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i <br /> I <br /> EXHIBIT D <br /> i <br /> I <br /> I <br /> Detention Calculations: <br /> I <br /> The total area of the La Porte Junior High School site is 26.8799 acres. Please note that this acreage i <br /> includes the area of La Porte Elementary School. <br /> The existing and proposed C values are based on calculations of pervious and impervious cover for each <br /> condition. Currently the site has 508,029 square feet of impervious cover. The proposed site will have a I <br /> final area of 549,829 square feet of impervious cover: <br /> 508,029 SF I <br /> Existing C = 1,170,891 sF=0.43 <br /> i <br /> Proposed C = 949,e29 SF _0.47 <br /> 1,170,891 SF <br /> I <br /> I <br /> The rainfall intensity for the 100 year storm is calculated using the TxDOT rainfall intensity-duration- <br /> frequency coefficients,e,b,and d,for a 30 minute time of concentration: <br /> e=0.706 j <br /> I <br /> I <br /> b=91 j <br /> d=7.9 <br /> i <br /> The resulting rainfall intensity is 7.0 in/hr. Calculating the 100 year peak flow for each condition: <br /> t <br /> Q=CiA,and iioo=7 in/hr <br /> I <br /> Existing Q=0.43 x 7 x 26.8799=80.91 CFS <br /> i <br /> i <br /> Proposed Q=0.47 x 7 x 26.8799=88.43 CFS <br /> i <br /> The City of La Porte detention storage requirement for new impervious cover is depicted on attached <br /> Figure 4. For the 26.8799 acre site a storage coefficient of 0.49 acre-feet per acre is required. <br /> I <br /> The increase in impervious cover: <br /> 549,829 SF—508,029 SF=41,800 SF <br /> 41,800 SF=0.9596 Acres <br /> i <br /> Detention Volume Required: <br /> 0.9596 Acres x 0.49 Acre-Feet/Acre=0.4702 Acre-Feet <br /> 0.4702 Acre-Feet=20,482 Cubic Feet j <br /> I <br /> i <br /> i <br /> i <br /> i <br />