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Alluvial Fan Flooding (1996)

Chapter: Indicators for Characterizing Alluvial Fans and Alluvial Fan Flooding

« Previous: Flooding Processes and Environments on Alluvial Fans
Suggested Citation:"Indicators for Characterizing Alluvial Fans and Alluvial Fan Flooding." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
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3
Indicators for Characterizing Alluvial Fans and Alluvial Fan Flooding

Alluvial fans and alluvial fan floods show great variability in climate, fan history, rates and styles of tectonism, source area lithology, vegetation, and land use. For this reason, it is essential that any investigation of alluvial fan flooding include careful examination of the specific fan for which information is needed. The committee recognizes that the extent of site-specific examination will be constrained by factors such as the amount of time and money allotted to the project, the tools available to the investigator, and the investigator's experience. As discussed in this chapter, however, much information can be gleaned from topographic and soil maps, as well as aerial photographs. Nevertheless, it is essential to do at least one field inspection of the fan that involves walking across its surfaces and along its channels. In general, the more fieldwork done, the better the understanding of the processes of flooding on the fan of interest.

According to the definition presented in Chapter 1, for regulatory purposes alluvial fan flooding is a flood hazard that on active parts of alluvial fans has a 1 percent chance of occurrence, and it is identified by flow path uncertainty and deposition and erosion below the hydrographic apex. The criteria used to assess whether an area is, or is not, subject to alluvial fan flooding must determine whether the flooding occurs on an alluvial fan and whether it is characterized by deposition, erosion, and flow path uncertainty below a hydrographic apex. For these reasons, the process of determining whether or not alluvial fan flooding can occur at a given location, and of defining the spatial extent of the 100-year flood, are divided into three stages:

  1. Recognizing and characterizing alluvial fan landforms.

  2. Defining the nature of the alluvial fan environment and identifying areas of active erosion, deposition, and flooding (as well as inactive areas).

  3. Defining and characterizing areas on active parts of alluvial fans that are subject to a 1 percent chance of occurrence (the 100-year flood), the FEMA mandate.

Progression through each of these stages results in a procedure that narrows the problem to smaller and smaller areas of uncertainty (Figure 3-1). In Stage 1, the landform on which flooding occurs must be characterized. If the location of interest is an alluvial fan, then the user progresses to Stage 2, in which those parts of the alluvial fan that are active and inactive are identified. The term active means those locations where flooding, erosion, and deposition have

Suggested Citation:"Indicators for Characterizing Alluvial Fans and Alluvial Fan Flooding." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

FIGURE 3-1 Three stages in the procedure to determine areas susceptible to alluvial fan flooding.

occurred on the fan in relatively recent time, and probably will continue to occur on that part of the alluvial fan. Those parts of the fan that have been active in relatively recent time can be identified depending on data availability for the site and money allotted to the project. (See Box 3-1.) Each active part of the alluvial fan also is characterized based on the dominant types of processes that result in flooding and sedimentation. Finally, in Stage 3, the evaluator determines where on those parts of the fan that are active the 100-year flood is possible. Progression through each of these stages will require the investigator to refer to a variety of maps, photographs, and other information sources (Table 3-1; also see Appendix B) and to do a significant amount of fieldwork to understand and characterize the alluvial fan flooding hazard.

Suggested Citation:"Indicators for Characterizing Alluvial Fans and Alluvial Fan Flooding." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

BOX 3-1 ''TIME" IN THE CONTEXT OF ALLUVIAL FAN FLOODING

It is not possible to give a precise definition to the phrase "relatively recent" as used in this report. Because of the variability among fans, the complexity of single fans, and the great range of ages present on fan surfaces, as well as tremendous diversity in the climate and geologic processes operating on fans, the committee is not able to specify a single time frame to use in defining whether or not a fan is active. Instead, such a judgment must be made on a site-specific basis.

At one extreme, the committee considers "recent" to be the past 10,000 years (the Holocene Epoch), which follows a particularly radical and widespread climate change at the end of the most recent Ice Age. At the other extreme, "recent" implies the time period over which there has been a relatively uniform range of environmental conditions that affect flood generation and channel behavior. Estimates of the probability of events occurring in the relevant, near-term future are based on the record of the "recent," homogeneous past. A problem exists, however, in that there often is no clear indication in most localities about how far back one should look in defining whether the record is relatively uniform, and this is in part why society is occasionally surprise by unforeseen flood hazards.

For purposes of the National Flood Insurance Program (NFIP), an arbitrary but reasonable decision was made to use, as a planning tool, the flood which has a 1 percent chance of occurring in any one year ("the 100-year flood"), which is usually a generally destructive flood in most areas of human settlement. Thus, the engineering perspective involves a timescale (a century) over which structures are typically designed to survive, while the geological perspective involves a longer time period with a greater range of geologic processes and environmental variability. The engineering perspective focuses on the regulatory requirements imposed by the NFIP; the geologic perspective focuses on geologic process. Both perspectives are important to understanding alluvial fan flooding. Examples of various attempts to determine ages of fans and fan components can be explored in-depth in the following references:

Bull, W. B. 1964. Geomorphology of segmented alluvial fans in western Fresno County, California. U.S. Geological Survey Professional Paper 352-E. Reston, Va.: U.S. Geological Survey.

Bull, W. B. 1968. Alluvial fans. Journal of Geologic Education 17(3):101–106.

Bull, W. B. 1977. The alluvial fan environment. Progress in Physical Geography 1:222–270.

Kellerhals, R., and M. Church. 1990. Hazard management on fans, with examples from British Columbia. In Alluvial Fans: A Field Approach. New York: John Wiley & Sons.

Lecce, S. A. 1990. The alluvial fan problem. In Alluvial Fans: A Field Approach. New York: John Wiley & Sons.

Machette, M. N. 1985. Calcic soils of the southwestern United States. Geological Society of America Special Paper 203. Boulder, Colo.: The Geological Society of America.

Markewich, H. W., and S. C. Cooper. 1991. One perspective on spatial variability in geologic mapping. In Spatial Variabilities of Soils and Landforms, M. J. Mausbach

Suggested Citation:"Indicators for Characterizing Alluvial Fans and Alluvial Fan Flooding." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
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and P. J. Wilding, eds. Soil Society of America Special Publication, no. 28.

Mausbach, M. J., and P. J. Wilding. 1991. Spatial Variability of Soils and Landforms. Soil Society of America Special Publication, no. 28.

Rhoads, B. L. 1986. Flood hazard assessment for land use planning near desert mountains. Environmental Management 10(1):97–106.

Zarn, B., and R. H. Davies. 1994. The significance of processes on alluvial fans to hazard assessment. Z. Geomorph. N. F. 38:487–500.

STAGE 1: RECOGNIZING AND CHARACTERIZING ALLUVIAL FAN LANDFORMS

Determining whether or not a Landform is an Alluvial Fan

The committee's definition of alluvial fan flooding specifically states that it occurs on alluvial fans. As a consequence, the first step in application of the definition is analysis of the area being considered for possible alluvial fan flooding. If this area does not meet the criteria for the definition of an alluvial fan, then it does not qualify for consideration of alluvial fan flooding. The committee defines an alluvial fan as "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 which has the shape of a fan, either fully or partially extended." These characteristics can be categorized as composition, morphology, and location, as follows.

