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1 <br /> SECTION II <br /> ' FEATURES AFFECTING LOCAL DRAINAGE <br /> AND SOUTH BOULDER CREEK OVERFLOWS <br /> FROM HIGHWAY 36 TO THE AREA OF THE WELLMAN DITCH <br /> ' GENERALFEATURES <br /> There are numerous features which influence South Boulder Creek flows arriving at <br /> the area of Highway 36 Bridge over the creek. Some of these are indicated on <br /> Drawing 3 in the back of this report and the watershed figure in Section III. <br /> There are approximately 93.7 square miles of mountainous watershed above Gross <br /> Reservoir, ranging in elevation from about 7300 feet to the continental divide <br /> (11 ,900 to 13,294 feet M.S.L.). Flooding at these altitudes is usually dominated <br /> by snowmelt runoff. Rainfall data is sparse and the NOAA atlas statistics the least <br /> reliable. <br /> Gross Reservoir captures much of the basin runoff and stores it with diversions <br /> from the west slope. Practically, most flood runoff events would probably be <br /> trapped for later release. Even with a conservative assumption that the reservoir <br /> was full at the beginning of the flood, as made by the Corps, the spillway and <br /> reservoir characteristics would greatly attenuate inflows and delay the upstream <br /> peak flows from joining downstream peak flows. <br /> Downstream of Gross Reservoir to Eldorado Springs there are approximately 19.4 <br /> square miles of steeply sloping mountainous watershed ranging in elevation from <br /> 5720 to 8900 feet M.S.L. The Eldorado Stream Gauge is about one mile upstream <br /> of Eldorado Springs. There the tributary area below Gross is 18.2 square miles. <br /> Such areas can experience significant extreme rainfall with resultant large peak <br /> flows. <br /> South Boulder Creek emerges from the mountains at Eldorado Springs and flows to <br /> Highway 93, gaining another 7.6 square miles of tributary area (27.0 square miles <br /> below Gross) of steep watershed ranging from 5400 to 8760 feet. <br /> As documented in the Phase I report, South Boulder Creek begins to have alluvial <br /> fan characteristics above Highway 93. Modest flood flows can spill from the main <br /> channel, which is generally on the east side of the valley, and travel along a west <br /> overflow path with two irrigation ditches (hereafter referred to as the West Valley <br /> QOverflow). One is the Dry Creek Ditch No. 2, which travels 6 miles to very near <br /> the South Boulder Creek confluence with Boulder Creek before rejoining South <br /> ' II - 1 <br />