Alluvial Fan Flooding (1996) / Chapter Skim
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Flooding Processes and Environments on Alluvial Fans
Flooding Processes and Environments on Alluvial Fans
Pages 29-50
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From page 29... ...
Alluvial fans occur in a wide range of environments, including the western and eastern mountains of the United States, western Canada, and various montane, arid, and volcanic regions around the world. In North America, most fans that have been subject to development are in the western mountainous regions.
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From page 30... ...
~,. ~ ~-= r ~ - _ A _ · _ ~V ~ ~1 ~ 01 ~O~ ~ ~1 _1 Because the frequency, triggering mechanisms, size, and sedimentation processes of debris flows are so different from those of water floods, and the morphology and other clues about the nature of the flooding hazard on the respective types of fans are so radically different, it is necessary to distinguish between streamflow fans (Bull, 1977)
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From page 31... ...
~7 ~ 1 FIGS 2-2 Where fans cohere Tom multiple source vaUeys, the On shape may not be obvious. The coalescing D~ and Doer Canon fans on a beads Hong the southern dopes of [he San Gabricl Mountains near Cucamonga, CaH~mia, flooded in annual 1969.
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From page 32... ...
This distinction is not widely made in the literature, where all fans are usually called al/? Ivia~fans' but it is important because recognition of the nature of flooding and sedimentation processes on a fan and an understanding of the difference in triggering mechanisms and therefore probabilities of debris flows and floods are crucial to the accurate interpretation and prediction of flood risk.
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From page 33... ...
Even quite large and well-defined channels can be abandoned if a flood breaches one of the channel banks and water flows overbank in depressions between old bar deposits on the fan surface, often eroding a deep channel headward up to the source channel, which is then diverted. Particularly large, kilometer-scale changes in the positions of flow paths and active sedimentation zones can occur without the channel occupying or shifting across intermediate positions if the channelized and the overbank flow cause sediment to be deposited within and close to the channel, raising the bed and the channel margins above the surrounding fan surface (Figure 2-4~.
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From page 34... ...
On streamflow fans where the sediment balance has turned negative, either at present or for some period in the recent past, the channels are deeper because sedimentation on their floors and margins is replaced by incision. The flow and sediment conveyance capacity increase because form roughness is less in the absence of aggressive bar growth.
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From page 35... ...
Debris Flow Fans Debris flow fans occur where strongly episodic sediment transport is triggered by collapse of an accumulation of weathered rock, soil, or sediment in a steep source region or by concentration of flow onto a steep accumulation of sediment that is then trenched rapidly in such a way that a high sediment concentration is developed with a mixture of sizes, including a significant proportion of fine sediment. The sediment-water ratio of the mixture must be so high that the flowing debris has a low permeability and water cannot drain out (upward)
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From page 36... ...
Thus, the debris flows that supply and mold any one fan have a probability distribution of discharges and theological properties, which determine the nature and magnitude of flood risk. Fortunately, these aspects of flood risk can be read from the morphology of the fan and its source basin.
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From page 37... ...
Like alluvial fans, debris flow fans are subject to varying amounts of deposition and parts or even much of the fan may be inactive under the present climate. For example, the debris flow fans emanating from the east side of the Sierra Nevada in the northwestern part of Owens Valley have more or less ceased to accumulate since the end of the last glaciation in the mountains, and the oldest parts of the fans date from previous glaciations.
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From page 38... ...
The lower, streamflow part of the fan has the characteristics of an alluvial fan described above, although there may also be a contribution of dilute debris flow deposition on these distal areas. An indication of the relative contributions of debris flows and water floods can be obtained through systematic identification and mapping of the distribution of the two types of sediments on the fan surface and in vertical sections along the sides of channels.
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From page 39... ...
FLOODING PROCESSES ON ALLUVIAL FANS Flooding on Streamflow Fans Since streamflow alluvial fans typically occur in arid and mountainous environments, one of the first difficulties encountered in the quantification of alluvial fan flooding processes is the magnitude-frequency relationship for flows supplied to the apex. The sparseness of hydrologic monitoring stations in such regions and the shortness of most records render most estimates of probable flood discharges highly uncertain.
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From page 40... ...
Even though velocities and depths are low, inundation by turbid water can be very destructive. Flooding on Debris Flow Fans Debris flows are dense (approximately 1.8 to 2.0 times the density of water)
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From page 41... ...
At the distal margins of debris flow fans, low-strength flows often spread widely in a manner similar to sheetflooding on streamflow alluvial fans (Figure 2-6~. A particularly hazardous situation arises on debris flow fans around active volcanoes because of the huge volumes of sediment that can be liquefied and the persistence of the liquefaction.
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From page 42... ...
Flooding on Composite Fans Composite fans are subject to both streamflow and debris flow flooding. The relative importance of each varies between fans, between positions on each fan, between individual
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From page 43... ...
After examining the debris flow generation potential of the source area, it is also possible to make an approximate calculation of the maximum conceivable transport of debris flows downfan. It is also possible through geologic mapping and dating of deposits and examination of source areas to determine whether the debris flow activity that may have built most of a fan is still active or will generate much smaller debris flows than during the period of intense fan building.
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From page 44... ...
The most permanent deposition typically begins at the toe and propagates both up the fan and below the toe where the slope typically diminishes. The evolution of the fan surface causes a difficult problem for the interpretation of field evidence concerning alluvial fan flooding and for the prediction of fixture flood risk.
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From page 45... ...
This would require a combination of geomorphologic survey (for low banks, elevated channels, zones of sediment accumulation, bank erosion and channel shifting, and topographic lows on the fan surface) and calculations of the heights of conceivable floods and debris flows.
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From page 46... ...
Rather, small distributary channels that in places appeared to be simple topographic lows conveyed overflow from the larger distributary channels or carried runoff from high-intensity rainfall directly on the fan surface. Many less active alluvial fans in the arid southwestern United States have developed soils with small, slightly incised stable flow paths that convey shallow high-velocity floodflow during major floods.
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From page 47... ...
Reported flood characteristics of the sample of fans include high-velocity water flows, debris flows, translatory waves, sheetflood, distributary flow, unstable and stable channel boundaries, movement of flow paths, stable flow paths, and alleviation on the unchanneled fan surface. The composite of these accounts of flooding shows a wide variability of processes and flood hazard in time and space, which places a premium on field inspection and interpretation of concrete evidence from each alluvial fan before a determination is made of flood risk.
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From page 48... ...
48 ALLUVIAL FANFLOODING Am FIGURE 2-7 In urbanized settings, such as on the Glendora fan in California. streets can influence the path of floods.
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From page 49... ...
1988. A probabilistic model for hazards related sedimentation processes on alluvial fans in Davis County.
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From page 50... ...
1992. The influence of debris-flow rheology on fan morphology, Owens Valley, California.
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