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Alluvial Fan Flooding (1996)

Chapter: Applying the Indicators to Example Fans

« Previous: Indicators for Characterizing Alluvial Fans and Alluvial Fan Flooding
Suggested Citation:"Applying the Indicators to Example Fans." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
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4
Applying the Indicators to Example Fans

Not all alluvial fans, or those geologic features that are commonly believed to be alluvial fans, are subject to alluvial fan flooding. To show how dramatically such sites can vary, the committee selected seven sites for in-depth analysis and applied the indicators presented in Chapter 3. The sites represent a wide range of flood processes, from unconfined water flooding and debris flows on untrenched active fans to confined water flooding in fully trenched inactive alluvial fans. Six alluvial fans in the western United States are used to illustrate different flood processes, and a group of fans in Virginia illustrate a particular type of flood hazard in the eastern United States (Figure 4-1). By applying the indicators to each of the example sites, the committee was able to see whether or not the site meets the criteria suggested earlier in the proposed definition of an alluvial fan. In addition, insights are gained about how the definition and the indicators function in the field, and the advantages and disadvantages to those who ultimately will have to apply the guidance in a regulatory context.

Each of the examples also represents a different amount of study (Table 4-1). The Arizona examples show the problems faced in major urbanizing areas where there is intense interest and resources to support detailed investigation. The California examples represent a modest amount of study that included a brief field reconnaissance of each fan and compilation of geologic, topographic, and soil maps and aerial photographs. A similar approach was used to characterize the Utah fan, including examination of many technical reports produced following unusual flooding of 1983 and 1984. An exhaustive study of technical literature was the basis to typify the Virginia fans, which illustrate that alluvial fan flooding is not strictly a western phenomenon. Four of the sites were inspected by the committee, and the other three sites were inspected by at least one committee member. However, the committee wants to emphasize that it did not conduct a thorough field investigation of any site, as would be required for regulatory purposes, and thus these examples are purely illustrative and not intended to influence decision making on these fans.

HENDERSON CANYON, CALIFORNIA

The Henderson Canyon alluvial fan, which is located in eastern San Diego County near Borrego Springs, California, is below a drainage basin of approximately 16.6 km2 (6.40 mi2) that

Suggested Citation:"Applying the Indicators to Example Fans." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

FIGURE 4-1 Six fans in the western United States are used to illustrate different flood processes, and a group of fans in Virginia illustrate a particular type of flood hazard in the eastern United States.

Suggested Citation:"Applying the Indicators to Example Fans." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

Table 4-1 Amount of Study, Sedimentation, Major Flood Processes, Flow Path Movement, and Relevant Comments for Example Fans (sites are listed in order from the West Coast of the United States)

Site

Amount of Study

Sedimentation

Major Process

Subject to Alluvial Fan Flooding

Comments

Henderson Canyon, California

Modest

Inactive and active

Water flood

Yes

Flooding confined to large trenches on relict fan. Sheetflooding on active fan. The use of maps, aerial photographs, soil surveys, and field reconnaissance is described in the example. See Appendix A.

Thousand Palms, California

Modest

Active

Water flood

Yes

Sheetflooding on fan. The general alignment of the fan has been altered by faulting.

Lytle Creek, California

Modest

Inactive

Water flood

No

Flooding confined to a single large trenched channel. See Appendix A.

Tortolita Mountains, Arizona

Extensive

Inactive

Water flood

No

Network of flow paths has appearance of active fan, but flow paths were stable during major flood. See Wild Burro alluvial fan in Appendix A.

Carefree, Arizona

Extensive

Inactive

Water flood

No

Flow is confined to network of trenched distributary channels with no evidence of flow path movement. The use of soil surveys by the Natural Resources Conservation Service is described in the example. See Appendix A.

Rudd Creek, Utah

Average

Active

Debris flow and water flood

Yes

Major debris flow in 1983 damaged or destroyed many homes. Episodes of debris flows are on the order of once every 100 to 1,400 years. See Wasatch Front alluvial fans in Appendix A.

Nelson County, Virginia

Average

Active

Debris flow and water flood

Yes?

Episodes of debris flows are on the order of once every 3,000 to 4,000 years.

Suggested Citation:"Applying the Indicators to Example Fans." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

heads on the eastern slopes of rugged mountains at an elevation of 1,420 m (4,659 feet) (Figures 4-2 and 4-3). Most of the mountainous basin above an elevation of 488 m (1,601 feet) is practically barren of vegetation, and runoff from the steep slopes is rapid. The region is tectonically active, but active faulting is generally located to the north and south of the basin and alluvial fan (Sharp, 1972). The alluvial fan is an example of an arid-clime composite fan with both relict debris flow and modern water flow processes where hazards on the relict fan have been significantly altered by geologically recent channel trenching.

Recognizing and Characterizing Alluvial Fans

Determining whether or not a Landform is an Alluvial Fan

This landform, known as the Henderson Canyon alluvial fan, was identified as an alluvial fan using the criteria defined in Chapter 3 for material composition, morphology, and location.

Composition

The site is identified as "sloping gullied land" of an alluvial fan on National Resource Conservation Service (NRCS, 1973) soil maps on a 7.5-minute series orthophoto base. It consists of alluvial sediments derived from igneous, sedimentary, and metamorphic rocks. Carrizo soil, which is gravelly sand derived from granitic alluvium and is also associated with alluvial fans, is shown on the lower fan near the valley. The type and relative position of the mapped soils suggest entrenched channels in a relict fan with an active fan downslope in the Carrizo soil.

Only upon field inspection of the alluvial fan was it clear that modern channels are deeply trenched into relict debris deposits of the sloping gullied land shown on the soil map (NRCS, 1973). Numerous massive mounds of debris flow deposits are composed of many 0.15-to 0.9-m (0.5 to 3.0-foot) boulders, and many of the fines have been washed from the debris matrix, forming sandy interlobe areas (Figure 4-4). Deposits are massive with distinct boundaries readily observed by field inspection; there is inverse grading and there is a concentration of large boulders at the snout of the deposited lobes.

Two major entrenched channels combine in the relict material to form a single entrenched channel that leads to modern alluvial deposits of gravelly sand and scattered cobbles. On the modern deposits, there is some stratification of thin beds that appear to have been deposited as large sheets. These loose and friable deposits are mapped as Carrizo-type soil. The fan is thus composed of relict and modern alluvial deposits like those of an alluvial fan.

Morphology

The site has the general appearance of a sector of a cone with concentric contour lines that are generally convex downslope and laterally confined in an "embayment" within the general alignment of a mountain front. The form and general bounds of the site can be readily identified on the available 7.5-minute series USGS topographic map (Borrego Palm Canyon Quadrangle). The landform is shaped like a partly extended fan that attenuates at the lateral bounds of a large valley to the east.

Location

The Henderson Canyon fan is located at a topographic break in lateral confinement at the upper end of the embayment. Many such breaks, although subtle, were observed by

Suggested Citation:"Applying the Indicators to Example Fans." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

FIGURE 4-2 Henderson Canyon drainage basin showing relict alluvial fan boundaries, location of active alluvial fan, location of apex of relict fan, and location of apex of active fan.

Hjalmarson and Kemna (1991) using channel profiles of the change in channel slope between topographic map contours. Hooke (1967) described this flattening and steepening of channel slope where confinement is lost at the apex (or intersection point). This break is not apparent on the channel profile defined using the USGS topographic map (Figure 4-3) possibly because the profile represents modern drainage effects. Rather, the surface of this relict fan is a few meters above the present trenched stream channel as observed on the USGS topographic map when used in conjunction with color-infrared aerial photographs obtained from the EROS Data Center of the USGS and orthophoto maps. The topographic break is located near the confluence of two major mountain streams at the upslope edge of relict alluvial deposits. There is a significant change in the surface texture at this location as shown on the aerial photographs and orthophoto maps.

Suggested Citation:"Applying the Indicators to Example Fans." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

FIGURE 4-3 Profile of Henderson Canyon relict alluvial fan near Borrego Springs, California.

FIGURE 4-4 View looking downslope and across to the south from center of Henderson Canyon relict alluvial fan at typical boulder mound 1.2 to 1.8 m (3.9 to 5.9 feet) high, March 21, 1995. Courtesy of H. W. Hjalmarson.

Suggested Citation:"Applying the Indicators to Example Fans." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×
Defining the Lateral Boundaries and Topographic Apex of the Alluvial Fan

The lateral bounds are at the toe of steep-bedrock mountain slopes that form the embayment. Below the mountain front the lateral bounds are defined by topographic ridges between fan drainage channels and the adjacent drainage channels. A swale-like drainage that traverses the fan at the western edge of Borrego Valley forms the fan toe. The fan toe closely coincides with the lower limits of the Carrizo soil unit shown on the NRCS soil survey maps. The general bounds of the alluvial fan can be readily identified on the available 7.5-minute series USGS topographic map (Borrego Palm Canyon Quadrangle).

Defining the Nature of the Alluvial Fan Environment

A few large channels and an area of active sedimentation are readily apparent on the color-infrared aerial photographs. The location of these channels coincided with evidence of trenched channels in the upper- and mid-fan areas, as suggested by the saw-toothed appearance of the contour lines on the USGS topographic map. The few large kinks in the contours that point upslope are typical of a fan surface with large incised channels. The field investigation revealed light grayish colored rock on the bed and banks of trenched channels, which is indicative of recent abrasion during sediment transport. The adjacent boulders in the debris lobes were lightly covered with rock varnish, which is indicative of a stable surface.

Field examination was needed to precisely locate the wide, flat hydrographic apex of the active alluvial fan located on the right, or south, side of the relict fan (Figure 4-2). The hydrographic apex is located at a gradual hydraulic expansion at the end of the large trenched channel.

Active Fan

There is active sedimentation below the hydrographic apex and active erosion above it. The average slope of the active fan is about 0.025 over a length of about 1.2 km (0.7 mi). The poorly defined channels are slightly braided, with large width-to-depth ratios in the upper fan. There are large sheetflood areas in the middle fan. The surface material appears to be very recent and has not developed a soil. The width of the active fan is approximately one-third the total width along the toe of the Henderson Canyon alluvial fan.

The lateral boundaries of the active fan were apparent on orthophoto and topographic maps at 1:2400 scale with 5-foot (1.52 m) contour intervals (furnished by the San Diego County Department of Public Works). These maps were available for the area below the major incised channels that included most of the active fan. The boundaries of the active fan area could be defined on these maps, especially when used in conjunction with the color-infrared aerial photographs. A few relict debris lobes also were indicated on the large-scale topographic maps, but most were attenuated and indistinct. Many sheetflood paths are clearly shown on the photographs, but the lateral bounds of sheetflood are indistinct. A field inspection was needed to distinguish between the sheetflood deposits of the active fan and the deposits below the

Suggested Citation:"Applying the Indicators to Example Fans." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

attenuated debris lobes of the relict fan. Both of these deposits are mapped as Carrizo-type soil (NRCS, 1973).

