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<br />through field inspections and collection of data on past <br /> <br /> <br />floods, a qualitative analysis of expected flood hazards c~n <br /> <br /> <br />be ~ade. This section identifies the key relationships <br /> <br /> <br />between characteristics, flood behavior. and hazards. <br /> <br />figure 4.3 provides a summary of the typical direction <br />(positive, negative. or site specific) and approximate <br />strength (high, moderate or minimal) of the correlation <br /> <br />between characteristics and hazards. for example, high fan <br />slope strongly correlates with a large scour hazard and with <br />a small problem over unpredictable flow paths. This matrix <br />is intended to be a qualitative guide to the identification <br />of hazards which may occur on fans. <br /> <br />Watershed, Slooe, Veaetation and Forest Fires <br /> <br />steep watersheds tend to concentrate runoff more rapidly and <br />produce short duration. high intensity floods. Sediment <br />production is also enhanced by the high velocities generated <br />on steep slopes. Conversely. watershed vegetation delays or <br />stores runoff and inhibits ero~ion. causing smaller <br />discharges of sediment and water. Forest fires cause abrupt <br />reductions in watershed vegetation and, consequently, can <br />cause large increases in sediment production and peak <br />discharges on fans. The stability of flow channels on the <br />fan can be seriously degraded by increases in sediment <br />production. <br /> <br />The severity of all the hazards increases with the magnitude <br />of sediment and water discharges from the watershed onto the <br />fan. fans associated with steep and/or poorly vegetated <br />watersheds have historically exhibited particularly large <br />inundation. sediment deposition and impact hazards. Mu~ <br />flows are nearly always associated with one or more of these <br />characteristics. <br /> <br />48 <br /> <br />