Composition

Alluvial fans are landforms constructed from deposits of alluvial sediments or debris flow materials.

To meet the criteria in the committee's definition of an alluvial fan, the landform of interest must be a sedimentary deposit, an accumulation of loose, unconsolidated to weakly consolidated sediments. In the following text, we use the term "alluvium" to refer to sediments transported by both streams and debris flows, but we emphasize that this is a grammatical convenience. On a particular fan, the distinction between these two forms of sediment transport is critical to a correct interpretation of the flood hazard.

Most sediments deposited during Quaternary time (2 million years ago to the present) still are loose and unconsolidated, as the processes of diagenesis that result in compaction, cementation, and lithification require millions of years to transform sediment to sedimentary rock. As a consequence, geologic maps commonly have a unit labeled "Qal" that conventionally is mapped in yellow and represents Quaternary alluvium. Determining whether or not a landform is an alluvial sedimentary deposit might be as simple as checking a published geologic map to see if the underlying material is mapped as alluvium. If a geologic map is not available, the user can

Suggested Citation:"Indicators for Characterizing Alluvial Fans and Alluvial Fan Flooding." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
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check Natural Resources Conservation Service (NRCS) soil maps or drilling and logging records from water wells. If none of these sources is available, field reconnaissance can be done to determine whether or not the landform consists of alluvial sediments.

Morphology

Alluvial fans are landforms that have the shape of a fan, either partly or fully extended.

To meet the criteria in the committee's definition of an alluvial fan, the landform of interest must have the shape of a fan, either partly or fully extended. Flow paths radiate outward to the perimeter of the fan. This criterion can be assessed with topographic maps. For example, in Figure 3-2a the landform downstream from the Lawton Ranch, Montana, has the shape of a fan that is nearly fully extended. This landform is known as the Cedar Creek alluvial fan and is a classic example of a fan with nearly ideal morphology.

Location

Alluvial fan landforms are located at a topographic break.

To meet the criteria in the committee's definition of an alluvial fan, the landform of interest must be 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 topographic apex. Figure 3-2 shows that the Cedar Creek alluvial fan begins at a topographic break, which in this case is a slightly embayed mountain front. As Cedar Creek exits its narrow bedrock canyon, it becomes less confined and is able to migrate more freely. Less confinement can lead to greater channel widths and smaller channel depths. As a result, the occurrence of deposition increases, and flow paths become more unstable.

Defining the Boundaries of an Alluvial Fan

Where are the toe and lateral boundaries of the alluvial fan?

Toe

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, or

  • smoother, gentler slopes of the piedmont plain.

Suggested Citation:"Indicators for Characterizing Alluvial Fans and Alluvial Fan Flooding." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
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TABLE 3-1 Data Sources for Information on Alluvial Fans

Agency or Source

Source Numbera

Topography

Surface Features

Land Cover

Land Use

Remotely Sensed

Aerial Photography

U.S. Department of the Interior

Bureau of Land Management

1

 

 

 

 

 

 

National Park Service

2

 

 

 

 

 

 

U.S. Geological Survey

3

 

 

 

 

 

 

U.S. Department of Agriculture

Agricultural Stabilization and Conservation Service

4

 

 

 

 

 

 

Forest Service

5

 

 

 

 

 

 

Natural Resources Conservation Service

6

 

 

 

 

 

 

U.S. Department of Commerce National Ocean Service

7

 

 

 

 

 

 

U.S. Army Corps of Engineers

8

 

 

 

 

 

 

Independent Federal Agencies

Federal Emergency Management Agency

9

 

 

 

 

 

 

Tennessee Valley Authority

10

 

 

 

 

 

 

National Archives and Records Administration

11

 

 

 

 

 

 

Library of Congress

12

 

 

 

 

 

 

Other agencies or sources:

State geologists

13

 

 

 

 

 

 

State floodplain management agencies

14

 

 

 

 

 

 

University libraries

15

 

 

 

 

 

 

County floodplain management agencies

16

 

 

 

 

 

 

Long-time residents

17

 

 

 

 

 

 

Newspapers

18

 

 

 

 

 

 

Technical journals

19

 

 

 

 

 

 

University theses

20

 

 

 

 

 

 

a See Appendix B.

Suggested Citation:"Indicators for Characterizing Alluvial Fans and Alluvial Fan Flooding." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
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Orthophotoquads

Satellite

Hydrologic

Flood

Hydrography

Water data

Floodplain

Subsurface

Geology

Soils

Other

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Suggested Citation:"Indicators for Characterizing Alluvial Fans and Alluvial Fan Flooding." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
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FIGURE 3-2 (a) Shaded relief map and (b) geologic map of Cedar Creek alluvial fan in Montana.

SOURCE: Reprinted with permission from Ritter et al. (1993).

Suggested Citation:"Indicators for Characterizing Alluvial Fans and Alluvial Fan Flooding." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

Such boundaries often can be identified on the basis of changes in the shapes of contour lines on topographic maps. For example, at the toe of a fan contour lines may become straighter or less concave when viewed downslope, although in the case of deeply dissected fans, contour lines may become more irregular and crenulated because of channel incision. The toe of the Cedar Creek1 alluvial fan (Figure 3-2a) is defined by Bear Creek along the fan's western margin, and by the much larger valley floor of the Madison River into which Bear Creek flows along the fan's northwestern margin. Streams draining the northern part of the fan are more deeply incised because the Madison River valley floor forms a lower base level for erosion than its tributary valley floor along Bear Creek.

The toe of some alluvial fans in arid regions is indicated by a relative increase in the amount, size, and type of vegetation because ground water is closer to the surface there than on the upper parts of the fan. The toe of some alluvial fans in humid regions may be indicated by relatively less vegetation because the recent deposits are less fertile than older sediments. A general sense of vegetation types often is indicated on topographic maps.

Lateral Boundaries

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. Crenulations in contour lines at fan boundary troughs can be observed along the margins of the Cedar Creek alluvial fan (Figure 3-2a).

Lateral boundaries of single alluvial fans commonly are distinct contacts between light-colored, freshly abraded, alluvial deposits and darker-colored, weathered deposits with well-developed soils on piedmont plains. Soils of active alluvial fans typically are less oxidized and lower in clay content than soils on older landforms. As a consequence, the younger soils generally are lighter colored and more friable. Color and texture changes often are pronounced on aerial photos or infrared remote sensing imagery. In areas with rock varnish formation,2 the lighter surfaces of recent alluvial fan deposits in contact with undisturbed varnished surfaces of older deposits form a distinct boundary or contact that readily is distinguished by the relative darkness of the ground on aerial photographs and by on-the-ground inspection. Dark, undisturbed surfaces of rock varnish are found on old piedmont and valley deposits throughout the Basin and Range province of the western United States.

In the case of multiple fans that coalesce to form bajadas, where deposits and reworked material of adjacent alluvial fans merge, the boundaries between adjacent fans may be less distinct than those of individual fans adjacent to streams, rivers, or smooth piedmonts, but generally are

1  

The committee has not visited the Cedar Creek fan and inspected its surface and deposits. It is used as an example here because it has been studied intensively by prominent geomorphologists, and thus much information is available regarding it. In addition, it is a classic alluvial fan in shape and history.

