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HomeMy WebLinkAboutDRAINAGE ANALYSIS Benjamin C. Osgood, Jr.,P.E. 16 Hillside Ave. Unit 3 Amesbury, MA 01913 January 15, 2011 John Simons, Chair North Andover Planning Board 1600 Osgood Street Building 20 Suite 2-36 North Andover, MA 01845 Re: Town Farm Soccer Field Dear.Mr. Simons: This Engineer has been retained by the North Andover Soccer Association to perform a drainage analysis for the proposed soccer field and parking lot construction at the Town Farm property. A brief report regarding the proposed project and the calculation methodology are as follows. Property Description: The site for the proposed project is 2.93 Acres in size and consists of existing scrub woods with a ground surface sloping gently from Dale Street to a wetlands to the north of the site. Drainage from the site drains from the Dale Street side of the site down the slope in to the wetlands at the bottom. Soils on the site are primarily Canton and Paxton soils which are classified as being well drained class B soils. The project will result in the disturbance of approximately 2.7 Acres of land and will result in the creation of approximately 1.8 Acres of playing field and approximately .4 Acres of new gravel parking lot to accommodate 52 cars. Drainage System Description: To control runoff from the proposed developed area a combination of open swales and a closed drainage systems consisting of two catch basins and a wet detention pond to capture stormwater flowing from the site and discharge it slowly via an outlet control structure that will regulate the discharge of the 1, 2, 10, 25 and 100 year design storms to less than existing rates. Stormwater Treatment: Although the site does not contain any impervious surfaces the drainage system is designed to provide treatment of runoff. Treatment is accomplished by the installation of deep sump catch basins, sediment forbay, and a wet detention pond. The detention pond is designed to hold 12 inches of water at all times to promote the growth of wetland plant species to assist in cleaning and treating storm water prior to being discharged from the pond. Town Farm Soccer Field Page 2 Dale Street, North Andover, MA January 15, 2011 Site Drainage Methodology: The estimation of flow rates and volumes were calculated utilizing NydroCad stormwater modeling software. The methodology is SCS TR-20, Type III rainfalls (1, 2, 10, 25 and 100 year events). The table below compares the pre development and post development runoff rates, Table 2 Peak Discharge Rates (All values shown are peak rates in CFS) `- _ Y � �Pre Deuel©piment Pbst Development Paint of Anatys�s 'Storm ; Rate Rate _ 1-year 0.09 0.08 2-year 0.44 0.18 North Limit of work 10-year 1.82 0.54 25-year 2.88 0.98 100-year 4.38 2.20 Post development flows are less than pre development flows in all of the analyzed storms. If you have any additional questions or comments please do not hesitate to contact this engineer, Respectfully submitted, Benjamin C. Osgood, Jr., .E.