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I I I I I I I 4 DRY CREEK FLOOD HAZARD AREA DELINEATION Background Information have a significant effect on downstream flows because of its off-channel location. Its storage volume is approximately 5,300 acre-feet and it can completely contain inflow due to the 100-year recurrence interval storm under normal operating conditions. The reservoir has a 40 foot high, 2,220 foot long dam in its northeast quadrant with outlet works leading to Dry Creek. The dam has an emergency spillway in natural sandstone which will convey discharge to the creek when it operates. However, some of this discharge will be intercepted by two irri- gation ditches which lie between the spillway and the creek. The spillway appears to be used as an access point to the reser- voir by a local recreation club. The State Engineer's records indicate that no potentially hazardous conditions are associated with the spillway or the dam. I I Dry Creek is a relatively small tributary of Boulder Creek originating near Base Line Reservoir and flowing towards the northeast for a distance of 6.2 miles. The creek has a tribu- tary basin of approximately 13.6 square miles and can receive base flow from either Base Line Reservoir or the New Dry Creek Ditch. Principal land uses in the basin are residential and agricultural with limited amounts of gravel mining. The channel is narrow and well defined throughout its entire length. The geologic flood plain is similarly narrow and con- fined in the upper reaches but becomes flatter and broader as the creek approaches its confluence with Boulder Creek. The basin contains numerous irrigation ditches with several having their point of diversion on Dry Creek. However 1 these all have been assumed as unable to intercept runoff for the pur- pose of flood hazard area delineation. Ten structures cross the creek between Base Line Reservoir and Boulder Creek. Hydraulics I I I I I I I I I I Hydrology Flood hydrology was derived by application of a synthetic unit hydrograph procedure using Urban Drainage and Flood Con- trol District design rainfall. The tributary basin was divided at Arapahoe Road for purposes of analysis. Figures 2 and 3 contain the storm hydrographs for the two subbasins as well as information on design rainfall and basin parameters. The parameters reflect anticipated future development of the basin based on planning projections obtained from Boulder County. Flood flows are 1,000 cfs above Arapahoe Road and 1,300 cfs below it. As with the Boulder Creek study reach 1 the ten structures crossing Dry Creek were examined to determine an assumed degree of blockage. Details of the structures are shown in the drawings at the end of this report. The Baseline Road bridge is the only structure capable of conveying flood flow without overtopping. An abandoned bridge in the channel immediately upstream of Valmont Drive has been disregarded because it is likely to washout completely under flood flow and will have no effect on the local flood hazard area. All other structures are of insufficient capacity to handle flow and will experience combined pressure and weir flow. Base Line Reservoir, owned by the Baseline Land and Reser- voir Company, has a drainage area of roughly 0.5 square miles including the surface area of the reservoir itself. It is located at the upper limit of the Dry Creek study and does not Flood water elevations were determined by using the Corps of Engineers' HEC-2 computer backwater program. These elevations are tabulated in Table 2. ~ Leonard Rice Consullrng Waler Engrneers, Inc. -~ -------~~---~~-~-~