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<br />DRAINAGE CRITERIA MANUAL <br /> <br />RUNOFF <br /> <br />The CUHP model is sensitive to the "percent impervious" parameter. Since . <br />the va 1 ue of th i s parameter depends, to a degree, on the dens ity of the <br />residential development, Figure 2-1 is provided to assist in estimating the <br />percent impervious values for low to medium density residential areas. It is <br />based on the observed trends in the urbanized portions of the Urban Drainage <br />and Flood Control District (39), and reflects the densities found in built-up <br />areas. Figure 2-1 does not account for parks or large open areas. <br /> <br />2.2 Depression Losses <br />Rain water that is collected and held in small depressions and does not <br />become part of the general surface runoff is called depression storage. Most <br />of this water eventually infiltrates or is evaporated. Depression losses also <br />include water intercepted by trees and bushes and water that is detained on <br />the surface and does not run off. The CUHP Method in Chapter 4, requires <br />numerical values of retention loss as input in calculating the effective <br />rainfall. Tabl e 2-1 can be used as a guide in estimating the amount of <br />depression retention storage to be used with CUHP. <br /> <br />. <br /> <br />TABLE 2-1 <br /> <br />TYPICAL DEPRESSION RETENTION FOR VARIOUS LAND COVERS <br />(all values in inches) <br />(For use with CUHP Method) <br />Depression & <br />Land Cover Detention Recommended <br />Impervious: <br />Large Paved Areas <br />Roofs - Flat <br />Roofs - Sloped <br />Pervious <br />Lawn Grass <br />Wooded Area and <br />Open Fields <br /> <br />0.05 - 0.15 0.1 <br />0.1 - 0.3 0.1 <br />0.05 - 0.1 0.05 <br />0.2 - 0.5 0.35 <br />0.2 - 0.6 0.4 <br /> <br />When an area is analyzed for depression retention storage, the pervious . <br />and impervious storage values for all parts of the basin must be considered <br /> <br />5-1-84 <br /> <br />'- <br />