2  

Rock varnish is a dark coating (from 2 to 500 microns thick) that forms on rocks at and near the Earth's surface as a result of mineral precipitation and eolian influx. The chemical composition of rock varnish typically is dominated by clay minerals and iron and/or manganese oxides and hydroxides, forming red and black varnishes, respectively. With time, the thickness of the coating increases if abrasion and burial of the rock surface do not occur. As a result, clastic sediments on alluvial fan surfaces that have been abandoned for long periods of time have much darker and thicker coatings of varnish than do younger deposits.

Suggested Citation:"Indicators for Characterizing Alluvial Fans and Alluvial Fan Flooding." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

marked by a topographic trough or ridge. Although it is difficult to separate young deposits on one fan from similar age deposits on a coalescing fan, it sometimes is possible to distinguish them based on different source basin rock types. For example, Bull (1963, 1964) defined fan boundaries in central California using contour maps, aerial photographs, and tests of the gypsum content of core hole samples. Bull found that the gypsum content of fan deposits derived from drainage basins underlain predominantly by clay-rich rocks was about five times that of fan deposits from drainage basins underlain predominantly by sandstone.

Boundaries of many alluvial fans are defined on 7.5-minute series orthophoto base maps by the NRCS. In the U.S. Southwest, typical NRCS soil series for alluvial fans include the Ramona, Soboba, Kinburn, and Anthony. Large areas where material from stream banks is freshly deposited and partly reworked during floods also are mapped, and smaller areas are identified as part of a particular series where the reworked material is located. For small alluvial fans less than about .8 km2 (.3 mi2), the detail of the mapped soil units on the 7.5-minute soil map series may not be sufficient to show many distributary channels and the fan boundaries. Soil maps used in conjunction with aerial photographs are an excellent means to define fan boundaries.

The nature and extent of alluvial fan flooding can be partially determined from published topographic, soils, and geologic maps and other sources of data. However, the committee emphasizes the importance of a field inspection by a qualified professional with experience and technical knowledge of geomorphology, slope stability, avalanche potential, flood hydraulics, flood hydrology, sedimentary facies, and alluvial fan processes. The general use of secondary information and the importance of field information is described in this chapter and in the examples described in Chapter 4.

STAGE 2: DEFINING THE NATURE OF THE ALLUVIAL FAN ENVIRONMENT AND IDENTIFYING THE LOCATION OF ACTIVE EROSION AND DEPOSITION

Most alluvial fans have parts that are active and parts that are inactive. Alluvial fan flooding occurs on active parts of alluvial fans.

In Stage 2, evidence is obtained that identifies areas of potential flooding. This step narrows the area of concern for Stage 3, which is the specification identification of the extent of the 100-year flood. Although 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 alluvial fan flooding now. In fact, in most of the United States it is possible to identify parts of alluvial fans that were actively constructed during Pleistocene time (about 2 million to 10,000 years ago) and parts that have been active (i.e., flooded) in the Halocene (the past 10,000 years). The reason that this broad distinction generally is straightforward and simple in practice is that the two time periods were identified and defined on the basis of different climatic conditions. The Halocene 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 Halocene (Figure 3-3). During glacial times, ice masses expanded and advanced, evaporation was low, and in the dry western U.S. ground water tables and stream discharges were high relative to interglacial times. As a result of these climatic differences, flooding and sedimentation occurred at different rates and magnitudes during the

Suggested Citation:"Indicators for Characterizing Alluvial Fans and Alluvial Fan Flooding." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

FIGURE 3-3 Quaternary period timescale illustrating oscillating climatic conditions from full glacial (cool) to interglacial (warm). SOURCE: Reprinted with permission from Skinner and Porter (1995).

Pleistocene and Holocene epochs. In many regions, the post-Pleistocene change of climate resulted in a reduction in the rate of sediment supply to fans, whether by streams or debris flows. As a result, the discharges presently available are able to move the sediment supplied on a lower slope than that formed during the Pleistocene, so fanhead incision is occurring on some fans.

One of the most common causes of the abandonment of large parts of an alluvial fan is a change in elevation of local streams. Elevation change can result from a change in climatic conditions or in rates of tectonism. Climatic change might result in a decrease in the size of large streams and/or lakes at the toe of the fan, as in the case of a change from braided, postglacial meltwater streams to smaller, meandering streams incised into the braided gravel deposits.

Suggested Citation:"Indicators for Characterizing Alluvial Fans and Alluvial Fan Flooding." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

A change in the rate of tectonic uplift along a mountain front also can result in abandonment of parts of alluvial fans. For example, a decrease in the rate of uplift at a mountain front relative to the fan could result in stream channel downcutting at the mountain front/fan apex over a period of time. As a consequence, the upper part of the fan would become entrenched and the area of active alluvial fan deposition would shift downfan3 (Figure 3-4). The opposite also can happen. In this case, an increase in the rate of uplift can result in rapid deposition at the fanhead and development of a young untrenched fan segment overlying the older fan surfaces.

It is clear from the examples of segmented fans that only certain parts of the fan, or segments, are active at any one time. In the entrenched fan, the distal segment downstream of the hydrographic apex is typically the active part of the fan. In the untrenched fan, the segment of the fan proximal to the source area, at the topographic break, is typically the active part of the fan. These examples, however, are simplistic in that few fans have only one active segment that is clearly distinguished from an older, inactive segment. More typically, fans must be mapped to identify surfaces of different ages, from youngest to oldest.

To determine what parts of a fan are active and inactive, the investigator must examine the whole fan using indicators of activity as described here.

Defining Active

Because it is not possible to predict with zero uncertainty where the next flood will occur on an alluvial fan, we resort to the traditional method of the geologist and rely on the dictum "the past, as preserved in the geologic record, is key to understanding the present and to predicting the future." Using this reasoning, the geologist concludes that the area of deposition on an alluvial fan shifts with time, but the next episode of flooding is more likely to occur where the most recent deposits have been laid down than where deposits of greatest antiquity occur.

Once the planner has incorporated this basic philosophy into efforts to identify those parts of the fan that are active, the next step is to decide what time period will be used to define active. As a conservative standard, in some areas it is easy to separate the parts of a fan that formed more than 10,000 years ago from those parts that formed during the past 10,000 years, the Holocene epoch. In many places in the southwestern United States, it is less easy to see a clear Pleistocene/Holocene change, but one can subdivide Holocene deposits on a much finer timescale. In areas with long records of flooding and aerial photographs that date back to about 60 years, it is possible to identify areas of historic flooding within the past 100 years.

The term active refers to that portion of a fan where flooding, deposition, and erosion are possible. If flooding and deposition have occurred on a part of a fan in the past 100 years, clearly that part of the fan is active. If flooding and deposition have occurred in the past 1,000 years, that part of the fan can be considered to be active. However, it becomes more difficult to determine whether or not a part of the fan that has not experienced sedimentation for (say) more

3  

Note that the Cedar Creek fan is similar to the entrenched fan shown in Figure 3-4, except that its toe is bounded by a river that transports sediment away from the distal part of the fan. As a result, a young fan segment is not forming or preserved at the fan toe.

