed in 1981-82 by the Cityof 
Seward. 

Flood Experience 

A storm in 1988 caused landslides in the canyon of the north branch, 
which temporarily dammed the stream, resulting in a surge-release flood 
event. Discharge derived from high water marks in the canyon after the debris 
dam was breached was 30,000 f13/s.This is estimated to be about a 
O.&percent chance exceedance event. This event flowed at an average of 3 
feet below the levee crests. Surveys indicated that, during this event, 3 to 4 
feet of sediment and woody debris were deposited in the lower floodway. 
These deposits forced a majority of the flow to impinge on the levee, causing a 
portion of it to fail during the falling limb of the hydrograph. No damage to the 
protected areas occurred, however, because flows were low by the time the 
levee was breached. Damage to the levees was repaired by using larger 
riprapon the levee face; however, no improvements to the levee toe were 
made. Between sta. 8+00and sta. 49+00,about 3.5 feet of scour was 
observed. In 1989, an estimated lo-percent chance exceedance event 
occurred. During this event, a log jam in the vicinity of the north diversion dike 
diverted the stream 90 degrees, causing it to impinge directly into the diversion 
dike, and scouring the toe and lower portions of the dike. The dike was again 
repaired with larger riprap.After the 1989 flood, a scour hole 4 feet deep was 
noted at the confluence of Fourth of July and GodwinCreeks. 

Basis of Design 

Hydrology. Only a few random-flow measurements are available on Fourth 
of July Creek. Flow-prediction equations based on multiple regression 
techniques were used in 1980 for the original levee design. Regression 
equations developed by the US. Forest Service were used. The levees were 
designed to withstand the i-percent chance exceedance flood event; the flow 
rate used is not known. Contemporary frequency analyses by the USACEplace the magnitude of this event at 8,600 p/s.Water surface profile 
computations using HEC-2 show that this event would flow 4 to 5 feet below 
the levee crests. 

Hydraulics. No specific design procedures were identified. From the 
information available, however, it appears that the levee heights were selected 
using step backwater computations, probably HEC-2. 

Chamer2. CASE STUDY REPORTS 


SedimenVDebris.During levee design, sediment samples were taken on the 

lower portion of the fan-delta and bed load computations were performed. 

Suspended sediment samples were also taken. The estimated suspended 

load for Fourth of July Creek and data from other Alaskan glacial streams were 
analyzed; these data correlated quite well with an empirical relation developed 
by Nordinfrom data on the Rio Grande. HEC-6 was used to estimate a 
channel size and configuration that would yield an equilibrium condition with 
regard to total sediment transport. It appears that this was done for the design 
flow event. The computed results indicated that the high proportion of larger-
sized material in the stream bed provides an armor layer after a small amount 
of scour at bank-full stage. The original design computations concluded that 
the total sediment load would pass through the leveed reach with little or no 
net scour or deposition. The upper basin has been logged several times in the 
past and stumps, slash, and trees are regularly carried downstream by the 
higher flows. This debris can and does run aground in bends and shallow 
areas, causing log jams and still-water areas, where deposition occurs. 
Changes in local energy gradients at these log jams can cause considerable 
local scour. The combination of deposition and scour in the area of the 
grounded debris can cause abrupt changes in flow direction. It appears that 
the original design did not address the ability of braided glacial streams to 
migrate laterally and scour locally. This problem may be particularly acute for 
floods that are less than the design flood where the entire width between the 

levees may not be occupied by high-velocity flows. 

2.8 Gold Creek Channelization Project 
Location: Juneau, Alaska 

Reference: Office Reports and Project Files, USACE,Alaska District, 
Anchorage, Alaska. 

Project Description 

The project consists of about 1,700 feet of reinforced concrete channel with 
intake and downstream energy-dissipation structures for the portion of Gold 
Creek passing through Juneau, which is located on an alluvial fan. Thickness 
of the concrete is 12 inches on the channel bottom and 10inches on the side 
walls. The average bottom slope of the channel is 90 ft/mile.The project was 
initially completed in March 1956 by the USACE.The City of Juneau is 
responsible for operation and maintenance. 

Chapter 2. CASE STUDY REPORTS 


Flood Experience 

The project deteriorated somewhat during its first 2 years of operation. The 
deterioration accelerated rapidly during the 3 year and failure of the structure 
was likely. The extent of the deterioration indicated a design and/or 
construction deficiency, so the project was rehabilitated by the USACE.In 
1962, two-thirds of the channel length was repaired and a minimum-sized 
debris basin (1 ,000~yd3)was excavated at the upstream end to capture larger 
sized material that was causing the channel deterioration. Periodic inspection 
showed that the portion of channel not repaired had continued to erode, with 
complete erosion of the bottom slab in some places, It was observed that the 
debris basin generally filled within 1week after ithad been cleaned. 

Test panels were installed in the channel in 1963 to help determine the 
best type of channel lining for the conditions, The five test sections were: 

1) Steel plate 

2) 70-lb rail with high strength concrete 

3) Steel armor grating embedded in high strength concrete 

4) Rubberized sheet tar 

5) Epoxy resin grout with high strength concrete 

Two weeks after application, the rubber tar section had failed completely. 
After 1 year, the epoxy resin grout panel showed one-half inch of wear and 
some loss of bond to the original concrete. Four months later, the steel plate 
was pitted and had lost anchors and fastenings. The steel rail and checker 
plate showed only minor wear. 

An inspection in 1964 identified large material, about 1 ftin diameter, 
deposited at the lower end of the channel that had obviously traversed the 
entire length of the paved reach. Repair of the channel and improvement of 
the headworks to eliminate the main source of debris was completed in June 
1968. Repair work to the remaining original channel lining was done in 1975. 
Repair work was again needed by 1979 and was completed in 1984. The 
debris basin has been enlarged to 5,000 yd3and is periodically cleaned out by 
the City of Juneau. Other than the necessity for periodic repair of the channel 
bottom, the project has performed well. 

Basis of Design 

Hydrology. Precipitation and stream-gaging data are available for the Gold 
Creek basin from the early 1900s. The original (1949) design was based on 
traditional flood frequency procedures using these data. Level of protection for 
the design was SPF. Use of subsequent data and reevaluation in the early 
1960s confirmed the SPF magnitude of 4,800 ft3/s.

Chmter2. CASE STUDY REPORTS 


Hydra&x.No specific design information was available; however it appears 
that the design was performed using USACEguidance available at that time for 
supercriiical flow channels. Note that the channel bottom was super-elevated 
on curves. 

Sediment/Debris. No specific design information was available. From the 
history of the project, however, itappears that the erosive capability of the 
large size material passing down the channel was underestimated. The repairs 
and modifications to the project were based upon observation and field testing. 

2.9 Wild Rose Development Project 
Location: Riverside County, California 

Reference: Flood Control Improvements on Alluvial Fans (Schall,
D.D.;Sender, D.W.; and Peairs, F.J., 1990). 

Project Desctiption

Wild Rose development was under construction when inspected in 
December 1990. As a new project, there is no flood experience to document. 
However, the referenced paper provides a good overview of the alternatives for 
the project and some general characteristics for those akernatives to be 
attractive. The project is being developed with a flood control channel to 
transport both debris and water through the development. The following 
presents some of the aiternative considerations as presented in the reference. 
The reference uses the word debris as an equivalent to sediment. 

Floodplain Management Alternatives 

According to the reference paper, there are four basic ways to manage 
development on alluvial fans: 

1) Mapping and zoning to prohibit development in the areas exposed to 
flood and debris flow; 

2) Developing confined channels that transport flood and debris flows 
safely through the development; 

3) Creating detention storage above the development for debris and 
transport of the relative clear water through the development; and 

Chapter 2. CASE STUDY REPORTS 


4) A combination of 2 and 3, where the debris basin is sized to contain 
debris from most events and, for larger events, the debris is transported 
downstream. 

Some of the key factors considered for each aiternative include: 

. Controlling development by zoning requires accurate mapping of the 
hazard area and a master plan for the entire alluvial fan. (The Upper 

Indian Bend Wash Regional Drainage and Flood Control Plan is an 
example of a master plan; however, itrepresents a combination solution 
as described in alternative 4.) Besides the uncertainty in defining the 
flood-prone area, the major problem with this alternative is the large 
land area that would be required to accommodate the potential flood 
and debris flow. The Wild Rose property did not have adequate land 
for a natural floodway or open space. 

Transporting both debris and water through a development must be 
carefully engineered, constructed, and maintained to be successful. A 
major problem is to maintain debris transport with the potential for 
deposition contributing to channel blockage and overflow. Maintaining 
the debris flow has the advantage of contributing sediment to the 
downstream channel system and, in coastal streams, the sediment 
supply to maintain beaches. 

. Using debris basins and downstream concrete-lined channels is a 
standard approach to flood control on alluvial fans, particularly in 
Southern California. The cost of constructing and maintaining the 
detention basin is generally high, and dam-safety issues are a concern. 
Dams located immediately upstream from urban development constitute 
a high risk. If debris loads for extreme events exceed design, the 
storage is lost and the potential for failure increases. Trapping debris in 
the reservoir can also cause downstream channel degradation due to 
the reduced inflow sediment load. 

. The combination of debris basin and flood-control channel is fairly 
common. This alternative usually provides debris control for smaller 
floods and transport for the larger floods; reducing the construction 
cost of the basin and the downstream hazard from a failure. Smaller 
debris basins would require more frequent cleaning to maintain their 
effectiveness. Temporary debris basins are frequently used in the 
Southern California watershed after a fire. 

Limited land area precluded the use of natural floodways at 
this project site. The potential site for a debris basin did not provide 
sufficient storage volume with a conventional earthfill structure. 

Chapter 2. CASE STUDY REPORTS 


Transport of debris through the channels was possible because the 
steep slope would provide high-velocity flow. The final design of the 
channel facilities were based on a bulking factor near 2, and 
maintenance of high channel velocity with minimum grade breaks and 
channel curves to minimize potential deposition of debris. The channel 
bottom and the lower side slopes were designed with extra concrete to 
provide a wearing surface and extra reinforcement to support the 
channel during a debris-flow event. 

Chapter 2. CASE STUDY REPORTS 


3. EXPERIENCE WITH FLOODCONTROL 
MEASURES 
Chapter 2 provided case studies that demonstrate some of the problems 
associated with the performance of flood-control measures. Additional 
information has been gleaned from other reports and books. The following 
sections summarize this experience for each structural flood-control measure. 
A general description of the measure and its application is provided. General 
experience and potential sources of failure are described and, if appropriate, 
recommendations on applicability are provided. 

3.1 Measures to Control Flood and Debris Flow 
Structural Flood-Control Measures 

Structural flood-control measures attempt to decrease the flow and/or 
decrease the flooding depth. Measures that decrease the flow include 
reservoirs and diversions, while measures that decrease flooding depth include 
channel alterations, levees, and floodwalls. 

Structural measures used to control flood and debris flows on alluvial fans 
have consisted primarily of the eight following: 

1) Debris Barriers
2) Debris Basins
3) Detention Basins
4) Flood-Control Channels
5) Diversions and Bypasses
6) Floodwalls
7) Levees
6) Floodwalls and Dikes

Debris Barriers Debris barriers are structures, usually located in the 
watershed, that stop or reduce the movement of debris down the channel 
system and onto the fan. 

