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.. . �Z` <br /> STORM WATER RUNOFF <br /> CALCULATIONS <br /> The entire existing building, gravel, and asphalt on the site has been identified as <br /> hardcover and the only additional hardcover being added is approximately 320 <br /> square feet in the southeast corner of the site. _ <br /> Area of new hardcover being added 320 SF or 0.007 acres <br /> Runoff calculations preformed using the WINTR55 software and rainfall amounts <br /> of 2.75" for the 2 yr. storm and 4.13" for the 10-year storm, yielded the following <br /> results: <br /> For this area in its present condition the peak runoff from a 2.5-year storm <br /> is 0.013 cfs and from a 10-year storm it is 0.025 cfs <br /> For the same area after paving, the peak runoff from a 2.5-year storm is <br /> 0.023 cfs and 0.043 cfs from a 10-year storm. <br /> The increase in the peak runoff caused by the new hardcover is 0.01 cfs <br /> and 0.02 cfs. This is a extremely minor increase and really below the <br /> ' capability of the software. <br /> Since these numbers are so small the total runoff for each of the two storm - <br /> events was determined using before and after R`s of 0.79 and 0.98 and the total <br /> increase in the total runoff was determine to be 14 CF for the 2.5 year storm and <br /> 21 CF for the 10-year storm. <br /> There is a fence encroaching on th property and after discussions with the City <br /> Council it was agreed not to disturb the fence or the trees behind it. Therefore, <br /> there is very little area available or any storm water storage. <br /> A small storage/filtration area with about 300 CF of storage is proposed along the <br /> edge of the new pavement at the southeast site exit. This is enough to store 60% <br /> of the runoff currently occurring during a 10-year storm event from a 7000 SF <br /> area of the parking lot.. The discharge of the storage area will be limited to the <br /> current 10-year discharge from existing 7000 SF parking lot that drains through <br /> this area. This discharge rate is 0.47 CFS or 210 gpm. A 4" orifice under a 6" <br /> head will discharge this amount and is provided for rate control. <br /> WATER QUALITY CONTROL � <br /> This site is totally developed and there is very limited opportunity for any type of <br /> water quality improvements. However, to provide some treatment, the runoff from <br /> the upper (north) parking area is being directed through either of two rock filter <br /> beds that will capture silt and debris before it is discharged onto County Road <br /> #15. Existing catch basin on CR 15 that received drainage from the site will be <br /> equipped with silt sacks or baskets during the construction period.. <br /> The lower (south) parking area presently drains South and Southwest through a <br /> grass and wooded area and this area will continue to do so. This is an effective <br /> � REC����� <br /> � � JUN 3 0 2010 <br /> CIT'Y f��'QRON(�► <br />