The upper (hydrographic apex) and lower limits (fan toe) of the active fan are approximately defined by the mapped limits of the Carrizo-type soil. However, based on a field inspection of recent sheetflood deposits, much of the toe is below the Carrizo soil at or slightly upslope of a transverse swale-like drainage at the western edge of Borrego Valley. The toe is indistinct, partly because the urban land has been altered by construction or obscured by structures, pavement, and golf courses.

Relict Fan

The mid-fan is composed of limey debris flow deposits that have been exposed along the entrenched channels. The exposed carbonates of the B horizon extend to depths of nearly 2 m (7.6 feet) along the steep banks of the entrenched channels and are indicative of soils that are more than 10,000 years old in this arid region (Machette, 1985).

There is no channel formation or other evidence of flow along the downfan side of the entrenched channel. Evidence of channel formation between the debris lobes is apparent only several hundred meters down the relict fan from the transverse channel. The few small channels in this area are formed by local runoff. The attenuated debris lobes in this area become undefined in the vicinity of the Carrizo soil downslope.

Mountainous Drainage Basin

Field observations of debris sources on the mountainsides revealed that most of the upper slopes are bare rock and apparently too steep for debris accumulation. The soil in the mountains is loamy coarse sand in texture and is sparse and shallow. The lower slopes of the mountains are covered with boulder debris that appears to be stable because the rock is covered with dark desert varnish and the slopes are less than the angle of repose of the rocks. No slumping was observed. There is one site of a geologically recent small debris slide on the southern side of the drainage basin where the hillslope is at least 38 degrees. This slide is apparent because the scar appears geologically fresh among the darkly varnished surrounding bedrock. Deposited rock from this slide is far from active channels and is not a significant source of material for debris flows down the fan. Little, if any, debris along the stream channels can be seen on aerial photographs. Because there is no known history of recent debris flows in the area and there is little evidence of geologically recent debris flow potential, large debris flows of the size that produced the many mounds are considered unlikely.

Storm of August 15–17, 1977

Tropical Storm Doreen produced from 7.6 to 12.7 cm (3.0 to 5 in) of rainfall in the vicinity of the Henderson Canyon alluvial fan. Most of the rain fell during a few hours on the evening of August 16, 1977, producing a peak discharge of 90.6 m3/s (3,200 feet3/s) at the apex

Suggested Citation:"Applying the Indicators to Example Fans." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

of the active alluvial fan about 0.8 km (0.5 mi) upslope of the De Anza Desert Country Club (San Diego County, 1977). This peak discharge was nearly equal to the 100-year flood based on methods described by Thomas et al. (1994). The flow split into two distinct paths upstream of the community. A short distance below the hydrographic apex, where the flow split the floodwaters became less confined and apparently coalesced as sheetflooding. The distribution of floodwater across the active fan at any particular time is unknown, but nearly all of the active fan was inundated at one time or another during the flood. Aerial photographs of flood remnants and hydraulic computations of peak discharge amounts suggest that floodwater covered most of the active fan at the time of the peak discharge. Large amounts of sand with gravel and a few small boulders were deposited throughout the community, and some floodflow passed through the Country Club, inundating farmland to the east. About 100 homes were damaged as previously effective drainage ditches and debris dams were overwhelmed by the floodwater and debris.

Floodwater from the drainage basin was conveyed in the incised channels on the relict fan to the hydrographic apex. Most of the basin flow was in the center channel and all downfan flow from the drainage basin was intersected by the channel, crossing the fan from the north.

Changes in Flow Path

A comparison of entrenched stream channels depicted on aerial photographs showed no discernible channel movement, enlargement, or formation on the relict fan. Three sets of aerial photographs obtained from the EROS Data Center of the USGS were used for the comparison. The photographs were good-quality black-and-white for 1954, poor-quality color-infrared for 1971, and excellent quality color-infrared for 1990. The large-scale orthophoto maps mentioned previously also were used for the assessment of changes in flow path.

Significant flow path change was not apparent on the active fan, but minor change of the sheetflood paths is suggested on the photographs and orthophoto maps. Because the paths are obscured by vegetation and possibly by eolian effects, the amount of movement is uncertain.

Characterizing Alluvial Fan Flooding Processes

Floods have eroded and apparently will continue to erode relict fan material and deposit it on the active fan. All the evidence points to the conclusion that the De Anza Desert Country Club, which is located on the lower portion of the active fan, is in the direct path of future sediment-laden water floods emanating from the 16.6-km2 (6.4-mi2) basin (Figure 4-2). The flood risk on the active fan is much greater than would be predicted by application of the FEMA procedure without recognition of the distinction between the active and the relict portions of the fan.

Defining Areas of Active Alluvial Fan Flood Hazard

Floodwater leaves the confines of the trenched channel at the hydrologic apex and spreads in two swales as sheetflood. The entire area of the active alluvial fan (Figure 4-2) is subject to alluvial fan flooding.

Suggested Citation:"Applying the Indicators to Example Fans." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

Where does Flow Depart from Confined Channels?

Below the hydrologic apex the flow paths are difficult to predict because flow is shallow and unconfined.

Where does Sheet Flow Deposition Occur?

Sheet flow deposition of sediment over most of the active fan, like that for the flood of August 16, 1977, can be expected during a single flood. Small amounts of sediment deposition on the upper active fan may cause changes in the paths of flow because the flood depths are small and flow is unconfined.

Where does Debris Flow Deposition Occur?

There is no evidence of recent debris flows.

Where are there Structures or Obstructions that Might Aggravate or Cause Alluvial Fan Flooding?

Nearly all of the area to the west of the hydrologic apex is within the Anza-Borrego Desert State Park and is not subject to development. The urban development of the De Anza Desert Country Club located on the lower portion of the active fan may alter flow paths and concentrate floodflow in streets and other open areas.

Where can the Flood Hazard be Mitigated by Means Other than Major Structural Flood Control Measures?

Only major structural controls will be effective because development is along the entire lower portion of the active fan.

Defining Areas of Nonalluvial Fan Flooding Hazard along Stable Channels

Much of the surface of the relict fan is above the level of flooding in the trenched channels and is not subject to alluvial fan flooding. Floodflow from the surrounding mountains is confined to trenched channels that traverse the relict fan. Fundamental hydraulic computations of channel capacity confirm this conclusion. Crude estimates of channel roughness, size, and slope using a hand level and surveying rod show that the channel capacity of the major channels is several times that needed to convey the 100-year flood, estimated using methods by Thomas et al. (1994), across the surface to the apex of the active fan.

Most of the runoff below the transverse channel has been from the relict fan itself, with a small amount from a small mountain basin to the north. Floodflow is less confined downfan in this area where the debris mounds are small and intermound areas have filled with sediment. Some sheetflooding and sedimentation along the toe of the relict fan are expected.

Determining the Type of Processes Occurring on the Active Parts of the Alluvial Fan

A brief field inspection of the deposited material of the active fan suggested the action of water flood processes in the following ways: Sheetflooding is indicated in the mid-fan area by the thin sorted beds of sand with silt and some gravel, which were loose and friable, and appeared continuous over large areas. The channels in the upper fan had very large width-to-depth ratios, indicating water flow. The deposits were permeable. There are no massive and unstratified deposits and no channels or debris mounds in the middle and lower fan that indicated debris flows. No indicators of debris flows (see Table 3-3 and Figure 3-8) were observed on the active fan.

Suggested Citation:"Applying the Indicators to Example Fans." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

THOUSAND PALMS WASH, CALIFORNIA

Thousand Palms fan is located in Riverside County, California. Runoff from the Little San Bernardino Mountains to the northeast is collected along the Indio Hills, mostly by Deception wash, a tributary to Thousand Palms wash, at the Mission Creek fault. The drainage area for Thousand Palms wash is 217 km2 (84 mi2). As the wash passes through the Indio Hills and crosses the San Andreas fault zone into the Coachella Valley, it flows onto a broad alluvial fan (Figure 4-5). There is a general lack of soil development and vegetation on the fan. Furthermore, the main channel loses definition shortly after passing through the apex.

Recognizing and Characterizing Alluvial Fans

The landform is identified on NRCS (1980) soil maps as an alluvial fan. The revised definition was applied to this example, and the landform was found to be an alluvial fan.

Determining whether or not a Landform is an Alluvial Fan

The fan-shape is apparent from topographic maps. Some of the soil material is from upstream alluvial fan deposits that have been removed by headcutting in response to strike-slip movement at the Mission Creek fault. The loose and friable deposits are in sheets or beds of sand and silt. The fan has classic concentric contours, but the center of the fan bends gradually to the left or east. Thus, the upper part of the fan faces southwest, while the lower part faces more to the south and east. This shape is related to the general morphology of the Coachella Valley, which lies to the south, and probably to slip faulting across the middle of the fan. There is considerable channel widening as Thousand Palms wash leaves the confines of the Indio Hills and crosses the San Andreas fault, where there has been vertical and lateral displacement.

The landform has the composition, morphology, and location to meet the committee's criteria for an alluvial fan.

Defining the Lateral Boundaries and Topographic Apex of the Alluvial Fan

The lateral bounds of the upper part of the fan are at the toes of steep slopes of older alluvial deposits that can be readily identified on the available 7.5-minute series USGS topographic map. Beyond the general alignment of the Indio Hills, the lateral bounds are the topographic trough lines on each side of the fan. These boundaries are swales and appear slightly concave downfan on the topographic map. The bounds generally correspond to the Carsitas and Myoma soils that are associated with alluvial fans (NRCS, 1980a). The western boundary is indistinct in places because of wind-blown deposits of sand and silt. The fan coalesces with a small fan to the west and with two small fans to the east.

Suggested Citation:"Applying the Indicators to Example Fans." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

FIGURE 4-5 Aerial photograph of Thousand Palms wash (1993). Courtesy of Aerial Fotobank, Inc.

Defining the Nature of the Alluvial Fan Environment

Active Fan

The topographic apex is located at the highest point on the fan where floodflow leaves the confines of a wide sand channel. The slope of the fan is approximately 2.5 percent near the apex and 0.5 percent at the margin where there is a transition to wind-blown sand dunes. The median

Suggested Citation:"Applying the Indicators to Example Fans." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

sediment grain size near the apex is approximately 1 mm (0.03 in) and, at the margin of the fan, 0.2 mm (0.007 in). No soil development was observed during a brief field inspection of the fan.

Relict Fan

There is no visible relict fan.

Mountainous Drainage Basin

The upper part of this complex basin is in the Little San Bernardino Mountains to the north. Three large alluvial fans have formed along the southern slopes of the mountains within the basin. Because of movement along the Mission Creek fault, the toes of the fans are gullied. Remobilized sediment from these fans is a major source of sediment for the Thousand Palms alluvial fan.