Chapter 3. EXPERIENCE WlTH FLOOD-CONTROL MEASURES 


Debris Basins. Debris basins are facilities that are designed to settle out, 
and provide storage for, coarse material and trash resuiting from a major 
storm. They are usually located at the upper ends of alluvial fans. 

Detention Basins. Detention basins are storage structures, usually 
uncontrolled, and are designed to reduce the peak flood flow from a 
drainage basin. Often one basin serves both purposes of debris and 
detention basin. The two are presented together in this report. 

Flood-Contol Channels. Flood-control channels are engineering works 
designed and constructed to pass flood discharges more efficiently than 
natural channels, thereby reducing flood stages. Flood-control channels 
usually follow the natural channel course. 

Diversions and Bypasses. Diversions and bypasses are man-made 
channels designed to provide additional flow capacity or to direct the flow 
away from developed areas. 

Fbodwalls. Floodwalls are vertical walls, usually made with reinforced 
concrete, oriented parallel to a stream to prevent overflows into developed 
areas. 

Levees. Levees are dikes, usually earthen and parallel to the stream, that 
are designed to prevent overflows into developed areas. 

Fbodwalls and Dikes. Floodwalls and dikes are sometimes used in 
conjunction with diversions. Also, they may be placed across the fan to 
divert the flow away from developed areas and reroute it toward local 
washes or sediment/debris-detention basins before disposing the excess 
flow into a nearby river or into the local storm drainage system. Examples 
of floodwalls and channel diversion systems are found in Clark County, 
Nevada, Bullhead City, Arizona and at Rancho Mirage, California. 

3.2 Debris Barriers 
Debris barriers are not flood-control measures in the strictest sense, they 
are barriers designed to block large materials like rocks, boulders, and floating 
debris (e.g., logs, brush, branches, trash, shopping carts and other large 
objects that may get entrained by high-velocity flows). Barriers can be a 
component of a flood-control plan; however, they do not significantly reduce 
flooding. The barriers can be permanently placed, or can be part of an 
emergency response after watershed fires or landslides, to retain an expected 
increase in debris loading. Southern California watersheds have many 
examples of both permanent and emergency applications. The Oakland Hills 

Chapter 3. EXPERIENCE WITH FLOOD-CONTROL MEASURES 


fire in November 1991, near Oakland, California, led to the immediate threat of 
possible landslides and debris flows with the rainy season about to occur. The 
SCS and the City of Oakland installed several different types of debris barriers 
throughout the burn area, including debris fences (chain link and wire mesh), 
silt curtains along the contours on steep slopes, log cribs, and small check 
dams. Debris barriers encompass several different forms, including fences, 
small walls, cribs, and check dams. 

Debris Fences. Debris fences are typically vertical beams (with or without 
connecting wire) anchored in a foundation perpendicular to the expected 
debris flow. Debris fences, by themselves, provide only incidental protection 
from debris during major events, and are not recommended as a general 
solution to debris-related damage. The primary purposes of fences are to 
retard the rate at which debris moves down the slope; to catch a portion of the 
debris; and to break up the flowing mass, thereby allowing the escape of air 
that may be trapped under the flow. Such trapped air can serve to reduce the 
friction between the flowing debris and the ground, resuiting in increased 
velocity. Debris fences often fail during a debris flow; however, if the mode of 
failure is a bending of the fence rather than an intact movement or sudden 
failure downslope, the fence will, generally, still slow the flow and trap a portion 
of the debris. 

The placement of the debris fence has a tremendous effect on the ability of 
the fence to retard the rate of the flow. If possible, several small debris fences 
should be constructed in the area where drainage concentrates to serve as 
debris collectors from small-slope movements and to slow the flow and break 
up the energy at the inception of the event, preventing the occurrence of 
larger-scale debris flows further downstream. Fences placed at the toe of 
steep slopes are likely to fail unless they are properly sized and reinforced, 
due to the high speeds at which the debris-laden flows can move and the 
high-impact loading that can occur. Debris fences must be inspected 
periodically and cleaned or repaired as necessary. Vandalism, such as cutting 
a fence with wire cutters, is sometimes more difficult to manage than the 
repairs following small- to intermediate-sized events. Debris fences are best 
applied in emergencies or as temporary measures, and should not be 
considered to be permanent flood-control structures (refer to Section 2.4). 

Debris Barrier Walls. Also known as "fire barriers, debris barrier walls are 
constructed in Southern California across canyon mouths, following watershed 
fires, in anticipation of debris flows induced by heavy winter rains on 
unvegetated hillsides. Numerous examples have been constructed by the Los 
Angeles County Flood Control District (LACFCD) in the San Gabriel Mountains. 
Debris barrier walls are typically constructed 7 to 15 feet tall, with 2-by-12 inch 
and 3-by-12 inch timbers supported by 60-lb rails set in a concrete foundation 
(LACFCD, 1979). Although these structures have been intended to provide 

Chapter 3. EXPERIENCE WITH FLOOD-CONTROL MEASURES 


temporary protection while protective natural vegetation is re-established in a 
burned watershed (lasting 3 to 5 years), in practice, they have proven to be 
quite durable. Examples of filled and unfilled fire barriers in good condition up 
to 15 years old are found in the San Gabriel Mountains near Glendora, 
California. Figure 3-1 shows a typical Southern California debris barrier wall. 

Figure 3-I Debris Barrier Wall in Southern California 

Crib Barriers. Crib barriers are constructed like small check dams across 
a channel. Concrete cribs have been used in Southern California by the U.S. 
Forest Service and by Los Angeles County as debris and flood-retarding 
structures. Cribs, which are periodically cleaned, function like small-debris 
basins. Crib structures are often constructed in series. The upper cribs, 
located in narrow canyons, may have limited access and, as a consequence, 
are usually not cleaned of debris. Upon filling, the area upstream from the crib 
becomes a heavily vegetated wetland and acts to retard flood flows by virtue 
of flatter slopes and higher hydraulic resistance. In addition, the filled cribs 
add stability to the toe of canyon side slopes, thus providing additional debris 
control. 

Current design and application of check dam structures in Southern 
California has evolved significantly from experience during the past 75 years. 
During the period of 1914 to 1920, more than 1,500 loose-rock check dams 

Chapter 3. EXPERIENCE WITH FLOOD-CONTROL MEASURES 


were constructed in Los Angeles area watersheds; however, subsequent floods 
demonstrated conclusively that the loose-rock check dams were unable to 
stand up to the impact and buoyancy forces of debris flows. Following the 
flood of 1920, rock-and-wire check dams, which utilized wire-bound rock 
mattresses, were constructed. The rock-and-wire check dams performed 
better, but the continued pounding of debris passing over the structures broke 
the wire mesh enclosing the rock mattresses. These structures were utilized 
until numerous failures during the New Years Flood of 1934 lead to their 
abandonment. 

In the late 1930s the U.S. Forest Service designed and built a series of 
mortar-rubble arch (up to 40 feet tall) and gravity dams. These structures 
have proven to be both durable and effective. In the 1940s the U.S. Forest 
Service utilized a wide variety of structures and materials, including soil 
cement, various metal structures, and concrete cribs, for the Arroyo Seco 
Flood-Control Project. This project has provided valuable cost and 
performance information on the alternatives employed. The concrete crib (see 
Figure 3-2 below) emerged from the Arroyo Seco Project as the most favorable 
of the various alternatives. In the 195Os, the LACFCD and the U.S. Forest 
Service joined forces to design and construct two experimental projects that 
included 79 concrete cribs (IACFCD, 1959). 

gure 3-2 Concrete Crib Barriers in Southern California 
Chapter 3. EXPERIENCE WITH FLOOD-CONTROL MEASURES 


Experience has shown that the grouted spillway section employed on the 
earliest structures could be damaged by debris. Recently constructed 
concrete cribs employ a cast-in-place reinforced spillway section that has 
proven to be more durable. During the past 40 years, Southern Californias 
experience has demonstrated that properly designed and constructed concrete 
cribs are an effective and durable type of debris barrier. Another application is 
crib walls built along highways to trap and block debris from entering the 
roadway. The California Department of Transportation (CALTRANS) Standard 
Plans has design details for concrete, steel, and timber crib wails (CALTRANS, 
1988). 

Retaining Walls. Retaining walls (6 to 10 feet high) are another common 
device for protecting single-lot structures in subdivisions. Deflection walls are 
another form of retaining wall, only placed at an angle other than 90 degrees 
to the direction of the slope. Observations of retaining and deflection wall 
performance during large flood events, generally, indicate that wall failures are 
usually associated with either poor construction methods or inadequate design 
(the failure to recognize the potential magnitude of impact and buoyancy 
forces). 

Retaining walls constructed of concrete block are commonly employed at 
this time due to reduced construction costs; however, they are the most 
susceptible to failure. Steel-reinforced, poured-in-place concrete retaining 
walls are far more durable than hollow block walls because they do not 
possess planes of weakness such as those that exist between concrete block 
and mortar. Also, proper placement of steel reinforcement in poured walls to 
resist tensile stresses is far more feasible than in block walls, and the bond 
between the steel and the concrete is superior in poured walls. 

3.3 Debris and Flood-Detention Basins 
Debris basins may be limited to trapping debris; however, most 
applications also provide flood detention. Typically, these basins are located 
in the watershed or near the apex of a fan where flow and debris are confined. 
The narrow width of the watershed canyon often provides the most attractive 
sight for locating a dam. Debris basins are often combined with an improved 
channel downstream from the debris basin. The West Magnesia Debris Basin 
and Channel provide standard project flood (SPF) protection to Rancho 
Mirage, California (USACE, 1988). Rancho Mirage below Magnesia Spring 
Canyon is often shown as a classic example of alluvial fan development (e.g., 
the cover photo on Alluvial Fans: Hazards and Management, (FEMA, 1989)). 
The Magnesia Springs fan was also the basis for a model study conducted by 
Anderson-Nichols (1981) for FEMA. As shown in Figure 3-3, the USACE 

Chapter 3. EXPERIENCE WITH FLOOD-CONTROL MEASURES 


FI 
gure 3-3 West Magnesia Spring Preferred Plan, Rancho Mirage, 

project consists of a debris basin with a spillway and concrete-lined flood-
control channel with an energy dissipator at the downstream end, where the 
channel enters the Whitewater River. 

The Magnesia Spring Canyon Debris Basin was required at the upstream 
end of the concrete-lined channel to ensure that the inlet capacity was not 
reduced due to sand deposition; to minimize the scour of the concrete lining 
caused by coarse sediments moving at high-velocity; and to ensure the 
functional adequacy of the outlet structure (USACE, 1983). The USACE, LOS 
Angeles District used traditional procedures to develop the SPF hydrology; 
however, to size and design the sediment retention basin, the engineers 
worked with geologists, soil scientists, and local flood-control personnel to 
evaluate soil erodibility and the potential sediment volume for the SPF. The 
Tatum Method (1963) was modified to better represent the sediment 
production for the basin conditions in the area. The sediment production 
estimate was based on soil type, vegetative cover, slope angle, soil erodibility, 
and an estimated IO-year Tatum burn recurrence condition. Computed values 
compared favorably with regional experience. The debris storage requirements 
were based on past experience that has found that the slope of material 
deposited after a major flood averages about one-half of the original slope. 

