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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />Road crossings with inadequate culvert and/or bridge openings often <br />cause channel constrictions and obstruction to flood flows. Elevated <br /> <br />roads in the flood plain act as barriers which raise the water surface <br /> <br />at some locations. Other road crossings are lower than the flood plain <br /> <br />and are over topped during flood stage. Sediment deposition and debris <br />are other factors which influence channel hydraulic characteristics and <br /> <br />subsequently affect the flood line location. <br /> <br />Velocities exceeding 3 feet per second combined with depths of 2 <br /> <br /> <br />feet or more are generally considered hazardous. These conditions would <br /> <br /> <br />exist at various locations on Coal Creek and Rock Creek during large <br /> <br />floods. Rapidly rising streams with associated deep flows are also <br />hazardous to an unsuspecting public, especially at road crossings. On <br />Coal and Rock Creek a lOa-year frequency flood will peak between 6 and <br />10 hours during a 24 hour storm. Duration of flooding will vary, <br />depending on the season in which flooding occurs. In the spring when <br />the streams are high with runoff from snowmelt, coupled with spring <br />rains, high water could persist for several days. During the late <br />summer months when the streams are normally low, runoff from thunderstorms <br />will recede in a few hours. <br /> <br />There are 2,466 acres of land in the study area subject to <br /> <br /> <br />inundation by the lOa-year flood. Of this amount, 1,807 acres and <br /> <br /> <br />659 acres are in the respective flood plains of Coal 'Creek and Rock <br /> <br /> <br />Creek. This includes 1,591 acres in Boulder County and 875 acres <br /> <br /> <br />in Weld County. <br /> <br />- 11 - <br />