Flood of 1977

An aerial photograph of the fan taken shortly after the flood of 1977 depicts paths of recent sheetflooding over much of the upper part of the fan. It is unknown if there was simultaneous inundation over the fan as suggested by the flow lines. About 1 mile below the apex to the southeast, the flow paths were separated by small islands of sand dunes. The dune islands become larger downfan. About 2 to 3 miles below the apex the flow becomes channelized. Because wind-blown sands obscure flow patterns following large floods, the widespread flooding depicted on this aerial photograph, on file at the Coachella Valley Water District, is a primary basis for concluding the fan is active and subject to sheetfloods.

Flow Path Changes

Flow path movement is likely during major floods because of the low transverse relief, undeveloped soils, and evidence of channel bed aggradation where coarse deposits may force water overbank and form new paths.

Characterizing Alluvial Fan Flooding and Sedimentation Processes

The braided flow paths appear uncertain just downstream of the topographic apex. Evidence of sheetflood is apparent from both field inspection and review of historical accounts of flooding on file at the Coachella Valley Water District. Water flood processes are suggested by the stratified deposits of silt, sand, and some gravel which are highly permeable and friable. There are also scattered cobbles in the upper fan. The channels in the upper and middle fan have very

Suggested Citation:"Applying the Indicators to Example Fans." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

large width-to-depth ratios. There are no debris mounds or unstratified cemented deposits that are characteristic of debris flows.

There are defined flow paths on the alluvial fan that convey floods of smaller-magnitude. The absence of vegetation and lack of soil development on the fan indicate that recent alluvial flood deposits are extensive. The area subject to sheetflooding closely corresponds to the Carsitas cobbly sand soil (NRCS, 1980a). Floods of large magnitude are subject to flow path uncertainty on the upper part of the fan.

The relatively steep fan slope and the absence of topographic confinement create a condition where measures such as set backs or elevation on fill may not reliably mitigate flood hazards on much of the fan. Structural mitigation of the flood hazard is required.

Thousand Palms Wash fits both the existing and the proposed definition for alluvial fan flooding.

LYTLE CREEK, CALIFORNIA

Lytle Creek is located in San Bernardino County, California. At the topographic apex, it drains approximately 50 square miles of the San Gabriel Mountains, which are composed of highly fractured rock at steep slopes. Erosion from the watershed produces a high yield of very coarse sediment. The fan slope is almost 3 percent. The main channel is incised as it leaves the mountains (Figure 4-6). Since the 1940s a series of spur dikes and levees have been built to confine the flows to a narrow corridor along the fan. Lytle Creek eventually combines with Cajon Creek before entering the Santa Ana River.

This fan is one of a series that consist of unconsolidated alluvial deposits to the south of the Sierra Madre fault zone. The piedmont is a complex bajada with several trenched and untrenched alluvial fans. For example, the Cucamonga fan, to the west, is deeply trenched, but the nearby Day and Deer creek fans, also to the west of this site, are not entrenched and have areas of active sedimentation and flooding.

Recognizing and Characterizing Alluvial Fans

This landform was identified as an alluvial fan using the committee's criteria (defined in Chapter 3) for material composition, morphology, and location. This landform was identified by both Eckis (1928) and the Natural Resources Conservation Service (1980b) as an alluvial fan.

Determining whether or not a Landform is an Alluvial Fan

The fan is composed of gravelly and bouldery granitic alluvium and other sediment including limestone, schist, and volcanic fragments of the San Gabriel Mountains to the north (Eckis, 1928). Eckis mapped the feature as dissected recent alluvium of an alluvial fan. The soil is composed of stony loamy sand deposited on alluvial fans (NRCS, 1980b).

The landform has the appearance of a cone nearly fully extended. The concentric contours are convex downslope, with some lateral restriction on the left or east side. The form and general

Suggested Citation:"Applying the Indicators to Example Fans." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

FIGURE 4-6 Aerial photograph of the head of Lytle Creek fan (1991). Courtesy of Eagle Aerial Photography.

bounds can be readily identified on the USGS 7.5-minute series topographic map (Devore Quadrangle).

The apex of this fan is at the southern front of the San Gabriel Mountains near the eastern edge of the Cucamonga scarp, which is associated with a few thousand feet of differential movement. A hint of differential vertical movement is suggested by the steepening of the elevation profile as the stream emerges from the confines of the V-shaped canyon upstream of Interstate

Suggested Citation:"Applying the Indicators to Example Fans." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

Highway 15. Much of the topographic break, however, also is associated with lateral unconfinement as the stream leaves the confines of the steep mountain canyon onto the piedmont.

Defining the Lateral Boundaries and Topographic Apex of the Alluvial Fan

The fan is broad and long, and the eastern lateral bounds are at the mountain in the upper part of the fan and at Cajon wash in the middle and lower parts. The western boundary is at a swale where the fan coalesces with the San Sevaine Canyon, Etiwanda Creek, and Day Creek fans. These bounds are roughly defined on the NRCS (1980b) soil survey maps, which identify several soils as those of an alluvial fan. The topographic apex is about 305 m (1,000 feet) within the mountain where the contours change from concave to convex. The apex is about 1830 m (6,000 feet) downstream of the U.S. Geological Survey streamflow gage.

Defining the Nature of the Alluvial Fan Environment

Active Fan

There is no active fan.

Relict Fan

The relict fan is being dissected by a modern fanhead trench that follows a regional fault. Differential vertical movement of this fault is suggested by different bank heights near the Santa Ana River a few miles downstream (Eckis, 1928). Because of this 2.5- to 5-m-deep (8- to 15-foot-deep) by about 610-m-wide (2,000-foot-wide) trench that dissects the relict fan from the mountains to Cajon Wash, no areas of this fan are subject to active sedimentation. The capacity of the trench is many times that needed to convey the 100-year discharge.

Mountainous Drainage Basin

A cursory examination of sediment accumulation and likelihood of slope failure suggests that there may be potential for a large sedimentation event such as a debris flow. For example, there is a remnant of a large slope failure to the west in the Day Creek basin, where there is little channel trenching and much of the fan is active. It has been a considerable time since a major sedimentation event, and it is possible that basin conditions are evolving toward such an event. A large event might be triggered by a large wildfire followed by heavy precipitation. A comprehensive assessment is beyond the scope of this example. However, tentatively, a large sedimentation event is considered so infrequent to be outside the realm of traditional hazard consideration.

Suggested Citation:"Applying the Indicators to Example Fans." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
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Past Floods

Historical flooding in Lytle Creek between the apex of the old fan at the front of the San Gabriel Range and the Santa Ana River to the south has been confined to the incised channel (also see Appendix A). The channel has cut into unconsolidated alluvial deposits from the canyon mouth near the apex of the old fan to the El Cajon Wash and even further downstream to near the confluence with the Santa Ana River. Some lateral movement of the steep cut banks is suggested by reports of bridge failures during past flooding at San Bernardino (McGlashan and Ebert, 1918) and reports of channel movement just above the canyon mouth (Troxell, 1942). The largest known discharge from the 119.9 km2 (46.3 mi2) drainage basin above the U.S. Geological Survey stream gaging station near Fontana, California (Number 11062000), was 1,017 m3 (35,900 feet3) per second on January 25, 1969 (Chin et al., 1991). The capacity of the channel about 2.4 km (1.5 mi) below the gaging station and just below the mountain front is about 3 times the magnitude of the 1969 flood.

Flow Path Changes

The present incised channel (1995) of Lytle Creek is very similar to the channel reported by Eckis (1928). DMA Consulting Engineers (1985) also reported that the flow path of major floods in upper Lytle Creek was unchanged from 1935 to 1969. A comparison of flow paths shown on aerial photographs of October 12, 1967 (DMA Consulting Engineers, 1985), and an aerial photograph of August 29, 1989, indicates no movement of the channel banks. Thus, using channel conditions suggested in the account of flooding by Troxell (1942), the channel of Lytle Creek has been deeply incised since at least 1862, and the path of flow has not changed.

The soil has a well-developed, grayish-brown surface layer of stony or gravelly loamy sand. The underlying material is brown, very stony sand to a depth of 1.5 m (5 feet). There is no active surface on the Lytle Creek alluvial fan.

Characterizing Alluvial Fan Flooding Processes

The Lytle Creek alluvial fan is not subject to alluvial fan flooding processes as defined by the committee. The incised channel conveys flows at shallow depths and high-velocity during floods. Flood control structures installed over the past 40 years have prevented large-scale channel migration. Although substantial erosion and deposition occur in the main channel, flow path uncertainty is minimal.

The floodplain is topographically bounded on both sides. This confinement is mostly because of channel incision and to a lesser degree because of the spur dikes and levees a few hundred feet above and below Interstate Highway 15. Without the presence of the constructed spur dikes and levees, flood mitigation measures such as setbacks and elevation on fill might not reliably mitigate the hazard.

Lytle Creek is entrenched in a classically shaped relict alluvial fan. It does not, however, fit the committee's definition of alluvial fan flooding. Sediment delivered by watershed erosion processes is conveyed through the fan, but large sedimentation events are tentatively considered

Suggested Citation:"Applying the Indicators to Example Fans." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
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remotely possible. The surface of the relict fan is resistant to erosion, and parts of the surface may be flooded by local runoff. This does not, by itself, mean that this area is subject to alluvial fan flooding, however.

TORTOLITA MOUNTAINS, ARIZONA

The Tortolita Mountain piedmont lies 20 km (12.4 mi) northwest of downtown Tucson, Arizona. The region is semiarid, with 280-mm (1.1-in) average annual precipitation. Average annual temperature is 20.3 degrees Celsius. Summer monsoonal precipitation results in intense floods. The area is characterized by sloping alluvial surfaces extending approximately 10 km (6.2 mi) from the front of the Tortolita Mountains to the floodplain of the Santa Cruz River (Figure 4-7). The highest elevation in the Tortolita Mountains is 1,533 m (5,030 feet). Elevation at its mountain front is approximately 900 m (2,953 feet), and the Santa Cruz valley floor elevation is approximately 600 m (197 feet). The Tortolita Mountain front was originally a steep fault scarp, but this range has been tectonically inactive since the late Tertiary. There is no modern fault scarp, and the mountain front is highly sinuous. Because this mountain front is inactive, deposition on the piedmont during the Quaternary has been controlled primarily by climate change (Fuller, 1990).

Recognizing and Characterizing Alluvial Fans

Determining Whether or Not a Landform is an Alluvial Fan

The Tortolita piedmont consists mainly of the dissected remnants of ancient fans. These do not meet the committee's definition of an alluvial fan. However, there are subunits of the piedmont that do display the criteria. An example is Cottonwood fan (see box in Figure 4-7).

Composition

The piedmont is mantled by alluvial sediments. The dissected fan remnants close to the Tortolita Mountain front are composed of gravel and boulders. The surfaces of these remnants are weathered to form desert pavements and soils. The latter include accumulations of fine material blown in by wind and the alteration products of chemical weathering over long periods of geologic time. These coarse-grained deposits decrease in grain size distally from the mountain front.

The alluvial fans that are inset into these dissected remnants are composed mainly of sand. This is because the fan apexes are not at the mountain front. The apexes are in the active stream channels that are incised through the old piedmont surfaces. The fine sediment is derived largely from the old soils that characterize the dissected fan remnants, which make up the divides between active stream channels cut into the piedmont.

