Laserfiche WebLink
<br />sediment which bulk the peak discharge. A concentra.tion of 40 <br />percent by volume would result in bulked peak discharge 1.67 times <br />greater than that for a water pea}~ discharge. In addition to <br />bulking the discharge, high sediment: concentrations can result in <br />a significant variation in the fluid properties of viscosity and <br />yield stress that control the floYl depth and velocity. <br /> <br />* FEMA I S methodology USElS the log Pearson <br />type III probability distribution to assess <br />peak discharge return periods. <br /> <br />Applicability of the log Pearson type III probability <br />distribution to alluvial fan flows i:s unproven. Its USE~ for flows <br />with high sediment concentration is questionable. Mudflows and <br />water floods can have substantially different peak discharges for <br />the same magnitude rainfall. <br /> <br />Summarizing, hazard delinea'tion on alluvial fans with <br />sUbdivisions, levees or otherwise al'tered flow paths requires more <br />detailed analyses than that prescribE!d by the FEMA guidelines. The <br />FEMA method is applicable only to idealized, uniform topography <br />alluvial fans, but there are no published guidelines on the limits <br />of its applicability. The method has numerous limitations and <br />caution should be exercised in applying it for the prediction of <br />hydraulics on urbanized fans, fans that have flood containment <br />structures such as levees or debris basins, or fans that exhibit <br />mud and debris flow characteristicf;. FEMA' s method should not be <br />used in the design of flood mitigat:ion. <br /> <br />Flood insurance which fails to reflect ac.tual levels of flood <br />hazards may encourage unsound devHlopment in flood-prone areas. <br />Addi tionally, poorly delineated flood hazard areas may require <br />disproportionately high payments from those in less flood-prone <br />areas. FEMA suggests that alternative methods should be applied in <br />the analysis of alluvial fan flooding where development or modified <br />flow paths will affect flow hydraulics. Accordinq to FEMA <br />Guidelines (page AS-I, 1985), <br /> <br />"In portions of alluvial fans in which natural alluvial <br />fan processes may not occur, such as in areas of <br />entrenched channels, areas protected by flood control <br />works, and heavily developed areas the study contractor <br />should exercise good engineering judgement in determining <br />the appropriate methodoloqy or combination of <br />methOdologies." <br /> <br />15 <br />