Suggested Citation:"Indicators for Characterizing Alluvial Fans and Alluvial Fan Flooding." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

FIGURE 3-4 Entrenched alluvial fan with deposition occurring at the distal part of the fan. SOURCE: Reprinted with permission from Bull (1977).

than 1,000 years really is active, that is, that there is some likelihood of flooding and sedimentation under the present climate. A systematic approach to the problem of estimating whether a place may be inundated after a long period of inactivity is suggested in Chapter 2, the section ''Change Over Time," which deals with processes by which flooding and deposition can migrate across an alluvial fan to invade places that have long been outside the zone of active deposition, even in the current climate. Since there is no clear analytical technique for making such projections, estimates of the probable spatial extent of inundation involve 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. The problem becomes even more difficult when one considers the likelihood that the environmental conditions affecting the generation of floods or debris flows in the source area may not have remained constant in time.

Judging where flooding and deposition might occur is particularly important in cases where land use patterns are substantially altered by human activity (e.g., the Wasatch Range, Utah) or where recent decades have been marked by more intense storm patterns. As an example

Suggested Citation:"Indicators for Characterizing Alluvial Fans and Alluvial Fan Flooding." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

of the latter, storms in southern Arizona have changed in this century from moderate-sized summer events with sources in the south to much larger fall and winter events coming from the west-southwest (Pacific Ocean). Some of this change in storm conditions is attributed to the increasing frequency of El Niño climatic events over the past few decades. In such cases, it probably is prudent to define active as more than just those parts of the fan that have been the sites of flooding and deposition in the past 100 or 1,000 years.

Identifying Areas of Flooding and Deposition on the Active Part of an Alluvial Fan

It is important to identify both active and inactive parts of the fan, because this provides a map of where flooding can occur as well as where it probably will not occur.

Preparing a Geomorphic Map of Different Age Fan Surfaces

Once a time period is chosen to represent the active part of a fan for the purpose of flood hazard assessment, the flood evaluator must determine which deposits are less than the chosen age. A simple place to start is to examine the historical record of flooding and sedimentation. Aerial photographs from different years can be compared to identify sites of deposition that are less than about 60 years in age. If humans have lived in the area, historical deposits often contain relicts of human activity, such as pieces of machinery, bottle caps, lumber, and scraps of metal.

These deposits can be examined and described to gain a good understanding of the nature of fresh deposits for that alluvial fan. The flood evaluator can map different deposits, placing them in relative chronological order from youngest to oldest, as mapping of the entire fan progresses. On a surface of essentially continuous deposition, gradational relations are the rule.

The product of this part of the investigation should be a basic geomorphic map of the entire fan, with particular emphasis on the active parts of the fan. An example of such a geomorphic map is shown for the Cedar Creek fan (Figure 3-2b). This map divides alluvial fan surfaces into different age categories, from as old as middle Pleistocene to as young as late Holocene. Soil profiles were described at different sites, and weathering characteristics such as those described below were used to assess the relative age of each surface. In this example, only a small part of the total fan can be considered to be active.

Morphologic and Weathering Criteria Used to Prepare a Geomorphic Map

A variety of properties can be used to separate deposits of different ages. These include features such as fan surface morphology and sediment weathering characteristics.

The surface of a recent deposit typically is irregular, whereas older deposits generally are smoother. Fresh stream-laid deposits commonly have bar and swale topography, whereas fresh debris flow deposits might have sharply defined levees along lateral margins. With time, loose, unconsolidated flood deposits weather, and some fine-grained material is added to the deposit from eolian influx. As deposits weather, clast size becomes smaller, and edges of deposits from individual flood episodes become more subdued. The net result is that the morphology of older surfaces becomes increasingly more subtle as micro- and macro-relief features are worn down. An

Suggested Citation:"Indicators for Characterizing Alluvial Fans and Alluvial Fan Flooding." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
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exception to this general rule occurs, however, as alluvial surfaces become so aged and smooth that runoff can collect and begin to incise the surface. In such cases, older alluvial fan surfaces are characterized by very smooth, dark surfaces dissected by narrow channels.

Weathering characteristics that have been used by many workers to determine relative ages of alluvial fan deposits include desert pavement, 4 rock varnish, B-horizon development in the soil profile, calcic-horizon development in the soil profile, and pitting and rilling of clasts (Bull, 1991; Cooke et al., 1993; Dorn, 1994). In general, each of these characteristics becomes more pronounced and better developed with time, although the rate of development is site-specific owing to the influence of factors such as climate, eolian influx, and parent rock type.

Weathering parameters such as desert pavement, rock varnish, calcic-horizon development, and pitting and rilling are more useful in arid and semiarid regions than in humid regions. The use of these parameters has been common in dry climates, however, because of the scarcity of datable organic material in alluvial deposits. In humid regions, such distinctions are often less obvious, but workers often have the additional benefit of organic material that can be dated with radiometric carbon methods for time periods in the range of interest. If organic matter can be obtained from shallow trenches in deposits on alluvial fans, the investigator need not rely on other more relative weathering parameters, although these can be used to supplement the radiometric age estimates.

In the Basin and Range province of Arizona, California, Nevada, and Utah, alluvial fans are common, and their deposits can be correlated from one fan to another on the basis of relative age criteria associated with morphologic and weathering characteristics. Christenson and Purcell (1985) identified eight characteristics that are useful in separating alluvial fan deposits into three general age categories throughout the region (Figure 3-5): young (less than 10,000 to 15,000 years), intermediate (10,000 to 700,000 years), and old (greater than 500,000 years). Their young category can be considered to be the active surface if the fan is not incised. The broad regional similarities in these deposits appear to be the result of Quaternary climatic changes (Christenson and Purcell, 1985). The eight characteristics identified and their nature for each of the three age groups are given in Table 3-2. Bull (1991) noted similar regional correlations over an even broader area that includes parts of northern Mexico and New Mexico.

In the eastern Grand Canyon, Hereford et al. (1995) were able to map three alluvial fan surfaces that range in depositional age from about 1000 B.C. (2950 years B.P.) to the present. Like Christenson and Purcell (1985), they referred to these deposits as older, intermediate, and younger (Figure 3-6), but their ages are 1 to 2 orders of magnitude younger. Although these surfaces are much more finely discriminated than those described by Christenson and Purcell (1983), the authors still were able to identify significant differences in weathering characteristics of each age surface (Table 3-3). Also, because of the young ages of the deposits, the authors were able to use archeological features and radiocarbon dating to supplement the weathering-related age criteria (Figure 3-7). In this example, only areas of channelized debris flows and younger debris flows could be considered to be active.

4  

Desert pavements are surfaces of tightly packed gravel that armor, as well as rest on, a thin layer of silt, presumably formed by weathering of the gravel. They have not experienced fluvial sedimentation for a long time, as shown by the thick varnish coating the pebbles, the pronounced weathering beneath the silt layer, and the striking smoothness of the surface, caused by obliteration of the original relief by downwasting into depressions (Ritter et al. 1995, pp. 252–253).

Suggested Citation:"Indicators for Characterizing Alluvial Fans and Alluvial Fan Flooding." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

FIGURE 3-5 Young, intermediate, and old alluvial fan deposits, Gila Mountains, southern Arizona. SOURCE: Reprinted with permission from Christenson and Purcell (1985).

Vegetation Criteria Used to Prepare a Geomorphic Map

Vegetation types often differ from an alluvial surface of one age to that of another. The reasons seem to be related to the texture and composition of the sediment, as well as to the abundance and availability of moisture in the sediments. For example, on a fresh alluvial deposit, incipient soils contain little organic carbon or clay. As a result, the soils are low in nutrients and have little water-holding capacity. Older deposits are more enriched in carbon and clay content and have higher water-holding capacities.

