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3.1 Hydrologio Analyses Hydrologio analyses were carried out to establish the peak discharge- frequency relationships for each flooding source studied in detail affecting the community. ' The discharges on the Big Tho~p~on River, the North Fork Big Thompson River, and their major tributaries were developed baeed upon a combination of gage records and' regression equations contained in the Colorado Water Conservation Board's Technioal Manual No.1, prepared by the U.S. Geological, Survey (Referenoe 10). The loca- tions of the stream gages analyzed and their respective years of record are as follows: Drake gage located on the Big Thompson River (47 years), Big Thompson River below Lake Estes (17 years), Big Thompson River above Lake _stes (27 years), and North Fork Big Thompson River at Drake (30 y~ars). The gage records were analyzed using the log-Pearson Type III distribution as recommended in the U.S. Water Resources Council Bulletin 17 (Reference 11), and the discharges were adjusted as recommended in Technical Manual No. 1 (Reference 10). . The Storm Water Management Model (SWMM) (Referenoe 12) was used to determine discharges on drainage basins that are representative of the smaller Big Thompson River land North Fork Big Thompson River tributaries. These disoharges ;were plotted on semi-log paper to develop discharge-drainage area curves for the region. These curves were entered with the drainage areas of the smaller tributaries, and the appropriate discharges:were tabulated. Discharges for the lower portiQn of the Big Thompson River near Loveland were provided by the COE in their Flood Plain Information report published in 1971 (Reference 13). The discharges for Buckhorn Cr~ek, Redstone Creek, and Black Canyon Creek were calculated based upOn the regression equations found in Technical Manual No.1 (Referenoe 10). Parameters needed for the regression equations were taken from U.S. Geological Survey topo- graphic maps at a scale of 112$0~000 (Reference 14), SCS County Soils Maps (Reference 15) and $ounty Land Use Maps (Reference 16). For Buckhorn Creek and Redston, Creek, discharges fOr each design point were calculated for the portions of the basin above 7,500 feet and below 7,500 feet, and!the largest discharge at each point was used. ~ The SCS performed the hydrOlogic analyses on Boxelder Creek in the vicinity of the Town of Wellington while compiling their Watershed Work Plan (Reference 8). Precipitation values were taken from National Weather Service Technical Papers 40 and 49 (Reference 8). Soil information was taken fro~ an unpublished detailed soil survey completed by the scs for Larim~r County. The SCS Technical Release No. 20 (Reference 17) was usedl to develop and route hydrographs based upon the information gathered. ~ 12