Alluvial Fan Flooding (1996) / Chapter Skim
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Indicators for Characterizing Alluvial Fans and Alluvial Fan Flooding
Indicators for Characterizing Alluvial Fans and Alluvial Fan Flooding
Pages 51-82
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From page 51... ...
A In tin the r1~;~;~;~ ~_~l :~ AL _ -' _ ~ r ~ ~ ~ ret alluvial fan flooding is a flood hazard that on active parts of alluvial fans has a 1 percent chance of occurrence, and it is identified by flow path uncertainty and deposition and erosion below the hydrographic apex. The criteria used to assess whether an area is, or is not, subject to alluvial fan flooding must determine whether the flooding occurs on an alluvial fan and whether it is characterized by deposition, erosion, and flow path uncertainty below a hydrographic apex.
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From page 52... ...
On what parts of the active alluvial fan is alluvial fan flooding occurring? (Refer to recent aerial photographs, soils maps, historical records, and detailed field mapping.)
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From page 54... ...
If this area does not meet the criteria for the definition of an alluvial fan, then it does not qualify for consideration of alluvial fan flooding. The committee defines an alluvial fan as "a sedimentary deposit located al a topographic break such as the base of a mour~tai'' front, escarpment, or valley side, that is composed of streamflow and/or debris flow sediments and which has the shape of a fall, either fully or partially extended.
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From page 55... ...
This landform is known as the Cedar Creek alluvial fan and is a classic example of a fan with nearly ideal morphology. Location All~vialfan landforms are located at a topographic break.
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From page 56... ...
. County floodplain management agencies Long-time residents _ .
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From page 57... ...
IND1CA TORS FOR CHARACTERlZlNG ALLUV1AL FANS 57 Orthophoto- Water quads Satellite Hydrologic Flood Hydrography data Floodplain Subsurface Geology Soils Other o O O O O O O O O O O O o o O O O O O O O o O O o O O O O O O O O O o O O O O O O O O O O o o o O O O O O O O O O O O
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From page 58... ...
geologic map of Cedar Creek alluvial fan in Montana. SOURCE: Reprinted with permission from Ritter et al.
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From page 59... ...
Dark, undisturbed surfaces of rock varnish are found on old Piedmont and valley deposits throughout the Basin and Range province of the western United States. In the case of multiple fans that coalesce to form bc~jadas7 where deposits and reworked material of adjacent alluvial fans merge, the boundaries between adjacent fans may be less distinct than those of individual fans adjacent to streams, rivers, or smooth piedmonts, but generally are ' The committee has not visited the Cedar Creek fan and inspected its surface and deposits.
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From page 60... ...
this does not mean that all parts are equally susceptible to alluvial fan flooding now. In fact, in most of the United States it is possible to identify parts of alluvial fans that were actively constructed during Pleistocene time (about 2 million to 10,000 years ago)
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From page 61... ...
61 Pleistocene and Holocene epochs. In many regions, the post-Pleistocene change of climate resulted in a reduction in the rate of sediment supply to fans, whether by streams or debris flows.
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From page 62... ...
In many places in the southwestern United States' it is less easy to see a clear Pleistocene/Holocene change, but one can subdivide Holocene deposits on a much finer timescale. In areas with long records of flooding and aerial photographs that date back to about 60 years, it is possible to identify areas of historic flooding within the past ~ 00 years.
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From page 63... ...
A systematic approach to the problem of estimating whether a place may be inundated aider a long period of inactivity is suggested in Chapter 2, the section " Change Over Time," which deals with processes by which flooding and deposition can migrate across an alluvial fan to invade places that have long been outside the zone of active deposition, even in the current climate. Since there is no clear analytical technique for making such projections, estimates of the probable spatial extent of inundation involve systematically applied judgment, and the combination of hydraulic computations and qualitative interpretations of geologic evidence concerning the recent history and probable future evolution of channel forms, as well as flooding and sedimentation processes.
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From page 64... ...
Identifying Areas of Flooding and Deposition on the Active Part of an Alluvial Fan It is important to identify both active and inactive parts of the fan, because this provides a map of where flooding can occur as wed as where it probably wiR not occ'`r. Preparing a Geomorphic Map of Different Age Fan Surfaces Once a time period is chosen to represent the active part of a fan for the purpose of flood hazard assessment, the flood evaluator must determine which deposits are less than the chosen age.
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From page 65... ...
If organic matter can be obtainer! from shallow trenches in clenosits on alluvial fans.
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From page 66... ...
, ~ , ~ . ,[ , ~ ~ ~ , ,~ ~ ; ~ ~ , ' ' , :FIGURE 3-5 Young, intermediate, and old alluvial fan deposits, Gila Mountains.