Chapter 3. EXPERIENCE WITH FLOOD-CONTROL MEASURES 


The capacity of the debris basin (150,000 yd3)was determined by calculating 
the volume between the excavated invert of the basin and the deposition slope 
projected upstream from the spillway crest at 0.5 of the natural slope, 

(USACE, 1983). 

The spillway and concrete channel sections were designed according to 
the guidelines defined in the USACE Hydraulic Design of Spillways (USACE, 
1965) and Flood-Control Channels (USACE, 1970, revised 1991). The 
rectangular spillway was designed to pass the probable maximum flood (PMF). 
The spillway length and height were based on studies of the relationship of 
debris storage, embankment height, spillway crest length and spillway 
transition length. The pool drain was located upstream from the spillway, with 
the intake tower 1 ft above the elevation of the assumed debris level at that 
location. The drain pipe was sized to operate under inlet control (not under 
pressure) and to drain the pool within 1day. 

Flood protection for the community of Rancho Mirage would not have been 
adequate without the addition of the West Channel Project. The western side 
of the development was subject to flooding from the western foothills, A series 
of diversion dikes divert water and sediments emanating from the western 
foothills into a diversion channel, which carries the water into a sediment-
retention basin that releases the water into the local storm drainage system 
and drains runoff generated on the surface of the developed fan. 

The SCS has developed several Debris/Flood Detention basins in Arizona. 
A review of the flood experience at Saddle Back Diversion indicated that the 
Flood-Reduction Structure (FRS) performed well (Chapter 2). This project and 
the Buckhorn Mesa Watershed flood-diversion and retention structures near 
Mesa, Arizona (SCS, 1984), are long, earthen, levee-like structures across the 
fan, capturing runoff from a portion of an alluvial fan. The Buckhorn-Mesa 
project also uses the levee, shown in Figure 3-4, to divert alluvial fan flow to 
the FRS. These structures would only be practical where there is sufficient 
open space to provide for flood inundation during flood events. Because these 
are dry dams, there is a potential for open-space land use within the storage 
zone of the reservoir. 

The SCS Earth Dams and Reservoirs Tech Release No. 60, describes 
design procedures and provides minimum requirements for planning and 
designing earth dams and associated spillways (SCS, 1985). The SCS also 
has Simplified Method for Determining Floodwater Retarding Storage, Tech 
Release No. 032, (SCS, 1966). The SCS has developed a covered riser that 
minimizes potential for debris blockage (from floating debris) on the outlet 
works; Tech Release No. 29 (SCS, 1965). Both the Saddle Back and 
Buckhorn Mesa projects have this type of outlet, see Figure 3-5. These outlets 
are designed for flood water, and would not be appropriate for debris basins, 

Chapter 3. EXPERIENCE WITH FLOOD-CONTROL MEASURES 


,.._:_
,::iFigure 3-4 Pass Mtn. Diversion Dam, Mesa, Arizona (Flow trapped on left 
diverted upward in picture.) 

Figure 3-5 Covered Riser Inlet at Signal Butte, Mesa, Arizona 

Chapter 3. EXPERIENCE WITH FLOOD-CONTROL MEASURES 


Debris basin outlets must be designed to pass the flow as the basin fills 
with sediment. The typical outlet works for a debris dam would be a circular 
tower with slotted openings around the circumference. Several of the projects, 
reviewed in Chapter 2, set the top of the inlet riser at least 1 ft above the 
emergency spillway elevation. If the riser is located some distance upstream 
from the spillway, the upstream slope of the sediment deposit should be 
considered. Several designs used a slope of one-half the natural upstream 
slope to estimate the surface slope of deposited sediment in the debris basin. 
The LACFCDs Design Manual Debris Dams and Basins (LACFCD, 1979) 
provides guidance for design, preparation of plans and specifications, and 
construction of debris dams and basins. 

The Central Arizona Project (CAP) canals are protected, in some locations, 
by a flood-retention structure across fan outwash areas, just above and parallel 
to the canal. The outlet works are simple culverts (referred to as overshoots) 
that carry the flood flow across the canal, preventing contamination of the 
canal water. Figure 3-6 shows the flood-retention structure and the inlet to a 
culvert crossing the CAP. The flow is then released into controlled or natural 
drainage systems below the canal. While the rural locations support using the 
natural drainage systems, more developed locations typically use a concrete-
lined flood channel. The local experience indicated that flood-control 
performance has been favorable; however, there have been some problems 
with settlement. The settlement is probably the result of consolidation of fan 

Figure 3-6 Flood Retention Structure and Inlet to Culvert Crossing Central 
Arizona Project, near Scottsdale, Arizona 

Chapter 3. EXPERIENCE WITH FLOOD-CONTROL MEASURES 


material caused by flood inundation. Similar problems have occurred along 
the California Aqueduct in the Central Valley of California. Portions of the 
aqueduct right-of-way were flooded to consolidate the material prior to project 
construction. 

The culvert outlets could only be used in those locations where sediment 
transported by the inflow is low; otherwise, the sediment would settle out from 
the inflow while it was in storage behind the embankment, and would tend to 
block the culvert inlet and reduce its flow capacity. Flow at the outlet would be 
an additional concern. When flow is concentrated, as it would be through the 
culverts, there is an increased potential for scour and sediment transport at the 
outlet. 

3.4 Flood-Control Channels 
Generally, flood-control channels have been developed as highly efficient, 
concrete-lined channels where right-of-way and real estate costs are high. The 
rectangular concrete channels around Rancho Mirage are examples. With 
lower right-of-way costs, larger trapezoidal-shaped channels of natural 
materials can be utilized, usually with lower construction costs. In alluvial fan 
areas, there is usually a problem with developing and maintaining unlined 
stable channels, The typical non-concrete channel will often require rock 
riprap bank protection, and possibly drop structures to maintain a flatter 
channel grade. While past design decisions were often based on the trade-off 
between construction cost and right-of-way and relocation costs, the increased 
concern over environmental and aesthetic values has expanded the 
considerations for project formulation. A third category of flood-control 
channels may be considered as multiple-purpose channels that provide wildlife 
habitat and/or support public recreational use of a portion of the flood-
conveyance area. The Indian Bend Wash of Scottsdale, Arizona (Figure 3-7) is 
an example of a multiple-purpose development. 

Because floods on alluvial fans tend to be heavily laden with sediment and 
debris, the question is whether to trap the sediment, usually in the headwater 
area, or transport the sediment through the channel. Heavy sediment loads 
and high-velocity flows are difficult to manage and often create extensive 
maintenance problems. Transported and deposited sediments also present 
significant maintenance problems for landscaped areas and park features in 
the flood-conveyance area. 

Even concrete channels can be heavily damaged by the sediment 
transported during a flood event and, at the outflow point, there is the impact 
of the transported sediment on the downstream drainage system. Flood-
control channels on alluvial fans must be designed with close attention given 

Chapter 3. EXPERIENCE WITH FLOOD-CONTROL MEASURES 


to the interception and storage or passing of sediment, particularly the coarser 
sizes moving at high velocities. 

Rock riprap is often used to maintain alluvial channels, especially when 
high velocities are expected. Figure 3-8 shows a section of Tahquitz Creek 
with riprap bank protection. The series of flood reports from Southern 
California (Sections 2.1 to 2.3) describe several causes for riprap failure; 
however, two, (1) toe failure due to undermining or movement, and (2) riprap 
failure caused by impinging flow from lateral inflow or poor channel alignment, 
seem to be the predominant causes of failure. 

Chapter 3 of EM 111 o-2-1601 provides design criteria for riprap protection. 
The USGS has developed a two-volume document, Rock Riprap Design for 
Protection of Stream Channels Near Highway Structures (USGS, 1986). 

Grouted riprap has been routinely used in the Denver Urban Drainage and 
Flood-Control District (DUD&FCD, 1990), particularly in applications where 
high velocities and tractive forces could pull away the rock in a typical dumped 
riprap section. The DUD&FCD Design Notes provides guidance on placing 
grouted riprap and boulders. Grouted riprap has also been used to protect 
diversion inlets, as shown in Figure 3-9, and drop structures at SCS and 
USACE projects. 

Chapter 3. EXPERIENCE WITH FLOOD-CONTROL MEASURES 


Figure 3-8 Tahquitz Creek, looking upstream from Sunrise Bridge, Palm 
Springs, California 

gure 3-9 Grouted Riprap Inlet into Signal Butte Floodway, Mesa, Arizona 

Chapter 3. EXPERIENCE WITH FLOOD-CONTROL MEASURES 


3.5 Diversions and Bypasses 
The typical flood-reduction diversion is a side weir or chute that is designed 
to divert a portion of the river flood flow, while the main channel carries the 
majority of the flow. The primary effect is to scalp the peak flow from the 
flood wave as it passes the diversion location. The diverted flow could 
temporarily be detained in a storage area, or transported through an alternative 
conveyance path. For alluvial fans, the diversion may be used to capture flood 
flow, distributed across the fan, and to divert it to a location where it is 
non-damaging, or to transport it through a controlled channel system. The 
Saddle Back Diversion in Maricopa County, Arizona (Section 2.5) is an 
example of the capture-and-dived application. In this application, the diversion 
channel also intercepted alluvial fan flow along the channel. While the flood-
retention structure and diversion channel functioned well, there were problems 
with sediment blocking the designed inflow points along the diversion channel. 
The planned Alamogordo Diversion Channel Project in New Mexico has 
sediment basins upstream from the diversion channel on most tributaries 
(USACE, 1987). 

The Day Creek Flood-Control Project in San Bernardino County is 
comprised of an SPF detention basin, a concrete channel, and a side weir to 
divert part of the flow to a spreading basin located parallel to the channel. 
Flow that remains in the channel is carried downstream to a detention basin. 
Flow leaving the basin is discharged into an open spreading ground for 
groundwater recharge. The concrete channel in this project is designed to 
carry water and sediments at velocities ranging from 60 to 70 ft/s.Because 
no major events have occurred since the completion of the project, it remains 
to be seen whether the concrete and design can withstand such high 
velocities. It is not recommended that channels be designed to carry water 
and sediment at high velocities. There are no proven design criteria for these 
kinds of high-velocity channels. The flood experience at the Lowell Creek 
Diversion in Seward, Alaska (Section 2.6) demonstrates the erosive power of 
the rock and sediment transported through the concrete- and steel-lined 
tunnel. 

The SCS Buckhorn Mesa project uses a combination of a dam across a 
portion of the alluvial fan and a diversion channel to transfer the intercepted 
flow into the flood retention structure, shown in Figure 3-10. The documented 
flood experience on this project occurred during construction; therefore, there 
has not been a significant test of the facility since construction was completed 
(SCS, 1984). The flood crossings over the Central Arizona Project are similar, 
with a long dam to intercept the fan flow and culverts to carry the flow across 
the canal, as shown earlier in Figure 3-6. 