Use and interpretation of diagnostic vegetation, just like the use and interpretation of desert pavement, varnish, or soil properties (e.g., clay or carbonate content) must be specific to the individual fan in question. For example, some mesquite species are riparian, but others can live anywhere in diminutive form. Palo Verde are more lush along waterways, but also can live well away from streams.

Suggested Citation:"Indicators for Characterizing Alluvial Fans and Alluvial Fan Flooding." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

TABLE 3-2 General Characteristics of Young (<10,000 to 15,000 years), Intermediate (10,000 to 700,000 years), and Old (>500,000 years) Alluvial Fan Deposits, Basin and Range Province, United States

Characteristic

Young

Intermediate

Old

Drainage pattern

Distributary; anastomosing or braided

Tributary; dendritic

Tributary; dendritic or parallel

Depth of incision

Less than 1 m

Variable (1 to 10 m)

Greater than 10 m

Fan surface morphology

Bar and channel

Variable, generally smooth and flat

Ridge and valley, most of surface slopes

Preservation of fan surface

Currently active

Incised, but well-preserved wide, flat divides

Basically destroyed, locally preserved on narrow divides

Desert pavement

None to weakly developed

None to strongly developed

None (surface destroyed) to strongly developed (surface preserved)

Desert varnish

None to weakly developed (most varnished clasts reworked from older surfaces or bedrock)

None to strongly developed

None (surface destroyed) to strongly developed (surface preserved)

B horizon

None to weakly developed

Weakly to strongly developed

None (surface destroyed) to strongly developed (surface preserved)

Calcic-horizon

None to weakly developed, CaCO3 disseminated throughout

Weakly to strongly developed

None, carbonate rubble on surface (surface destroyed) to strongly developed petrocalcic horizon (surface preserved)

 

SOURCE: Reprinted with permission from Christenson and Purcell (1985).

Plant type, as well as vegetation density and diversity, is associated with surface age. Some plant species are riparian (ironwood), others are xerophitic (cacti), and others are completely intolerant of moist soil (e.g., saguaro). Vegetation density and diversity are low (but not nonexistent) in streambeds, become most dense and diverse for intermediate-age surfaces (middle to late Holocene), and become less dense and less diverse for old ridge-and-ravine surfaces. Streamflow limits vegetation in the channels by scour and removal of plants and their root support systems, but promotes vegetation on low terraces by watering them with overbank flow and water infiltrated into the bed. Vegetation is limited on old surfaces because they receive only direct rain, are often erosional, and can be less fertile (carbonate soil cropping out at the surface, for example).

On the Tortolita piedmont, in Arizona, surface age and vegetation are related in the following manner, with the most dominant plant listed first (Pearthree et al., 1992):

Suggested Citation:"Indicators for Characterizing Alluvial Fans and Alluvial Fan Flooding." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
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FIGURE 3-6 Geologic maps of debris flow fans in the eastern Grand Canyon. SOURCE: Hereford et al. (1995).

Suggested Citation:"Indicators for Characterizing Alluvial Fans and Alluvial Fan Flooding." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
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TABLE 3-3 Surface Weathering Characteristics of Clasts on Fan-Forming Debris Flows, Eastern Grand Canyon

Characteristic

Debris Flow Age Category

 

Younger

Intermediate

Older

Carbonate coatings, underside of clasts

None

Stage I, discontinuous, thin, <0.1 mm

Stage I, discontinuous to continuous, thin, <0.5 mm

Splitting, spalling, and granular disintegration of sandstone clasts

None

Slight

Common

Tafoni

None

Present

Present and well- developed

Limestone-clast pittinga

None to incipient, <1 mm

Present, 1.47 to 4.04 mm

Present, 4.74 to 9.97 mm

Rilling of limestone-clasts

None

None

Present on 5 percent of clasts

Rock varnish, sandstone clasts

Absent to incipient on 50 percent of clasts

Present on all 50 to 100 percent of clasts, brown to dark brown

Well-developed on all clasts, dark brown to black

a Average depth of solution pits measured with a depth micrometer; number of individual measurements = 1,156 and 1,301 of intermediate-age and older, respectively. Measurements made on the surfaces of 149 and 96 intermediate-age and older clasts. SOURCE: Hereford et al. (1995).

Late Holocene

Ironwood, grasses (after rainy season), Palo Verde, mesquite, bushes

Early Holocene

Palo Verde, ironwood, cholla, bushes

Latest Pleistocene

Bursage bushes, Palo Verde, cholla

Late Pleistocene (100 ka)

Saguaro, cholla, Palo Verde, bursage

Because recent deposits are likely to be within a zone of frequent flooding, it is unlikely that mature vegetation will occur on historical deposits. It sometimes is possible to find evidence of flood damage on vegetation, thus providing a clear means of identifying parts of fans that recently have been active.

Types of Alluvial Fan Flooding

Alluvial fan flooding, as described in the committee's definition, is characterized by flow path uncertainty below the hydrographic apex and caused by abrupt deposition of sediment, proximity to a sediment or a debris flow source area, sufficient energy to carry coarse sediment at shallow flow depths, and the absence of topographic confinement which may allow higher flows to initiate a new, distinct flow path of uncertain direction. Although such flooding occurs on the active part of an alluvial fan, the fact that an area is defined as active does not mandate that it also

Suggested Citation:"Indicators for Characterizing Alluvial Fans and Alluvial Fan Flooding." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

FIGURE 3-7 Time stratigraphy, physical stratigraphy, and archeological chronology, eastern Grand Canyon. SOURCE: Hereford et al. (1995).

is subject to 100-year alluvial fan flooding. Riverine flooding also occurs along the channels of many alluvial fans, especially those that are deeply incised. Even though flood hazards happen to be on alluvial fan landforms, they should be dealt with by FEMA under the guidelines established for river floodplains.

Identifying those parts of the active part of alluvial fans that are susceptible to alluvial fan flooding also requires examination of the types of flooding as recorded by flood deposits. The

Suggested Citation:"Indicators for Characterizing Alluvial Fans and Alluvial Fan Flooding." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

final result of such investigation should be a map of areas of different types of flood hazards. This map is not the same as a Flood Insurance Rate Map, which is a map of different flood hazard zones. The map recommended here is one that delineates the boundaries of areas of all types of flood hazards that occur on the active parts of the alluvial fan. Such a map requires the identification of locations where flow paths are uncertain, where erosion and deposition are likely to occur, where channels with confined flow exist, where channel avulsions have occurred or might occur, where sheet flow occurs, where debris flows occur, and where channelized streamflow with overbank flooding occurs. Mapping similar to that recommended here has been done, for example, for the Alberta Creek fan in Canada (Kellerhals and Church, 1990). Such maps can be included in flood insurance studies.

Defining Flooding along Stable Channels

It is not uncommon for active parts of fans to contain stable channels that will not be susceptible to alluvial fan flooding. These channels might become unstable in the downstream direction, as in the case of entrenched alluvial fans. On the other hand, unstable channels can become stable in the downstream direction, as in the case of the dissected toe of the Cedar Creek alluvial fan shown earlier (Figure 3-2a). FEMA maps of alluvial fans should strive to indicate those channels susceptible to riverine flooding as well as those areas prone to alluvial fan flooding.