Morphology

The Tortolita piedmont consists of old alluvial surfaces that slope away from the mountain front. The surfaces were formed by ancient coalescing alluvial fans but are not considered fans today under the committee's definition because they lack the characteristic fan-shape.

Suggested Citation:"Applying the Indicators to Example Fans." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

FIGURE 4-7 Geomorphologic map of alluvial surfaces on the southwestern piedmont of the Tortolita Mountains. CF indicates Cottonwood Fan. Dot pattern shows areas subject to active modern flooding (Unit b). The other units are old, inactive parts of the piedmont. SOURCE: Reprinted with permission from Field (1994).

Suggested Citation:"Applying the Indicators to Example Fans." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

Several active washes extend from major canyons in the Tortolita Mountains and are deeply incised into the old piedmont surfaces. As these washes extend beyond the ancient piedmont surfaces, they fan out radially. The pattern is obvious in active channelways visible on aerial photographs and recognized by historical documentation of active streamflow. An example is Cottonwood fan (Figure 4-8). This clearly shows a fan-shape.

Location

The Tortolita piedmont extends to the prominent topographic break of the Tortolita Mountains. However, this break is not associated with fan morphology but rather with the morphology of dissected old piedmont surfaces.

The Cottonwood fan extends headward to a confined channel that is incised into the old dissected piedmont surface.

Defining the Lateral Boundaries and Topographic Apex of the Alluvial Fan

The Tortolita piedmont does not have clearly defined lateral boundaries. The old piedmont surfaces extend back up the embayed elements of the mountain front (Figure 4-7). However, away from the mountain front these old dissected surfaces coalesce to form an apron sloping away from the mountains.

Cottonwood fan is bounded laterally by the depositional channels that head back to its upstream source. Lateral to these channels are either (1) old piedmont surfaces at higher elevations or (2) active channels that head back to other sources. The apex of the Cottonwood fan is clearly seen where the fan channels converge headward into a single-thread (Figure 4-8).

Defining the Nature of the Alluvial Fan Environment

The extent of active channels on the Tortolita Mountain piedmont can be determined from geomorphologic study of aerial photographs, soil development, and topography (Figure 4-7). The active areas, frequently inundated by modern floods, have poorly developed soils, distributary drainage paths, and a lack of desert pavement or rock varnish. These areas are inset at lower relative elevations in relation to adjacent ancient piedmont surfaces. The latter show strong soil development, well-developed dendritic drainage (dissecting the old surfaces), closely packed desert pavement, and well-developed rock varnish. Detailed discussion of these distinguishing criteria is provided by Pearthree et al. (1992).

CAREFREE, ARIZONA

Carefree fan is located in central Arizona about 6.5 km (4.0 mi) south of the town of Carefree and a few kilometers north of Phoenix. Above the apex, the 11.1-km2 (4.2-mi2) long, narrow drainage basin heads on a relatively gently sloping pediment at 1,000-m (3,281-foot) elevation in desert vegetation. The apex is 14.5 km (9 mi) southwest of the head of the basin at an elevation of 649 m (2,129 feet). The area of the 7.1-km-long (4.4-mi-long) alluvial fan below the apex is 9.9 km2 (3.8 mi2) (Figure 4-9). Such pediments and alluvial deposits in central and

Suggested Citation:"Applying the Indicators to Example Fans." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

FIGURE 4-8 Cottonwood fan (see box in Figure 4-7), showing active fan features mapped from aerial photographs, historical accounts, and geomorphologic survey. SOURCE: Reprinted with permission from Field (1994).

Suggested Citation:"Applying the Indicators to Example Fans." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

southern Arizona are predominantly modern sediment transport surfaces. Relatively inactive alluvial fans are common on these geologically and hydraulically complex surfaces, where fan apexes typically are on the dissected piedmont near the lower edge of a pediment.

Recognizing and Characterizing Alluvial Fans

Determining whether or not a Landform is an Alluvial Fan

This landform was identified as an alluvial fan using the committee's criteria (defined in Chapter 3) based on material composition, morphology and location. The crucial morphologic requirement was the planametric shape, because any transverse convex shape was lost to modern erosion.

Composition

Most of the landform is alluvium, which has weathered to a developed gravelly loam and gravelly sandy loam on a fan terrace (Camp, 1986). Along the drainage ways are younger soils that are poorly developed sandy and gravelly loam (Anthony-Arizo and Antho-Carrizo-Maripo soil units, Figure 4-9).

Morphology

The site has a general cone-shape, but any transverse convex shape has been lost over much of the fan through erosion and coalescence with adjacent fans. Trenched distributary channels radiate downslope toward the west from the apex where the feeder channel leaves the confines of a gully. These channels are not readily apparent on USGS 7.5-minute topographic maps (Currys Corner and Cave Creek Quadrangles) or Natural Resources Conservation Service soil maps on a 7.5-minute series orthophoto base (Camp, 1986). The shape of the complex network of entrenched channels is clearly observed on large-scale topographic maps at 1:2,400 scale with 2 foot (0.61 m) contour intervals (furnished by the Flood Control District of Maricopa County, Arizona), especially when used in conjunction with color-infrared aerial photographs obtained from the EROS Data Center of the USGS. The landform is shaped like a partly extended fan that ends near a major stream channel. The stream channel appears to truncate the fan.

Location

The landform is at the lower edge of a gradual transition zone between a pediment and bajada where floodflow leaves the confines of a gully and spreads laterally into two channels that are entrenched into old alluvium. A second small topographic break is located on the pediment about 4.8 km (3 mi) upstream where a small channel leaves the otherwise finite drainage basin bounds. This upstream diffluence is above the transition zone and is not considered part of this fan. Thus, the topographic break for this fan is downslope of all pediment remnants on the piedmont (Figure 4-9).

Defining the Lateral Boundaries and Topographic Apex of the Alluvial Fan

The topographic apex is located at the topographic break described above (Figure 4-9). The lateral boundaries of the fan were estimated using the large-scale topographic maps in conjunction with aerial photographs. The boundaries shown in Figure 4-9 generally are located at

Suggested Citation:"Applying the Indicators to Example Fans." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

FIGURE 4-9 Carefree alluvial fan, showing distributary channels, soil units, and estimated extent of 100-year flood.

Suggested Citation:"Applying the Indicators to Example Fans." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

topographic ridges shared with adjacent fans. Mapping of the fan boundaries started at the apex and proceeded downslope. The lateral boundaries, which obviously are estimated downslope of any tributaries and distributaries, tend to be perpendicular to the general alignment of the topographic contours.

The fan boundary at the toe is indistinct and transitional between the dominantly distributary drainage pattern of the fan and the dominantly entrenched-tributary drainage pattern adjacent to Cave Creek. Below the fan toe the lower portion of the piedmont merges with Cave Creek to the west.

Defining the Nature of the Alluvial Fan Environment

The sediment balance appears to have turned negative, especially on the upper parts of the fan, where channel sedimentation has been replaced by erosion in the geologically recent past. In the lower parts of the fan the balance appears less negative (less net erosion), as suggested by two small areas of sheetflow and sediment deposition beyond the banks of entrenched channels during recent flooding. The depth of channel downcutting is restricted by a calcic soil horizon (> 10,000 years old), which in places is highly cemented (Figure 4-10). Thus, the grade of the channels is controlled by developed calcic soil.

The longitudinal profile of the basin and alluvial fan is slightly concave (Figure 4-11), with a longitudinal slope of 0.016 at the toe and 0.019 at the apex. The relief ratios of the fan and drainage basin are 0.018 and 0.024, respectively, and are small for fans in Arizona (Hjalmarson and Kemna, 1991). The small relief ratio of the basin suggests that debris flows are unlikely.

The braided pattern of the distributary channels may have resulted when the old sediment was deposited, and the channel trenching possibly was initiated by postglacial climate change. Because the area is tectonically inactive (Rhoads, 1986), climate change is a plausible cause of the channel incision. There is evidence in other areas of channel entrenchment during a single storm, but the interlacing drainage pattern and rather uniform hydraulic geometry of the channels indicate that entrenchment and stabilization took place over a long period. Incision of Cave Creek, the base-level stream to the west, has resulted in minor tributary headcutting below the toe of the fan. The headcutting has little effect on the fan because the grade of the dominant distributary channels is controlled by resistant blocks of calcic soil above the Cave Creek channel. The interfluves are much wider near the toe, with little transverse relief except at the few incised distributary channels.

Accounts of Flooding

Meteorologic records and published accounts of major storms and floods indicate there have been at least three notable floods in the area during this century. The floods were on October 23, 1956, June 22, 1972, and October 6, 1993. Runoff and sediment information, furnished mostly by the Flood Control District of Maricopa County, is summarized for the second and third floods.

Suggested Citation:"Applying the Indicators to Example Fans." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

FIGURE 4-10 View looking upstream at distributary channel where the grade is controlled by a calcrete mass located above the backpack. The channel bed abruptly rises approximately 0.3 m at this spot. The outside dimension of the square frame is 1.5 feet (0.46 m). Courtesy of H. W. Hjalmarson.

Storm of June 22, 1972

Heavy rains in amounts of more than 100 mm (3.9 in) fell in nearby mountains within a 2-hour period. Although the storm center was to the south of Carefree fan, there was considerable runoff in the area. The peak discharge for Indian Bend wash, which drained 360 km2 (138.8 m2), of which Carefree fan is a part, was 595 m3/s (21,015 feet3/s) and is the highest peak since at least 1922. Unit peak discharges determined by the USGS for small mountainous basins to the south were from 5.77 m3/s/km2 (527.60 feet3/s/mi2) to an unusually large 37.2 km3/s/km2 (23.1 mi3/s/mi2). Unit peak discharge for the pediment above the Carefree fan undoubtedly was smaller because rainfall amounts were less and orographic effects were unlikely.

Photographs of floodflow and deposited sediment at road dip crossings suggest that there was a considerable amount of runoff and sediment movement in and through Carefree fan. Coarse sand bed material was deposited in road dip crossings along tributary streams at Scottsdale Road (Figure 4-9). Although dip crossings act as sediment traps as floodflow expands hydraulically (loses kinetic energy), the large deposits clearly show that large amounts of sediment moved onto the fan. There were no accounts of deposition in major areas of distributary channels on the

Suggested Citation:"Applying the Indicators to Example Fans." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

FIGURE 4-11 Profile of Carefree alluvial fan in Arizona.

piedmont plain, but there were large areas of sheetflow and undoubtedly deposition in lower parts of the piedmont plain, mostly below and to the south of Carefree fan. A few meters of bank erosion along distributary channel banks incised in younger deposits was observed. It is significant that there are no known accounts of flow path movement on the piedmont plain where the Carefree fan is located.