64 Chapter 3. EXPERIENCE WlTH FLOOD-CONTROL MEASURES 


Fi gure 3-10 Signal Butte Flood Retention Structure, Mesa, Arizona 
3.6 Muttipurpose Diversion Structures 
Nouri (1987) describes the design of cost-effective multipurpose diversion 
structures that provide flood detention during peak-flow periods for a specified 
range of recurrence intervals, while providing an open park-and-play area for 
the community the rest of the time. A special inlet and outlet design (Nouri, 
1997) also allows a controlled amount of bed material load to be transported 
through the diversion pipe to the downstream channel in order to provide 
enough channel stabilizing sediment load into the downstream channel. 
Figure 3-11 shows a schematic drawing of the components of the overflow 
structure and how excess flows are released vertically into the detention basin 
(a park or play ground) while the bed load continues through the pipe outlet to 
the downstream channel. 

The Nouri design has been implemented at two project sites in Orange 

County, California, where downstream channels are alluvial riparian streams. 
The goals of the design were to: 

. Maintain downstream channel stability 

. Reduce peak discharges along downstream reaches 

. Use the detention area as a community park or golf course 

Chapter 3. EXPERIENCE WITH FLOOD-CONTROL MEASURES 


Eleven multi-purpose basins employing overflow structures such as those 
shown in Figure 3-11 have been designed and constructed within the Newport 
Coast Community Development. Those basins are constructed within two 18hole 
golf courses and parks. The basins not only have maintained 
downstream channel stability, but have also allowed the transport of sediment 
to the coast for beach sand replenishment. This was one of the design constraints 
because the beaches in the project area are deficient of sediment. 

Another multi-purpose basin employing a similar overflow structure, as 
shown in Figure 3-11, has been in operation along Handy Creek in the City of 
Orange since 1986. The design has maintained the stability of Handy Creek, 
which was degrading prior to construction of the multi-purpose basin. Large 
debris can plug this type of detention structure, requiring maintenance; 
however, in areas where the likelihood of large debris accumulation is small, 
these types of basins appear to function well. 

gure 3-11 Details of a Multi-purpose Diversion Structure 
Chapter 3. EXPERIENCE WITH FLOOD-CONTROL MEASURES 


3.7 Levees and Floodwalls 
Generally, the levees associated with the reviewed flood-control channels 
have been small portions of the improved flood-control channels. No floodwalls 
were reviewed during this investigation. Figure 3-12 shows a low 
concrete wall on the outside bend of West Magnesia Canyon Channel at 
Rancho Mirage. The channel is protected with gunite-rock riprap. 

The flooding consequence for events exceeding a high levee or floodwall 
design capacity could be more catastrophic than the event would have been 
under non-leveed conditions due to the potential failure of the wall or levee 
releasing the high flood stage in a rapid-failure scenario. Instead of the area 
becoming gradually flooded as the flood wave passes down the channel 
system, a sudden release of flood flow would result from a levee or floodwall 
failure. There would be little time for evacuation and other emergency actions. 

The issue of channel stability, described above, is applicable to levees 
because most levees are constructed from the same material as the channel. 

gure 3-12 Low Concrete Wall on Outside Bend of Magnesia Canyon 
Channel, Rancho Mirage, California 

Chapter 3. EXPERIENCE WITH FLOOD-CONTROL MEASURES 


Care must be taken to protect the levee faces and toes from erosion by the 
high-velocity, sediment-laden flows (Whitehouse & McSaveney, 1990). Rock 
riprap is frequently used for protecting the channel side of the levee; however, 
there may be strong local interest in a more aesthetically pleasing alternative 
when the stream runs through an urban environment. For high-velocity 
channels, concrete lining, soil cement, or grouted rock riprap, is often used to 
protect the channel and levee. With the low relative roughness of concrete 
and soil cement, a more efficient channel can be developed, which tends to 
reduce right-of-way costs. 

Geotextiles (sometimes referred to as geofabrics) have been used to 
protect the levee banks against high-flow velocities. On alluvial fan areas 
where the soil is fine and where there is a scarcity of well graded riprap 
material, the geofabric prevents possible piping of the finer materials from the 
levee through the rock riprap. 

3.8 Summary 
There are few documented flood events for structural alluvial fan flood-
control projects. This review was expanded to include existing projects in 
alluvial environments. While alluvial fans may be different under natural 
conditions, the performance (or problems) of flood-control measures on alluvial 
fans should be similar to other high-velocity alluvial streams. Every situation 
and project is unique in some way; however, some general conclusions can be 
drawn from the documented experience. 

The traditional flood-reduction project study evaluates the trade-off between 
flood detention and more efficient flood conveyance. On alluvial fans, the 
problem expands to include storage and transport of sediment and debris. As 
described in Section 2.9, the basic structural choices are: 

Develop confined channels that transport the flood and debris safely 
through, or around, the development 

. Develop storage above the development for debris and transport 
relatively clear water through, or around, the development; or 

. A combination of the two. 

Flood-Control Channels 

Well-engineered and -constructed concrete flood-control channels have 
generally performed well, as examples in the Los Angeles basin floods show. 

Chapter 3. EXPERIENCE WITH FLOOD-CONTROL MEASURES 


Small concrete drainage structures have been severely damaged during flood 
events (e.g., the Las Vegas flash flood described in Section 2.4). 

Sediment deposits in channels, without debris basins, were found at 
tributaries, and at other locations where there was a decrease in energy slope. 
Any situation that will cause the inflowing water to slow will probably produce 
sediment deposition. A rule of thumb to consider is to never design a project 
that requires the flows to change direction or change velocity quickly in a short 
distance. Sediment erosion, deposition, channel avulsion, and possible 
structural failure may result from attempting to force flows to change their 
course abruptly. 

Problems with alluvial flood-control channels, that were not overtopped, 
were generally of two types: (1) destruction of riprap protection, with 
subsequent project feature damage, and (2) sediment deposits decreasing 
flow capacity. Riprap and other forms of bank protection tended to fail at the 
edges (i.e., at inlet, outlet, or toe protection). Failure of the toe protection due 
to excessive scour occurred in several projects. Riprap failures also occurred 
due to impinging flow from tributaries, or from flow meander within the 
channel. Similar problems also occurred on levee protection. 

In an overview of stability problems with flood-control channels (USACE, 
1990), two major potential stability problems were identified: 

 When depths are increased but the original slope is maintained, 
velocities at the higher discharges will be increased and the bed and 
banks may erode, especially if bank stability previously depended on 
cohesive sediment deposits, armoring or vegetation that was removed 
in the enlargement process. 

. When the cross-section enlargement is too large, and there is a 
substantial sediment transport load, "ther cross section may partly infill 
with sediment deposits and the calculated flood capacity may not be 
achieved without maintenance. 

Levees may also cause channel sedimentation in streams with high 
sediment loads by restricting transport and deposition of sand on the overbank 
areas. More sand is then retained in the channel to deposit further 
downstream in reaches of flatter slope. This may initiate a progressive 
upstream-advancing aggradation of the bed. Also, thick deposition of finer 
suspended sediment on the berm between the river bank and the levee 
(occurring mainly during flood recessions) may overload the bank to cause 
slump failures, (ibid, p. 3-5). 

Chapter 3. EXPERIENCE WITH FLOOD-CONTROL MEASURES 


Diversions 

Sediment deposits were also identified as a problem at the inlets of 
diversions and interceptor channels in cases where there was ponding at the 
inlet. Again, the decrease in energy slope allowed sediment to deposit to a 
depth that obstructed flow. Also, the interceptor channel at Saddleback 
(Section 2.5) had inlets located based on apparent flow paths on the fan. 
However, there was shifting of the flow paths during the reported flood event, 
and the new flow paths did not align with the inlets constructed along the 
interceptor channel. 

Stability problems on diversions can be difficult to predict, and depend on 
the method used to divert the flow. Sedimentation may occur in the main 
channel, in the diversion, or in both, as the sediment-carrying capacity of both 
is likely to be less than that of the existing channel. The division of sediment 
between the two channels is not necessarily proportional to the division of flow. 
Further sedimentation problems may arise if there are substantial downstream 
inflows of sediment that the reduced flows are unable to transport, (USACE, 
1990). 

When flood flows are diverted into a channel, but the channel is not 
deliberately modified to accommodate the increased discharges, serious 
erosional problems may ensue. The channel tends to respond by widening 
and deepening, and by flattening slope through upstream degradation and 
downstream gradation, (ibid, p. 3-9). 

Detention Storage 

In general, sediment and debris basins have worked well. The critical 
factors are size, outlet works, and downstream protection. The bigger the 
storage capacity, the better the chance for the project to perform well (not be 
exceeded) during a flood event, Even those projects that did fill were 
considered successful by the reviewers because they prevented the stored 
volume from moving downstream. If a debris basin is effective, the 
downstream channel must be protected from the relatively clearer outflow. 
Also, channel protection is usually required in the vicinity of the overflow 
spillway. 

Debris Barriers 

Debris barriers have also been effective in keeping some of the debris from 
moving downstream, as shown by their extensive use in Southern California. 
While they are not considered effective for flood control, they should be 
considered as a component of a total plan. Even when full, they have been 

Chapter 3. EXPERIENCE WITH FLOOD-CONTROL MEASURES 


credited for reducing the stream slope and thus reducing the rate of debris 
movement downstream. Localized debris barriers have also been proposed 
for protection of subdivisions from mudflow in Colorado (Mears, 1977). 

Analysis Problem 

The Base Flood for determining flood-prone areas is the 1 -percent-chance 
flood. The problem is to define the flood (including possible flow bulking) and 
evaluate the performance of the flood-reduction measures to determine 
whether an area is protected from flooding by this design event. 

While the analysis of flood hydrology is difficult due to a lack of recorded 
data in many alluvial fan areas, the prediction of sediment yield and transport 
is extremely difficult. The analytical methods used are highly dependent on 
regional data and experience. Additionally, the analysis must model the flow 
and debris movement through the area to be developed. The natural 
conveyance channels are often unstable, and there is considerable uncertainty 
in the prediction of the size and location of the channel during and after a 
flood event. Channel avulsions are common during large events. 

Every factor affecting the nature of flood and debris problems, plus the 
development and its susceptibility to flooding, affect the feasibility of flood-
reduction options, There is no cookbook approach to developing an effective 
flood-reduction project. Planning and design of flood-control structures on 
alluvial fans must always consider the effect of all possible f/ows on the 
structure as we// as the effects the structure may have on the flow /oca//y and 
downstream. While the FIA criteria are based on the i-percent-chance flood, 
the proper design of any flood-reduction project must consider project 
performance for the entire range of floods, including floods larger than the 
Base Flood. 

Chapter 3. EXPERIENCE WITH FLOOD-CONTROL MEASURES 


4. REFERENCES
Anderson-Nichols (1981) Flood Plain Management Tools for Alluvial Fans, 
Final Report to FEMA, November 1981. 

Beaty, C.B. (1990), Anatomy of a white mountains debris-flow -The making of 
an alluvial fan, alluvial fans, Alluvial Fans: A Field Approach, ed. by A. H. 
Rachoki and M. Church, John Wiley & Sons, 1990. 

CALTRANS (1986), Standard Specifications, California Department of 
Transportation, North Highlands, California, January 1988. 

