Guidelines and Specifications for Flood Hazard Mapping Partners - Appendix G: Guidance for Alluvial Fan Flooding Analyses and Mapping
Summary of Changes to Appendix G, 

Guidance for Alluvial Fan Flooding Analyses and 
Mapping 

The Summary of Changes below details changes to Appendix F that were made subsequent to 
the initial publication of these Guidelines in February 2002. These changes represent new or 
updated guidance for Flood Hazard Mapping Partners. 

 

Date 

Affected 
Section(s)/Subsection(s)

Description of Changes 

April 2003 

None 

No changes representing new or updated guidance 
were made. 

 


Appendix G 

Guidance for Alluvial Fan Flooding Analyses and 
Mapping 

G.1 Introduction [February 2002] 

Alluvial fans, and flooding on alluvial fans, show great diversity because of variations in 
climate, fan history, rates and styles of tectonism, source area lithology, vegetation, and land use. 
Acknowledging this diversity, the Federal Emergency Management Agency (FEMA) developed 
an approach that considers site-specific conditions in the identification and mapping of flood 
hazards on alluvial fans. This approach, summarized herein, was first documented in Guidelines 
for Determining Flood Hazards on Alluvial Fans. 

Investigation and analysis of the site-specific conditions may require knowledge in various 
disciplines, such as geomorphology, soil science, hydrology, and hydraulic engineering. 
Although the scope of study may constrain the degree of site-specific consideration undertaken, 
field inspections of the alluvial fan must be conducted. 

According to Section 59.1 of the National Flood Insurance Program (NFIP) regulations, the 
current definition of Alluvial Fan Flooding means 

flooding occurring 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, unpredictable 
flowpaths. 

FEMA will revise the current definition under Section 59.1 to be consistent with the approach 
described in this Appendix and specifically to eliminate reference to similar landforms. The 
process described in this Appendix is intended for flooding only on alluvial fans as described 
below. 

As interim guidance in the determination of similar landform, unless the landform under 
investigation meets the three criteria under Stage 1 for composition, morphology, and location, 
the landform is not considered to be similar. 

This Appendix provides guidance for the identification and mapping of flood hazards occurring 
on alluvial fans, irrespective of the level of fan forming activity. The term alluvial fan flooding 
encompasses both active alluvial fan flooding and inactive alluvial fan flooding. Each type of 
alluvial fan flooding is described below. 


Active alluvial fan flooding occurs only on alluvial fans and is characterized by flow path 
uncertainty so great that this uncertainty cannot be set aside in realistic assessments of flood risk 
or in the reliable mitigation of the hazard. 

An active alluvial fan flooding hazard is indicated by the following three related criteria: 

1. Flow path uncertainty below the hydrographic apex; 
2. Abrupt deposition and ensuing erosion of sediment as a stream or debris flow loses its 
ability to carry material eroded from a steeper, upstream source area; and 
3. An environment where the combination of sediment availability, slope, and topography 
creates an ultrahazardous condition for which elevation on fill will not reliably mitigate 
the risk. 

Inactive alluvial fan flooding is similar to traditional riverine flood hazards, but occurs only on 
alluvial fans. Inactive alluvial fan flooding is characterized by flow paths with a higher degree 
of certainty in realistic assessments of flood risk or in the reliable mitigation of the hazard. 
Unlike active alluvial fan flooding hazards, an inactive alluvial fan flooding hazard is 
characterized by relatively stable flow paths. However, like areas of active alluvial fan flooding, 
inactive alluvial fan flooding may be subject to sediment deposition and erosion, but to a degree 
that does not cause flow path instability and uncertainty. 

An alluvial fan may exhibit both active and inactive alluvial fan flooding hazards. The hazards 
may vary spatially or vary at the same location, contingent on the level of floodflow discharge. 
Spatially, for example, upstream inactive portions of the alluvial fan may distribute floodflow to 
active areas at the distal part of the alluvial fan. Hazards may vary at the same location, for 
example, with a flow path that may be stable for lower flows, but become unstable at higher 
flows. 

An example of an alluvial fan that exhibits both active and inactive alluvial fan flooding is 
depicted in Figure G-1. In this example, the area between the topographic apex and the 
hydrographic apex (apex definitions will be discussed below) would be considered inactive 
alluvial fan flooding because this reach is characterized by a stable, entrenched channel which 
can convey the 1-percent-annual-chance (100-year) flood discharge without overbank flooding. 
The area below the hydrographic apex would be considered active alluvial fan flooding because 
this area is characterized by flow path uncertainty, abrupt deposition, and ensuing erosion of 
sediment as the channel loses its competence to carry material eroded from a steeper, entrenched 
upstream source area. 


Figure G-1. Alluvial Fan With Entrenched Channel Leading to Active Deposition. 

Figure G-1. Alluvial Fan With Entrenched Channel Leading To Active Deposition 
at Distal Part of the Fan. Original Published as Figure 3-2 in Alluvial Fan Flooding 
(National Research Council, 1996). Reproduced with Permission From the 
National Research Council; Annotations Added by FEMA. 

 


G.2 Analysis Approach [February 2002] 

Through the approach for alluvial fan flooding identification and mapping documented herein, 
FEMA seeks to identify whether (1) the area under study is an alluvial fan and (2) which 
portions of this area, if any, are characterized by or subject to active alluvial fan flooding. After 
these steps, various methods unique to different situations can be employed to analyze and 
define the 1-percent-annual-chance (100-year) flood within the areas of alluvial fan flooding 
identified on the alluvial fan. Thus, the approach for the identification and mapping of alluvial 
fan flooding can be divided into three stages. 

Stage 1Recognizing and characterizing alluvial fan landforms; 
Stage 2Defining the nature of the alluvial fan environment and identifying active and 
inactive areas of the fan; and 
Stage 3Defining and characterizing the 1-percent-annual-chance (100-year) flood 
within the defined areas. 

Each of these stages is described in detail in this Appendix. Additional information also can be 
found in a National Research Council report entitled Alluvial Fan Flooding (National Research 
Council, 1996). 