Storm of October 6, 1993

Tropical Storm Norma converged over central Arizona with a cold front associated with a strong Pacific low-pressure system and produced an average of 4.3 cm (1.7 in) of rainfall over Carefree fan and drainage basin. Flow was confined in nearly all distributary channels and flow divided and combined at all major channel forks and joins. Some sheetflooding and deposition in shallow overflow areas were observed in places near the fan toe. Measurements and estimates of peak discharge and runoff at 35 inflow, outflow, and internal channels show a peak discharge of 1.76 m3/s (62.16 feet3/s) in a single channel from the 10.9-km2 (4.2 mi2) drainage basin at Scottsdale Road and a total peak discharge of 13.0 m3/s (459.2 feet3/s) from 15 channels near the fan toe. Near the center of the fan, the total peak discharge at 11 tributary and distributary channels was 10.6 m3/s (374.4 feet 3/s). These data show that a significant part of the runoff and peak discharge at the toe of Carefree fan was from the fan itself.

Storm runoff within the bounds of the Carefree fan and its drainage basin is directly related to total storm rainfall. The simple linear relation indicates approximately 41 mm (1.6 in) of rainfall were needed to produce runoff and the proportion of rainfall that entered the larger

Suggested Citation:"Applying the Indicators to Example Fans." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

tributary and distributary channels increased with the amount of rainfall. For example, the relation indicates that for 4.3 cm (1.7 in) of rainfall only about 0.11 percent ran off but at a total storm rainfall of 4.6 cm (1.8 in) about 0.32 percent ran off.

Flow Path Movement

The tree-lined distributary channels indicate that the flow paths are fixed and in a condition of relative stability. Many of the large Palo Verde and mesquite trees along the channels are visible on aerial photographs taken on September 7, 1941, March 8, 1953, and March 30, 1991. A comparison of stream channels depicted on these good-quality large-scale aerial photographs revealed no change in the location of flow paths on Carefree fan. Rhoads (1986) also found no major changes in the form of channel networks in the general region for a 30 year historical period using several sets of aerial photography.

Characterizing Alluvial Fan Flooding Processes

Floodflow from the basin is conveyed through a long channel bounded by a narrow drainage basin to the apex. The drainage basin is not expected to produce large flood peaks because (1) the basin has a general ''banana" shape with an average width of 770 m (2,530 feet), or only 5 percent of the basin length and (2) the first-order streams convey flow to only a few long second-order streams. Below the apex, floodflow enters the network of incised distributary channels located between stable ridges within the alluvial fan. The rainfall and runoff data for the storm of October 6, 1993, show that large amounts of discharge originated on the fan. The abnormally large fraction of the peak discharge that originated on the fan for that particular storm was partly because of an uneven rainfall distribution over the fan and basin. This suggests that the peak flood discharges on the lower part of the fan would be greater than predicted with normal application of the FEMA procedure (FEMA, 1990) as that procedure does not take into account runoff from the fan.

A simplifying flood characteristic is that avulsions, if any, are rare because there is little alluviation. Flood characteristics, however, are complex because (1) floodwater may be confined to defined channels or may inundate large portions of land between channels, (2) floodflow is generated from rainfall on the fan itself, (3) floodwater is lost to infiltration within channels, and (4) peak discharge is reduced by attenuation as floodflow entering the fan divides into channels and is temporarily stored on the fan surface. Also, distributary channels may be subject to scour and fill, while the interfluves separating the channels are composed of developed soils that resist lateral channel movement. Because flow paths are confined by stable interfluves and there is little alluviation, there currently is no active flooding on Carefree fan.

Defining Areas of Active Alluvial Fan Flooding Hazard

There are no areas on the Carefree fan where flow paths are expected to change.

Suggested Citation:"Applying the Indicators to Example Fans." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

Where does Flow Depart from Confined Channels?

Floodflow typically is confined within and adjacent to the trenched channels.

Where does Sheet Flow Deposition Occur?

During the largest recent flood of October 6, 1993, there was minor overbank flooding at two low-lying areas of the lower fan. On this basis and because of the relatively small topographic relief, sheetflow with some sediment deposition might be expected during major floods in the 4.0-km2 (2.5 mi2) area defined as "sheetflow in low-lying areas" (Figure 4-9).

Where does Debris Flow Deposition Occur?

There is no evidence of debris flows.

Where are there Structures or Obstructions that Might Cause or Aggravate Alluvial Fan Flooding?

There are no structures or obstructions that might cause flow path movement. Some bank erosion in young soils may result at a few locations where structures are near the banks of distributary channels. Structures in geomorphically active areas (young soils along the distributary channels) should be avoided.

Where can the Flood Hazard be Mitigated by Means other than Major Structural Flood Control Measures?

Low-density development with restriction of structures to the stable ridges of old soils between the distributary channels and the elevation of structure floors form an effective means of mitigating the flood hazard.

Defining Areas of Nonalluvial Fan Flooding Hazard along Stable Channels

In general, except during major floods, floodwater is confined to the entrenched channels. Flooding of approximately 6 percent of the 9.9-km2 (3.8 m2) alluvial fan area, mostly along the 30 km (18.6 mi) of defined channels, is estimated to have a probability of 1 percent in any year. Most of the discharge of the 100-year flood, estimated using methods of Thomas et al. (1994), will be conveyed in the channels with some shallow overbank flow commonly adjacent to the channels. Some overbank flow probably will combine with direct runoff of nearby tributary channels that have formed on the fan.

Determining the Types of Processes Occurring on the Active Parts of the Alluvial Fan

Recent unconsolidated deposits along the margins of the distributary channels are stratified. These young soils are composed of several alternating layers of silt, sand and small gravel and very thin layers of clay. There are no large unstratified deposits or large marginal levees of very coarse material associated with debris flows. Local small levee deposits of coarse sand and fine gravel on gently sloping channel banks were apparent at two sites following the October 6, 1993, flood. These small levees were less than 50 m (164 feet) long and are associated with the remobilization and movement of bed sediment of the distributary channels. Thus, water flood processes are dominant and channel deposits of coarse sand can remobilize as debris masses for short distances.

Suggested Citation:"Applying the Indicators to Example Fans." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×
Using Soil Maps to Define the Nature of the Fan and the Extent of Flooding

NRCS soil survey reports with 7.5-minute orthophoto maps depicting types of soils are useful for rapid assessment of active and inactive areas on fans (Cain and Beatty, 1968). For example, the latest available map of soil units (Camp, 1986) shows that most of Carefree fan is a terrace with well-developed soil profiles at fairly shallow depths: the lower part of the fan is Momoli soil and most of the middle and upper parts are Pinaleno soil (general soil map, Figure 4-9). Within these areas of predominantly old soils are units of predominantly young soils such as the Anthony-Arizo unit along the stream channel in the upper par of the fan. The young soils correspond to areas along stream channels where sediment may be subject to fluvial erosion and deposition or where horizons have not yet developed. Thus, significant low-cost information is gleaned from a cursory examination of the soil survey report, which shows that Carefree fan is not active.

To demonstrate the value of more detailed mapping of soil units, part of Carefree fan was recently mapped by the NRCS on aerial photographs at a scale of approximately 1:5,000 (Cathy E. McGuire, soil scientist, NRCS, written communication). The detailed mapping typically showed a close correlation between young soils and flood channels as defined using hydraulic methods (detailed soil map and flood map, Figure 4-9). Stable interfluves of older soil are also closely correlated with areas above defined flood levels. Thus, for Carefree fan the detailed soil surveys by the NRCS, which typically are not published, were useful but not essential for the assessment of flood characteristics.

RUDD CREEK, UTAH

Rudd Creek Canyon lies on the face of the western slopes of the Wasatch front in Davis County, Utah, north of Salt Lake City at the community of Farmington. The steep 1.8-km2 (0.7 mi2) basin rises about 1,150 m (3,773 feet) above the fan apex and is underlain by bedrock and covered with grasses, scrub oak, and mountain mahogany vegetation. The Wasatch fault, located near the mountain front, is near the apex of the fan.

A massive sedimentation event that consisted of several debris flows over the course of approximately a week originated in this steep canyon in the late spring of 1983 during rapid runoff from snowmelt. The debris flows, which included an especially large debris flow, caused damage to 35 structures on the fan; 4 homes near the topographic apex were completely destroyed, and 15 other homes were severely damaged. This event has been the subject of several published reports and has a substantial amount of documentation, including a video tape (Costa and Williams, 1984) of one of the debris flows. The video tape shows massive angular boulders being rapidly pushed or rafted down the steep canyon toward the alluvial fan.

The area is complex because the occurrence of large debris flows on Rudd Creek may be so infrequent as to be outside the realm of traditional hazard consideration. Also, debris flows result from the triggering or release of sediment that has accumulated to threshold levels over hundreds of years. Future debris flows probably will be smaller than the 1983 event until sediment again accumulates to threshold capacity and is released to the fan below. Clearly, assessment of debris accumulation in the drainage basin should be part of hazard mitigation as described by Keaton (1995).

Suggested Citation:"Applying the Indicators to Example Fans." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

Recognizing and Characterizing Alluvial Fans

Determining whether or not a Landform is an Alluvial Fan

On the basis of the criteria established by the committee, Rudd Creek is an alluvial fan. Its composition, morphology, and location all fit the requirements, as detailed below.

Composition

The upper slopes are coarse debris fan deposits. The soil of the upper part of the fan is Kilburn cobbly sandy loam that occurs on short, slightly convex slopes of alluvial fans, mainly along the channel of intermittent streams (NRCS, 1968). The soils along the fan are characterized by the NRCS as occurring on alluvial fans in the uplands. The parent materials are alluvium and colluvium derived mainly from gneiss, quartzite, and granite.

Morphology

The site is shaped like a fully extended fan with a general convex shape and concentric contour lines. Locally irregular contour lines depict streets and other urbanization. The debris basin near the apex was constructed following the 1983 debris flow (Figure 4-12).

Location

The depositional zone begins at a topographic break associated with the Wasatch fault where the stream channel loses definition. The zone extends downslope to Farmington Creek, which forms the toe of the fan.

Defining the Lateral Boundaries and Topographic Apex of the Alluvial Fan

The fan is on a bajada and the lateral bounds generally are at swales where the fan coalesces with adjacent fans. Construction of a debris basin as well as urbanization on the fan has past altered the natural bounds. Inspection of the soil map indicates that alluvial deposits extend along the toe of the steep slopes equidistant from the mouth of the canyon. The topographic apex is readily apparent in Figure 4-12.

Defining the Nature of the Alluvial Fan Environment

Active Fan

Geologic assessment of sedimentation indicates that seven major debris flows have occurred during the Holocene (Keaton et al., 1988), with two of the flows occurring in the 140 years (historical period). About 63,000 m3 (2.2 million feet3) of sediment was deposited on the fan in 1983. Deposited debris in the upper part of the fan is remobilized and transferred downfan by water floods.

Suggested Citation:"Applying the Indicators to Example Fans." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

FIGURE 4-12 Topographic map on orthophoto base of Rudd Creek Canyon. The debris basin shown near the center of the map was built where homes were destroyed during the 1983 debris flow. Courtesy of Aero-Graphics, Inc.

