6525 HARDESON RD BASE FILE 2016-01-01 MF Import

DETENTION CALCULATIONS The detention calculations were perfortned using Engenious System's Inc. computer program Water Works. The detention pond was designed to Stream Protec:tion Star.dards (SPS) using a S.C.S. Type IA, 24-hour rainfall distribution. After determining the theoretical volume required per SPS, the actual volume of live storage required was computed by applying the appropriate volume correctinn factor (:actor of safety) per Department of Ewlogy standards. The factor of safety is baced upon the amount of impervious cover in the developed condition. When developed, the site will be approximately 78% impervious. Thus, according to Figure 1-3.1 (please see Appendir) a volume correction factor of 42% is required. To allow for construction inaccuracies, an additional5% construction factor of safety will be added. Therefore, the detention facility was designed to accommodate a 47% factor of safety. A brief summary of the results of the :lc�ention calculations for the basin is provided on the following pages. Actual Water Works output (i.e. basin summaries, stage-storage/ discharge tables, level pool routing tables, etc.) for the basin is located in the Appendix of this report. Please reference the Exisli�rg and Developed Condiliat Maps in the Appendix of this report along with the table of S.C.S. curve numbers. According to Washington State isopluvials (please see Appen:ix), the estimated precipitation values for the 2-year, ]0-year, and 100-year, 24-hour storm events are as follows: 1.55 inches, 2.20 inches, and 3.20 inches. Currently the site is cleared and graded. In order to accurately compute the runoff release rates, the "existing" conditions for the site were modeled as young second growth forest or brush using original slopes ranging from 1/2% to 24% per the previously known existing topography. Storvn Drnir�nge Xepcnr For Kmg L•xfrusion - Pnge 5 I