Each stage must be addressed and thoroughly documented during the analysis process. Because 
each stage builds on the previous stage and because of the complexity of many alluvial fans, the 
Mapping Partner who undertakes the analysis and mapping of alluvial fan flooding must 
coordinate closely with the FEMA Regional Project Officer (RPO) and FEMA Headquarters 
(HQ) from the onset of the study. The progression of the process is shown in Figure G-2. 

Progression through each of the stages results in a procedure that narrows or divides the problem 
to smaller and smaller areas. In Stage 1, the landform on which the flooding occurs must be 
characterized. If the location of study is an alluvial fan, the Mapping Partner proceeds to Stage 2 
to identify which parts of the alluvial fan are active or inactive. Finally, in Stage 3, the Mapping 
Partner performing the analysis must use various methods to define and analyze the 1-percent-
annual-chance (100-year) flood within each identified area of alluvial fan flooding. Progression 
through these stages requires a variety of maps and photographs, as well as a significant amount 
of field work and analysis to fully understand the flood hazard. The Mapping Partner may need 
to consult with geologists, geomorphologists, and/or soil scientists during each stage. 


 

Stage 1: 
Recognizing and Characterizing Alluvial Fan Landforms
* Is the landform a sedimentary deposit composed of alluvium or debris-flow deposits? (Refer to surficial geologic and soils maps.)
* Does the landform have the shape of a fan? (Refer to topographic maps.)
* Is the landform located at a topographic break? (Refer to topographic maps.)
* Where are the lateral boundaries of the fan? (Refer to topographic and soils maps, surficial geologic maps, and aerial photographs.)
Stage 2:
Defining Active and Inactive Areas of Erosion and Deposition
* What parts of the alluvial fan are still active?
* What parts are inactive but subject to flooding? (Refer to aerial photographs, topographic and soils maps, surficial geologic maps, and historical records in a preliminary assessment to plan a more detailed field investigation.)
Stage 3:
Defining the 100-Year Flood Within the Defined Areas
* Determine method of analysis (deterministic, probabilistic or geomorphic) based on assumptions, limitations and recommended applications.
* To what extent and degree is alluvial fan flooding occurring within the defined areas? (Refer to recent aerial photographs, topographic and soils maps, historical records, and detailed field mapping to support analysis.) 

 

Figure G-2. Three Stages of the Process To Identify and Map Alluvial Fan 
Flooding. Original Published in National Research Council, 1996, Figure 3-1; 
Amended by FEMA. 

 

 

 

 

 


G.2.1 Stage 1: Recognizing and Characterizing Alluvial Fan 
Landforms [February 2002] 

As defined in this Appendix, alluvial fan flooding occurs only on alluvial fans. Therefore, the 
first stage of the process is to determine whether the landform in question is an alluvial fan. If, 
after following the guidelines in this subsection, the Mapping Partner concludes that the 
landform is not an alluvial fan, then the methods described in this Appendix are not intended for, 
or necessarily applicable to, the landform in question. 

An alluvial fan is a sedimentary deposit located at a topographic break such as the base of a 
mountain front, escarpment, or valley side, that is composed of streamflow and/or debris flow 
sediments and has the shape of a fan, either fully or partially extended. These characteristics can 
be categorized by composition, morphology, and location as discussed in Subsections G.2.1.1, 
G.2.1.2, and G.2.1.3. 

G.2.1.1 Composition [February 2002] 

Alluvial fans are landforms constructed from deposits of alluvial sediments or debris flow 
materials. These deposits, alluvium, are an accumulation of loose, unconsolidated to weakly 
consolidated sediments. Alluvium refers to sediments transported by either streamflow or debris 
flows. Geologic maps and field reconnaissance can be used to determine whether the landform 
is composed of alluvium. 

G.2.1.2 Morphology [February 2002] 

Alluvial fans are landforms that have the shape of a fan, either partly or fully extended. Flow 
paths may radiate outward to the perimeter of the fan; however, drainage may exhibit a range of 
patterns such as dendritic, anastomosing, and distributary. Topographic maps and aerial photos 
can be used to assess this criterion. 

G.2.1.3 Location [February 2002] 

Alluvial fan landforms are located at a topographic break where long-term channel migration and 
sediment accumulation become markedly less confined than upstream of the break. This locus 
of increased channel migration and sedimentation is referred to as the alluvial fan apex. 

The topographic apex is at the extreme upstream extent of the alluvial fan landform. The 
hydrographic apex is the highest point on the alluvial fan where there exists physical evidence of 
channel bifurcation and/or significant flow outside the defined channel; its location may be 
either coincidental with, or at a point downstream of, the topographic apex as seen in Figure G-1. 
The hydrographic apex may depend on the discharge and may vary with the magnitude of the 
flooding event. 


 

G.2.1.4 Defining Toe and Lateral Boundaries [February 2002] 

The distal terminus, or toe, of an alluvial fan commonly is defined by: 

A stream that intersects the fan and transports deposits away from the fan; 
A playa lake; 
An alluvial plain; and 
Smoother, gentler slopes of the piedmont plain. 

Such boundaries can often be identified on topographic maps by changes in contour lines or 
identified on aerial photographs or by field inspection as changes in vegetation as a result of 
sediment changes or increased water table depth. 

Lateral boundaries of alluvial fans are the edges of deposited and reworked alluvial materials. 
The lateral boundary of a single alluvial fan typically is a trough, channel, or swale formed at the 
lateral limits of deposition. The lateral boundary also may be a confining mountainside. 

Lateral boundaries of single alluvial fans can often be identified as a contact of distinct 
differences between light-colored, freshly abraded, alluvial deposits and darker-colored, 
weathered deposits with well-developed soils on piedmont plains. Care should be taken to 
ensure that the contact is not simply a divide between older and more recent deposits of the 
alluvial fan. 

The lateral boundaries of alluvial fans that coalesce with adjacent alluvial fans are generally less 
distinct than those of single alluvial fans. These lateral boundaries may be marked by a 
topographic trough or ridge. It is sometimes possible to distinguish between surfaces of adjacent 
alluvial fans based on different source-basin rock types. Defining the lateral boundaries of 
coalescing fans will likely require additional fieldwork, use of surficial geologic and soils maps, 
and consultation with a geomorphologist or soil scientist. 