Suggested Citation:"Applying the Indicators to Example Fans." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×
Relict Fan

Although major debris flows have been infrequent during the past 10,000 years, the entire fan would be considered active.

Mountainous Drainage Basin

The steep, rugged drainage basin is heavily vegetated. This vegetation protects the watershed from erosion and landslides, which are a source of rock for debris flows. In the absence of wildfires and overgrazing, the accumulation of colluvium in the canyon is slow. Accumulation to threshold levels where heavy moisture can trigger movement of the colluvium and form major sediment events takes approximately 1,400 years. Because much of this colluvium was removed in 1983, and deposited on the alluvial fan, many years of accumulation may be needed before threshold levels are again reached. However, the frequency of major debris flows may be nonstationary because of changed land use practices.

The amount of sediment available for debris flows is related to the length of channels in the basin. When progressive sediment accumulation approaches the threshold that leads to sedimentation events on the fan, about 10 to 12 yards3/foot (7.6 to 9.8 m3/m) of channel length of colluvium is stored in the basins. This amount of unit storage is common for basins along the Wasatch front in Davis County (Sidney Smith, floodplain manager of Davis County, oral communication, 1995).

Sedimentation Event of 1983

The debris flows of 1983 were generated by mobilization of colluvium in the main channel of the basin as described above and by Lowe (1993) and Mathewson and Keaton (1990). This is the largest event recorded for the watershed since Davis County began observations in 1847. Similar snowmelt in 1984 produced very little debris in comparison.

Flow Path Changes

The 1983 debris flows obliterated the preflow paths of flow, but the paths of debris flow and subsequent floodflow were restricted by the urban development on the fan. For example, several homes blocked debris and shielded downslope areas.

Characterizing Alluvial Fan Flooding Processes

Defining Areas of Active Alluvial Fan Flooding Hazard

Where does Flow Depart from Confined Channels?

The 1983 debris flows were not confined below the topographic apex as described above.

Suggested Citation:"Applying the Indicators to Example Fans." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

Where does Sheetflood Deposition Occur?

There appears to be little sheetflooding except in the mid-fan area where remobilized debris flow deposits are deposited by water flows.

Where does Debris Flow Deposition Occur?

The entire fan might be subject to flooding if the newly constructed debris basin were overwhelmed by a large debris flow. The 1983 flows remained in the channel until they reached the apex. Sheetflow and deposition will occur downstream of the debris basin if it is filled and the outflow channel blocked. Studies by Keaton (1995) and Keaton et al. (1988) have determined that the debris basin should be able to hold all of the sediment from most events. Urbanization of the fan has altered drainage patterns and most flooding is likely to occur along the streets.

Alluvial fan flooding emanating from the canyon now will be confined by the debris basin and overflows will most likely be diverted by Farmington Canyon Road and other local streets.

Where are there Structures or Obstructions that Might Aggravate or Cause Alluvial Fan Flooding?

Structures below the debris basin form obstructions, and streets become flow paths.

Where can the Flood Hazard be Mitigated by Means other than Major Structural Flood Control Measures?

Possible methods of mitigating debris flow hazards include avoidance, source area stabilization, transportation-zone (debris flow track between the source area and the depositional zone) modification, and defensive measures in the depositional zone (Lowe, 1993). Because of the many residential structures on the fan, a warning alarm might be used to avoid loss of life. Also, source area stabilization through maintenance of dense vegetation, including prevention of basin wildfires, can mitigate the flood hazard. Maintenance of the debris basin, a major structural control, is fundamental.

Determining the Type of Processes Occurring on the Active Parts of the Alluvial Fan

The approximately 30-foot-high (10-m-high) snout of Rudd Creek debris flow was low in clay and high in clast content suggesting a cohesionless flow with considerable intergranular collision (Shanmugan, 1996; Keaton et al., 1988). The video tape of a subsequent smaller flow shows large, rafted, angular boulders being rapidly pushed down the steep canyon. The actual peak water flow during this sedimentation even has been estimated to be 85 feet3/s (2.4 m3/s), a seemingly insignificant amount of flow. The fan is formed by about seven massive debris flows in the Holocene and many small debris flows. Deposited debris has been at several locations, including some above the apex. The coarse deposits have been remobilized and transported downfan, where some sediment is carried away at Farmington Creek at the fan toe.

HUMID REGION ALLUVIAL FANS

Alluvial fans and alluvial fan flooding are not limited to semiarid sites in the western United States; indeed, both are prevalent throughout the Appalachian Mountains of the eastern United States, from Tennessee to New Hampshire (Kochel, 1987). Although numerous landforms that would be described as alluvial fans exist in the Appalachian Mountains, many

Suggested Citation:"Applying the Indicators to Example Fans." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

appear to be inactive or to have active parts that are much smaller than the overall landform. For example, an extensive apron of alluvial fan deposits mantles the western slopes of the Blue Ridge in Virginia, yet evidence to date seems to indicate that flooding has not occurred on any of the coalescing fans. In fact, the overlapping margins of the fans, where sediments are finest, are the sites of deeply incised channels. The stratigraphy and morphology of deposits on the now abandoned interior fan surfaces indicate that fluvial processes once crossed the fans. Some workers have attributed the large, relict landforms to past climates, and in particular to warmer and wetter conditions during Tertiary time (> 2 million years ago).

A common type of alluvial fan in the Appalachians has been characterized by episodic debris flow deposition throughout the Holocene epoch. These fans are very small (<1 km2 (0.6 mi2)), thin (deposits 5 to 20 m (16.4 to 65 feet) thick), steep (>10¹ longitudinal profile), and elongate in comparison to arid region fans (Williams and Guy, 1973; Mills, 1983; Mills and Allison, 1994; Kochel, 1987). The characteristics of these fans can be attributed to the dominant fan-forming mechanism in the region: debris flows that develop from rainfall-generated debris avalanches emanating from small, low-order channels draining steep mountain slopes (Figure 4-13a and 4-13b) and to the fact that they have accumulated slowly, probably only during the Holocene. Storms that have triggered flooding and deposition on small debris fans have included Hurricane Camille (1969, Virginia); Hurricane Juan (1985, Virginia and West Virginia); and the June 1995 storms in Madison County, Virginia. Recent hurricanes have caused devastating result such as

  • deep gullying along and upstream from fan apexes;

  • debris slides and avalanches along steep channels that feed into small, steep fans; and

  • substantial scour and deposition along larger channels into which steeper mountain streams deposit their water and sediment.

Population is growing in the East as elsewhere, and the alluvial plains surrounding low to moderately high mountains are prime spots for development. Developers consider the piedmonts to be safer than the valley floors, but during hurricanes their safety is dependent on the stability of slope deposits upstream from the piedmont. Most debris avalanches originate in colluvial hollows, or bowl-shaped concavities that collect sediment between debris-flushing events and concentrate subsurface water flow during storms (Figure 4-14). Although such sites could be identified and mapped by a professional geomorphologist, no attempt is usually made to locate these areas of potential catastrophe. As a result, homes have been built on debris fans downslope from hollows that are in the process of being charged with soil that could be released, perhaps during the next intense rainfall (see Figure 4-15 for an example of a home that was destroyed by a debris flow produced by evacuation of a colluvial hollow).

In the following pages, examples of several small debris fans in Nelson County, Virginia, are considered according to the procedure used by the committee for identifying areas of potential alluvial fan flooding1 (Figure 4-16). Fans in this area have been studied and described by Williams and Guy (1973) and Kochel (1987). In 1969, during Hurricane Camille, about 50

1  

The figures and field data used in this discussion are actually a composite from several debris fans in the Lovingston area of Nelson County, rather than a single, small fan. The work of Kochel (1987) includes analysis of more than 1 dozen fans in west-central Virginia and concludes that all are very similar in their depositional histories and processes.

Suggested Citation:"Applying the Indicators to Example Fans." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

FIGURE 4-13 (Top) Block diagram of a debris avalanche that terminates on a debris fan. (Bottom) Debris avalanches that formed during Hurricane Camille, 1969, on hillsides terminating on two debris fans along the Virginia piedmont. East Branch of Hat Creek in lower left foreground removes sediment and water from the toes of the fans. Note that both debris fans are elongate, and active streams are found along the margins on the right in the photo. SOURCE: (Top) Reprinted with permission from Clark (1987). (Bottom) Williams and Guy (1973). Original photograph courtesy of Virginia Division of Mineral Resources (photographer T. M. Gathright II).

Suggested Citation:"Applying the Indicators to Example Fans." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

FIGURE 4-14 Schematic model of a colluvium-filled hollow, illustrating (a) an evacuated hollow with thin soils developed on hillsides and concentration of subsurface water flow in the trough of the hollow floor, and (b) the thick accumulation of colluvium and slope wash in the hollow after several hundred years. SOURCE: Reprinted with permission from Ritter et al. (1995).

percent of the surface area of many fans was flooded and altered by sedimentation from debris flows. As described below, these fans would be mapped and classified as potentially hazardous according to the procedures recommended in this document.

Recognizing and Characterizing Alluvial Fans

Determining whether or not a Landform is an Alluvial Fan
Composition

Examination of geologic maps alone does not provide enough information to identify the landform because the debris fans are not mapped separately from the bedrock mountain slopes; thus field work must be used to determine whether or not the landform is

Suggested Citation:"Applying the Indicators to Example Fans." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

FIGURE 4-15 (Top) The town of Lovingston, Virginia, is developed on a debris fan at the base of a debris avalanche that failed during Hurricane Camille on August 20, 1969. (Bottom) This clubhouse located on a debris fan was destroyed during a Blue Ridge, Tennessee, storm in 1973. At the time of the storm, the clubhouse was unoccupied. Upstream failure in the colluvial hollow resulted from failure of an access road fill built on the hollow.

SOURCES: (Top) Williams and Guy (1973); photograph by Ed Rosenberry. (Bottom) Reprinted with permission from Clark (1987).

Suggested Citation:"Applying the Indicators to Example Fans." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

FIGURE 4-16 Debris avalanches and debris fans in Nelson County, Virginia, that formed during the 1969 hurricane. Courtesy of Virginia Division of Mineral Resources; photograph by T. M. Gathright II.

composed of alluvial sediments. Field studies of small debris fans in Nelson County indicate that the fans are composed solely of angular, very poorly sorted, mud matrix-supported gravels that range in size to as large as 5 m (16.4 feet) in maximum dimension (Kochel, 1987). Because these sediments are unconsolidated and have characteristics that indicate deposition from streams or debris flows, the composition of the landforms meets the criteria for the definition of an alluvial fan.

Morphology

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. Examination of a plane table map for one small fan in Nelson County (Figure 4-17) illustrates that the landform has the shape of a partly extended fan. Low-altitude photographs indicate that all of these piedmont landforms have an elongate fan-shape.

