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R!D pond will mitigate the abo�,�e storms to what flowed offsite before <br />development. Field research shows that the offsite(downsream) analysis area <br />is not sensitive and will not be impacted by this site development with the <br />detention pond construction. <br />Part II Development Design <br />A. Detention Volume Calculations <br />Impervious Area Calculations: <br />The following is a calculation of impervious surFaces for both existing <br />and developed conditions on the back portion of the site that will be developed. <br />It will be input into the Waterworks software to determine the detention volume <br />for the 2, 10 and 100 year-24 hour storm from this development. <br />Existin4 conditions: <br />Pervious. Surface: <br />Ex. = 82,600 sq ft or 1.90 Ac. <br />Impervious SurFace is none for the existing conditions. <br />Total Site area = 1.90 acres of forest at Cn=81 for soil type "C" for this <br />area per SCS soil Survey for Snohomish County. <br />Post develoament conditions: <br />Pervious Surtace: <br />Dev. 1.90 Ac at Cn =89 for a totally <br />graveled area of 1.9 acres and for Cn for graveled parking lots is 89. <br />Since no impervious surfaces are being added, again the impervious <br />area is zero for the devetoped conditions. <br />The longest route that water travels on site will be the same after <br />development since a layer of earth was added all over the site. The longest <br />time water travels is from the northeast corner going to the southwest corner as <br />sheet flow for 294 feet. The actual flow time under existing conditions, using <br />Ns of 0.24 for dense grass becomes 50.02 minutes at approximately 2% slope <br />across the site. The time of concentration for developed conditions used gravel <br />becomes 14 minutes. The Waterworks software determines the peak 2, 10 <br />and 100 year storm peak flows on site in Basin B by creating six input <br />hydrographs See page 1 and 2 of Appendix A. The Waterworks <br />