G.2.2 Stage 2: Defining Active and Inactive Areas [February 2002] 

During Stage 1, the Mapping Partner conducting the analysis identified whether the landform in 
question is an alluvial fan. During Stage 2, the Mapping Partner will seek to delineate areas of 
the alluvial fan that are active or inactive in the deposition, erosion, and unstable flow path 
flooding that builds alluvial fans. The activities in Stage 2 have been designed to narrow the 
area of concern for Stage 3, which is the specific identification of the extent of the 1-percent-
annual-chance (100-year) flood. 

 


Although active alluvial fan flooding has occurred on all parts of an alluvial fan at some time in 
the geologic past in order to construct the landform itself, this does not mean that all parts are 
equally susceptible to active alluvial fan flooding now. Also, flooding may be occurring on 
inactive areas of the alluvial fan. 

In most of the United States, it is possible to identify parts of alluvial fans that were actively 
constructed during the Pleistocene epoch (approximately 2 million to 10,000 years ago) and parts 
that have been active (i.e., flooded) during the Holocene epoch (the past 10,000 years). The 
reason that this broad distinction generally is possible is that the two epochs were identified and 
defined on the basis of climatic conditions. 

The Holocene epoch is a time of interglacial warm conditions, whereas the Pleistocene epoch 
was marked by repeated full glacial, cool conditions alternating with warm interglacials like that 
of the Holocene epoch. As a result of these climatic differences, flooding and sedimentation 
occurred at different rates and magnitudes during the Pleistocene and Holocene epochs. The 
impacts of these climatic changes on alluvial fan formation can be inferred from geologic, 
geomorphic, and soil data. 

A change in the rate of tectonic uplift along a mountain front can also result in abandonment of 
parts of alluvial fans. For example, a decrease in the rate of uplift at a mountain front relative to 
the alluvial fan could result in stream channel downcutting at the mountain front/alluvial fan 
apex over a period of time. As a consequence, the upper part of the fan would become 
entrenched, and the active area of deposition would shift downfan. 

G.2.2.1 Identification of Active Areas [February 2002] 

The term active refers to that portion of an alluvial fan where deposition, erosion, and unstable 
flow paths are possible. If flooding and deposition have occurred on a part of an alluvial fan in 
the past 100 years, clearly that part of the fan can be considered to be active. 

Historic records, photographs, time-sequence aerial photography, and engineering and 
geomorphic information may support this conclusion. If flooding and deposition have occurred 
on a part of an alluvial fan in the past 1,000 years, for example, that part of the fan may be 
subject to future alluvial fan flooding. 

This conclusion may only be supported by geomorphic information, however. It becomes more 
difficult to determine whether a part of the fan that has not experienced sedimentation for more 
than 1,000 years actually is active, that is, that there is some likelihood of flooding and 
sedimentation under the present climate conditions. 

Because there is no clear analytical technique for making such projections of the estimates of the 
spatial extent of inundation, Stage 2 analysis involves systematically applied judgment and the 
combination of hydraulic computations and qualitative interpretations of geologic evidence 
concerning the recent history and probable future evolution of channel forms, as well as flooding 
and sedimentation processes. It must be kept in mind, however, that the intent of Stage 2 is to 
narrow the area of concern with regard to active deposition, erosion, and unstable flow paths 


over a period of time generally exceeding 100 years. Therefore, the combination of engineering 
and geomorphic analyses, both qualitative and quantitative, provide an indication of the 
approximate spatial extent of possible inundation over a relatively long time period (i.e., several 
thousand years). During Stage 3, the Mapping Partner that performs the detailed study shall 
determine the floodplain limits associated with the 1-percent-annual-chance (100-year) flood. 

G.2.2.2 Identification of Inactive Areas [February 2002] 

For a given area of the alluvial fan, if the situations described in Subsection G.2.2.1 do not exist, 
then the area is considered inactive and not subject to the deposition, erosion, and unstable flow 
path flooding that builds alluvial fans. Inactive areas may be subject to flooding though, most 
notably within entrenched channels. 

Evidence of inactive areas may include armoring along the margin of the area bordering active 
areas, older vegetation, and the lack of change in flow paths viewed over the aerial photographic 
record. This evidence, though, does not preclude the area from possibly being classified as an 
active area as a result of changes in, or conditions within, adjacent active areas. 

Older alluvial fan surfaces are considered active if any of the following are true: 

 The recently active sedimentation zone is migrating into the older surface. 
The elevation difference between the recently active sedimentation zone and the older 
surface is small relative to flood, deposition, and debris depths conceivable in the current 
regime of climate, hydrology, or land use in the source area. 
Upstream of the site, there is an opportunity for avulsions that could lead channels or 
sheet floods across the older surface. 
G.2.2.3 Identification Process [February 2002] 

Once a relative time period is chosen (e.g., <1,000 years) to help evaluate the active areas of an 
alluvial fan, the analyst must determine relative ages for the morphologic features on the alluvial 
fan. Indicators of land surface age for Stage 2 are based on relative age indicators. Absolute 
(numerical) dating techniques, such as radiocarbon dating, are generally beyond the scope of 
many studies. 

Detailed soils and surficial geological maps, when available, provide useful delineation of soil 
types and surface ages. An examination of the historical record of flooding and deposition can 
enhance the information gained from the soils map. Aerial photographs from different years can 
be used to identify sites of deposition. Field examination of morphologic features on the alluvial 
fan surface, particularly noting evidence of human activity (recent or archaeological) or 
weathering characteristics such as desert pavement, rock varnish, B-horizon development in the 
soil profile, calcic-horizon development, and pitting and rilling of clasts may also provide 
relative age information. 


Density and type of vegetation can provide useful clues to the age of an alluvial fan surface area. 
Texture and composition of the sediment, in addition to the water-holding capacity, relate to the 
surface vegetation. Fresh alluvial deposits contain little organic carbon or clay and, as a result, 
do not promote vegetation growth. Vegetation is limited on older surfaces because they receive 
only direct rain, are often erosional, and can be less fertile (carbonate soil cropping out at the 
surface, for example). Intermediate-age surfaces (middle to late Holocene) contain the most 
dense and diverse vegetation. Use and interpretation of diagnostic vegetation, like the use and 
interpretation of desert pavement, varnish, or soil properties, are generally specific to the 
individual fan in question. Within a geographic region, however, surface characteristics of 
alluvial fans may be correlated from one fan to another. 