Suggested Citation:"Applying the Indicators to Example Fans." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

FIGURE 4-17 Stratigraphy and sedimentology of older and younger (1969) deposits on the Valentino debris fan. (a) Plane table map with locations of cross sections A-A' (proximal) and B-B' (distal). (b) Cross sections were measured and described in stream bank exposures and trenches dug on the fan surface. Radiocarbon ages indicate that older deposits are early Holocene and mid-Holocene in age. (c) Texture and matrix composition vary with depth, enabling the investigator to discriminate boundaries between debris flow events. These differences are illustrated in (c). (d) In this photo of a stratigraphic section, note the coarse, angular deposits from Hurricane Camille above the glove and two older deposits with paleosols indicated by the shovel. SOURCES: (a), (b), and (c) are reprinted with permission from Kochel (1987); (d) is reprinted with permission from Kochel and Johnson (1994).

Suggested Citation:"Applying the Indicators to Example Fans." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×
Location

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. The Valentino fan, like other small debris fans in Nelson County, is located at a topographic break in slope along the eastern flank of the Blue Ridge Mountains.

Defining the Toe, Lateral Boundaries, and Topographic Apex of an Alluvial Fan

The gradients of the lower parts of the fans are gentler than those at the fan apexes, as can be seen from the greater spacing of contour lines in Figure 4-17a. The small debris fans in Nelson County (Figure 4-18) typically are encircled by slightly incised streams, many of which wrap around the toes of the fans (see also the shape of the ephemeral stream on the right in Figure 4-17a). These encircling streams, many of which are ephemeral, form the lateral boundaries and toes of the fans in Nelson County and can be identified on contour maps.

The topographic apex of the Valentino fan occurs approximately at the boundary between grass-covered slopes and tree-covered, steep hillsides. This is also the point where flow in the channel becomes unconfined and more uncertain and thus is coincident with the hydrologic apex.

Defining the Nature of the Alluvial Fan Environment and Identifying the Loci of Active Sedimentation

Defining Active

The committee recommends that the term active be used to refer to that time period during which sedimentation and flooding are possible in the current regime of climate and watershed conditions. In Nelson County, evidence is available to document when and how often episodic debris flow flooding and deposition have occurred. Furthermore, because it is clear that debris flows are associated with evacuation of hollows that must be full or close to full with colluvium before failure, it is possible for the investigator to have some understanding of the likelihood of activity on a given slope and its downstream depositional fan (Reneau et al., 1986).

In Nelson County, debris flow deposition and flooding have occurred about three times in the past 11,000 years, the Holocene (Kochel, 1987). Most fans have been constructed from deposits of three different ages, the oldest of which rests on late Pleistocene (˜13,000 years B.P.) solifluction deposits overlying bedrock and has been radiocarbon dated at about 11,000 years at two sites. A younger deposit that has a mid-Holocene age (6,340 years B.P.; Figure 4-17b) is sandwiched between the basal unit and the historical deposit left by Hurricane Camille in 1969. From these data, geologists estimate that recurrence intervals for episodic debris flow deposition in the area are on the order of >3,000 to 4,000 years (Kochel, 1987), although from a stochastic hydrologist's perspective it is important to note that these events are not random in time.

Kochel (1987) provides evidence in support of the hypothesis that debris flow activity in the area was initiated by the Pleistocene-Holocene climatic transition, and in particular by the

Suggested Citation:"Applying the Indicators to Example Fans." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

FIGURE 4-18 A debris flow fan and recent drastically deposited rubble from the headwaters of the North Prong of Davis Creek that formed during Hurricane Camille in 1969. SOURCE: Williams and Guy (1973). Original photograph courtesy of the Virginia Division of Mineral Resources (photographer T. M. Gathright II).

Suggested Citation:"Applying the Indicators to Example Fans." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

onset of incursion of tropical air masses and moisture into the central Appalachians. Incursion of tropical moisture was concurrent with retreat of the polar front as Pleistocene glacial conditions waned and ice masses withdrew northward. These moist air masses can become locked in the steep, rugged terrain of the central Appalachians, unleashing intense rainfall events over short time periods. Many workers have documented that some colluvial hollows throughout the region are primed and prone to failure during such storms (Hack and Goodlett, 1960; Clark, 1987; Kochel, 1987; Jacobson, 1993).

For all the reasons summarized here, it seems evident that the past 11,000 years has been a time of active albeit sporadic debris fan growth and development in west-central Virginia. If no historical flooding had occurred, and the youngest deposits throughout the region were mid-Holocene in age, an investigator might be tempted to consider only the past few thousand years as critical to assessing the potential for flooding. However, historical flooding and fan sedimentation have occurred, and all deposits indicate long recurrence intervals that probably reflect the amount of time necessary to replenish the sediment supply in colluvial hollows. As a consequence, in this case a time period of 11,000 years is chosen as the best representation of whether or not a fan is active. (One must keep in mind, however, that numerous alluvial fans on the western flanks of the Blue Ridge have no historical or Holocene deposition and thus would be mapped as completely inactive according to this choice of time unit.)

An additional reason for this choice of time unit is the possibility that human activities now increase the potential of evacuation of colluvial hollows. As both tourism and urbanization are increasing in the region, it is probable that the potential for activity on a debris fan has increased.

Identifying Areas of Flooding and Deposition for the Time Period Chosen to Represent the Active Part of an Alluvial Fan

The alluvial fans in Nelson County are very small, and mapping by Kochel (1987) has indicated that single deposits can be traced across the entire fanhead area (Figure 4-17b). In addition, photographs of flooding during Hurricane Camille indicate that areas comprising up to 50 percent of the total fan area were flooded.

Finally, some photographs indicate that human-engineered structures and developments affected the paths of flow on mid- and lower fan areas. Therefore, it is prudent to map the entire fan as active, unless it can be demonstrated that part of a fan is of such high relief and resistance relative to the channelways that it is unlikely to be affected. If a fan did not have flooding and sedimentation during Hurricane Camille, it may be more likely to be flooded during the next large storm because upslope colluvial hollows have not been evacuated in several thousand years.

Defining and Characterizing Areas of Alluvial Fan Flooding on Active Parts of Alluvial Fans

Defining Areas of Alluvial Fan Flooding Hazard
Identifying Areas where Flow Departs from Confined Channels (i.e., where Flow Paths are Uncertain)

For the same reasons as stated earlier, all parts of the small fans in Nelson County

Suggested Citation:"Applying the Indicators to Example Fans." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

appear to be susceptible to alluvial fan flooding. As can be seen in Figures 4-13b, 4-15, 4-16 and 4-18, flow paths are highly unpredictable and prone to expansion and shifting. Because of the small size and low relief of the fan surfaces, channel migration is possible on any part of the fan.

Identifying Areas where Sheetflood Deposition Occurs

Sheetflood deposition can be seen commonly at the toes of the fans in Nelson County (see Figures 4-13b and 4-16), especially where the fans grade into floodplains of larger streams, and thus have very low slopes.

Identifying Areas where Debris Flow Deposition Occurs

In Nelson County, all deposition except the sheetflood deposits on the lower parts of the fans is the result of debris flows, as demonstrated by analyses of stratigraphy in the fans (Williams and Guy, 1973; Kochel, 1987).

Identifying Areas where Structures or Obstructions Might Aggravate or Cause Alluvial Fan Flooding

Evidence of structural controls that aggravated or caused alluvial fan flooding in Nelson County can be seen in Figure 4-19, where an access road with fill built across a colluvial hollow might have triggered collapse of sediment in the hollow because of water collection and diversion. Outside of Nelson County, along the Potomac River where other examples of humid region alluvial fans occur, a good example of the migration of channel flow along a highway can be seen (Figure 4-20).

Defining Areas of Nonalluvial Fan Flooding Hazard along Stable Channels

None of the channels on the alluvial fans in Nelson County appear to be stable; thus all flooding is deemed to be alluvial fan flooding.

SUMMARY

Judging whether alluvial fans are subject to flooding is not necessarily simple. The intent of these examples is to illustrate and inform, not to second guess past decisions. Four out of the seven examples fit the revised definition of alluvial fan flooding, (i.e., Henderson, Thousand Palms, Rudd Creek, Nelson County). However, although Lytle Creek, Tortolita, and Carefree are alluvial fans, they do not meet the active fan criteria and are therefore not subject to alluvial fan flooding even though they exhibit some characteristics that distinguish them from riverine flooding. Lytle Creek illustrates a site where a large, wide trench cuts the fan from the topographic apex to Cajon Creek along the distal east side of the fan, thereby isolating adjacent parts of the fan from active sedimentation and alluvial fan flooding. Carefree illustrates a site where the network of incised distributary channels transport sediment through the fan and the fan presently is inactive. The dissected, coalesced fan remnants along the west side of the Tortolita Mountains function to transport sediment, are not aggrading, and do not have the shape of an alluvial fan. The seven sites illustrate the complex nature of flooding on alluvial fans and piedmonts and the advantage of using a systematic approach to flood hazard assessment such as suggested by this committee.

Suggested Citation:"Applying the Indicators to Example Fans." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

FIGURE 4-19 Human activities can increase the potential for release of materials, as shown by this mass movement initiated along an access road in an area damaged during the Blue Ridge, Tennessee, storm of 1973.

SOURCE: Reprinted with permission from Clark (1987).

Applying the committee's definition to field examples reveals that there is a choice to be made between having a very inclusive definition that consists of cases such as alluvial fans, deltas, and braided alluvial washes; or a somewhat exclusive definition that leaves out all nonalluvial fan cases along with those that display certain characteristics but not others. The first alternative would include sites like the Tijuana River (Box 4-1 and Figure 4-21) and involves changing the name alluvial fan flooding in the current regulations to uncertain flowpath flooding and adopting the committee's definition thereof. If the definition is to include cases that fit only one or two criteria, the distinction between alluvial fan and nonalluvial fan must be dropped because it would be inconsistent to include (1) less severe situations where fans are inactive and not subject to the committee's definition of alluvial fan flooding (i.e., and thus not include alluvial fans such as Lytle Creek and Carefree) and (2) nonalluvial fans with stable but distributary paths of flow like much of the western slope of the Tortolita Mountains. This will result in a more inclusive definition. The second alternative is to keep the term alluvial fan flooding in the regulations, clarify that it applies only to alluvial fans, and adopt the committee's definition thereof. This will result in a more exclusive definition.

Suggested Citation:"Applying the Indicators to Example Fans." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

FIGURE 4-20 Debris fan at mouth of Nelson Run, tributary to North Fork South Branch Potomac River, Virginia. Flow along North Fork is from right to left; field of view is 750 m (2,461 feet) wide. Note diversion of tributary flow along road (to the right along base of mountain slope), which then moved back across the fan to reach the North Fork. SOURCE: Jacobson (1993).

This choice is to be made by FEMA based on policy, the tolerance for uncertainty in NFIP mapping procedures, and the resources available to restudy those areas that might fit a more inclusive definition. Regardless, the selection of either alternative will result in a better, more precise definition that can be directly applied to the physical processes associated with a given flooding source.

