03/17/94 Public Hearing @ 7:36 - Laserfiche WebLink

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12. The Site Plan Development Permit will require a maintenance guarantee to ensure the survival of the plantings for a 2 year period. ~ The plan proposed converting 1.05 acres of the 1.15 acre site to an impermeable sutfi~e. The majority of the site drainage is proposed to be directed to the west curbline of Ebony Street via three paved flumes. The storm sewer in Ebony Street in turn conveys the runoff to the Mississipi River. The westernmost 32 feet of pavement is directed to drain westbound toward the property line of the City park. Drainage is a major concern on this site plan fi.om two separate criteria; water quality standards set by the local water management organization and the capacity of the existing storm water system to handle the volume of runoff generated. The Lower Rum River Water Management Organization (LRRWMO) requires sedimentation prior to discharge into a Class A waterbody which is the ciassification of the Mississippi River. In addition to the drainage requirements of the WMO, the adequacy of the existing storm sewer servicing the property must be considered. The commercial area served by the storm sewer to which the proposed development would discharge includes approximately 9.25 acres. This includes the land uses south of Highway #10 between Feldspar and Dolomite Street. The proposed development covers an area of 1.15 acres or 12% of the total drainage area. The capacity of the existing storm sewer is 14 cfs. If approved in the manner proposed, the expanded self-service storage facility would have a total impervious area., of 3.5 acres. This would generate enough runoff to equal the capacity of the existing stormwater pipe for a 2 year storm. If the capacity of the storm sewer were to be allocated based upon area, the discharge from the proposed development should be limited to 1.7 cfs. As proposed, the runoff from this facility from a 2 year storm would be 3.8 cfs. However, since the runoff from the existing self storage facility already exceeds its proportionate share of the capacity, I would recommend that the aforementioned discharge limitation be allowed on the condition that the existing self storage be reduced through the installation of detention volume when concrete curb is installed to meet the 1997 facility requirements. The amount of volume needed to achieve a discharge of 1.7 cf.s could be achieved by constructing a two foot deep vee sump area along the eastern property line. Such a sump could be made mowable by utilizing a maximum side slope of 4:1 and would occupy a width of approximately 20 feet. A revised drainage plan dated February 23, 1994 was received which shows one-half of the runoff generated to be diverted to the west into theCity Park. There is a berm in close proximity to the sites western boundary in the City park which carries a paved park trail, beyond which is a graded depression covered by a drainage easement. A culvert would need to be placed through the berm to reach this easement. In addition, the capacity of the detention area of the drainage easement was paid for by the developers of the Rivenwick