Detailed topographic maps (i.e., 2-foot contour interval) are instrumental in identifying potential 
avulsion areas and in delineating the boundaries of areas subject to different flood, deposition 
and debris flow depths. Topographic maps also can be used to identify older alluvial surfaces 
within active zones that are not subject to flooding. 

Areas of question noted during the analysis of maps and aerial photographs should be closely 
examined during the field inspection. All flow paths should be walked to verify the active and 
inactive areas that have been delineated. Stage 2 is complete when the analyst has defined and 
delineated all active and inactive areas of deposition, erosion, and unstable flow path flooding, as 
well as adjacent inactive fan areas. All inactive areas with stable flow path flooding and all 
active areas may be considered floodprone, but through Stage 2, the degree to which these areas 
are floodprone is not yet known. The delineated floodprone areas of Stage 2 should 
approximate the largest possible extent of the 1-percent-annual-chance (100-year) flood. 

G.2.2.4 Types of Alluvial Fan Flooding [February 2002] 

Several types of flooding occur on alluvial fans. The most common ones are flooding along 
stable channels, sheetflow, debris flow, and unstable flow path flooding. 

Flooding Along Stable Channels 

A deeply entrenched channel or network of channels often is subject to inactive alluvial fan 
flooding. This type of flooding usually occurs within distributary flow systems that were formed 
during climatic or tectonic conditions different from the present. This flooding can occur at the 
head of the alluvial fan but become unstable downstream. Conversely, unstable channels can 
become stable in the downstream direction; this can occur because of headcutting into the toe as 
a result of changing hydraulic conditions downstream from the toe. Human intervention, directly 
by channel modification or indirectly by land-use change, can create stable channels. 

Sheetflow 

Some parts of alluvial fans are characterized by sheetflow, which is the flow of water as broad 
sheets that are completely unconfined by any channel boundaries. Sheetflow might occur where 
flow departs from a confined channel and no new channel is formed. It might also occur where 
several shallow, distributary channels join together near the toe of a fan and the gradient of the 


fan is so low that the flows merge into a broad sheet. Because such sheetflows can carry high 
concentrations of sediment in shallow water and follow unpredictable flow paths, they are 
classified as active alluvial fan flooding. 

Sheetflows generally occur on downslope parts of fans, where channel depths are low and the 
boundaries of channels become indiscernible. They are also more common at distal locations 
because of the likelihood of fine-grained sediments and shallow groundwater; during prolonged 
rainfall, the ground can become saturated, resulting in extensive sheet flooding as runoff arrives 
from upslope. Fine-grained sediments can aggravate the likelihood of sheetflow because some 
clay minerals swell when wet, forming an impermeable surface at the beginning of a rainstorm. 

Debris Flow 

 

Some parts of alluvial fans are characterized by debris flows, flows with a very high 
concentration of sediment in relation to water. Debris flows pose hazards that are very different 
from those of sheetflows or water flows in channels. Identifying those parts of alluvial fans 
where debris flow deposition might occur requires the examination of deposits from past flows. 
Debris flow deposits can be distinguished from fluvial deposits by differences in morphology, 
depositional relief, stratigraphy, and clast fabric. Exposures in channel banks can be examined 
and can be supplemented with shallow trenches in different deposits. 

Unstable Flow Path Flooding 

Active areas of an alluvial fan will generally be characterized by unstable and uncertain flow 
path flooding. This type of flooding usually creates a single channel just below the apex, but 
splits into multiple channels as it proceeds down the alluvial fan. These channels are subject to 
deposition and bank or bottom erosion that cause channel migration, avulsion, and the formation 
of new channels. Areas subject to this type of flooding are characterized by shallow, braided or 
distributary, sand- to gravel-bed channels. Recently formed channels may have less established 
vegetation, such as trees, than older channels in the same general area. 

G.2.3 Stage 3: Defining the 100-Year Flood Within Defined Areas 
[February 2002] 

FEMA uses the 100-year flood, the flood having a 1-percent chance of being exceeded in any 
given year, to delineate Special Flood Hazard Areas (SFHAs) on NFIP maps. In the preceding 
discussion of Stages 1 and 2, methods of identifying alluvial fan landforms and areas of active 
and inactive deposition, erosion, and unstable flow path flooding were described. During Stage 
3, the Mapping Partner that performs the detailed study will determine the severity and will 
delineate the extent of the 1-percent-annual-chance (100-year) flood within any floodprone area 
identified during Stage 2. 

The broad spectrum of alluvial fan landforms and types of flooding illustrates, as previously 
discussed, the futility of developing a cookbook method to apply to all fans in all geographic 
areas. The analysis of the flood hazards on alluvial fans therefore requires a flexible approach 


that is based on site-specific evaluations. Several methods for quantifying the 1-percent-annual-
chance (100-year) flood are presented in the following sections and are summarized in Table G-
1. Not all methods are appropriate for all situations. The assumptions and limitations of each 
should be carefully considered in deciding which methods to apply to particular areas of an 
alluvial fan. 

Sample maps resulting from the application of some of the available methods are included as 
Figures G-5 through G-13. 

G.2.3.1 Risk-Based Analysis [February 2002] 

The U.S Army Corps of Engineers provided a framework that may be used to analyze flood 
hazards on alluvial fans using the principles of risk-based analysis in Guidelines for Risk and 
Uncertainty Analysis in Water Resources Planning (U.S Army Corps of Engineers, 1992). This 
method uses the total probability equation that will be discussed in detail in Subsection G.2.3.2. 
The degree of uncertainty associated with a prediction of a given flood scenario is assessed by 
bringing to bear evidence derived from geomorphologic and other studies. This method tracks 
the effects of the error associated with a calculation to provide a confidence band in ensuing 
predictions of flood-hazard severity. 