Identifying Areas Where Sheetflow Deposition Occurs

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 can be classified as alluvial fan flooding processes if they occur on alluvial fans. Sheetflow generally occurs 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 ground water; during prolonged rainfall, the ground can become saturated, resulting in extensive sheetflooding as runoff arrives from unslope. 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.

Identifying Areas Where Debris Flow Deposition Occurs

Some parts of alluvial fans are characterized by debris flows. Debris flows pose hazards that are very different from those of sheetflows or water flows in channels (see Chapter 2). 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

Suggested Citation:"Indicators for Characterizing Alluvial Fans and Alluvial Fan Flooding." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

deposits by differences in morphology, depositional relief, stratigraphy, and clast fabric (Figure 3-8; Table 3-4). Exposures in channel banks can be examined and can be supplemented with shallow trenches in different deposits. In an example of a channel bank exposure described by Hereford et al. (1995) in the eastern Grand Canyon, debris flow deposits are interbedded with streamflow gravels, but can be distinguished by the differences in stratigraphy and clast fabric (Figure 3-9).

STAGE 3: DEFINING AND CHARACTERIZING AREAS OF 100-YEAR ALLUVIAL FAN FLOODING

For FEMA to carry out the mandates of the National Flood Insurance Program (NFIP), areas that are subject to flooding during a 100-year flood—that is, areas subject to a 1 percent chance of flooding in any year—must be identified. The two previous sections described methods of identifying landforms subject to alluvial fan flooding and the active portions of the fan that are subject to flooding. But identification of possible hazard areas is only the first step. The third step, one that is critical for floodplain managers and regulators, is to determine the severity and to delineate the extent of the 100-year flood, that is, the area exposed to a 1 percent risk of flooding in any given year. Although field work and study of aerial photographs and topographic maps are essential for carrying out the three stages necessary to identify alluvial fans and stable and unstable components of fans, the three-stage analysis can be quantified by the use of hydrologic methods. Although it is beyond this committee's scope and resources to explore in detail the numerous methods that have been developed to evaluate flood hazards, it is appropriate to give a general overview of the methods available to delineate the actual flood hazards on a fan. Thus this section briefly addresses the techniques, the types of analysis, and the appropriate perspectives that may be of assistance in the delineation of the hazards on alluvial fans and explores their potential for assisting FEMA in its mapping responsibilities. This discussion is not intended to be a complete exploration of all the methodologies that have been developed for hydraulic analyses, but rather it is a general introduction to several methods currently in use. In the future FEMA might consider conducting a detailed review of these methods and how they are applied

The mapping of flood risks for purposes of the NFIP is based on the flooding that is likely to result from an event that has the probability of occurrence of 1 percent in any given year, an event more commonly known as the 100-year flood. Within relatively stable river systems, it has been a standard practice to delineate the 100-year floodplain using a modeling technique based on the assumption that the flow is clear water and the hydraulic conditions are such that flow is gradually varied. In many instances, this technique also is used to model more dynamic systems with some acknowledgment of its limitations, because the areas of hazard within a river valley are usually apparent and confined to a geologic floodplain.

Areas subject to alluvial fan flooding often are not as readily apparent as those subject to riverine-type flooding. The physical characteristics of the fan-shape also make the use of simplifying assumptions seem less logical and therefore less acceptable. Active alluvial fans are changeable, and erosion and deposition occur to some degree with most events. Inactive fans may also have flow paths that are unconfined and subject to uncertainty largely because of the numerous channel forks and joins.

Suggested Citation:"Indicators for Characterizing Alluvial Fans and Alluvial Fan Flooding." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

FIGURE 3-8 Morphologic and stratigraphic characteristics of different flow types developed from an example fan in England. SOURCE: Reprinted with permission from Wells and Harvey (1987).

When floodwater contains a significant amount of sediment or the flooded area is subject to scour and deposition, the flow behavior becomes less predictable. High concentrations of sediment and debris in flowing water can cause it to behave differently than clear water flows. Some of these differences, such as the unit weight, are quantitative in nature. Other differences, such as the vertical velocity distribution for a debris flow, display qualitative differences when compared to clear water.

Suggested Citation:"Indicators for Characterizing Alluvial Fans and Alluvial Fan Flooding." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

TABLE 3-4 Summary of Morphologic and Sedimentologic Field Criteria for Distinguishing Facies Types

Facies Type

Morphology

Depositional Relief (m)

Texture and Stratification

Characteristics of Clast Fabric

Debris flow (D1)

Lobate to digitate; narrow; steep front and flanks; flat tops with low relief pressure ridges

High (0.8–1.5)

Matrix-rich (muddy); matrix-supported clasts; poorly sorted; bmax range 80–210 mm; stratification absent

Elongate clasts oriented parallel to flow boundary forming a push fabric

Dilute debris flow (D2)

Thin lobate; broad, flat top; gentle lobe fronts and flanks

Moderate (0.3–0.5)

Matrix-rich; matrix-supported clasts; poorly sorted; bmax range 60–230 mm; stratification absent

None observed

Transitional flow deposits (T1)

Stacked lobes; broad small superimposed mounds; small collapse depressions

High (0.5–1.5)

Clast support with no matrix in upper few centimeters; matrix (sandy) increases with depth bmax typically <180 m; moderately sorted; stratification present

Collapse packing

Fluvial boulder bar and lobes (S1)

Linear bars to transverse lobes

Moderate to high (0.5–0.8)

No matrix; clast support; front-to-tail sorting; bmax typically >200 mm

Imbrication

Fluvial longitudinal bar (S2)

Linear bars

Moderate (0.2–0.5)

Clast support; matrix (sandy) increases with depth; market front-to-tail sorting; more poorly sorted than type S1; bmax typically <120 mm

Strong imbrication

Fluvial sheet deposits (S3)

Broad and flat; some fan-shaped; subdued bar and swale forms

Low (–0.1)

Clast support; little matrix (sandy); well stratified; normal grading in some strata; moderate sorting in each stratum; bmax typically <100 mm

Weak imbrication

 

SOURCE: Reprinted with permission from Wells and Harvey (1987).

Suggested Citation:"Indicators for Characterizing Alluvial Fans and Alluvial Fan Flooding." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

FIGURE 3-9 Interbedded debris flow and streamflow gravel, eastern Grand Canyon. SOURCE: Hereford et al. (1995).

AVAILABLE METHODS OF ANALYSIS

To investigate flood hazards, there are three general categories of interest: clear water flows that can be analyzed with traditional hydraulic methods, hyperconcentrated sediment flows that can be analyzed to a great extent by sediment transport theory, and debris flows that can be assessed by various empirical methods such as the bulking factor, the Bingham model, and other methods.

Appendix 5 of FEMA 37, Guidelines and Specifications for Study Contractors (1995), describes a method for delineating the boundaries of flood hazards on a fan-shaped surface. This method, however, is the cause of some confusion. The method considers the conditional probability of the occurrence of a flood with a given magnitude, taking a certain path through the spatial domain, and inundating a point of interest. The equation that allows one to apply this method is called the total probability equation. Its purpose is to compute, for example, when the combined probability of two events is equal to 0.01. The events can be the occurrence of a flood, the failure of a levee, the coincidence with a different flood, the chance that floodwaters take a certain flow path, and so on. The purpose of using this method is to account for uncertainty when it cannot be easily set aside.

