FLOOD04616 - Laserfiche WebLink

Home Browse Search

of 10, 50, 100, and 500 years have been selected as having special sig- nificance for flood plain management and for flood insurance premium rates. The analyses reported here reflect current conditions in the watersheds of the flooding sources. 3.1 Hydrologic Analyses Hydrologic analyses were carried out to establish the peak discharge- frequency relationships for floods of the sele~ted recurrence intervals for each stream studied in detail in the community. Discharges were computed for the Little Thompson River by the U_S. Army Corps of Engineers (Reference 4). The records for the Berthoud gaging station were analyzed using a log-Pearson Type III analysis (Reference 5)_ The period of record for the gage is 17 years_ The gage is located approximately 14 miles west of Milliken. The peaks from these analyses were applied to unit hydrographs that were developed based on regional relationships. The hydro graphs were routed through three reaches to get the peak discharges at Milliken. The SO- and SOD-year peak discharges were plotted using log- probability and 10- and lOa-year discharges, Peak discharge-drainage area relationships for the Little Thompson River are shown in Table 1. Table 1_ Summary of Discharges Drainage Area (Square Miles) Peak Discharges (Cubic Feet per Second) 10-Year 50-Year laO-Year SOO-Year Flooding Source and Location Little Thompson River At Milliken 200 1630 3600 4800 8400 3_2 Hydraulic Analyses Analyses of the hydraulic characteristics of streams in the commu- nity were carried out to provide estimates of the elevations of floods of the selected recurrence intervals along each stream studied in the community. Starting water-surface elevations for Milliken were comour0d by the slope-area method. Cross section data for the Little Thompson River were obtalned from field surveys. All bridges and backwater-producing structures were surveyed to obtain elevation data and structural geometry. 4