Table G-1. Methods for Defining the 1-Percent-Annual Chance (100-Year) Flood 
Within Floodprone Areas Defined During Stage 2 

METHOD 

ASSUMPTIONS 

LIMITATIONS 

RECOMMENDED 
APPLICATIONS 

FIGURE 
NUMBER 

Risk-Based 
Analysis 

Refer to Guidelines for Risk and 
Uncertainty Analysis in Water 
Resources Planning (USACE, 1992). 

 

 

 

FAN 
Computer 
Program 

Flooding in rectangular channel; 
critical depth, erosion of rectangular 
channel banks until the change in 
width divided by the change in depth 
equals 200; the probability density 
function of a discharge occurring at 
the apex is log-Pearson Type III; the 
frequency of flood events for various 
recurrence intervals, i.e., 2-year 
through 500-year, can be adequately 
defined; equal probability along 
contour arcs (random flow paths); 
(also provides for multiple channels at 
normal depth, assuming total width is 
3.8 times the single-channel width) 

Fluvial (as opposed 
to debris flow) 
formed fan, 
unstable flow paths 

Highly active, conical 
fans 

G-5 

Sheetflow 

Broad, unconfined, shallow flooding 

Not for use in areas 
of undulating 
terrain 

Shallow flooding across 
uniformly sloping 
surfaces 

G-6 

Hydraulic 
Analytical 
Methods 

Stable flow path, uncertainty is to a 
degree that may be disregarded 

Not for use with 
active alluvial fan 
flooding 

Entrenched stable 
channel networks, 
constructed channels, 
urbanized areas 

G-7 and G-
13 

Geomorphic 
Data, Post-
Flood Hazard 
Verification, 
and Historical 
Information 

Relies primarily on qualitative 
information, post-flood verification, 
historical data, and interpretive studies 

Approximate 
method 

Alluvial fans with little 
or no urbanization 

G-8 and G-9 

Composite 
Methods 

As identified in the sections referring 
to the methods being applied 

Must integrate 
multiple methods 
into one result 

Floodprone areas that 
contain unique physical 
features in some 
locations or have areas 
varying in levels of 
erosion and migration 
activity 

G-10, G-11, 
and G-12 


 

Figure G-5. Sample Map Generated From Alluvial Fan Analysis Using FAN Computer Program. This map appeared as Example 1 in Guidelines for Determining Flood Hazards on Alluvial Fans (FEMA, 2000). 

Figure G-5. Sample Map Generated From Alluvial Fan Analysis Using FAN 
Computer Program. This map appeared as Example 1 in Guidelines for 
Determining Flood Hazards on Alluvial Fans (FEMA, 2000). 

 


 

 

Figure G-6. Sample Map Generated From Alluvial Fan Analysis Using Sheetflow Analysis Methods. This map appeared as Example 9 in Guidelines for Determining Flood Hazards on Alluvial Fans (FEMA, 2000). 

Figure G-6. Sample Map Generated From Alluvial Fan Analysis Using Sheetflow 
Analysis Methods. This map appeared as Example 9 in Guidelines for 
Determining Flood Hazards on Alluvial Fans (FEMA, 2000). 

 

 

 

 

 

 


 

Figure G-7. Sample Map Generated From Alluvial Fan Analysis Using Hydraulic Analytical Methods. This map appeared as Example 2 in Guidelines for Determining Flood Hazards on Alluvial Fans (FEMA, 2000). 

Figure G-7. Sample Map Generated From Alluvial Fan Analysis Using Hydraulic 
Analytical Methods. This map appeared as Example 2 in Guidelines for 
Determining Flood Hazards on Alluvial Fans (FEMA, 2000). 

 


 

 

Figure G-8. Sample Map Generated From Alluvial Fan Analysis Using Geomorphic Data, Post-Flood Hazard Verification Data, and Historic Information. This map appeared as Example 3 in Guidelines for Determining Flood Hazards on Alluvial Fan s (FEMA, 2000). 

Figure G-8. Sample Map Generated From Alluvial Fan Analysis Using 
Geomorphic Data, Post-Flood Hazard Verification Data, and Historic Information. 
This map appeared as Example 3 in Guidelines for Determining Flood Hazards on 
Alluvial Fan s (FEMA, 2000). 

 


 

 

Figure G-9. Sample Map Generated From Alluvial Fan Analysis Using Geomorphic Data, Post-Flood Hazard Verification, and Historic Information (Administrative Floodway Shown). This map appeared as Example 4 in Guidelines for Determining Flood Hazards Alluvial Fans (FEMA, 2000). 

Figure G-9. Sample Map Generated From Alluvial Fan Analysis Using 
Geomorphic Data, Post-Flood Hazard Verification, and Historic Information 
(Administrative Floodway Shown). This map appeared as Example 4 in 
Guidelines for Determining Flood Hazards Alluvial Fans (FEMA, 2000). 

 


 

 

Figure G-10. Sample Map Generated From Alluvial Fan Analysis Using Composite Methods (Geomorphic Data and Hydraulic Analytical Methods). This map appeared as Example 5 in Guidelines for Determining Flood Hazards on Alluvial Fans (FEMA, 2000). 

Figure G-10. Sample Map Generated From Alluvial Fan Analysis Using 
Composite Methods (Geomorphic Data and Hydraulic Analytical Methods). This 
map appeared as Example 5 in Guidelines for Determining Flood Hazards on 
Alluvial Fans (FEMA, 2000). 

 


 

 

Figure G-11. Sample Map Generated From Alluvial Fan Analysis Using Composite Methods (Geomorphic Data and Hydraulic Analytical Methods); Zone AH Shown. This map appeared as Example 6 in Guidelines for Determining Flood Hazards on Alluvial Fan s (FEMA, 2000). 

Figure G-11. Sample Map Generated From Alluvial Fan Analysis Using 
Composite Methods (Geomorphic Data and Hydraulic Analytical Methods); Zone 
AH Shown. This map appeared as Example 6 in Guidelines for Determining Flood 
Hazards on Alluvial Fan s (FEMA, 2000). 

 


 

 

Figure G-12. Sample Map Generated From Analysis Using Composite Methods (Geomorphic Data, Hydraulic Analytical Methods, and FAN Computer Program). This map appeared as Example 7 in Guidelines for Determining Flood Hazards on Alluvial Fans (FEMA, 2000). 

