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The Yrjanainen and Warren method of detention facility design is the same method <br />used by king County. ":Nis method can be used for tributary areas up to 30 acres. <br />Where tributary areas are greater than 30 acres, the Soil Conservation Servi method <br />must be used. Also, in general, on -site detention will not be required in the lower <br />one-third of any drainage basin. A step by step description of the Y do W method <br />follows: <br />Step 1. Runoff Coefficient <br />The runoff coefficient, "c", represents the fraction of rainfall that contributes <br />to runoff during a rainstorm. The more intensely an area is developed, <br />the higher its c will be. Table 1 lists several c factors for varying ground <br />cover conditions; it can be used as a guide for selecting your runoff coefficient. <br />runoff coefficients oftentimes will have to be "area weighted". For example, <br />if a site has 1 acre of paved parking and rooftops and 1 acre of landscaping, <br />the area weighteu c would be: 1 (0.9) + 1 (0.2). In the rest of the discussion, <br />2 <br />Cexist and Cdev will refer to the runoff coefficients before and after <br />development, respectively. <br />