DUD&FCD (1990) Grouted Riprap and Boulder installations flood Hazard 
News, Vol. 20, No. 1, Denver Urban Drainage and Flood-Control District, 
Denver, Colorado, December 1990. 

Engineer Team (1980), Report on Levee Failures and Distress, San Jacinto 
River and Bautista Creek Channel, Riverside County, Santa Ana River 
Basin, California. Prepared for the USACE, Los Angeles District, Los 
Angeles, California, August 1980. 

FEMA (1989), Alluvial Fans: Hazards and Management, FEMA-165, Federal 
Emergency Management Agency, Washington, DC., February 1989. 

French, R.H. (1987) Hydraulic Processes on Alluvial Fans, Elsevier Science 
Publishers, Amsterdam, 1987. 

Katzer, T.L.; Glancy, P.A.; Harmsen, L. (1976), A Brief Hydrologic Appraisal of 
the July 3-4, 1975, Flash Flood in Las Vegas Valley, Nevada, Office Report, 

U.S. Geological Survey, Carson City, Nevada, 1976. 
LACFCD (1959), Report on Debris Reduction Studies for Mountain Watersheds 
of Los Angeles County, Los Angeles County Flood-Control District, Los 
Angeles, California, November 1959. 

LACFCD (1979), Design Manual, Debris Dams and Basins, Los Angeles 
County Flood-Control District, Los Angeles, California, 1979. 

LACFCD (1979), Report on Debris Reduction Studies For Mountain 
Watersheds, Dams and Conservation Branch, Los Angeles County Flood-
Control District, Los Angeles, California, May 1979. 

Chapter 4. REFERENCES 


Lecce, S.A. (1990), The Alluvial Fan Problem, Alluvial Fans: A field Approach, 
ed. by A. H. Rachoki and M. Church, John Wiley & Sons, 1990. 

MacArthur, R.C. (1983), Evaluation of the Effects of Fire on Sediment Delivery 
Rates In A Southern California Watershed, Proceedings of the D/3. Simons 
Symposium on Erosion and Sedimentation, Colorado State University, Fort 
Collins, Colorado, July 27-29, 1983. 

MacArthur, R.C.; Hamilton, D.L.; Harvey, M.D.; Kekaula, H.W. (1992), Analyses 
of Special Hazards and Flooding Problems in Tropical Island 
Environments, Proceedings Environmental Engineering Sessions, Water 
forum 92, Baltimore, Maryland, August 2-6, 1992. 

Mears, A.I. (1977) Debris-Flow Hazard Analysis and Mitigation, An Example 
From Glenwood Springs, Colorado, Colorado Geological Survey, 
Department of Natural Resources, State of Colorado, Denver, Colorado, 
1977. 

Moore, CM.; Wood, W.J.; Renfro, G.W. (1960), Trap Efficiency of Reservoirs, 
Debris Basins, and Debris Dams, Journal of the Hydraulics Division, ASCE, 
Vol 86, Paper No. 2374, HY2, pp 69-87. 

Nouri, Hasan (1987), Diversion Structure for Peak Flow Reduction, 

Proceedings of Irrigation Systems for the 27st Century, Irrigation & Drainage 

Division, ASCE, Portland, Oregon, July 28-30, 1987. 

PSIAC (1968) Report of the Water Management Subcommittee on Factors 
Affecting Sediment Yield in the Pacific Southwest Areas and Selection and 
Evaluation of Measures for Reduction of Erosion and Sediment Yield, 
Sedimentation Task Force, Pacific Southwest Inter-Agency Committee, 
October 1968. 

Rachocki, A. (1981), Alluvial Fans -An Attempt at an Empirical Approach, ed. 
by John Wiley & Sons, 1981. 

Reel, D.V.; Bond, L.S. (1988) Flooding in the Desert -Taking Control, Public 
Works Journal, Vol. 119, No.13, Box 688, Ridgewood, New Jersey, 
December 1988. 

Schall, D. D.; Bener, D.W.; Peairs, F. J. (1990) Flood-Control Improvements 
on Alluvial Fans, ASCE Conference Proceedings, Hydraulics/Hydrology of 
Arid Lands, 1990. 

Chapter 4. REFERENCES 


SCS (1965) Hydraulics of Two-Way Covered Risers, Technical Release No. 
29, USDA, Soil Conservation Service, June 1965. 

SCS (1966), Procedures for Determining Rates of Land Damage, Land 
Depreciation, and Volume of Sediment Produced by Gully Erosion, 
Technical Release No. 32, USDA, Soil Conservation Service, Washington, 
DC, 1966. 

SCS (1984), Flood Report, Buckhorn Mesa Watershed Maricopa and Pinal 
Counties, Arizona, for Storm of July 17 8 18, 1984, USDA, Soil 
Conservation Service, Phoenix, Arizona, November 21, 1984. 

SCS (1985), Earth Dams and Reservoirs, Technical Release No. 60, USDA, 
Soil Conservation Service, Washington, DC., 1985. 

SCS (1987), Saddleback Diversion -Harquahala Valley Watershed, Maricopa 
County, Arizona, Engineering Report, USDA, Soil Conservation Service, 
Phoenix, Arizona, February 1987. 

Tatum, Fred E. (1963) A New Method of Estimating Debris Storage 
Requirements for Debris Basins, USACE, Los Angeles District, Los 
Angeles, California, 1963, published in Proceedings of the Federal Inter-
Agency Sedimentation Conference, pp. 886-898, 1963. 

USACE (1938) Report on Engineering Aspects, Flood of March 1938, 
USACE, Los Angeles District, Los Angeles, California, August 1938. 

USACE (1965), Hydraulic Design of Spillways, EM 1110-2-1603, Washington, 
DC., 31 March 1965. 

USACE (1971) Additional Guidance for Riprap Channel Protection CH 1 ,I ETL 
1110-2-120, Washington, DC, 14 May 1971. 

USACE (1974), Report on Engineering Aspects, Floods of January and 
February 1969 in Southern California, USACE, Los Angeles District, Los 
Angeles, California, 1974. 

USACE (1983) Magnesia Spring Canyon -Detailed Project Report for Flood 
Control, Riverside, County, USACE, Los Angeles District, Los Angeles, 
California, 1983. 

USACE (1987), Alamogordo Diversion Channel Project, Supplement to 
General Design Memorandum No. 2, USACE, Albuquerque District, 
Albuquerque, New Mexico, June 1987. 

Chapter 4. REFERENCES 


USACE (1988), Special Report-Whitewater River Flood Warning System and 
Preparedness Plan, Whitewater River Basin, USACE, Los Angeles District, 
Los Angeles, California, October 1988. 

USACE (1990), Hydrologic Engineering Requirements for Flood Damage 
Reduction Projects, ER 1110-2-700 DRAFT, Washington, D.C., 
30 December 1990. 

USACE (1990), Stability of Flood-Control Channels, DRAFT, EC 1110-8-l 
(FR), Washington, D.C., January 1990. 

USACE (1991), Hydraulic Design of Flood-Control Channels, EM 1110-2-1601, 
Washington, D.C., 1 July 1970, revised 1991. 

USGS (1986), Rock Riprap Design for Protection of Stream Channels Near 
Highway Structures, Vol. 1 -- Hydraulic Characteristics of Open Channels, 
Water-Resources Investigations Report 86-4127, U.S. Geological Survey, 
Sacramento, California, 1986. 

USGS (1986), Rock Riprap Design for Protection of Stream Channels Near 
Highway Structures, Vol. 2 -- Evaluation of Riprap Design Procedures, 
Water-Resources Investigations Report 86-4128, U.S. Geological Survey, 
Sacramento, California, 1986. 

Vanoni, Vito A., ed. (1975), Sedimentation Engineering, American Society of 
Civil Engineers -Manuals and Reports on Engineering Practice, No. 54, 
New York, New York, 1975. 

Whitehouse, I.E.; McSaveney, M.J. (1990) Geomorphic appraisals for developments 
on two steep, active alluvial fans, Mt. Cook, New Zealand, Alluvial 
Fans: A Field Approach, ed. by A. H. Rachoki and M. Church, John Wiley 
& Sons, 1990. 

76 Chapter 4. REFERENCES 


Appendix A

GLOSSARY


Appendix A. GLOSSARY

AGGHADATION The geologic process by which stream beds, floodplains, 
and the bottoms of other water bodies are raised in elevation by the 
deposition of material eroded and transported from other areas. It is the 
opposite of degradation, 

ALGORITHM A procedure for solving a mathematical problem in a finite 
number of steps that frequently involves repetition of an operation. A step 
by step procedure for solving a problem or accomplishing an end. A set of 
numerical steps or routines to obtain a numerical output from a numerical 
input. 

ALUVIAL CHANNEL OR STREAM A stream which flows within bed and 
banks that are composed of appreciable quantities of the material which it 
transports. Such streams are sometimes referred to as authors of their 
own geometry because of the interaction between the streams flow 
characteristics (hydraulic and hydrologic) and its shape, both plan form 
and cross sectional. 

ALLUVIAL DEPOSfT Clay, siit, sand, gravel, or other sediment deposited by 
the action of running or receding water. 

ALLUVIAL FAN A conical, or fan shaped deposit at the base of a mountain 
range where the mountain stream encounters the lesser slope of the valley 
floor. The deposits are generally coarse and alluvial fans most often occur 
in arid and semi-arid regions where streamflow is ephemeral and 
vegetation cover sparse. 

ALLUVIAL FAN FLOODING Alluvial fan flooding is flooding that occurs on the 
surface of an alluvial fan or similar landform which originates at the apex 
and is characterized by high-velocity flows; active processes of erosion, 
sediment transport, and deposition: and unpredictableflow paths. Some of 
the flood hazards associated with alluvial fan flooding are flash flooding, 
unpredictable flow paths, and a high velocity of flow coupled with the 
material of the landforms being highly susceptible to erosion. 

Appendix A. GLOSSARY 


ALLUVIUM A general term for all detrital deposits resulting directly or 
indirectly from the sediment transported by (modern) streams, thus 
including the sediments laid down in river beds, floodplains, lakes, fans, 
and estuaries. 

ANCHOR A series of bolts or metal straps used to secure a structure to its 
footings or foundation wall so that it will not be displaced by flood or wind 
forces. 

APEX A point on an alluvial fan or similar landform below which the flow-path 
of the major streams that formed the fan becomes unpredictable and 
alluvial fan flooding occur. 

ARMORING The process of progressive coarsening of the bed layer by 
removal of fine particles until it becomes resistant to scour. The coarse 
layer that remains on the surface is termed the armor layer. Armoring is a 
temporary condition; higher flows may destroy an armor layer and it may 
re-form as flows decrease. 

AVULSION A rapid change in channel direction and form that occurs during 
catastrophic, rare floods. 

A-ZONE See Special Flood Hazard Area. 

BACKFLOW VALVE See Check Valve 

BANK MIGRATION Lateral or horizontal movement of the banks of a 
streamcourse. 

