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. . . DRAINAGE CRITERIA MANUAL RUNOFF 2. EFFECTIVE RAINFALL FOR CUHP The method described in this chapter is for use in conjunction with the Colorado Urban Hydrograph Procedure (CUHP) and does not apply to the Rational Method because in the Rational Method the abstracti ons are accounted for in the runoff coefficient C. However, the information presented here can aid the engineer in selecting a reasonable runoff coefficient for the Rational Method. Effective rainfall is that portion of precipitation which runs off the land to drainageways after a rainstorm. Those portions of precipitation that do not reach drainageways are called abstractions, and include: interception by vegetation, evaporation, infiltration, storage in all surface depressions, and long-time surface retention. The total design rain falling to the earth may be obtained from the Rainfall part of the Manual. This chapter illustrates a method for estimating the amount of rainfall that actually becomes runoff whenever a design rainstorm is used. 2.1 Pervious-Impervious Areas All parts of a basin can be considered either pervious or impervious. The pervious part of a drainage basin is that area where water can readily infiltrate into the ground. The impervious part is the area that does not readily allow water to infiltrate into the ground, such as areas that are paved or covered with buildings and sidewalks. In urban hydrology the percent of pervious and the percent of impervious land is important. As urbanization occurs the percent of impervious area increases and the rainfall-runoff re 1 ati on changes s i gnifi cantly. The total amount of runoff volume normally increases, the time to the runoff peak rate decreases, and the peak runoff rates increase as the area urbanizes. When analyzing an area for design purposes, the probable future percent of impervious area must be estimated. A complete tabulation of recommended va lues of the total percent of impervi ousness to be used with the CUHP is provided in Table 3-1 of "RATIONAL METHOD". Although some of the runoff models use "connected impervious" or "effective impervious" areas in estimating effective rainfall, the CUHP runoff model was developed using the "total impervious" area in the watershed. Thus, all references to impervious area and all calculations in this Manual are based on total impervious areas. 5-1-84