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<br />RMinfMll Intensitv <br /> <br />Intense. short duration thunderstorms are common occurences <br />in the semi-arid regions of the west and are responsible for <br />the flash floods which cause most of the property damage on <br />fans. The variation in duration and intensity of rainfall <br />between fans is substantial, and contributes to the highly <br />variable behavior of fan floods. <br /> <br />Lan Sloce <br /> <br />Surface slopes on fans vary from 1-2X on fans where sediment <br /> <br /> <br />and water production in the watershed is relatively low <br /> <br /> <br />(e.g. Boise) to 20-30X on fans built by debris flows (e.g., <br /> <br /> <br />Glenwood Springs). Fan slope is determined by long-term <br /> <br /> <br />watershed characteristics and. in turn. has a pronounced <br /> <br /> <br />effect on flood dynamics. <br /> <br />Fan Geomorchic BiMs. Entrenchment. and Entrance Anole <br /> <br />The shape and structure (morphology) of a fan indicates both <br />the history of the fan (past watershed conditions and <br />floods) and the likely behavior of future flood flows. If a <br />fan exhibits a topographic variation which confines the flow <br />to one part of the fan, it can be thought of as having a <br />geomorphic bias. flow can be similarly confined by an <br />incised channel or entrenchment which carries the entire <br />flood from the apex to a point downslope. Since water <br />velocities are typically very high at the fan apex during a <br />flood, the direction of the channel entering the fan <br />strongly influences the path of flooding down the fan. <br />Figure 3.2 illustrates two typical fan morphologies. <br /> <br />Fan VeaQt~tion <br /> <br />The type and density of fan vegetation are likely to be <br /> <br />31 <br />