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I I I I I I I I I I I I I I I I I I I 9 Runoff Surface runoff from a basin equals total rainfall minus basin retention and losses. Basin retention includes temporary pond- ings, temporary interception of rainfall and the like that de- lay surface runoff. Rainfall losses include phenomena such as infiltration and evaporation that reduce the amount of water that reaches the drainage channel. An important consideration in estimating basin retention and losses is the appraisal of antecedent moisture conditions on the watershed. Denver area rainfall records indicate the major rainfall events are fre- quently preceded by rains, so that the design storm losses should be calculated in fairly high antecedent moisture conditions. Basin retention and losses also depend on basin soil character- istics and the extent of urbanization. The soils in the Lake- wood Gulch and McIntyre Gulch basins have low rates of absorption. Typical rates used are as follows: 1. Infiltration, 0.38 to 0.60 inches per hour, depending on extents of grass cover and storm intensity. 2. Surface Retention, 0.20 to 0.40 inches average reten- tion depth depending on ground slope and density of ground cover. would occur from unmodified watershed areas. Under future urbanized conditions, it is expected that about 49 percent of the area will be covered by impervious surfaces. The degree of urbanization in any sub-basin is important in hydro graph analysis. A high degree of urbanization results in extensive impervious areas and generally greater runoff than Hydrographs Hydrograph shape depends on a number of watershed characteris- tics. The configuration of the watershed and its slope are two of the most important characteristics. A short, wide water- shed can be expected to have a high rate of peak discharge that develops quickly, while a long, narrow watershed will have a relatively lower peak discharge that develops more slowly. A steeply sloped watershed will have a relatively higher peak rate of discharge that develops more quickly than would a more gently sloping watershed. Peak rates from upstream water- shed areas may be reduced as they pass through long, narrow, gently sloping watersheds because of the effect of natural chan- nel and flood plain storage capacity. A synthetic unit hydro- graph method was used to develop flood hydro graphs for the 24 hydrologic points in the Lakewood and McIntyre Gulch basins. The unit hydrograph method used is the Colorado Urban Hydrograph Procedure as described in the Drainage Criteria Manual. In this method, it is first necessary to determine the area, stream length, location of the centroid, and the average slope of each