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I I I I I I I I I I I I I I I I I I I SECTION III HYDROLOGY A thorough hydrological investigation has been completed on the Slaughterhouse Gulch drainage basin. As required by the contract, flood hydrographs were determined for the 2-5~lO-lOO- year storm frequencies. In the hydrologic evaluation, only the projected land uses were considered. Hydrographs were computed to define the peak rate of runoff at the selected design points, as well as the volume of runoff characteristic of the storm frequency and the type of development within the basin. The method used to compute the flood hydrographs is the Colorado Urban Hydrograph Procedure. This method is well outlined in the Runoff section of the Urban Storm Drainage Criteria Manual (USDCM) and, therefore, will not be discussed In detail in this report. A. DESIGN POINTS To facilitate the evaluation of intermediate points along the gulch, four design points were selected within the study area. These design points permitted a more detailed evaluation of the flooding problems within the basin and along the approxi- mately 1.75 mile study reach. Sheet 2 of 8 in Appendix B, Basin Boundaries, shows the location of the design points and the corresponding subbasins. The basin boundaries have been determined primarily on the basis of natural topography, field inspection and storm sewer locations. Table I provides the physical data for each of the subbasins and the channel reach between each design point. The peak storm runoff rate was computed for developed conditions at each design point and is provided on discharge probability profile charts which will be discussed in a later section. B. BASIN PARAMETERS To facilitate the computation of the design hydrographs for each of the selected storm frequencies, each basin must be physically described in terms of tributary area, basin length, length to basin centroid, degree of perviousness throughout the basin, and the selection of other representative coeffi- cients typical of the area. This information has been deter- mined for each of the basins and is tabulated in Table II. All the physical parameters of the basin have been measured from available USGS quadrant maps. Information on the degree of development within the basin, as well as the type of land use, has been obtained from available aerial photographs and field inspection. The projected development information has been determined from information on available planning and zoning maps. 111-1