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<br />humans. People commonly underestimate the dangers <br /> <br /> <br />associated with rapid flow and may not take appropriate <br /> <br /> <br />protective action. There is often insufficient warning of <br /> <br /> <br />an impending flash flood to allow such measures on fans, <br /> <br /> <br />even if people understand the dangers. <br /> <br />In general. velocities decrease with distance from the fan <br />apex; however, local flow restrictions, drainage channels, <br />and even streets may carry water at velocities exceeding 10 <br />feet per second at the toe of the fan. Because the flood <br />pathways are unpredictable and erratic on many fans. the <br />presence of high velocities cannot be forecast effectively <br />except where incised channels exist. <br /> <br />Unoredictable flow Paths <br /> <br />In normal riverine floods. the chann~l and overbank areas <br />are cle~rly defined and are. relatively stable from one flood <br />event to another. The path of flow on alluvial fans is <br />characterized by unpredictable directions from one flood to <br />another and during a single flood, making the design of <br />flood protection efforts difficult. The unpredictable <br />nature of fan floods begins at the apex and extends to the <br />toe for fans with smooth surfaces. When entrenched channels <br />and/or topographic constrictions on the flow exist on a fan, <br />the location of flood flows in these areas will be <br />stabilized. Hence. the severity of this hazard is dependent <br />on fan morphology. <br /> <br />4.4 Effect Q.i Lill and Watershed Characteristics <br /> <br />The fan and watershed characteristics discus~ed in Section <br />3.2. due to thelr influence on flood dynamics. have a major <br />impact on the location and severity of flood ha=ards on <br />alluvial fans. Once these characteristics are identified <br /> <br />47 <br />