Figure G-12. Sample Map Generated From Analysis Using Composite Methods 
(Geomorphic Data, Hydraulic Analytical Methods, and FAN Computer Program). 
This map appeared as Example 7 in Guidelines for Determining Flood Hazards on 
Alluvial Fans (FEMA, 2000). 

 

 


 

 

Figure G-13. Sample Map Generated From Alluvial Fan Analysis Using Hydraulic Analytical Methods (Two-Dimensional Flow Model). This map appeared as Example 8 in Guidelines for Determining Flood Hazards on Alluvial Fans (FEMA, 2000). 

Figure G-13. Sample Map Generated From Alluvial Fan Analysis Using Hydraulic 
Analytical Methods (Two-Dimensional Flow Model). This map appeared as 
Example 8 in Guidelines for Determining Flood Hazards on Alluvial Fans (FEMA, 
2000). 


G.2.3.2 Analysis Using FAN Computer Program [February 2002] 

Assumptions, limitations, and recommended applications for the FAN Computer program are as 
follows: 

 Assumptions: flooding in rectangular channel; critical depth; erosion of rectangular 
channel banks until the change in width divided by the change in depth equals -200; the 
probability density function of a discharge occurring at the apex is log-Pearson Type III; 
the frequency of flood events for various recurrence intervals, i.e., 2-year through 500-
year, can be adequately defined; equal probability along contour arcs (random flow 
paths); also provides for multiple channels at normal depth, assuming total width is 3.8 
times the single channel width 
 Limitations: fluvial (as opposed to debris flow) formed fan, unstable flow paths 
 Recommended Applications: highly active, conical fans 

 

 

The FAN computer program provides one method of analyzing the flood hazards on alluvial 
fans. The methodology used by the FAN program defines the risk of inundation at any particular 
location by applying the definition of the 1-percent-annual-chance (100-year) flood through the 
theorem of total probability. The methodology itself is broader than the use within the FAN 
program. Let H be a random variable denoting the occurrence of flooding at a particular 
location. That is: 

1 if the location is inundated 

H = 

0 if the location is not inundated 

Then the probability of the location being inundated by a flood above a given magnitude, say q0, 
is: 

formula 

where 

 Q = random variable denoting the magnitude of the flood 

PH|Q(1,q) = conditional probability that the location will be inundated, given that a flood 
of magnitude q is occurring 

fQ(q) = probability density function (PDF) defining the likelihood that a flood of a 
magnitude between q and q+dq will occur in any given year 


The FAN computer program provides one method of analyzing the flood hazards on alluvial 
fans. The methodology used by the FAN program defines the risk of inundation at any particular 
location by applying the definition of the 1-percent-annual-chance (100-year) flood through the 
theorem of total probability. The methodology itself is broader than the use within the FAN 
program. Let H be a random variable denoting the occurrence of flooding at a particular 
location. That is: 

1 if the location is inundated 

H = 

0 if the location is not inundated 

Then the probability of the location being inundated by a flood above a given magnitude, say q0, 
is: 

formula 

where 

 Q = random variable denoting the magnitude of the flood 

PH|Q(1,q) = conditional probability that the location will be inundated, given that a flood 
of magnitude q is occurring 

fQ(q) = probability density function (PDF) defining the likelihood that a flood of a 
magnitude between q and q+dq will occur in any given year 

Equation (1) only defines whether a location is within an SFHA and does so in terms of the 
parameter q0. For riverine flooding, q0 represents an elevation, and PH|Q(1,q) is 1 if the elevation 
of the location is less than q0 and 0 if it is greater than q0. At a given location (point on a cross 
section), there is a one-to-one relationship between the discharge being conveyed by the stream 
and the elevation of the surface of the floodwater (i.e., the rating curve for the cross section). 
For riverine flooding, solving Equation (1) reduces to defining the discharge-frequency 
relationship for the reach of the stream under consideration (hence the notation q0 to denote 
magnitude). 

As in riverine analysis, the PDF describing frequency of the magnitude of flooding for alluvial 
fan flooding is taken to be the discharge-frequency relationship of the contributing drainage 
basin. Unlike riverine analysis, PH|Q(1,q) does not simplify to 0 or 1, because there is 
uncertainty in the flow path. The FAN program provides energy depths and velocities relating to 
discharge for use in defining the flood hazard. 


The FAN program uses the assumptions outlined below. Where noted with an asterisk (*), these 
assumptions may be adjusted for observed field conditions; however, the FAN program does not 
readily accommodate these adjustments. 

This methods assumptions are as follows. Floods on alluvial fans are at liberty to expend 
energy to create the most efficient path to convey the water and sediment load. That path is 
shallow and approximately rectangular in cross section. Energy is expended through sediment 
movement until the minimum energy possible is reached. In short, the reasoning is that a flood 
flows at critical depth and is confined to a rectangular path. The flow path would not widen 
indefinitely but, instead, would reach a point where it would stabilize. From empirical data, of 
which there are very little, that point is taken to be where the rate of change of topwidth per 
change in depth (dW/dd) is 200 (* may be adjusted). 

The reasoning leads to the one-to-one relationships: 

 (2) d = 0.106 q to the one fifth

 (3) v = 1.506 q to the one fifth

 

where 

d = specific energy in feet 

v = velocity in feet per second 

q = discharge in cubic feet per second (cfs) 

The conditional probability in Equation (1) accounts for the uncertainty in the path of a flood 
with a given magnitude. Even if the path of the flood can be predicted with reasonable certainty, 
the magnitude of the flood at a particular location may not be so certain, as deposition or scour in 
shallow channels may greatly affect the direction of flow at channel splits. Many alluvial fans 
exhibit a channel network. The capacities of the individual channels as well as the capacities of 
the networks in aggregate vary from almost negligible to more than the 1-percent-annual-chance 
(100-year) flood discharge. The treatment of the uncertainty in a given discharge being 
exceeded at a particular location given the discharge somewhere else [PH|Q(1,q)] varies. 

