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<br />\ <br />. ~ <br /> ~ EN HILLS <br /> MEMORr'\NlllJM <br /> DATE: October 3 I, 2006 <br /> TO: James Lehnhoff, City Planner <br /> FROM: Kristine Giga, Civill:ngineer <br /> SUlUECT: Planning else #06-028: Site Plan Review <br /> Per your request, I have reviewed the site plan for 3413 Lake Johanna Boulevard \vith regard to drainage <br /> and runof1~ <br /> Storm sewer systems in resickntial neighborhoods arc designed to accept a cel1ain amount runoff from <br />. driveways and rooftops. This site is being designed !()r a total of 25% impervious area, even though this <br /> t)l)(' of development is allowed up to 35% impervious, Given this information, and the fact that not all of <br /> the impervious area will drain towards Fain-'iew A VCI1ue, there should be minimal increase in runoff onto <br /> the public right of way and into the storm sewer system, It should be nOll:d, however. that stonn sewer <br /> systems are designed to a standard lO-year storm event, which is 4 2 inches of rainEt!! in :1 24-hour <br /> period. Designing systems f()f extreme rain events, such as the IOO-year stOllll (6 inches in a 24 hour <br /> period) or shorter, but more intense stonns, is not cost-effective. Residents may see the street flood during <br /> these greater storm events, but it won't be caused solely by the addition of impervious area from this <br /> development. <br /> The underdrain proposed in the rain garden is designed to convey water to the storm sewer system during <br /> wetter peliods \vhen soils are saturated and cannot infiltrate all of the water. The water \vill have <br /> undergone some filtration, filtering through the plants and soils in the rain garden bdi.m' reaching the <br /> underdrain. The pipe connecting the rain garden to the storm seWTr is also perforated, which will allow <br /> water to infiltrate into the soils en route to the catch basin as weIl. Water that docs reach the catch basin <br /> (and ultimately, I.ake Johanna) via the rain garden and underdrain will be of improved water quality. <br />. <br /> \",J\1t:ln), illl.'l,IIS\~lIlkllhills\PR& I'Vv"',f';ngineering\2(IOt, ,I'lannlllg ( 'aSt's"t H,,-02g,_ 343 ,_I akc_,Jo doc <br />