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. . . . . . . . . . . . . - . . . . . . . . . . . . . . . . . . . . . . I. I I. . . . . - 19 02/23/00 SF Composite Methods Assumptions: as identified in the sections referring to the methods being applied Limitations: must integrate multiple methods into one result Recommended Applications: floodprone areas that contain unique physical features in some locations or have areas varying in levels of erosion and migration activity Site-specific conditions on alluvial fans may lend themselves to the use of multiple or combined methods previously described for the determination of flood hazards. For example, in areas that contain manmade conveyance channels or deeply entrenched stable channels, one can combine the results of traditional hydraulic computer programs with methods for analyzing ac.rive areas. Coordination with the Regional PO and Headquarters is required in developing a study plan. 6 ADDITIONAL INFORMATION 6A Sediment Transport The following section regarding sediment transport is included as supplemental information for the analysis of alluvial fans. Sediment transport analyses are generally required for alluvial fan studies and revisions. The boundaries of the stream channel are usually soil material with a given resistance to erosion. Bed material can range from large boulders to very fine clay particles. In general terms, sediment can be cohesive, including clay, silt, and mixtures, or noncohesive, including sand, gravel, and larger particles, Transport of noncohesive materials is strongly dependent on particle size. The entire size distribution of the material is needed to ascertain its erodibility. The bond between particles in cohesive soil dictates its resistance to erosion and is far more important than size distribution. However, size becomes important once the material has been eroded and is transported by the flow. An important sediment transport process is the development of an armor layer in beds containing gravel and cobbles. Water flowing over the mixture of sand and coarser material lifts the smaller grains and leaves an upper layer or armor of large particles. This armor protects the underlying sediment from further erosion and controls the subsequent behavior of sediment transport. A flood event oflarge magnitude can disturb the protective layer, and the armoring process will start again. Sediment transport exerts substantial control over morphology and channel geometric configuration. An indicator of this influence is the sediment transport rate, which is the rate at which material moves in the stream as quantified in units of weight per unit time. The transport rate is closely dependent on the water discharge. There are two classification methods to describe the sediment load in a stream. The first one divides the load into bed load and suspended load. The bed load is that portion of the sediment that moves along the bottom by sliding, rolling, or saltation. The