Suggested Citation:"Indicators for Characterizing Alluvial Fans and Alluvial Fan Flooding." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

The use of the total probability equation is not limited to alluvial fans, and it is used by other federal agencies in addition to FEMA (NRC, 1995). The method of solving the total probability equation proposed by Dawdy (1979) has been used in the preparation of several FIRMs in the western United States. This method assumes that all areas of the fan are subject to flooding and that there is a fixed relationship between flooding depth and discharge. These assumptions apply when there is absolute uncertainty regarding how floods will occur. The advantages of these assumptions are that they are reproducible, they lend themselves to uncomplicated regulatory implementation, and, in certain situations, they are the easiest assumptions to defend. FEMA has developed a computer program called FAN (FEMA, 1990) that incorporates these assumptions, and it provides this program to contractors charged to delineate alluvial fan flooding.

When it comes to mitigation and the implementation of floodplain management regulations, however, it may be appropriate to review the assumption of complete uncertainty. There may be historical flow paths that are preferred during small floods. From a mitigation perspective, it would make sense to reinforce these paths rather than ignore them. The current Flood Insurance Rate Maps (FIRMs) that have been prepared using the procedure recommended by FEMA are a statement of complete uncertainty. These FIRMs then are not necessarily useful to floodplain mangers and regulators (who are often unaware of the procedures followed to identify the hazard) to assist them in determining the hazards on a particular fan area.

Since the decision on how or whether to solve the total probability equation is usually made by the flood insurance study contractor, the safe, default assumption of complete uncertainty is typically embraced to save, among other things, time. Most of the alluvial fan areas examined by the committee, however, show obvious, preferred flow directions. Alternative solutions to the total probability equation can be applied to these areas, but the guidelines in FEMA 37 (1995) are not clear about this and suggest only that the default assumption should be a starting point. Furthermore, permission to deviate from this assumption must be obtained in writing from FEMA.

All flooding sources have uncertainty. There is an apparent contradiction between the existing definition of alluvial fan flooding, which is very inclusive, and the actual method being used to delineate the hazard, which is limited to fan-shaped landforms. Flood behavior is predictable within the expected range of uncertainty. When the uncertainty can no longer be set aside but must be dealt with directly to achieve a reasonable result, then the total probability equation becomes a useful method for delineating flood hazards. The applicability of the method, however, does not mean that an area is subject to alluvial fan flooding. It is merely a way of expressing uncertainty.

FEMA has not developed guidelines on the general solution of the total probability equation. The committee recommends consideration of the use of Guidelines for Risk and Uncertainty Analysis in Water Resources Planning (USACE, 1992) for specific guidelines on how to apply the method.

The principles of risk-based analysis (USACE, 1992) provide a framework for a more general and realistic way to identify areas subject to flooding with an annual probability of 1 percent. 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 (for example, an alluvial fan with a series of branching channels). Figure 3-10 shows a flow diagram for conducting an analysis of diverging channels by considering various scenarios. Figure 3-11

Suggested Citation:"Indicators for Characterizing Alluvial Fans and Alluvial Fan Flooding." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

FIGURE 3-10 Analysis of flow path uncertainty considering possible scenarios.

Suggested Citation:"Indicators for Characterizing Alluvial Fans and Alluvial Fan Flooding." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
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FIGURE 3-11 Conditional nonexceedance probability estimation with event sampling.

Suggested Citation:"Indicators for Characterizing Alluvial Fans and Alluvial Fan Flooding." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

shows an example of estimating conditional nonexceedance probability using event sampling.

The broad spectrum of types of flooding that can occur and that have been observed on alluvial fans illustrates the futility of developing a ''cookbook" method to apply to all fans in all geographic areas. Reviews of current research and discussions with local officials charged with regulating development in alluvial fan flooding areas indicate a prevailing preference for analysis of the flood hazards based on site-specific evaluations. The types of information that should be gathered include both geomorphic and process considerations. For example, some flow associated with channels meets the criteria for alluvial fan flooding, even though it occurs in channels in much the same way that riverine flow occurs in channels. The difference between the two is that in alluvial fan flooding the channels are likely to shift position with time and flows often abandon one channel to form another, resulting in much unpredictability regarding the locations of future flow paths. The following questions can provide guidelines to identify areas where flow paths are uncertain and flow is likely to leave confined channels to move in unpredictable directions:

  • Where is there evidence of recent channel shifting?

  • Where is there evidence of recent channel avulsion?

  • Where is there evidence of the potential for channel avulsion?

  • Where has channel geometry changed markedly in recent time?

Because alluvial fan flooding is associated with high rates of erosion, sediment transport, and deposition, it is common for such flows to shift position as sediment is dropped and forms obstructions to the flow. In some events, previous channels are completely blocked by deposits, and a new channel is formed. This process is known as avulsion and can be identified from aerial photos or field mapping by the presence of topographic lows (abandoned channels), the upstream parts of which filled with sediment. Areas of potential channel avulsion sometimes can be identified from construction of longitudinal and cross-fan profiles, because avulsion is likely to occur in places where sedimentation has raised the channel floor surface to a level that is nearly as high as the surrounding surface of the alluvial fan.

In addition, human modification of alluvial fan surfaces and urban development on alluvial fans have resulted in cases where human-made obstructions themselves have been the cause of alluvial fan flooding. For example, construction of culverts to divert water from one part of a fan to another sometimes results in rapid sedimentation downstream from the mouth of the culvert. The result can be that alluvial fan flooding then occurs in an area that might not have been mapped as susceptible to this type of flooding before human alteration of the landscape. Special attention is needed to identify areas where engineered works might aggravate or cause alluvial fan flooding during the time period designated as active by the investigator.

Specific steps that should be followed before undertaking any final delineation of alluvial fan flooding hazards include detailed office and field reviews of historical information and the evaluation of the present landform. Initial office procedures include the review of topographic maps and aerial photographs to determine the location and the morphology of the landform to determine whether it is a true alluvial fan. Other data that should be gathered early include historical maps and old photographs to document channel changes, changes in channel morphology, and the areas of the fan that may be classified as either active or inactive. Soil and geologic mappings should be examined to confirm the relative geologic age of fan deposits. Climatologic data and appropriate hydrologic analyses will be needed to determine the magnitude

Suggested Citation:"Indicators for Characterizing Alluvial Fans and Alluvial Fan Flooding." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

and frequency of flooding to be addressed. Aerial photographs and geologic information of the catchment area will provide indications of the amount of sediment and debris that can be delivered to the fan.

Field investigations by a trained observer should include gathering information on elevation differences across the fan and in a transverse direction if detailed topographic maps are not available. Vegetation types, soil characteristics, and the presence of desert varnish should be added to the office maps to confirm the active or inactive portions of the fan. Observations and measurements of channel conditions should be made to determine areas of possible avulsion. Detailed inspection of diffluences or abandoned channels should indicate the most likely flow paths. The results of the initial office and field investigations should provide sufficient information to direct the final analysis.

