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I I I I I I I I I I I SECTION III HYDROLDGICAL SUMMARY GENERAL CDMMENTS 1. Determination of rainfall that would most 1 ikely occur during an event with a given return period by use of isohyetal (line of equal depth rainfall) depth-duration-frequency maps that were synthesized during the formation of the Manual. This provides a basic input for the runoff computation. This information was analyzed for the South Lakewood Gulch watershed and found to be relatively constant, con- sidering the relatively small size of the basin. 2. Field inspection and analysis of data pertaining to stream lengths, impervious cover, slopes, and other physical phenomena pertaining to runoff flow. This information is presented in Table 11-1. 3. Determination of runoff flows based on the data established in 1 and 2 above. The primary method applicable in the Manual is the Colorado Urban Hydrograph Procedure (CUHP) which is based on a synthetic unit hydrograph developed by Snyder in 1938. The extent of present flooding and the sizing and character of major drainage works and other urban storm drainage facilities are determined by the magnitude of runoff. Furthermore, the design of the facilities must be based upon expected sediment problems, including both degradation and aggradation, and water quality problems. Any master plan must be based on adequate and thorough hydrological studies if the plan is to have a significant value and establish a firm factual basis. Hydrological studies must take into consideration volume of runoff, as well as peak rates of flow. It is when the volume charac- teristics are known that one realizes the value of detention storage, both pond type and channel storage, to reduce the peak rates of flow. Runoff flows were determined at many locations for t . d f 5 10 re urn perlo so, , 50 and lDO years so that a determination could be made concerning the storm frequency for which drainage facilities should be designed. This section highlights basic information. SOILS, GEOLOGY, AND INFILTRATION Soils in the South Lakewood Gulch basin are made up mostly of the Denver, Englewood, and Nunn series as defined by the Soil Conservation Service (SCS). These soils are generally deep and well-drained, formed either in clayey alluvium or loamy sediments on terraces, fans, and uplands. The typical surface layer is from 6 to 10 inches of silty clay or clay loam. The sub- soil is clay loam or clay, from 23 to 37 inches thick, and overlies silty clay or clay loam that extends to 60 inches or more. Slopes generally vary from 0 to 12 percent, and the natural vegetation is mostly grass. The bed- rock geology upon which most of the surficial deposits lie is the Denver Formation. It is composed of conglomerate, sandstone, siltstone, and com- pact clay, and is about 400 to 500 feet thick. I I I I I I I I The magnitude of a flood can be judged by its maximum rate of flow, even though the maximum rate may last for only ten minutes. The frequency of a flood is judged by its return period. The return period is defined as the average interval of time within which a given event will be equalled or exceeded once. Thus, a flood having a return period of 100 years has a 1 percent probability of being equalled or exceeded in any given year. 111-1 For the purpose of computing subbasin hydrographs by the CUHP method, the infiltration values recommended in the Urban Storm Drainage Criteria Manual were used. The soils as described above are typical of the general soil types in the Lakewood area, and correlate well with soil types used in previous studies on Lena Gulch and Ralston/Leyden Creeks. For those studies both SCS runoff curves and actual infiltration tests indicated saturated soil infiltration rates of about 1/2 inch per hour (Ref. 3) The approach taken by the Denver Regional Council of Governments (DRCOG) Urban Storm Drainage Criteria Manual in the determination of runoff flows is: 111-1