Suggested Citation:"Applying the Indicators to Example Fans." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

BOX 4-1 WHEN IT IS NOT A FAN BUT IT ACTS LIKE ONE: TIJUANA RIVER, CALIFORNIA

The Tijuana River flows to the Pacific Ocean in San Diego County, California. Most of the 1,700-square-mile drainage area is in Mexico. Figure 4-21 was taken shortly after a large flood in 1993 and illustrates the effects of sediment deposition during a flood, which caused the river to split into more than one channel. This process resulted in an uneven distribution of flow across the floodplain and significant deviations from predicted base flood elevations.

The Tijuana is a coastal stream and clearly not an alluvial fan. However, this flooding source exhibits the characteristics of the existing NFIP definition of alluvial fan flooding: uncertain flow paths, high-velocity flows that create new flow paths through erosion, shallow flow depths, and high sediment transport rates, which cause significant damage by depositing sand in and around structures. Applying the committee's proposed definition and field indicators to this case yields the following findings

  • Is it an alluvial fan? No. The Tijuana River is a coastal stream grading into a coastal delta.

  • Nature of the fan environment: There is a perennial low-flow channel that presently appears to be the main channel. Riparian vegetation is present both in the channel and on other parts of the floodplain.

  • Sediment transport: Coarse sediment yielded by watershed erosion processes has deposited long before it reaches this part of the river, which has a slope of approximately 0.2 percent.

  • Topographic confinement: The floodplain is topographically bounded on the north and south. Due to natural flooding processes, however, there is considerable uncertainty associated with how flow paths are laterally distributed during a flood.

  • Characterizing flooding processes: Review of historical floods since the mid-19th century indicates that the low-flow channel has shifted its location numerous times. Early maps also indicate that there used to be several low-flow channels along the floodplain. Flows do not spread evenly across the floodplain but rather form a number of concentrated conveyance regions. Application of the traditional flood paradigm (using HEC-2) has been somewhat successful even though it fails to account for bank erosion, avulsion, scour, and other aspects of the actual flooding processes.

  • Structural mitigation required: Floodway setbacks, elevation on fill, or similar measures may not necessarily be adequate to mitigate the flood hazard.

Although this flooding source is topographically confined, natural sediment transport processes introduce considerable uncertainty into the prediction of stage-discharge relationships for the Tijuana River. The presence of multiple flow paths and the uncertainty they introduce does not, by itself, mean that this area is subject to alluvial fan flooding because the area is not an alluvial fan. The Tijuana River floodplain does, however, fall into the category of uncertain flow path flooding. It would seem reasonable that more stringent rules might apply to the mitigation of flood hazards for this case.

Suggested Citation:"Applying the Indicators to Example Fans." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

FIGURE 4-21 Tijuana River north of the Mexico border (1993). Courtesy of Aerial Fotobank, Inc.

REFERENCES

Bryant, B. 1988. Geology of the Farmington Canyon complex, Wasatch Mountains, Utah. U.S. Geological Survey Professional Paper 1476. Washington, D.C.: Department of the Interior.


Cain, J. M., and M. T. Beatty. 1968. The use of soil maps in the delineation of floodplains. Water Resources Research 4(1): 173–182.

Suggested Citation:"Applying the Indicators to Example Fans." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
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Camp, P. D. 1986. Soil Survey of Aguila-Carefree Area, Parts of Maricopa and Pinal Counties, Arizona. U.S. Soil Conservation Service Report. Washington, D.C: Department of the Interior.

Chin, E. H., B. A. Aldridge, and R. J. Longfield. 1991. Floods of February 1980 in Southern California and Central Arizona. U. S. Geological Survey Professional Paper 1494. Washington, D.C.: Department of the Interior.

Clark, G. M. 1987. Debris slide and debris flow historical events in the Appalachians south of the glacial border. Pp. 139–155 in Reviews in Engineering Geology, Volume VII. Boulder, Colo.: The Geological Society of America.

Costa, J., and G. Williams. 1984. Debris flow dynamics. U. S. Geological Survey Open-File Report 84-606, 22-1/2 minutes (videotape). Reston, Va.: U.S. Geological Survey.

DMA Consulting Engineers. 1985. Alluvial Fan Flooding Methodology—An Analysis. Federal Emergency Management Agency Contract EMW-84-C-1488. Washington, D.C.: FEMA.


Eckis, R. 1928. Alluvial fans of the Cucamonga district, Southern California. Journal of Geology 36:224–247.


Federal Emergency Management Agency (FEMA). 1990. Fan, An Alluvial Fan Flooding Computer Program . Washington, D.C.: FEMA.

Fields, J. J. 1994. Surficial Process; Channel Change, and Geological Methods of Flood Hazard Assessment on Fluvially Dominated Alluvial Fans in Arizona. Ph.D. dissertation, University of Arizona.

Fuller, J. E. 1990. Misapplication of the FEMA alluvial fan model: a case history. Pp. 367–377 in Hydraulics and Hydrology of Arid Lands, R. H. French, ed. New York: American Society of Civil Engineers.


Hack, J. T., and J. C. Goodlett. 1960. Geomorphology and Forest Ecology of a Mountain Region in the Central Appalachians: U.S. Geological Survey Professional Paper 347. Reston, Va.: U.S. Geological Survey.

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.

Hjalmarson, H. W., and S. P Kemna. 1991. Flood Hazards of Distributary-Flow Areas in Southwestern Arizona. U.S. Geological Survey Water Resources Investigations Report 91-4171. Reston, Va.: U.S. Geological Survey.

Hooke, R. L. 1967. Processes on arid-region alluvial fans. J. Geol. 75:438–460.


Jacobson, R. B., ed. 1993. Geomorphic studies of the storm and flood of November 3–5, in the Upper Potomac and Cheat River basins in West Virginia and Virginia: U.S. Geological Survey Bulletin 1981 . Reston, Va.: U.S. Geological Survey.


Keaton, J. R. 1995. Dilemmas in regulating debris flow hazards in Davis county, Utah. Pp. 185–192 in Environmental and Engineering Geology of the Wasatch Front Region, W. R. Lund, ed. Publication 24. Salt Lake City: Utah Geological Association.

Keaton, J. R., L. R. Anderson, and C. C. Mathewson. 1988. Assessing Debris Flow Hazards on Alluvial Fans in Davis County, Utah. Final report for U.S. Geological Survey Landslide Hazard Reduction Program, Agreement No. 14-08-0001-A0507. Reston, Va.: U.S. Geological Survey.

Suggested Citation:"Applying the Indicators to Example Fans." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
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Kochel, R. C., and Johnson, R. A. 1984. Geomorphology and sedimentology of humid-temperate alluvial fans, central Virginia. Pp. 109–122 in Gravels and Conglomerates, E. Koster and R. Steel. Canadian Society of Petroleum Geologists Memoir 10.

Kochel, R. C. 1987. Holocene debris flows in central Virginia: Pp. 139–155 in Reviews in Engineering Geology, Volume VII. Boulder, Colo.: The Geological Society of America.

Lowe, M. 1993. Debris Flow Hazards: A Guide for Land Use Planning, Davis County, Utah. U.S. Geological Survey Professional Paper 1519. Reston, Va., U.S. Geological Survey.


MacArthur, R. C. and D. L. Hamilton. 1986. Corps of Engineers efforts in modeling mudflows. Pp. 549–554 in Proceedings of a Western State High Risk Flood Areas Symposium, Improving the Effectiveness of Floodplain Management in Arid and Semi-Arid Regions. Madison, Wisc.: Association of State Floodplain Managers.

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.

Mathewson, C. C., and J. R. Keaton. 1990. Multiple phenomena of debris flow processes: A challenge for hazard assessments. Pp. 549–554 in Proceedings of the International Symposium, Hydraulics/Hydrology of Arid Lands. New York: Hydraulics Division of American Society of Civil Engineers.

McGlashan, H. D., and F. C. Ebert. 1918. Southern California floods of January 1916. U.S. Geological Survey Water-Supply Paper 426. Reston, Va.: U.S. Geological Survey.

Mills, H. H. 1983 Piedmont evolution at Roan Mountain, North Carolina. Geografisika Annaler 65A:111–126.

Mills, H. H., and J. B. Allison. 1994. Controls on the variations of fan-surface age in the Blue Ridge Mountains of Haywood County, North Carolina. Physical Geography 156:465–480.


Natural Resources Conservation Service (NRCS). 1968. Soil Survey of Davis-Weber Area, Utah . U.S. Department of Agriculture. Washington, D.C.: U.S. Department of Agriculture.

Natural Resources Conservation Service (NRCS). 1973. Soil Survey of San Diego County, California. Washington, D.C.: U.S. Department of Agriculture.

Natural Resources Conservation Service (NRCS). 1980a. Soil Survey of Riverside County, California, Coachella Valley Area. Washington, D.C.: U.S. Department of Agriculture.

Natural Resources Conservation Service (NRCS). 1980b. Soil Survey of San Bernardino County—Southwestern Part, California. Washington, D.C.: U.S. Department of Agriculture.


Patton, P. C., and V. R. Baker. 1976. Morphology and floods in small drainage basins subject to diverse hydrogeomorphic controls. Water Resources Research 12(5):941–952.

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


Reneau, S. L., W. E. Dietrich, D. J. Donahue, A. J. T. Juli, and M. Rubin. 1986. Late Quaternary History of Colluvial Deposition and Erosion in Hollows, Central California Coast ranges. Geological Society of America Bulletin 102(7):969–982.

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

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

Suggested Citation:"Applying the Indicators to Example Fans." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
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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. Sacks, 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.

San Diego County. 1977. Storm Report for Tropical Storm Doreen, August 15–17, 1977. San Diego: County of San Diego Department of Sanitation and Flood Control.

Shanmugan, G. 1996. High-density turbidity currents: Are they sandy debris flows? Journal of Sedimentary Research 66(1):2–10.

Sharp, R. V. 1972. The Borrego Mountain Earthquake of April 9, 1968. U.S. Geological Survey Professional Paper 787. Reston, Va.: U.S. Geological Survey.


Thomas, B. E., H. W. Hjalmarson, and S. D. Waltemeyer. 1994. Methods for Estimating Magnitude and Frequency of Floods in the Southwestern United States. U.S. Geological Survey Open-File Report 93-419. Reston, Va.: U.S. Geological Survey.

Troxell, H. P. 1942. Floods of March 1938 in Southern California. U.S. Geological Survey Water-Supply Paper 844. Reston, Va.: U.S. Geological Survey.


Waananen, A. O. 1969. Floods of January and February 1969 in Central and Southern California. U.S. Geological Survey Open-File Report. Reston, Va.: U.S. Geological Survey.

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

Williams, G. P., and H. P. Guy. 1973. Erosional and depositional aspects of Hurricane Camille in Virginia, 1969. U.S. Geological Survey Professional Paper 804. Reston, Va.: U.S. Geological Survey.

Williams, S. R., and M. Lowe. 1990. Process-based debris flow prediction method. Pp. 66–71 in Hydraulics/Hydrology of Arid Lands, R. H. French, ed. New York: American Society of Civil Engineers.

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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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