How do we obtain the runoff coefficient for before and after construction of a linear project, and how do we calculate the percent of impervious area before and after construction?
There are many published sources for the values of the runoff coefficient, C. Useful and comprehensive examples can be found in: Handbook of Applied Hydrology; A Compendium of Water Resources Technology by Ven Te Chow, Editor in Chief, 1964; the Erosion and Sediment Control by Steven J. Goldman, et al. 1996; and the ABAG Manual of Standards for Erosion and Sediment Control, May 1995 (Table 2 page 4.4)Some judgement is required to select the appropriate C value within the ranges presented in the published tables. In general, for pervious surfaces, large areas with permeable soils, flat slopes, and dense vegetation should have the lowest C values. Small areas with dense soils, impervious surfaces, moderate to steep slopes, and sparse vegetation should be assigned the highest C values (1986. Goldman, Steven J. et al. The calculation of percent of site imperviousness as required on the NOI form involves a simple areal estimation of how much the ground has been covered by impervious surface
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