FLOOD06009 - Laserfiche WebLink

Home Browse Search

conditions existing 1n the community at the time of completion of this study. Maps and flood elevations will be amended periodically to reflect future changes. 3.1 Hydrologic Analyses Hydrologic analyses were carried out to establish peak discharge- frequency relationships for each flooding source studied by detailed methods affecting the community. For the original study the SCS performed the hydrologic analyses of Coal Creek and Boxelder Creek while compiling their Watershed Work Plan (Reference 4). Precipitation values were taken from U.S. Weather Bureau Technical Papers 40 and 49 (References 6 and 7, respectively). Soil information was taken from the unpublished detail soil survey completed by the SCS for Larimer County (Reference 8). The SCS Program for Project Formulation, Hydrology was used to develop and route hydrographs (Reference 9). At the time the original Flood Insurance Study was completed, only two of the three reservoirs above Wellington had been constructed; therefore, based on the information supplied by the SCS, the storage for the third reservoir, B-2, was not considered in the discharge computation. The discharges used for Coal Creek are based on conditions. The unregulated discharges have been account for the water which will be intercepted by Ditch, located near the northern corporate limits of The Windsor Ditch diverts the 10-year flood; therefore, shown for the 10-year flood in this report. pre-project reduced to the Windsor Well ington. no data are Hydrologic data used for the restudy of Boxelder Creek were provided to the SCS by the CWCB. The original source of the discharge-frequency values was the Simons, Li, & Associates, rnc.. study "Cooper Slough, 80xelder Creek Master Drainageway Planning Study - 1981" (Reference 2). The data were reviewed and concurred in by the CWCB and the SCS. The methodology for developing the discharge data was the "Storm Water Management Model" (SWMM) , as developed by the Environmental Protection Agency, and revised by the Missouri River Division, U.S. Army Corps of Engineers (CDE), 1973 (Reference 10). Hydrologic data were based on conditions that reflected final completion of the Boxelder Creek Watershed project. A general assumption in the hydrologic data is that irrigation ditches and canals that cross the basin do not intercept flood waters from the drainages they cross. The logic behind this assumption is that the ditches and canals come into the basin full from upstream runoff and; therefore, have no capacity to intercept additional runoff. Peak discharge-drainage area relationships for Boxelder Creek and Coal Creek are shown in Table 1. 5