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<br />avulsing channels found on steep :sloped alluvial fans. FEMA' s <br />single channel method also does no'l: account for the varia1:ion in <br />channel geometry in a down-fan direlction. <br /> <br />'" FEMA' s single channel method assumes a <br />critical depth flow relgime. <br /> <br />The critical flow depth assumption for single channel flow <br />according to Dawdy (1979) provides "...the most efficient movement <br />of water and sediment down the fan. III 'rhis hypothesis has not been <br />verified either analytically or eKperimentally. French (1986) <br />cites two references that indicate <1:ha1: flows on alluvial fans are <br />often supercritical. Supercritical flow is supported by evidence <br />observed in fan characteristics, in debris deposits and in the <br />computer simulation of flow hydraulics. Considering the variable <br />slope and roughness of alluvial fans and the transient nature of <br />flood flows, an assumption of critical flow may be <br />misrepresentation of the peak discharge flow regime. <br /> <br />'" Predicted flow hydraulics are assumed to <br />represent average values. <br /> <br />The FEMA methodology predicts 'average' velocity and flow <br />depths on alluvial fans. variation in flow hydraulics can be <br />substantial depending on the location on the fan. The design of <br />structural mitigation should be ba!;e!d on maximum hydrostatic and <br />hydrodynamic forces determined from predictions of maximum flow <br />depths and velocity. <br /> <br />* <br /> <br />FEMA's method <br />attenuation or <br />discharge as the <br />fan. <br /> <br />assUJ:nE!S there is no <br />sur<Iing of the peak <br />flow progresses over the <br /> <br />Flood wave attenuation and surging can cause large variation <br />in the flow depth and velocity as the flow progresses over the fan. <br />"On the lower part of the alluvial fan it is not reasonable to <br />expect the model to be accuratEI!, particularly if there is <br />infiltration and storage of flood flows on the alluvial <br />fan..." (Dawdy, et. al., 1989). Variable roughness and topography, <br />fan rainfall and obstructions effect the peak discharge as it <br />propagates down-fan. On large alluvial fans, rainfall runoff can <br />increase the peak discharge, but in the FEMA procedure, the <br />"...runoff from the watershed area below the apex is neglected..." <br />(Dawdy, et. al., 1989). <br /> <br />* Fan hydraulics are assumed to be unaffected <br />by sediment concentration. <br /> <br />The FEMA method predicts flow hydraulics for water only, mud <br />and debris are not considered. The inclusion of sediment "...would <br />affect the flow depths and velocities." (Dawdy, et. al., 1989). <br />Most alluvial fan flows have relativlely high concentrations of <br /> <br />14 <br />