1025 90TH ST SW 2018-01-02 MF Import

I. Site Description: Existing Conditions The existing site is occupied by a single family residence, garage and driveways. The existing site is mainly lawn and at one time was lawn and landscaping. The existing drainage flow is cast to west at an approximate grade of 1 percent. From the site site the drainage enters a drainage ditch created by the adjacent property. The water flow is then south to north and then easterly across the adjacent property within an underground pipe system. t. The proposed development is to constiuct—a�-um't con3otfllttiumprojecL'n tw , buildings. The drainage from these units will enter a Retention/Detention wet 2 year storm will be pumped to the discharge point. The 10 aad 100 year storms or�bination ofa•pump and orifice di;y�^tge -• 04 11�- - S (n then the'I retained { - ) The combined discharge will conform closely to the release rate of the existing 10 year storm event during the 10 year developed storm and the 100 year at the 100 year. The usual discharge of 1ii the existing 2 year storm event at the 2 year storm will not be met because of constraints in choosing the smallest pump (10 gpm minimum); even if a standard multiple orifice were chosen the discharge rates would have been greater due to selecting a minimum sized orifice to prevent plugging. The Waterworks-Stormshed program was used for these calculations. III. Pond Sizing The selected pond is a concrete vault 20' x 27.5' long. The attached calculations show the printout out and how the sizing was obtained- 20' x 20'(sizing used in calculations). The added volume is the correction factor for impervious surface. The completed project will have 63% impervious. This translates into a 36% increase in the Retention/Detention volume. Using the length of 20', a length of 27.2 feet is required: use 27.5 feet. IV. Water Quality The six month storm contributes 1271 cubic feet of storage. The first bay will be 3.6 feet deep, 9 feet wide and 20 feet long and has a volume of 648 cubic feet. The other two bays are 14.5 feet wide by 2 feet deep and 20 feet long for a volume of 580 feet There is an additional 2.5 feet of dead storage in Catch Basin #2 which is a 5 foot diameter manhole. The additional volume is 49 cubic feet. The total volume of dead storage is 1277 cubic feet NOTE: A VEHICLE WASHING AREA WILL NOT BE PROVIDED. THEREFORE, HOSE BIBS IN THE FRONT OF THE UNITS WILL BE PROHIBITED AND WASHING OF VEHICLES WILL BE PROHIBITED f