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From page 67... ...
Alluvial Fan Deposits, Basin and Range Province, United States Characteristic Drainage pattern Depth of incision Fan surface morphology Young Distributary; anastomosing or braided Less than 1 In Bar and channel Intermediate Tributary; dendritic Variable (1 to 10 m) Variable, generally smooth and flat Old Tributary; dendritic or parallel Greater than 10 m Ridge and valley, most of surface slopes Preservation of fan surface Currently active Lncised, but well-preserved Basically destroyed, locally wide, flat divides preserved on narrow divides Desert pavement None to wealthy developed None to strongly developed None (surface destroyed)
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From page 68... ...
Intermediate age Colorado River alluvium E' Older and eolian send reworked from alluvium Younger ~ Erosional scarp Reconstructed margin of intermediate-age debris fan FIGURE 3-6 Geologic maps of debris flow fans in the eastern Grand Canyon. SOURCE: Hereford et al.
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From page 69... ...
It sometimes is possible to find evidence of flood damage on vegetation, thus providing a clear means of identifying parts of fans that recently have been active. Types of Alluvial Fan Flooding Alluvial fan flooding, as described in the committee's clefinition, is characterized by flow path uncertainty below the hydrographic apex and caused by abrupt deposition of sediment, proximity to a sediment or a debris flow source area, sufficient energy to carry coarse sediment at shallow flow depths, and the absence of topographic confinement which may allow higher flows to initiate a new, distinct flow path of uncertain direction.
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From page 70... ...
TUentif~ing those parts of the active part of alluvial fans that are susceptible to alluvial fan flooding also requires examination of the types of flooding as recorded by flood deposits.
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From page 71... ...
It might also occur where several shallow, distributary channels join together near the toe of a fan and the gradient of the fan is so low that the flows merge into a broad sheet. Because such sheetflows can carry high concentrations of sediment in shallow water and follow unpredictable flow paths, they can be classified as alluvial fan flooding processes if they occur on alluvial fans.
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From page 72... ...
Active alluvial fans are changeable, and erosion and deposition occur to some degree with most events. Inactive fans may also have flow paths that are unconfined and subject to uncertainty largely because of the numerous channel forks and joins.
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From page 73... ...
^ ~ eras - ~ ~A`,AA ~ ~ ~ ~ 80 ~ Ba. ;e obsc red Interlobe depression Degraded levee 0 °0.< ~ O~O\P Remnant clasts O 0 w00 ° DILUTE DEBRIS FLOWS TRANSITIONAL DEPOSITS FLUID ·RUN OUT' PHASE [D2]
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From page 74... ...
matrix-supported clasts; oriented parallel.to and flanks; flat tops poorly sorted; hmax range flow boundary with low relief 80-210 mm; stratification forming a push pressure ridges absent fabric Dilute debris flow Thin lobate; broad, Moderate Matrix-rich; matrix None observed (D2) flat top; gentle lobe (0.3-0.5)
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From page 75... ...
(1995~. AVAILABLE METHODS OF ANALYSTS 75 To investigate flood hazards, there are three general categories of interest: clear water flows that can be analyzed with traditional hydraulic methods, hyperconcentrated sediment flows that can be analyzed to a great extent by sediment transport theory, and debris flows that can be assessed by various empirical methods such as the bulking factor, the gingham model, and other methods.
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From page 76... ...
FEMA has developed a computer program called FAN (FEMA, 1990) that incorporates these assumptions, and it provides this program to contractors charged to delineate alluvial fan flooding.
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From page 77... ...
. I A QT Flow path uncertainty exists at Point A
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From page 78... ...
No | Increment # of ~ nonexceedances ~_Yes ,? ~; Conditional probability =# nonexceedance/ number of samples FIGURE 3-11 Conditional nonexceedance probability estimation with event sampling.
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From page 79... ...
In addition, human modification of alluvial fan surfaces and urban development on alluvial fans have resulted in cases where human-made obstructions themselves have been the cause of alluvial fan flooding. For example, construction of culverts to divert water from one part of a fan to another sometimes results in rapid sedimentation downstream from the mouth of the culvert.
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From page 80... ...
A FIRM showing alluvial fan flooding hazards mapped considering complete uncertainty is of little use for floodplain management. By making a conservative trade-ofl: in favor of all possibilities, this type of FIRM ignores the importance and the more threatening hazard of flow in existing channels and historical flow paths and conversely penalizes safer areas..
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From page 81... ...
1964. Alluvial fans and near surface subsidence in western Fresno County, California.
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From page 82... ...
1987. Sedimentologic and geomorphic variations in stormgenerated alluvial fans, Howgill Fells, northwest England.
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