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<br />Obstruction in the flood plain, natural or man-made, influence the depth <br /> <br /> <br />and width of the area flooded. Trees and brush growing along the <br /> <br /> <br />streams retard the out of channel flood flows causing backwater and <br /> <br /> <br />greater depths of flooding. Road crossings with inadequate culvert <br /> <br /> <br />and/or bridge openings often become channel constrictions and obstruc- <br /> <br /> <br />tion to flood flows. Elevated roads in the flood plain act as barriers <br /> <br />Rapidly rising streams are also hazardous to an unsuspecting public, <br /> <br />especially at road crossings. On Cottonwood Creek a 100-year frequency <br /> <br />flood will peak in about 7 hours at Buena Vista. <br /> <br />which raise the water surface at some locations. Other road crossings <br /> <br />are lower than the flood plain and are over topped during flood stage. <br /> <br /> <br />The effects of culvert and bridge constrictions are graphically displayed <br /> <br /> <br />on the flood profile drawings, Exhibits A-I through A-5c. Trees, brush, <br /> <br /> <br />and debris washed out during the periods of high runoff, are carried <br /> <br />Duration of flooding will vary, depending on the season in which flood- <br /> <br /> <br />ing occurs. In the spring when the streams are high with runoff from <br /> <br /> <br />snowmelt, coupled with spring rains, high water could persist for <br />several days. During the late summer months when the streams are <br /> <br />normally low, runoff from thunderstorms will recede in a few hours. <br /> <br />Flood Hazard Areas <br /> <br />downstream into bridges and culverts. These obstructions act as dams, <br /> <br /> <br />backing the water upstream, until they break loose, causing a sudden <br /> <br /> <br />surge or wall of water with additional destructive force. Obstructions <br /> <br /> <br />caused by debris,clogging culverts and bridges are variable, resulting <br /> <br /> <br />in a multitude of flow conditions which are unpredictable for any given <br /> <br /> <br />flood event. The potential effect of debris clogging are not included <br /> <br />The total flood hazard area on all reaches subject to inundation by the <br /> <br /> <br />100-year flood under present conditions is about 154 acres. The area <br /> <br /> <br />increases to about 192 acres for the SOD-year flood. The following <br /> <br />tabulation for areas flooded is by study reach. <br /> <br />Flood Hazard Area <br /> <br />in the flood profile determinations. <br /> <br />Study Reach <br />No. <br /> <br />100-Year Flood <br />Acres <br /> <br />SOD-Year Flood <br />Acres <br /> <br />Flood water flowing at high velocities creates dangerous conditions. <br /> <br />Velocities exceeding 3 feet per second with depths of 2 feet or more are <br /> <br /> <br />generally considered hazardous. Those conditions would exist at various <br /> <br /> <br />locations in the study area, especially in the stream channel of Cotton- <br /> <br /> <br />wood Creek. Most of the flood plain outside of the stream channel is <br /> <br />2 <br /> <br />6.6 1/ <br />11.4l! <br />52.1 .!/ <br />62.5 <br />21.5 <br />154.1 <br /> <br />6.7 l! <br />17.0 l! <br />65.S]} <br />79.9 <br /> <br />3 <br />4 <br /> <br />5 <br /> <br />22.7 <br /> <br />subject to shallow flooding and the flows are less than 2 feet in depth. <br /> <br />Study Area Total <br /> <br />192.1 <br /> <br />l! Does not include channel area. <br /> <br />10 <br /> <br />9 <br />