BASE FLOOD EVALUATION (BFE) The evaluation for which there is a l-
percent chance in any given year that flood levels will equal or exceed it. 
The BFE is determined by statistical analysis for each local area and 
designated on the Flood Insurance Rate Maps. It is also known as the 
1 00-year Flood 

BED FORMS Irregularities found on the bottom (bed) of a stream that are 
related to flow characteristics. They are given names such as dunes, 
ripples, and antidunes. They are related to the transport of sediment 
and interact with the flow because they change the roughness of the 
stream bed. An analog to stream bed forms are desert sand dunes 
(although the physical mechanisms for their creation and movement may 
be different). 

Appendix A. GLOSSARY 


BED LOAD Material moving on or near the stream bed by rolling, sliding, and 
sometimes making brief excursions into the flow a few diameters above the 
bed, i.e. jumping. The term saltation is sometimes used in place of 
jumping. Bed load is bed material that moves in continuous contact with 
the bed; contrast with SUSPENDED LOAD. 

BED LOAD DISCHARGE The quantity of bed load passing a cross section in 
a unit of time, i.e. the rate. Usually presented in units of tons per day. May 
be measured or computed. See BED LOAD. 

BED MATERIAL The sediment mixture of which the moving bed is composed. 

BED MATERIAL DISCHARGE OR LOAD The total rate (tons/day) at which bed 
material is transported by a given flow at a given location on a stream. It 
consists bed material moving as both bed load and suspended load. 

BED ROCK A general term for the rock, usually solid, that underlies soil or 
other unconsolidated, surficial material. 

BERM A bank or mound of earth, usually placed against a foundation wall. 

BORROW AREA An area where material has been excavated for use as fill at 
another location. 

BOUNDARY ROUGHNESS The roughness of the bed and banks of a stream 
or river. The greater the roughness, the greater the frictional resistance to 
flows; and, hence, the greater the water surface elevation for any given 
discharge. 

BRAIDED CHANNEL A stream that is characterized by relatively shallow 
interlaced channels divided by islands or bars. Bars which divide the 
stream into separate channels at low flows are often submerged at high 
flow. 

BREAKAWAY WALLS Walls enclosing the are below an elevated structure 
that are designed to break away before transmitting damaging forces to the 
structure and its foundation. Breakaway walls are required by NFIP 
regulations in coastal high-hazard areas (V-Zones) and are recommended 
in areas where flood waters could flow at significant velocities (usually 
greater than four feet per second) or could contain ice or other debris. 

BUILDING CODE Regulations adopted by local governments that establish 
standards for construction, modification, and repair of buildings and other 
structures. 

Appendix A. GLOSSARY 


CAULKING Material used to fill joints in a structure, such as around windows 
or doors. 

CHANNEL A natural or artificial waterway which periodically or continuously 
contains moving water. 

CHANNEL STABILIZATION A stable channel is neither progressively 
aggrading nor degrading, or changing its cross sectional area through 
time. It could aggrade or degrade slightly, but over the period of a year, 
the channel would remain similar in shape and dimensions and position to 
previous times. Unstable channels are depositing or eroding in response 
to some exterior conditions. Stabilization techniques consist of bank 
protection and other measures that work to transform an unstable channel 
into a stable one. 

CHECK VALVE A type of valve that allows water to flow one way, but 
automatically closes when water attempts to flow the opposite direction. 

CLAY See TABLE A-l, 

CLOSURE A shield made of strong material, such as steel, aluminum or 
plywood, used to temporarily fill in gaps in floodwalls, levees, or sealed 
structures that have been left open for day-to-day convenience at entrances 
such as doors and driveways. 

COASTAL HIGH-HAZARD AREA Designated as V-Zone on Flood Insurance 
Rate Maps, this is that portion of the coastal floodplain subject to storm 
driven velocity waves of three feet or more in height. 

COBBLES See TABLE A-1. 

COHESIVE SEDIMENTS Sediments whose resistance to initial movement or 
erosion is affected mostly by cohesive (chemical) bonds between particles. 

COLUMN Upright support units for a building set in predug holes and 
backfilled with compacted material. Columns will often require bracing in 
order to provide adequate support. They are also known as posts, 
although they are usually of concrete or masonry construction. 

CONCENTRATION OF SEDIMENT The dry weight of sediment per unit 
volume of water-sediment mixture, i.e. mg/l. (Note: In earlier writings, 
concentration was calculated as the ratio of the dry weight of sediment in a 
water-sediment mixture to the total weight of the mixture multiplied by 
l,OOO,OOO. It was expressed as parts per million, i.e. ppm. Either method 
gives the same result, within 1 percent, for concentrations up to 16,000 

Appendix A. GLOSSARY 


mg/l. A correction is needed for concentrations in excess of that value.) 
The conversion to mg/l (milligrams per liter) from ppm (parts per million) is 
as follows: 

weight of sediment x1,OOO,OWmg// = Kx Gopni)= Kx 
weight of water-sediment mixture

where K is a correction factor. 

CONCRETE MASONRY UNIT (CMU) Blocks of concrete used in construction. 

CONSOUDATION The compaction of deposited sediments caused by grain 
reorientation and by the squeezing out of water trapped in the pores. 

CONVEVANCE A measure of the carrying capacity of the channel section. 
Flow is directly proportional to conveyance for steady flow. From 
Mannings equation, the proportionality factor is the square root of the 
energy slope. 

CRAWL SPACE Low space below the first floor of a house, where there has 
not been excavation deep enough for a basement, but where there is often 
access for pipes, ducts, and utilities. 

CRITICAL DEPTH If discharge is held constant and the water depth allowed 
to decrease, as in the case of water approaching a free overfall, velocity 
head will increase, pressure head will decrease, and total energy will 
decrease toward a minimum value where the rate of decrease in the 
pressure head is just counterbalanced by the rate of increase in velocity 
head, This is the critical depth. More generally, the critical depth is the 
depth of flow that would produce the minimum total energy head, and it 
depends on cross section geometry and water discharge. 

CRITICAL FLOW The state of flow where the water depth is at the critical 
depth and when the inertial and gravitational forces are equal. 

CRITICAL SHEAR OR TRACTNE FORCE The critical tractive force is the 
maximum shear force on the bed that will not cause movement of the 
material forming the channel bed on a level surface. 

CROSS SECTION Depicts the shape of the channel in which a stream flows. 
Measured by surveying the stream bed elevation across the stream on a 
line perpendicular to the flow. Necessary data for the computation of 
hydraulic and sediment transport information. 

Appendix A. GLOSSARY 


DEBRIS IMPACT LOADS Sudden loads induced on a structure by debris 
carried by flood water, Though difficult to predict, allowances for impact 
loads must be made when floodproofing a structure. 

DEGRADATION The geologic process by which stream beds, floodplains, 
and the bottoms of other water bodies are lowered in elevation by the 
removal of material from the boundary. It is the opposite of aggradation. 

DEPOSITION Raising of the stream bed by settlement of moving sediment 
that may be due to local changes in the flow, or during a single flood 
event. 

DEBRIS FLOW A mass movement of large size material such as boulders 
with little water visible. They are characterized by a steep front several feet 
high and typically move in surges down an alluvial fan. 

DISTRIBUTARIES Diverging streams which do not return to the main stream, 
but discharge into another stream or the ocean. 

DOMINANT DISCHARGE A particular magnitude of flow which is sometimes 
referred to as the channel forming discharge. Empirical relations have 
been developed between equilibrium stream width, depth, and slope and 
dominant discharge. It has been variously defined as the bank full flow, 
mean annual discharge, etc. 

DRY FLOODPROOFING A floodproofing method used in areas of low level 
flooding to completely seal a home against water. 

ELEVATION The raising of a structure to place it above flood waters on an 
extended support structure. 

ENTRAINMENT The carrying away of the material produced by erosive action 
from bed and banks. 

EPHEMERAL Existing or continuing for a short time; transitory or temporary. 

EQUIUBRIUM LOAD The amount of sediment that a system can carry for a 
given discharge without an overall accumulation (deposit) or scour 
(degradation). 

EROSION The wearing away of the land surface by detachment and 
movement of soil and rock fragments through the action of moving water 
and other geological agents. 

Appendix A. GLOSSARY 


EXISTING CONSTRUCTION The structures already existing or under 
construction prior to the effective date of a communitys floodplain 
management regulations. 

FALL VELOCITY The falling or settling rate of a particle in a given medium. 

FEDERAL EMERGENCY MANAGEMENT AGENCY (FEMA) This agency was 
created in 1978 to provide a single point of accountability for all federal 
activities related to disaster mitigation and emergency preparedness and 
response. 

FEDERAL INSURANCE ADMINISTRATION (FIA) The governmental unit, a part 
of the Federal Emergency Management Agency, that administers the 
National Flood Insurance Program. 

FILL Materials such as earth, clay, or crushed stone which is dumped in an 
area and compacted to increase ground elevation. 

FLASH FLOOD A flood that crests in a short length of time and is often 
characterized by high velocity flow. It is often the result of heavy rainfall in 
a localized area. 

FLOOD (For NFIP flood insurance policies) A partial or complete inundation 
of normally dry land areas from 1) the overland flood of a lake, river, 
stream, ditch, etc; 2) the unusual and rapid accumulation of runoff of 
surface waters; and 3) mudflows or the sudden collapse of shoreline land. 

FLOOD FRINGE That portion of the floodplain that lies beyond the floodway 
and serves as a temporary storage area for flood waters during a flood. 
This section receives waters that are shallower and of lower velocities than 
those of the floodway. 

FLOOD HAZARD BOUNDARY MAP (FHBM) The official map of a community 
that shows the boundaries of the floodplain and special flood hazard areas 
that have been designated. It is prepared by FEMA using the best flood 
data available at the time a community enters the emergency phase of the 
NFIP. It is superseded by the FIRM after a more detailed study has been 
completed. 

FLOOD INSURANCE RATE MAP (FIRM) The official map of a community 
prepared by FEMA that shows the Base Flood Elevation, along with the 
special hazard areas and the risk premium zones for flood insurance 
purposes. Once it has been accepted, the community is part of the regular 
phase of the NFIP. 

Appendix A. GLOSSARY 


FLOOD INSURANCE STUDY (FIS) A study performed by any of a variety of 
agencies and consultants to delineate the special flood hazard areas, base 
flood elevations and risk premium zones. The study is funded by FEMA 
and is based on detailed site surveys and analysis of the site-specific 
hydrologic characteristics. 

FLOODPWN Normally dry land adjacent to a body of water, such as a river, 
stream, lake, or ocean, which is susceptible to inundation by floodwaters. 

FLOODPWN MANAGEMENT A program of corrective and preventive 
measures for reducing flood damage, including but not limited to flood 
control projects, floodplain land use regulations, floodproofing or retrofitting 
of buildings, and emergency preparedness plans. 

FLOODPROOFING Any combination of measures taken on a new or existing 
structure for reducing or eliminating flood damages. 

FLOOD ROUTING The process of tracing, by calculation, the course and 
character of a flood as it progresses through a river reach or a reservoir. 

FLOODWALL A constructed barrier of resistant material, such as concrete or 
masonry block, designed to keep water away from a structure. 

FLOODWAY The central portion of the floodplain that carries the greatest 
portion of the waterflow in a flood. Obstructions in the floodway will result 
in increased flood levels upstream. 

