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Last modified
11/23/2009 1:21:56 PM
Creation date
10/4/2006 9:11:52 PM
Metadata
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Template:
Floodplain Documents
County
Denver
Community
All
Stream Name
All
Title
Determining Flood Hazards on Alluvial Fans
Date
7/17/1999
Prepared For
State of Colorado
Prepared By
FEMA
Floodplain - Doc Type
Educational/Technical/Reference Information
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<br />I <br /> <br />I <br />I <br />I <br />I <br />I <br /> <br />20 <br /> <br />07/16/99 <br /> <br />5F Composite Methods <br /> <br />Assumptions: as identified in the sections referring to the methods being applied <br /> <br />Limitations: must integrate multiple methods into one result <br /> <br />Recommended Applications: floodprone areas that contain unique physical features in some <br />locations or have areas varying in levels of erosion and <br />migration activity <br /> <br />I <br />I <br />I <br />I <br /> <br />Site-specific conditions on alluvial fans may lend themselves to the use of multiple or combined <br />methods previously described for the determination of flood-hazards. For example, areas that <br />contain manmade conveyance channels or deeply entrenched stable channels can combine the results <br />of traditional hydraulic computer programs with methods for analyzing active areas. Coordination <br />with the Regional PO and Headquarters is required in developing a study plan. <br /> <br />6 ADDITIONAL INFORMATION <br /> <br />6A Sediment Transport <br /> <br />The following section regarding sediment transport is included as supplemental information for the <br />analysis of alluvial fans. Sediment transport analyses are generally required for alluvial fan studies <br />and revisions. <br /> <br />I <br />I <br />I <br />I <br />I <br /> <br />The boundaries of the stream channel are usually soil material with a given resistance to erosion. <br />Bed material can range from large boulders to very fine clay particles. ill general terms, sediment <br />can be cohesive, including clay, silt, and mixtures, or noncohesive, including sand, gravel, and larger <br />particles. Transport of non cohesive materials is strongly dependent on particle size. The entire size <br />distribution of the material is needed to ascertain its erodibility. The bond between particles in <br />cohesive soil dictates its resistance to erosion and is far more important than size distribution. <br />However, size becomes important once the material has been eroded and is transported by the flow. <br /> <br />An important sediment transport process is the development of an armor layer in beds containing <br />gravel and cobbles. Water flowing over the mixture of sand and coarser material lifts the smaller <br />grains and leaves an upper layer or armor of large particles. This armor protects the underlying <br />sediment from further erosion and controls the subsequent behavior of sediment transport. A flood <br />event oflarge magnitude can disturb the protective layer and the armoring process will start again. <br /> <br />I <br />I <br />I <br /> <br />Sediment transport exerts substantial control over mOJphology and channel geometric configuration. <br />An indicator of this influence is the sediment transport rate, which is the rate at which material <br />moves in the stream as quantified in units of weight per unit time. The transport rate is closely <br />dependent on the water discharge. There are two classification methods to describe the sediment <br />load in a stream. The first one divides the load into bed load and suspended load. The bed load is <br />that portion of the sediment that movcs along the bottom by sliding, rolling or saltation. The <br /> <br />I <br />
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