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<br />l <br /> <br />3. A~LUVIAL fAN DESCRIPTION <br /> <br />Alluvial (depositional) formations have topographic, <br />morphologic, and hydrologic characteristics which differ <br />substantially from the characteristics of most river <br />valleys. Since the hydraulic processes and the extent of <br />flood hazards on fans depend on these characteristics, a <br />summary of the geologic processes which form and reshape <br />fans is included. <br /> <br />3.1 Fan Formation and Modification <br /> <br />Three types of alluvial landforms are often confused in <br />flood studies; and yet have strikingly different flood <br />behavior: alluvial fans, alluvial aprons, and washes. The <br />differences between these landforms lie in how and when they <br />develop. All three st~uctures form at the base of steep, <br />highly erodible mountaln masses which are subjected to high <br />intensity, short duration rainfall events. Such rainfall <br />events dislodge large amounts of sediment and transport the <br />sediment to the valley floor through ravines and channels. <br />When the flood flows leave the confinement of the rock walls <br />of the ravine, the water spreads out, becomes more shallow, <br />and drops the majority of its suspended sediment load onto <br />the valley floor. Over geologic time these deposits form a <br />segment of a cone with its apex at the mountain front. <br />Lines of maximum topographic slcpe radiate away from the <br />apex and terminate at the valley floor, where original <br />valley slopes again dominate. Characteristic slopes of fans <br />depend on several factors, including size of sediment <br />particles and concentration of sediment in the discharge <br />from the watershed. <br /> <br />24 <br />