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Flood Control District. The CUHp inputs include rainfall data and other basin information such as surface and soil character- istics. This procedure is discussed further in a later section. RAINFALL Rainfall represents the rain falling on the study area during a selected storm frequency. The rainfall depth-duration pattern determined for the 2-, 10-, and 100-year frequencies is given on Figure 4-1 and Tabl~ 4-1. The Figure 4-1 graph is based on the isohyetal depth, d~ration, frequency maps for the Denver region given in the Urban Storm Draina e Criteria Manual. One location of ra~n a was used for t ese four small basins to average rainfal~ for all parts of the basins. This location, as identified on Figure 4-1, is in Section 27, T5S, R69W. The 100-ye&r storm frequency wa~ chosen as the design rainfall. Table 4-2 ~ists the 100-year fr~quency distributed, ~earranged rainfall increments that were u~ed for the flood hyd~ograph development. RUNOFF The CUHP W&S used to derive run~ff resulting from the design rainfall. The CUHP calculates effective rainfall (rainfall minus losses), determines a unit hydrograph and then establishes the design or local storm hydro~raph, as discussed below. Effective Rainfall Effective ~ainfall is the storm water that reaches the drainage facilities after a rainfall. The difference between rainfall and effective rainfall is due to losses called abstractions. Abstractions pertain to the deg~ee of impervious-perVious surface area, depression and detention losses, and infiltration. Effective ~ainfall was calculated for each subarea for all three storm frequencies. Impervious-Pervious Surface Area. Most rainfall is absorbed by the pervious areas in a basin (grassed areas or natural cover) or becomes runoff in the impervious areas (paved areas or buildings). Table 2-1 of the "Runoff" section in the Urban Storm Drainaqe Criteria Manual was used to calculate the percent of imperviousness. The table lists the percent of pervious and impervious area associated with specific land uses. The land uses shown earlier on Figure 3-1 for each basin's existing and potential development were used to determine the percent of imperviousness in this study. Depression and Detention Losses. Depression and detention storage losses are amounts of water that fall as rainfall but 4-2 I I I I I I I I I I I I I I I I I I I