The least complex treatment (used in the FAN program) follows from the reasoning that the 
topography of the area is the result of deposition that occurred during the past. If that process 
continues, then, over the long term, the probability of every point on a contour being inundated 
is the same. That is, PH|Q(1,q) is uniformly distributed and, for a given point, is approximately 
the width of the flood path divided by the width (the "contour width") of the area subject to 
flooding at the elevation of that point (* may be adjusted). This method assumes that all areas of 
the alluvial fan are subject to flooding and that there is a fixed relationship between flooding 
depth and discharge. 


In general, these assumptions apply when there is absolute uncertainty regarding how floods will 
occur. Thus, for the FAN program, under the simple conditions, 

P hq(1,q) = (w(q)) / (W fax) = (9.408 q to the two fifths) / (w fax) (4) 

where 

w(q) = width of the path conveying q cfs 

Wfan = contour width 

The contour width, Wfan , is shown in Figure G-3. The resulting flood insurance risk zones are 
depicted in Figure G-4. The functional form of Equation (4) is a consequence of the reasoning 
leading to Equations (2) and (3) and is presented here for demonstrative purposes, not as the only 
form possible. 

Figure G-3. Fan and Single-Channel Widths 

Figure G-3. Fan and Single-Channel Widths 


Figure G-4. Flood Insurance Risk Zones Respective to Figure G-3 

Figure G-4. Flood Insurance Risk Zones Respective to Figure G-3 

The FAN program provides for the situation where flows are near normal depth in multiple 
channels. Program output includes results for this situation in addition to the single channel at 
critical depth. The results are then applied based on observed field conditions. More 
information is provided in FAN: An Alluvial Fan Flooding Computer Program Users Manual 
and Program Disk (FEMA, 1990). The FAN program is available online through the FEMA 
Flood Hazard Mapping Web site at http://www.fema.gov/fhm/dl_fnprg.shtm. 

G.2.3.3 Sheetflow Analysis Method [February 2002] 

Assumptions, limitations, and recommended applications for the sheetflow analysis method are 
as follows: 

 Assumptions: broad, unconfined, shallow flooding 
Limitations: not for use in areas of undulating terrain 
Recommended Applications: shallow flooding across uniformly sloping surfaces 

 

 

Guidance on the analysis and mapping of shallow flooding is provided in Appendix E of these 
Guidelines. Although Appendix E indicates that Mapping Partners are not to use the procedures 
in that Appendix for the analysis of alluvial fan flooding, the approach established by this 
Appendix enables the use of those methods described in Appendix E, except for highly active 
conical fans that are studied using the FAN program. 


G.2.3.4 Hydraulic Analytical Methods [February 2002] 

Assumptions, limitations, and recommended applications for hydraulic analytical methods are as 
follows: 

Assumptions: stable flow path, uncertainty is to a degree that may be disregarded 
Limitations: not for use with active alluvial fan flooding 
Recommended Applications: entrenched stable channels and channel networks, 
constructed channels, urbanized areas 

 

 

For inactive, yet floodprone areas, the Mapping Partner that performs the alluvial fan analysis 
may use riverine hydraulic analytical methods. Where flow paths are stable and flow is 
reasonably confined, standard hydraulic engineering methods, such as backwater computations, 
may be used to define the elevation (or depth), velocity, and extent of the 1-percent-annual-
chance (100-year) flood. Hydraulic methods may also be used for stable channel networks when 
applicable. For example, relict alluvial fans or inactive fans with stable channels, as determined 
by a geomorphic analysis, may be subject to flow splits throughout the distributary system that 
exists. Hydraulic modeling can generally handle split-flow analyses through stream junctions of 
this type. 

In general, for stable channels on alluvial fans, physically based methods that consider site 
processes and hydraulics, such as channel geometry, grade and roughness, and channel bank and 
bed material are preferred. Where precise computations of water-surface profiles using energy 
and momentum based methods may not be feasible based on the scope of the study, the use of 
normal depth calculations for definition of approximate floodplain boundaries for the 1-percent-
annual-chance (100-year) flood may be warranted. 

Appendix C of these Guidelines provides guidance for hydraulic analytical methods. Several 
methods applicable to conditions found on alluvial fans are described. These methods include 
two-dimensional water-surface models, modeling techniques of streams with supercritical flow 
regimes, and split-flow analysis. 

Two-dimensional models may be appropriate for determining flood hazards on an alluvial fan. 
Different two-dimensional models may be particularly useful in the analysis and modeling of 
some or all of the following situations: flows that contain a high amount of sediment, unconfined 
flows, split flows, mud/debris flows, and complex urban flooding. For use in defining flood 
hazards for the NFIP, all hydraulic models must meet the conditions of Paragraph 65.6 (a) (6) of 
the NFIP regulations. 

One-dimensional sediment transport models or the methods described in Section G.3 are also 
useful for the analysis of conditions on alluvial fans. 

 


G.2.3.5 Analysis Using Geomorphic Data, Post-Flood Hazard Verification, and 
Historical Information [February 2002] 

Assumptions, limitations, and recommended applications for alluvial fan flooding analyses 
performed using geomorphic, post-flood hazard verification, and historical information are as 
follows: 

Assumptions: relies primarily on qualitative information, post-flood hazard verification, 
historical data, and interpretive studies 
Limitations: approximate method 
Recommended Applications: alluvial fans with little or no urbanization 

 

 

 

The geomorphic approach is for active alluvial fans where deposition, erosion, and unstable flow 
paths are possible. Traditional engineering methods, as described in Subsection G.2.3.4, 
generally are inappropriate for areas with these hydraulic characteristics. Probabilistic methods, 
as described in Subsection G.2.3.2 and contained in the FAN computer program, also contain 
inherent limiting assumptions that may not adequately represent field conditions and may not be 
applicable to many active alluvial fans. 

 

In some situations, the Mapping Partner may use the information collected during Stage 2 to 
delineate an approximate floodplain on an alluvial fan. In situations where geomorphic field 
investigations, coupled with historical documentation, and documentation of hydrologic and 
hydraulic characteristics of flood event(s) (post-flood hazard verification) are available, an 
approximate flood hazard delineation is possible. 

