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<br />C-12 <br /> <br /> <br />In thc lower region of the fan, flood flows split and form multiple channels. For <br />purposes of this procedure, the concept of a single eguivalent channcl is used to compute <br />flood depths and velocities. Normal flow conditions are considered to exist in the multiple <br />channel region due to the relativcly flatter slopes. <br /> <br />The probability of a point being flooded in a given flood event decreases from the <br />apcx to thc toc of the fan because the downslope widening of the fan surface provides a <br />greatcr arca ovcr which a channel of given width may occur. <br /> <br />b. Depth of Flooding <br /> <br />For flood mapping purposcs, the depth of flooding computed on alluvial fans is the <br />depth of flow (dcpth of channel) in the channel that carries a given dischargc to thc toe <br />of the fan surface. <br /> <br />c. Velocity of Flooding <br /> <br />For alluvial fan flood mapping, the velocity of flooding computcd for alluvial fan <br />flood mapping is the velocity of flow in thc channel that carries thc given discharge to <br />the toe of the fan surface. <br /> <br />d. A vulsions <br /> <br />During major floods on active alluvial fans, peak flows may abruptly abandon one <br />channel that had been formed during the flood, and form a new channel This phe- <br />nomenon, termed an avulsion, can cause a significant incrcase in thc probability of flood- <br />ing at a given point on a fan bccause of the increased channel widths that may cross a <br />given contour during a given flood event. The treatment of avulsions is an important fac- <br />tor in the application of the methodology presented in this Appcndix. <br /> <br />3. FLOOD HAZARD ZONES <br /> <br />Special flood Hazard Areas on alluvial fans arc identified as Zone AO with thc <br />following definition: <br /> <br />'Zone AO: Zone AO is the flood insurance rate zone that corresponds to the areas <br />of 100-ycar shallow flooding (usually sheet flow on sloping terrain) where <br />average depths are between I and 3 fcct, Average whole-foot depths derivcd <br />from the dctailed hydraulic analyses are shown within this zone, <br /> <br />. Exceptioll to the 3-foot depth limit for zolle AO is permitted for alluvial failS whell <br />approved by the PO, <br /> <br />The Spccial Flood Hazard Area on each alluvial fan is subdivided into separate <br />AO zones with similar depths and velocities. Zones are delineatcd that have dcpths or <br />vcloeities diffcring by an average of 1.0 foot in depth or 1.0 foot per second (fps) in <br />velocity. <br /> <br />In areas of coalcscent alluvial fans, separate depth-frequency rclationships should <br />be developed for each source of flooding and combined based on the probability of the <br />union of independent events. <br />