Alluvial Fan Flooding (1996) / Chapter Skim
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Applying the Indicators to Example Fans
Applying the Indicators to Example Fans
Pages 83-132
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From page 83... ...
The sites represent a wide range of flood processes, from unconfined water flooding and debris flows on entrenched 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)
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From page 84... ...
84 ye a)
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From page 85... ...
See Wasatch Front alluvial fans in Appendix A Nelson Average Active Debris flow Yes?
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From page 86... ...
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 TICS, 1973~.
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From page 87... ...
T~'~' ~-LL.IF' DR.4lt'JA';=E EASIrl eOULdCiAP''r' RELIC ALLLJ'v'IAL F.~.N B6ILlNGiAPY :~< aTC'EAt`1l-HANNEL APEX OF RELIC ALLLl ~IP.L FAN t: APEX OF ACTIVE ALLL~VI.~.L FAN O 111 ; [a; rat, :93 1 2 WILOf~lETERS 1 1 ACTIVE ALLV'`.''IAL FAN l I ~l -,! RILES n 1 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.
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From page 88... ...
~Apex of active fan ~-~ -L .~ ~ 4 5 6 7 !
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From page 89... ...
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.
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From page 90... ...
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.
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From page 91... ...
Because the oaths 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.
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From page 92... ...
There is no evidence of recent debris flows. Where are there structures or obstructions that might aggravate or cause alinvial fan flooding?
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From page 93... ...
Recognizing and Characterizing Alluvial Fans The landform is identified on NRC S (1980) soil maps as an alluvial fan.
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From page 94... ...
. ALL WILL FAN FLOODING FIGURE 4-5 Aerial photograph of Thousand Palms wash (1993)
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From page 95... ...
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.
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From page 96... ...
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.
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From page 97... ...
bounds can be readily identified on the USGS 7.5-minute series topographic map (Devore Quadrangle)
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From page 98... ...
Geological Survey streamflow gage. Defining the Nature of the Alluvial Fan Environment Active Fan Relict Fan There is no active fan.
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From page 99... ...
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)
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From page 100... ...
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.
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From page 101... ...
.. ·~4T~ :;^ r nor ~32° 28' N SANTA (RUZ RIVER FlOODP~IN [I Active fans end thnnnels, 0-5 ka Weakly dissected inactive fans, 5-20 ha Moderately dissever inactive fans, 20-]
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From page 102... ...
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)
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From page 103... ...
103 Cottonwood Fan FIGURE 4-8 Cottonwood fan (see box in Figure 4-7) , showing active fan features mapped from aerial photographs, historical accounts, and geomorphologic survey.
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From page 104... ...
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.
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From page 105... ...
NOMOLI O PINALENO C] PINANT-TRENANT |DETAILED SOIL [;1API ~ , ~ ~ ~ ~ ~ E ~ IN ROAD or At- IFLOOD MAP| ., ~ ~1 0 2000m 1 1 1 J~-~MA\~ ROAD LEGEND BOUNDARY OF ALLIJ`..(IAL FAN _- TRIBUTARY STREAM hi, INFLOW TO OR OUTFLOW FROM ALLUVIAL FAN : DISTRIBUTARY CHANNEL WITH ESTIMATED EXTENT OF 100-YEAR FLOOD SHEETFLOW IN LOW LYING AREAS FIGURE 4-9 Carefree alluvial fan, showing distributary channels, soil units and estimated extent of 100-year flood.
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From page 106... ...
. The small relief ratio of the basin suggests that debris flows are unlikely.
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From page 107... ...
. Unit peak discharge for the pediment above the Carefree fan undoubtedly was smaller because rainfall amounts were less and orographic effects were unlikely.
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From page 108... ...
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.
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From page 109... ...
Defining Areas of Active Alluvial Fan Flooding Hazard There are no areas on the Carefree fan where flow paths are expected to change.
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From page 110... ...
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 elective 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.
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From page 111... ...
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.
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From page 112... ...
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)
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From page 113... ...
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.
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From page 114... ...
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.
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From page 115... ...
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)
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From page 116... ...
, 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 413a and 4-13b)
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From page 117... ...
(Bottom) Debris avalanches that formed during Hurricane Camille, 1969, on hillsides terminating on two debris fans along the Virginia Piedmont.
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From page 118... ...
(1995~. percent of the surface area of many fans was flooded and altered by sedimentation from debris flows.
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From page 119... ...
(Bottom) This clubhouse located on a debris fan was destroyed during a Blue Ridge, Tennessee, storm in 1973.
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From page 120... ...
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.
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From page 121... ...
(c) Texture and matrix composition vary with depth, enabling the investigator to discriminate boundaries between debris flow events.
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From page 122... ...
In Nelson County, evidence is available to document when and how open 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~.
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From page 123... ...
APPLYING THE INDICT TORS TO EXAMPLE FANS 123 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~.
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From page 124... ...
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 upsIope colluvial hollows have not been evacuated in several thousand years. Defining and Characterizing Areas of Alluvial Fan Floocling on Active Parts of Alluvial Fans Defining Areas of Alluvial Fan Flooding Hazard Identifying areas where flow depaffsirom confined channels (i.e., where flow paths are uncertain)
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From page 125... ...
. 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.
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From page 126... ...
This will result in a more inclusive definition. The second alternative is to keep the term allavialfan flooding in the regulations, clarify that it applies only to alluvial fans, and adopt the committee's definition thereof.
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From page 127... ...
127 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.
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From page 128... ...
726 ~ ~ -- 1 ~S~S~S~S~S~S~S~S~S~S~S~SSSSSSSSS;SSSSSSS~SSSSS~S~SSSSSSSSSSSSSSSSSSS~SSSS.,. ss~s~ss~s~ |
From page 129... ...
U.S. Geological Survey Professional Paper 1476.
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From page 130... ...
1988. Assessing Debris Flow Hazards on Alluvial Fans in Davis County, Utah.
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From page 131... ...
1987. Holocene debris flows in central Virginia: Pp.
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From page 132... ...
1996. High-density turbidity currents: Are they sandy debris flows?
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