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! I I I I ! I I I I I I I I I I I I 12. The Si~ Plan Development Permit will requir~ a malutenance ~arant~e to ensure the survival bf thc plantings for a 2 year p~iod. ~ Thc plan proposed eonveaxing 1..05 ae~s of thc 1..1.5 ae~ site to an impaq..?able surfac~. }Thc majority of thc site drainage is proposed to be ~ to thc west eurblinc of Ebony S[re~t via three paved flumes. Thc storm sewer in Ebony Street in un'n conveys the runoff td thc Mississipi River. ' The wes~mmost 32 feet of pavement is directed to drain westbound toward the propexty line of ~e City park. Drainag~ is a major concern on this sit~ plan from two separate criteria; wate~ quality standardt; s~t by thc local water management organization and the capacity of the existing storm w~tex system to handle the volume of runoff generated. The Lo{vet l~um River Water Management Organization (LRRWMO) requires sedimentation prior to discharge into a Class A wat~-rbody which is the classification of the Mississip~pi PAver. Ym additi9n to the drainage requirements of the WMO, the adequacy of the existing storm sewea' sea!vicing the propea'ty must be considered. The commel'~fi~.l area s~,ved by the storm sewer tofwhich the proposed development would disetmrge includes approximately 9.25 acres. T'ttis includes the land uses south of Highway #10 between Feldspar and Dolomite Street. '~e proposed development covers aa area of 1.15 acres or 12% of the total draiaagetarea. The capacity of the existing store{ sewer is 14 c£s. ~ approved in the manner ,~o~ _posexi, 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 stormwa~?r pipe for a.2 year storm. · .:.:.s: -...~, ,... .;!h.C: . .. -:-~:~ If the casaeity of the storm sewer were to be allocated based upon ar~ the discharge from the propqsed development should be limited to 1.7 cfs. As proposed, the nmoff from this facility f~m a 2 year storm would be 3.8 els. However, since the runoff from the existing self storage facility already exceeds its proportionate shar~ of the capacity, I would recomme~nd that the aforementioned discharge limitation be allowed on the condition that the exis '.t~ng self storage be reduced through the installation of detention volume when concrete ~:urb is installed to meet the 1997 facility requirements. The amolant of volume needed to aehleve a discharge of 1.7 c~ could be achieved by constru~g 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 feel A revised drainage plan dated February 23, 1994 was received which shows one-half of the runo~ generated to be diverted to the west into the'City Park. There is a berm in close pro×~m~ty, to the sites western boundary ia the City park which carries a paved park trail, beyond ~hich is a graded depression covered by a drainage easement. A culvert would need to b~ placed through the berm to reach this easement. Lq addition, the capaeity of the detentioniarea of the drainage easement was paid for by the developers of the Rivenwick