By combining quantitative data on an actual flood event, historical information and photographs 
of other flood events, time-sequence aerial photography documenting recent activity or 
inactivity, and field investigation of the morphologic characteristics and relative ages of the fan, 
an approximate (Zone A) flood hazard delineation may be warranted. 

For many alluvial fans, the various flood indicators (Stage 2 information) provide limited or 
partial information. Because the flood assessment of active alluvial fans is more uncertain than 
more traditional flood assessment, the Mapping Partner that perform the analysis must document 
all assumptions and limitations well and consider these assumptions and limitations in the 
overall evaluation. 


G.2.3.6 Analysis Using Composite Methods [February 2002] 

Assumptions, limitations, and recommended applications for alluvial fan flooding analyses 
performed using composite methods are as follows: 

Assumptions: as identified in the sections referring to the methods being applied 
Limitations: must integrate multiple methods into one result 
Recommended Applications: floodprone areas that contain unique physical features in 
some locations or have areas varying in levels of erosion and migration activity 

 

 

 

Site-specific conditions on alluvial fans may lend themselves to the use of multiple or combined 
methods previously described for the determination of flood hazards. For example, in areas that 
contain manmade conveyance channels or deeply entrenched stable channels, the Mapping 
Partner can combine the results of traditional hydraulic computer programs with methods for 
analyzing active areas. The Mapping Partner that performs the analysis must coordinate with the 
FEMA RPO and with FEMA HQ staff during the development of the study plan. 

 


G.3 Additional Information on Sediment Transport 
[February 2002] 

This section regarding sediment transport is included as supplemental information for the 
analysis of alluvial fans. Sediment transport analyses are generally required for alluvial fan 
studies and revisions. 

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

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

Sediment transport exerts substantial control over morphology and channel geometric 
configuration. An indicator of this influence is the sediment transport rate, which is the rate at 
which material moves in the stream as quantified in units of weight per unit time. The transport 
rate is closely dependent on the water discharge. 

Two classification systems are used describe the sediment load in a stream. The first 
classification system divides the load into bed load and suspended load. The bed load is that 
portion of the sediment that moves along the bottom by sliding, rolling, or saltation. The 
suspended load is comprised of all of the material carried in suspension. 

The second classification system divides the sediment load into wash load and bed-material 
load. The wash load is comprised of very fine materials, clay and silt, rarely found in the bed. 
The wash load does not depend on the carrying capacity of the stream but on the amount 
supplied by the watershed. The bed-material load is comprised of all of the material found in 
the bed. Some of it will move very close to the bottom, but some may be found in suspension. 

Quantification of sediment transport is fraught with uncertainty because of the complexity of the 
phenomenon and its inherent spatial and temporal variability. Existing mathematical 
representations have relied heavily on experimental results. 


The available sediment transport formulas have been grouped according to the approach used to 
derive them. Three major approaches have been used: shear stress, power, and parametric. 
Formulas also can be grouped according to the component of the total load they attempt to 
quantify: bed load, suspended load, or bed-material load. Table G-2 summarizes some of the 
more commonly used formulas; however, it is not intended to be a complete listing. 

Table G-2. Sediment Transport Formulas and Classifications 

Criteria 

Grouping 

Sediment Transport Formula 

Criteria 

Grouping 

DuBoys (1879) 

Shields (1936) 

Einstein Bed Load(1950) 

Einstein Suspended Load 
(1950)

Meyer-Peter-Muller (1948) 

Einstein-Brown (1950) 

Parker et al. (1982) 

Engelund-Hansen (1967) 

Ackers-White (1973) 

Yang (1972) 

Colby (1964) 

Approach 

Shear Stress 

x 

x 

x 

 

x 

x 

x 

 

 

 

 

 

Power 

 

 

 

 

 

 

x 

x 

x 

 

 

Parametric 

 

 

 

 

 

 

 

 

x 

Load Component 

Bed Load 

x 

x 

x 

 

x 

x 

x 

 

 

 

 

 

Suspended Load 

 

 

 

x 

 

 

 

 

 

 

 

 

Bed-Material Load 

 

 

 

 

 

 

 

x 

x 

x 

x 

 

Despite the intense efforts expended in the development of these formulas, evaluation against 
field data indicates that they commonly overpredict or underpredict sediment loads by orders of 
magnitude of actual measured sediment transport rates. This discrepancy is likely a result of 
imperfect knowledge of the physics of sediment transport and also of the extensive variability 
and heterogeneity in hydrologic and geologic factors. 

For these reasons, no one formula is better than the others. Mapping Partners, who must have 
sufficient field experience to make decisions regarding the method to use and how to map the 
results obtained using that method, must select a sediment transport formula based on how well 
the conditions of the problem at hand match the assumptions underlying the formula. If possible, 
Mapping Partners should verify the applicability of the formula with site-specific field data. 


G.4 References [February 2002] 

Chang, H. H., Fluvial Processes in River Engineering, New York: John Wiley & Sons, 1988. 

Dawdy, D.R., Flood Frequency Estimates on Alluvial Fans, Journal of the Hydraulics 
Division, ASCE, Proceedings, Vol. 105, No. HYII, pp. 1407-1413, November 1979. 

Federal Emergency Management Agency, FAN: An Alluvial Fan Flooding Computer Program 
Users Manual and Program Disk, September 1990. 

Federal Emergency Management Agency, Guidelines for Determining Flood Hazards on 
Alluvial Fans, February 2000. 

Gomez, B., and M. Church, An assessment of bed load sediment transport formulae for gravel 
bed rivers, Water Resources Research, Vol. 25, No. 6, p. 1161-1186, 1989. 

National Research Council, Committee on Alluvial Fan Flooding, Alluvial Fan Flooding, 
Washington, DC: National Academy Press, 1997 

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

U.S. Department of the Army, Corps of Engineers, Guidelines for Risk and Uncertainty Analysis 
in Water Resources Planning, Report 92-R-1, Fort Belvoir, Virginia, 1992. 

Yang, C. T., and S. Wan, Comparison of selected bed-material formulas, ASCE Journal of 
Hydraulic Engineering, Vol. 117, p. 973-989, 1991.