FLOW DURATION CURVE A measure of the range and variability of a 
streams flow. The flow duration curve represents the percent of time 
during which specified flow rates are exceeded at a given location. This is 
usually presented as a graph of flow rate (discharge) vs. percent of time 
that flows are greater than, or equal to, that flow. 

FOOTING The enlarged base of a foundation, wall, pier, or column, designed 
to spread the load of the structure so that it does not exceed the soil 
bearing capacity. 

FOUNDATION The underlying structure of a building, usually constructed of 
concrete, that supports the foundation walls, piers, or columns, 

FOUNDATION WALLS A support structure that connects the foundation to the 
main portion of the building, or superstructure. 

FREEBOARD An additional amount of height used as a factor of safety in 
determining the design height of a floodproofing or retrofitting method to 

A-l 0 Appendix A. GLOSSARY 


compensate for unknown factors such as wave action. Certain guidelines 
and restrictions apply for establishing freeboard on levees and floodwalls in 
NFIP areas. 

FREQUENCY The number of repetitions of a periodic process in a certain 
time period. 

GEOLOGIC CONTROL A local rock formation or clay layer that limits (within 
the engineering time frame) the vertical and/or lateral movement of a 
stream at a particular point. Note that man-made controls such as drop 
structures also exist. 

GEOLOGY A science that deals with the history of the earth and its life, 
especially as recorded in rocks. 

GEOMORPHOLOGY The study of landform development under processes 
associated with running water. 

GRADATION The proportion of material of each particle size, or the frequency 
distribution of various sizes, constituting a particulate material such as a 
soil, sediment, or sedimentary rock. The limits of each size are chosen 
arbitrarily. Four different gradations are significant: the gradation of the 
suspended load, the gradation of the bed load, the gradation of the 
material comprising the bed surface, and the gradation of material beneath 
the bed surface. 

GRADATION CURVE 

Sediment samples 
usually contain a range 
of grain sizes, and it is 
customary to break this 
range into classes of 
percentages of the total 
sample weight 
contained in each 
class. After the 
individual percentages 
are accumulated, a 
graph, the gradation 
curve, shows the grain 
size vs. the 
accumulated percent of material that is finer than that grain size. These 
curves (see example above) are used by movable boundary models to 
depict the bed sediment material properties (e.g., grain size distribution of 
the bed material). 

Appendix A. GLOSSARY A-l 1 


GRAIN SHAPE FACTOR See PARTICLE SHAPE FACTOR. 

GRAIN SlZE See PARTICLE SIZE. 

GRAlN SlZE DISTRIBUTION (GRADATION) A measure of the variation in 
grain (particle) sizes within a mixture. Usually presented as a graph of 
grain diameter vs. percent of the mixture that is finer than that diameter. 

GRAVEL See TABLE A-i. 

HUMAN INTERVENTION The required presence and active involvement of 
people to enact any type of floodproofing or retrofitting measure prior to 
flooding. 

HYDRAULIC MODEL A physical scale model of a river used for engineering 
studies. 

HYDRAULICS The study and computation of the characteristics, e.g. depth 
(water surface elevation), velocity and slope, of water flowing in a stream or 
river. 

HYDRODYNAMIC LOADS Forces imposed on an object, such as a structure, 
by water moving around it. Among these loads are positive frontal 
pressure, against the structure; drag effect, along the sides; and negative 
pressure on the downstream side. 

HYDROGRAPH A graph showing, for a given point on a stream or conduit, 
the discharge,water surface elevation, stage, velocity, available power, or 
other property of water with respect to time. 

HYDROLOGY The study of the properties, distribution, and circulation of 
water on the surface of the land, in the soil, and in the atmosphere. 

HYDROSTATIC LOADS Forces imposed on a surface, such as a wall or floor 
slab, by a standing mass of water. The water pressure increases with the 
square of the water depth. 

INCIPIENT MOTION The flow condition at which a given size bed particle just 
begins to move. Usually related to a threshold shear stress. 

INFLOWING LOAD CURVE See SEDIMENT RATING CURVE. 

IN SITU In (its original) place. 

A-l 2 Appendix A. GLOSSARY 


INTERIOR GRADE BEAM A section of a floor slab that has a thicker section 
of concrete to act as a footing to provide stability under load-bearing or 
critical structural walls. 

LEVEE A barrier of compacted soil designed to keep flood water away from a 
structure. 

LIFT A layer of soil that is compacted before the next layer is added in the 
construction of a fill pad or levee. 

LOCAL SCOUR Erosion caused by an abrupt change in flow direction or 
velocity. Examples include erosion around bridge piers, downstream of 
stilling basins, at the ends of dikes, and near snags. 

MANNINGS EQUATION The empirical Mannings equation commonly applied 
in water surface profile calculations defines the relationship between 
surface roughness, discharge, flow geometry, and rate of friction loss for a 
given stream location. 

MANNINGS n-VALUE The coefficient of roughness with the dimensions of T x 
L.It accounts for energy loss due to the friction between the bed and 
the water. In fluvialhydraulics (movable boundary hydraulics), the 
Mannings n value includes the effects of all losses, such as grain 
roughness of the movable bed, form roughness of the bed, bank 
irregularities, vegetation, bend losses, and junction losses. Contraction 
and expansion losses are not included in Mannings n, but are typically 
accounted for separately. 

MATHEMATICAL MODEL A model that uses mathematical expressions (i.e., a 
set of equations, usually based upon fundamental physical principles) to 
represent a physical process. 

MEANDERING STREAM An alluvial stream characterized in planformby a 
series of pronounced alternating bends. The shape and existence of the 
bends in a meandering stream are a result of alluvial processes and not 
determined by the nature of the terrain (geology) through which the stream 
flows. 

MEAN SEA LEVEL The average height of the sea for all stages of the tide, 
usually determined from hourly height observations over a I9-yearperiod 
on an open coast or in adjacent waters having free access to the sea. 

Appendix A. GLOSSARY A-l 3 


MITIGATION To make restitution for adverse project impacts. 

MODEL A representation of a physical process or thing that can be used to 
predict the processs or things behavior or state. 

Examples: 
A conceptual model: If I throw a rock harder, it will go faster. 
A mathematical model: F=ma 
A hydraulic model: Columbia River physical model. 

MOVABLE BED MODEL Model in which the bed and/or side material is 
erodible and transported in a manner similar to the prototype. 

MUD FLOW Debris laden water originating on a steep slope carrying such 
large concentrations of sediment, particularly sands and finer sizes, that it 
forms a fluid much denser than water and is capable of transporting 
boulders which are buoyed up by the viscous flow. 

NATIONAL FLOOD INSURANCE PROGRAM (NFIP) The federal program, 
created by an act of Congress in 1968, that makes flood insurance 
available in communities that enact satisfactory floodplain management 
regulations. 

NORMAL DEPTH The depth that would exist if the flow were uniform is called 
normal depth. 

NUMERICAL MODEL A numerical model is the representation of a 

mathematical model as a sequence of instructions (program) for a 
computer. Given approximate data, the execution of this sequence of 
instructions yields an approximate solution to the set of equations that 
comprise the mathematical model. 

ONE-DIMENSIONAL ENERGY EQUATION This equation has the same form 
as the Bernoulli Equation and the same terms are present. In addition, an 
cx term has been added to correct for velocity distribution. 

ONE HUNDRED (100) YEAR FLOOD The flood elevation that has a i-percent 
chance of being equal or exceeded in any given year. It is also known as 
the Base Flood elevation. (See Base Flood Elevation.) 

Appendix A. GLOSSARY 


OVERBANK In a river reach, the surface area between the bank on the main 
channel and the limits of the floodplain. See figure below: 

Examples of Overbanks 
PARAMETER Any set of physical properties whose values determine the 
characteristics or behavior of something. 
PARTICLE SHAPE FACTOR The particle shape factor of a perfect sphere is 

1 .O and can be as low as 0.1 for very irregular shapes. It is defined by: 
where: 

a,b,c = the lengths of the longest, intermediate, and 
shortest, respectively, mutually perpendicular axes 
on a sediment particle. 

PARTICLE SIZE A linear dimension usually designated as diameter, used to 
characterize the size of a panicle. The dimension may be determined by 
any of several different techniques, including sedimentation sieving, 
micrometric measurement, or direct measurement. 

PERMEABILITY The property of a soil that permits the passage of water 
under a gradient of force. 

PIER An upright support member of a building, with a height limited to a 
maximum of three times its least lateral dimension. It is designed and 
constructed to function as an independent structural element in supporting 
and transmitting building and environmental loads to the ground. 

Appendix A. GLOSSARY A-l 5 


PILE An upright support member of a building, usually long and slender in 
shape, driven into the ground by mechanical means and primarily 
supported by friction between the pile and the surrounding earth. Piles 
often cannot act as individual units, and require bracing to other pilings. 

PLANFORM The shape and size of channel and overbank features as viewed 
from directly above. 

POST Long upright support units for a building, set in predug holes and 
backfilled with compacted material. Each post usually requires bracing to 
other units. They are also known as columns, although they are usually 
made of wood. 

PROTOTYPE The full-sized structure, system process, or phenomenon being 
modeled. 

QUALITATIVE Relating to or involving quality or kind. 

QUANTITATIVE A specific measurement of a quantity or amount. 

RATING CURVE See STAGE-DISCHARGE CURVE. 

REACH (1) The length of a channel, uniform with respect to discharge, 
depth, area, and slope, e.g., study reach, typical channel reach or 
degrading reach, etc. (2) The length of a stream between two specified 
gaging stations. 

REGULATORY FLOODWAY As referenced in a floodplain management 
ordinance, this is the portion of the floodplain needed to discharge the IOO-
year flood without increasing the flood elevation by more than a designated 
height, usually one foot. 

RELOCATION The moving of a structure from a flood area to a new location, 
normally to one where there is no threat of flooding. 

REPLICATE To duplicate (a statistical experiment). 

RETROFITTING Floodproofing measures taken on an existing structure. 

RETROFLOODPROOFING See Retrofitting 

RIPPLE Small triangular-shaped bed forms that are similar to dunes but have 
much smaller heights and lengths of 0.3 m or less. They develop when the 
Froude number is less than approximately 0.3. 

Appendix A. GLOSSARY 


RIPRAP Broken stone, cut stone blocks, or rubble that is placed on slopes to 
protect the from erosion or scouring caused by flood waters or wave 
action. 

ROUTING MODEL A model (see MATHEMATICAL MODEL and NUMERICAL 
MODEL) for performing flood routing (see FLOOD ROUTING). 

SAND See TABLE A-l. 

SATURATION The degree to which voids in soil are filled with water. 

SCOUR The enlargement of a flow section by the removal of boundary 
material through the action of the fluid in motion. 

SECONDARY CURRENTS (OR FLOW) The movement of water particles on a 
cross section normal to the longitudinal direction of the channel. 

SEDIMENT (1) Particles derived from rocks or biological materials that have 
been transported by a fluid. (2) Solid material (sludges) suspended in or 
settled from water. A collective term meaning an accumulation of soil, rock 
and mineral particles transported or deposited by flowing water. 

SEDIMENTATION A broad term that pertains to the five fundamental process 
responsible for the formation of sedimentary rocks: (1) weathering, (2) 
detachment, (3) transportation, (4) deposition (sedimentation), and (5) 
diagenesis; and to the gravitational settling of suspended particles that are 
heavier than water. 