SUMMARY

The previous three stages demonstrate that flood risk on alluvial fans is not unpredictable, but rather that it is predictable with varying degrees of uncertainty. The assumption of a uniform risk (FEMA, 1995) or complete uncertainty across an alluvial fan can be used as a formalized guess that allows one to delineate risk on the FIRM using a straightforward technique. This technique may be reasonable for the delineation of hazards on certain alluvial fans. The method proposed by Dawdy (1979) is an insightful application of the total probability equation. Although the assumptions used to solve the equation may vary for each situation, the method itself is sound and quite general.

A FIRM showing alluvial fan flooding hazards mapped considering complete uncertainty is of little use for floodplain management. By making a conservative trade-off in favor of all possibilities, this type of FIRM ignores the importance and the more threatening hazard of flow in existing channels and historical flow paths and conversely penalizes safer areas.

The FEMA Guidelines and Specification for Study Contractors (1995) asserts that flow paths for alluvial fan flooding are unpredictable and the assumption of uniform uncertainty must be used in the hazard delineation unless written approval is sought. Approaching the wide range of alluvial fan flooding conditions from the inflexible perspective of this special case is part of the reason for the conflict surrounding this matter.

The committee recommends that all efforts at mapping start with the existing channel. For situations where there is an entrenched channel on an alluvial fan, the uncertainty may be set aside. However, elsewhere the uncertainty associated with flow path direction might cause one to select FEMA's uniform risk method. For the majority of the cases, however, consideration of specific, foreseeable scenarios based on stages 1 and 2 make the most sense.

For some undissected fans, the assumption of uniform flow path uncertainty may apply. Such cases are not in the majority, and yet they are the only cases where the computer program FAN (FEMA, 1990) might be applicable.

Suggested Citation:"Indicators for Characterizing Alluvial Fans and Alluvial Fan Flooding." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

REFERENCES

Bull, W. B. 1963. Alluvial fan deposits in western Fresno County, California. Journal of Geology 71:243–351.

Bull, W. B. 1964. Alluvial fans and near surface subsidence in western Fresno County, California. U.S. Geological Survey Professional Paper 437-A. Reston, Va.: U.S. Geological Survey.

Bull, W. B. 1977. The alluvial fan environment. Progress in Physical Geography 1(2):222–270.

Bull, W. B. 1991. Geomorphic Response to Climatic Change. New York: Oxford University Press.


Christenson, G. E., and C. Purcell. 1985. Correlation and age of Quaternary alluvial fan sequences, Basin and Range province, southwestern United States. Pp. 115–122 in Soils and Quaternary Geology of the Southwestern United States. GSA Special Paper 203. Boulder, Colo.: The Geological Society of America.

Cooke, R., A. Warren, and A. Goudie. 1993. Desert Geomorphology. London, England: University College London Press.


Dawdy, D. R. 1979. Flood frequency estimates on alluvial fans. American Society of Civil Engineers Journal of the Hydraulics Division 105(HY11):407–1413.

Dorn, R. I. 1994. The role of climatic change in alluvial fan development. Pp. 593–615 in Geomorphology of Desert Environments, A. D. Abrahams and A. J. Parsons, eds. London, England: Chapman and Hall.


Federal Emergency Management Agency (FEMA). 1990. FAN: An Alluvial Fan Flooding Computer Program, User's Manual and Program Disk. Washington, D.C.: FEMA.

Federal Emergency Management Agency (FEMA). 1995. Guidelines and specifications for study contractors. Document no. 37, Appendix 5: Studies of alluvial fan flooding, Washington, D.C.: FEMA.


Hereford, R., K. S. Thompson, K. J. Burke, and H. C. Fairley. 1995. Late Holocene debris fans and alluvial chronology of the Colorado River, Eastern Grand Canyon, Arizona. U.S. Geological Survey Open-File Report 95-97. Reston, Va.: U.S. Geological Survey.

Hydrologic Engineering Center (HEC). 1990 HEC-2 Water Surface Profiles, User's Manual. Davis, Calif.: U.S. Army Corps of Engineers Water Resources Support Center.


Keaton, J. R. 1988. A Probabilistic Model for Hazards-Related Sedimentation Processes on Alluvial Fans in Davis County. Ph.D. dissertation. Texas A&M University, College Station.

Kellerhals, R., and M. Church. 1990. Hazard management on fans, with examples from British Columbia. In Alluvial Fans: A Field Approach, A. H. Rachocki and M. Church, eds. New York: John Wiley & Sons.


MacArthur, R. C. 1983. Evaluation of the effects of fire on sediment delivery rates in a southern California watershed. In Proceedings of the D. B. Simons Symposium on Erosion and Sedimentation, Colorado State University, Fort Collins. July 27–29, 1983.


National Research Council. 1995. Flood Risk Management and the American River Basin: An Evaluation. Washington, D.C.: National Academy Press.


Pearthree, P. A., K. A. Demsey, J. Onken, K. R. Vincent, and P. K. House. 1992. Geomorphic Assessment of Flood-Prone Areas on the Southern Piedmont of the Tortolita Mountains, Pima County, Arizona . Arizona Geological Survey Open-File Report 91-11. Tucson, Ariz.: Arizona Geological Survey.

Suggested Citation:"Indicators for Characterizing Alluvial Fans and Alluvial Fan Flooding." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

Ritter, J. B., J. R. Miller, Y. Enzel, S. D. Howes, G. Nadon, M. D. Grubb, K. A. Hoover, T. Olsen, S. L. Reneau, D. Sack, C. L. Summa, I. Taylor, K. C. N. Touysinhthiphonexay, E. G. Yodis, N. P. Schneider, D. F. Ritter, and S. G. Wells. 1993. Quaternary evolution of Cedar Creek alluvial fan, Montana. Geomorphology 8:287–304.

Ritter, D. F., R. C. Kochel, and J. Miller. 1995. Process Geomorphology, 3rd Ed. Dubuque, Iowa: Times Mirror Higher Education Group.


Skinner, B. J., and S. C. Porter. 1995. The Blue Planet. New York: John Wiley & Sons.


U.S. Army Corps of Engineers (USACE). 1992. Guidelines for Risk and Uncertainty Analysis in Water Resources Planning. Report 92-R-1. Fort Belvoir, Va.: USACE Water Resources Support.


Wells, S. G., and A. M. Harvey. 1987. Sedimentologic and geomorphic variations in storm-generated alluvial fans, Howgill Fells, northwest England. The Geological Society of America Bulletin 98:182–198.

Suggested Citation:"Indicators for Characterizing Alluvial Fans and Alluvial Fan Flooding." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
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Alluvial fans are gently sloping, fan-shaped landforms common at the base of mountain ranges in arid and semiarid regions such as the American West. Floods on alluvial fans, although characterized by relatively shallow depths, strike with little if any warning, can travel at extremely high velocities, and can carry a tremendous amount of sediment and debris. Such flooding presents unique problems to federal and state planners in terms of quantifying flood hazards, predicting the magnitude at which those hazards can be expected at a particular location, and devising reliable mitigation strategies. Alluvial Fan Flooding attempts to improve our capability to determine whether areas are subject to alluvial fan flooding and provides a practical perspective on how to make such a determination. The book presents criteria for determining whether an area is subject to flooding and provides examples of applying the definition and criteria to real situations in Arizona, California, New Mexico, Utah, and elsewhere. The volume also contains recommendations for the Federal Emergency Management Agency, which is primarily responsible for floodplain mapping, and for state and local decisionmakers involved in flood hazard reduction.

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