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- FLOOD RELATED STUDIES - In March 1984 , Simons , Li and Associates , Inc . (SLA) completed a Major Drainageway Planning Study for the confluence area of Boulder and South Boulder A "Flood Plain Information" report was published by the Army Corps of Engineers Creeks (Reference 6) . The SLA study ended just above the C&S Railroad crossing in August of 1969 (Reference 2). This report included floodplain analysis for (which is the downstream limit for this study). Results from the SLA report were ' the Intermediate Regional Flood (approximately 100-year flood) and Standard utilized to establish starting water surface elevations for the current study. Project Flood (between a 200-year and 500-year flood) . Peak discharges for these events were 5 ,000 cfs and 9,400 cfs , respectively. Flood profiles were developed - HYDROLOGIC AND HYDRAULIC DETERMINATIONS - based on stream and valley cross-sections surveyed in 1968. Flooded areas were defined on USGS 7.5 minute series topographic maps with contour intervals ranging Hydrologic Analysis from 10 to 40 feet. A hydrologic analysis was completed by the U.S. Army Corps of Engineers to define In December 1973 , R. W. Beck and Associates published a Major Drainageway the magnitude of the 10- , 25- , 50- , 100- , and 500-year flood events along South Planning Study outlining major components and preliminary designs for flood Boulder Creek. Discharge-probability relationships for South Boulder Creek were ' control improvements along South Boulder Creek (Reference 3). Included in the developed using the Environmental Protection Agency' s Storm Water Management R. W. Beck and Associates study effort was the evaluation of hydrology and Model (SWMM) . hydraulics and delineation of the 100-year floodplain on 1-foot contour topographic mapping. The flood control measures were designed for a 100-year To model hydrologic response, the 132 square mile South Boulder Creek basin was ' discharge of 5 ,600 cfs. The U.S. Bureau of Reclamation's backwater analysis divided into 236 subcatchments averaging 358 acres in size. Rainfall input data model was utilized to evaluate the hydraulics. for the SWMM model was obtained from the National Oceanic and Atmospheric Administration's Atlas 2, Volume 3, Colorado, dated 1973. Rainfall hyetographs ' In November 1977 , Leonard Rice Consulting Water Engineers completed a hydraulic having 6-hour storm durations were input into the model for each respective analysis and floodplain definition along South Boulder Creek and Dry Creek No. 2 frequency storm. Adjustments for depth-area and expected probability were ' Ditch in the vicinity of the Flatiron Industrial Park (Reference 4). The report reflected in all hyetographs. The "Standard Project Storm" breakdown included evaluation of the C&S Railroad bridge openings on both Dry Creek No. 2 distribution procedure outlined in Civil Works Engineer Bulletin 52-8 was used to Ditch and South Boulder Creek. Results indicated that a split flow condition distribute the twelve 30-minute hyetograph values. would occur with approximately two-thirds of the South Boulder Creek discharge ' passing into the Dry Creek No. 2 Ditch drainageway. Soil infiltration rates of 1 .0 inch per hour in the mountains and 0.5 inch per hour in the plains were used for the pervious areas in the hydrologic model . In August 1978, the Federal Insurance Administration published a Flood Insurance Selected loss rates were consistent with other hydrologic studies made in this Study for unincorporated areas of Boulder County (Reference 5) . Hydrologic and region. The Manning's "n" coefficient used for channel and overbank generally hydraulic analyses for this study were performed by the U.S. Soil Conservation ranged from .045 to .10, respectively. ' Service in August 1974. The Army COE HEC-2 water surface profile model was utilized in this study with a 100-year flood peak of 5 ,000 cfs. The impact of Gross Reservoir, a 43,060 acre-foot reservoir constructed in 1955 to provide water conservation storage, was evaluated by routing storm runoff through the reservoir pool . Significant reductions in peak discharges resulted with the various frequency routed storms. The 6-hour, 10- , 25- , 50- , and ' 6