SEDIMENTATION DlAMETER The diameter of a sphere of the same specific 
weight and the same terminal settling velocity as the given particle in the 
same fluid. 

SEDIMENT DISCHARGE The mass or volume of sediment (usually mass) 
passing a stream cross section in a unit of time. The term may be 
qualified, for example; as suspended-sediment discharge, bed load 
discharge, or total-sediment discharge. See SEDIMENT LOAD. 

SEDIMENT LOAD A general term that refers to material in suspension and/or 
in transport. It is not synonymous with either discharge or concentration, 
It may also refer to a particular type of load; e.g. total, suspended, wash, 
bed, or material. 

SEDIMENT PARTICLE Fragments of mineral or organic material in either a 
singular or aggregate state. 

Appendix A. GLOSSARY A-l 7 


SEDIMENT RATING TABLES Tables which relate inflowing sediment loads to 
water discharge for the upstream ends of the main stem, tributaries, and 
local inflow points. 

SEDIMENT TRANSPORT (RATE) See SEDIMENT DISCHARGE. 

SEDIMENT TRANSPORT FUNCTION A formula or algorithm for calculating 
the sediment transport rate given the hydraulics and bed material at a 
cross section. Most sediment transport functions compute the bed material 
load capacity. The actual transport may be less than the computed 
capacity due to armoring, geologic controls, etc. 

SEDIMENT TRANSPORT ROUTING The computation of sediment movement 
for a selected length of stream (reach) for a period of time with varying 
flows. Application of sediment continuity relations allow the computation of 
aggradation and deposition as functions of time. 

SEDIMENT TRAP EFFICIENCY See TRAP EFFICIENCY. 

SETTLlNG VELOCITY See FALL VELOCITY. 

SHAPE FACTOR See PARTICLE SHAPE FACTOR 

SHEAR INTENSlTY A dimensionless number that is taken from Einsteins bed 
load function. It is the inverse of Shields parameter. 

SHEAR STRESS Frictional force per unit of bed area exerted on the bed by 
the flowing water. An important factor in the movement of bed material. 

SHIELDS DETERMINISTIC CURVE A curve of the dimensionless tractive force 
plotted against the grain Reynolds number. For example, 

U.
DJv

where: 
u*= turbulent shear velocity 
D,= characteristic or effective size of the grains or 
roughness elements 
V kinematic viscosity) and which is used to help 
determine the CRITICAL TRACTIVE FORCE. 

A-l 8 Appendix A. GLOSSARY 


SHIELDS PARAMETER A dimensionless number referred to as a 
dimensionless shear stress. The beginning of motion of bed material is a 
function of this dimensionless number. 

where: 
TC= critical tractive force 
*Is= specific weight of the particle 
Yspecific weight of water 
D,= characteristic or effective size of the grains or 

roughness elements 

SIEVE DIAMETER The smallest standard sieve opening size through which a 
given particle of sediment will pass. 

SILT See TABLE A-l. 

SILTATION An unacceptable term. Use sediment deposition, sediment 
discharge, or sediment yield as appropriate. 

SIMULATE To express a physical system in mathematical terms, 

SINUOSITY A measure of meander intensity. Computed as the ratio of the 
length of a stream measured along its thalweg (or centerline) to the length 
of the valley through which the stream flows. 

SLAB ON GRADE A structural design where the first floor sits directly on a 
poured concrete slab which sits directly on the ground. 

SORTING The dynamic process by which sedimentary particles having some 
particular characteristic (such as similarity of size, shape, or specific 
gravity) are naturally selected and separated from associated but dissimilar 
particles by the agents of transportation. Also, see GRADATION. 

SPECIAL FLOOD HAZARD AREA Portion of the floodplain subject to the IOO-
year flood, also known as the A-Zone. In coastal regions, this area is 
subject to velocity wave action of less than three feet. 

Appendix A. GLOSSARY A-l 9 


n 
(4 b) 
Split Flow Examples 
SPLIT FLOW Flow that I 
leaves the main river flow 
and takes a completely 
different path from the 
main river [Case (a)]. Split 
flow can also occur in the 
case of flow bifurcation 
around an island [Case 

WI. 

STABLE CHANNEL A stream 
channel that does not 
change in platform or bed 
profile during a particular 
period of time. For 
purposes of this glossary 
the time period is years to 
tens of years. 

STAGE-DISCHARGE (RATING) CURVE Defines a relationship between 
discharge and water surface elevation at a given location. 

STEADY STATE MODEL Model in which the variables being investigated do 
not change with time. 

STILE A set of stairs to allow access over an obstruction, such as a floodwall. 

STREAM GAGE A device that measures and records flow characteristics such 
as water discharge and water surface elevation at a specific location on a 
stream. Sediment transport measurements are usually made at stream 
gage sites. 

STREAM POWER The product of bed shear stress and mean cross-sectional 
velocity at a cross section for a given flow. 

STREAM PROFILE A plot of the elevation of a stream bed vs. distance along 
the stream. 

STRUCTURAL MAT SLAB The concrete slab of a building which includes 
structural reinforcement to help support the buildings structure. 

SUBCRITICAL FLOW The state of flow where the water depth is above the 
critical depth, Here, the influence of gravity forces dominate the influences 
of inertial forces, and flow, having a low velocity, is often described as 
tranquil. 

Appendix A. GLOSSARY 


SUBSTANTML IMPROVEMENT Any repair, reconstruction, or improvement of 
a structure, the cost of which equals or exceeds 50 percent of the market 
value of the structure either: a)before the improvement is started, or b) if 
the structure has been damaged and is being restored, before the damage 
occurred. 

SUPERCRITICAL FLOW The state of flow where the water depth is below the 
critical depth, inertial forces dominate the gravitational forces, and the flow 
is described as rapid or shooting. 

SUSPENDED BED MATERIAL LOAD That portion of the suspended load that 
is composed of particle sizes found in the bed material. 

SUSPENDED LOAD Includes both suspended bed material load and wash 
load. Sediment that moves in suspension is continuously supported in the 
water column by fluid turbulence. Contrast with BED LOAD. 

SUSPENDED-SEDIMENT DISCHARGE The quantity of suspended sediment 
passing a cross section in a unit of time usually given in tons/day. See 
SUSPENDED LOAD. 

TAIL WATER The water surface elevation downstream from a structure, such 
as below a dam, weir or drop structure. 

THALWEG The line following the lowest part of a valley, whether under water 
or not. Usually the line following the deepest part or middle of the bed or 
channel of a river. 

TOTAL SEDIMENT DISCHARGE The total rate at which sediment passes a 
given point on the stream (tons/day). See TOTAL SEDIMENT LOAD. 

TOTAL-SEDIMENT LOAD (TOTAL LOAD) Includes bed load, suspended bed 
material load, and wash load. In general, total sediment load cannot be 
calculated or directly measured. 

TRACTIVE FORCE When water flows in a channel, a force is developed that 
acts in the direction of flow on the channel bed. This force, which is simply 
the pull of water on the wetted area, is known as the tractive force. In a 
uniform flow, the equation for the unit tractive force (i.e., the average value 
to the tractive force per unit wetted area) is: 

where: 

Appendix A. GLOSSARY 


= unit tractive force
zzunit weight of water
the hydraulic radius
S

the slope of the channel. 

TRANSPORTATION (SEDIMENT) The complex processes of moving sediment 
particles from place to place. The principal transporting agents are flowing 
water and wind. 

TRANSPORT CAPACITY The ability of the stream to transport a given volume 
or weight of sediment material of a specific size per time for a given flow 
condition. The units of transport capacity are usually given in tons per day 
of sediment transported passed a given cross section for a given flow. 
Transport capacity for each sediment grain size is the transport potential for 
that size material multiplied by the actual fraction of each size class present 
in the bed and bank material. 

TRANSPORT POTENTIAL Transport potential is the rate at which a stream 
could transport sediment of a given grain size for given hydraulic 
conditions if the bed and banks were composed entirely of material of that 
size. 

TRAP EFFICIENCY Proportion of sediment inflow to a stream reach (or 
reservoir) that is retained within that reach (or reservoir). Computed as 
inflowing sediment volume minus outflowing sediment volume divided by 
inflowing sediment volume. Positive values indicate aggradation; negative 
values, degradation. 

TRIBUTARY A river segment other than the main stem in which sediment 
transport is calculated. More generally, a stream or other body of water, 
surface or underground, that contributes its water to another and larger 
stream or body of water, 

TURBULENCE In general terms, the irregular motion of a flowing fluid. 

UNMEASURED LOAD Equipment used to measure sediment transport by 
sampling the concentration of suspended sediment cannot operate close to 
the stream bed. The material moving below the lowest point which the 
sampler can reach is termed unmeasured load. 

VENTING A system designed to allow flood waters to enter an enclosure, 
usually the interior of the foundation walls, so that the rising water does not 
create a dangerous differential in hydrostatic pressure. This is achieved 
through small openings in the wall, such as a missing or rotated brick or 
concrete block, or small pipe. 

Appendix A. GLOSSARY 


V-ZONE See Coastal High Hazard Area 

WASH LOAD That part of the suspended load that is finer than the bed 
material. Wash load is limited by supply rather than hydraulics, What 
grain sizes constitute wash load varies with flow and location in a stream. 
Sampling procedures that measure suspended load will include both wash 
load and suspended bed material load. Normally, that is of sediment 
particles smaller than 0.062 mm. 

WATER COLUMN An imaginary vertical column of water used as a control 
volume for computational purposes. Usually the size of a unit area and as 
deep as the depth of water at that location in the river. 

WATER DISCHARGE See STREAM DISCHARGE. 

WATERSHED A topographically defined area drained by a river/stream or 
system of connecting rivers/streams such that all outflow is discharged 
through a single outlet. Also called a drainage area. 

WEIR A small dam in a stream, designed to raise the water level or to divert 
its flow through desired channel. A diversion dam. 

TABLE A-l 
Scale for Size Classification of Sediment Particles

CLASS NAME MILLIMETERS MICRONS PHI VALUE 
Boulders 1 256 < -0 
Cobbles 256 64 -0to -6 
Gravel 64 2 ._-6 to -1 
Very coarse sand 2.0 1.0 2000 1000 -1 too 
Coarse sand 1.o 0.50 1000 500 oto+1Medium sand 0.50 0.25 500 250 i-1to +2Fine sand 0.25 0.125 250 125 +2to +3Very fine sand 0.125 0.062 125-62 +3to +4Coarse silt 0.062 0.031 62 31 +4to +5Medium silt 0.031 0.016 31 -16 +5to +6Fine silt 0.016 0.008 16 -a +6to +7Very fine silt 0.008 0.004 0-4+7to +6Coarse clay 0.004 0.0020 4-2 +a to +9Medium clay 0.0020 0.0010 2-1+9to +10Fine clay 0.0010 0.0005 10.5 +10to fllVery fine clay 0.0005 0.00024 0.5 0.24 +11to f12Colloids <0.00024 <0.24 >f12

TABLE A-l is taken from EM 111 o-2-4000, March 1988 

Appendix A. GLOSSARY GPO 583.075/10003

A-23