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I I I I I I I I I I I I I I I I I I I 5 TABLE 2 BASIN CHARACTERISTICS Area Stream length Slope Future % Reach No. (acres) (Feet) (F eet/Feet) Impervious 1 292 7,800 0.023 13 2 330 6,700 0.007 31 3 748 3,600 0.006 37 TOTAL 1,370 18,100 0.014* 30* *Weighted average for entire basin I I I. HYDROLOGY Runoff flows were determined at various locations for return periods of 2-, 5-, 10-, 50-, and 100-years so that a determination could be made concerning the storm frequency for which drainage facilities should be designed. A modified computer runoff simulation model, the Stormwater Management Model (SWMM) as modified by the Missouri River Division, U.S. Army Corps of Engineers, was used to determine the peak runoff of each return period for the Marston lake North drainage basin. The basin was sub-divided into 39 sub-catchments, each having a gutter or channel draining it. The basin boundary map is shown in Drawin9 No.3 at the end of this report. Rainfall The rainfall intensity was assumed to be uniformly distributed throughout the drainage basin. The rainfall depths for return periods of 2-, 5-, 10-, 50-, and 100-years were obtained from Rainfall/Runoff Information publ i shed by the Denver Regional Council of Governments and the Urban Drainage and Flood Control District in May 1972. The time incremental rainfall for various frequencies is presented in Table 3. Soils and Infiltration Soils within the Marston lake North drainage basin are primarily of the Denver, Englewood and Nunn formations, as defined by the Soil Conservation Service (SCS). These soils are characterized by sand, gravel, silt and clay, ranging in depth of O-feet to 60-feet. The slopes in the area vary from 0 to 10 percent, and the natural vegeta- tion is mostly grass. The bedrock which underlies the surficial soils is the Denver formation and is composed of sandstone, siltstone and clay conglomerate. For the purpose of computing sub-catchment hydrographs by SWMM, the infiltration losses were calculated using Horton's equation with maxi- mum and minimum infiltration rates of 1.8 inch per hour and 0.5 inch per hour respectively and a decay rate of 0.00115.