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I I I I I I I I I I I I I I I I I I I at Sunset, and practically everything at Copper Creek. Computations made 18 years later produced estimates of the peak discharge in Boulder that ranged from 9,000 cubic feet per second to 13,600 cubic feet per second. Flood of September 4, 1938 - A large stonn system produced general rains over all of eastern Colorado. The largest amount of precipitation occurred in the mountains where over six inches was reported west of E1dorado Springs. Boulder reported 3.62 inches of precipitation from August 31 to September 4 with 2.32 inches falling on September 2. E1dorado Springs located in the South Boulder Creek basin had 4.42 inches of rainfall. A maximum discharge of 4,410 cubic feet per second occurred near the mouth of Boulder Creek at noon on September 3, 1938. Flood of May 6-8, 1969 - Thi s flood was the resul t of long durati on rainfall. Precipitation was heaviest in the mountains, part of ~Jhich fell as snow. In the Boulder and South Boulder Creek basins, the rainfall continued at a moderate rate for nearly four days. Total precipitati on for the stonn amounted to 7.60 inches at Bou1 der and 9.34 inches at the Boulder Hydroelectric Plant located about three miles up the canyon from Boulder. Precipitation amounts totaled 8.11 inches at El dorado Spri ngs and 10.05 inches at Gross Reservoi r on South Bou1 der Creek. Peak fl oodi ng occurred on the 7th of May on both Boul der and South Boul der Creeks. The gaugi ng stati on at Orodell recorded a peak discharge of 1,220 cubic feet per second. In Boulder, local inflow increased the Boulder Creek peak discharge to an estimated 3,000 cubic feet per second. The peak discharge on South Boulder Creek at E1dorado Springs was 1,690 cubic feet per second. Flooding below the confluence of these two streams extended over large portions of the floodplain. HYOROLOGIC AND HYDRAULIC DETERMINATIONS. Flood Characteristics Floods in the Boulder Creek basin are produced by intense rainfall during either isolated convectional or general frontal stonn systems. In addition, flows nonnally increase during the mountain snowmelt and runoff period in May and June which when augmented by rainfall runoff increases the severity of flooding. Hydrologic Analysis The hydrologic analysis for Boulder Creek and for Founni1e Creek was performed by the U.S. Anny Corps of Engineers, Omaha District and consisted of detennining the magnitude of the 10-, 50-, 100-, and 500-year floods. Si nce the fragmentary gaugi ng record for Boul der Creek at Boulder was considered too short to be a reliable basis for estimating the di scharge probabi 1 i ty at Bou1 der and si nce the long-tenn records for Boulder Creek at Orodell does not include the flows on Founnile Creek, whi ch accordi ng to hi stori cal i nfonnati on, has been the major source of flooding in Boulder, the discharge probability relationships for Boulder Creek and Founnile Creek were developed usi ng the EPA' s Stonn Water Management Model (SWf.1M) (Reference 9). This model was calibrated using data collected from the flood hydrograph of May 1973 for the uncontrolled area of Cherry Creek located downstream from Cherry Creek Dam. Based upon this calibration, the hydrologic model was applied to the Boulder Creek and Founnile Creek basins. The resulting discharge probability re1 ati onships were compared wi th that from the short record on Bou1 der Creek at Boulder and with regional criteria prepared by the U.S. Geological Survey. 5