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I I I I I I I I I I I I I I I I I ,I I 22. a. 1. Continued. ihe predevelopment rate of dtsch~rge for the 100-y!ar 24-hour event is expected to be approximately 35.6 cfsl w~th a total runoff volume of 8.07 acre-'!et. The post-developr.ent rate of discharge will be regulated at or below the predevelopment rate. This ts consistent with the Rice Creek Watershed POltcy. In order tc ~chieve discharge rate regulation. on- sita runoff storage basins are propo;.ed. The proposed method of rate control utllizes a combination of direct and regulated di5charge from the site. Approximately 11.7 acres are pro- posed to discharge directly from the s1te. The direct discharge for the lOO-year frequency event is estimated to be apprOx1wately 27.4 cfs. The corresponding regulated peak discharge from the retehtion basin's rate is proposed to be less than or equal to 8.2 cfs. Approximately 21.1 acres of the project site are proposed to be routed through on-s1te retention factl,t;es. The approximate volune of on-site storage for the IOO-year 24-hour event Is expected to be approx1ma~ely 1.7 to 2.0 acre-feet. During the final design process. the critlcal event will nep.d to be analyzed to deter- ~t~. the final design volume of the on-site sedimentation and rate control basins. in general. runoff from natural watershed'\ is relatively clean. wtth low concentrations o~ suspended sol1ds and nutrients. In a natural watershedJ runoff percolates lnto the ground. evaporates. or flows overland to stre~T.s, ~hannels. lakes. and ponds. As a watershe'l is developed, Impervious surface area 1~ Increased by pavem~nt and butld1ngs. This red1lces vegetation and permeable :loC area. which r/~duces per- colatto~ and evaporation. Runoff rate and volwr~ ~re 1ncreased and runoff Quali..! 1s reduced. Polluta~ts resulting from h~n d:t1v1ty accumulate in the wa iarshed and al'~ carried to rec! 1 vl"9 waten by stonn runoff. ihese pollu~ants commonly lncludp phOSphOrQ~s and nttrogen from lawn ftrttlizers. heavy metals, oil and grease from auto- mobiles. sollds such a~ dtrt. silt. and sa~d from road and parking surfaces. and chlorlde~ from deicing chemlcals. The ~roposed deve:opment ~tJl utiltze stonn water retention basins for rat~ control. These basins sho~ld be designed to maximize the olstance between the Inlet an1 the outlet from the basin. The rate regulation system sh~ulJ be designed to settle out sand, grit and when pract1cable. particles with diMneters greater than 0.01 rom. A skl~~r should be provided to remove floatables such ~s oil. gre~s~. leavesl and general debris. Proper bastn and !klmMer desl~n and ma1ntenance can lmprove stonm water quality. After dls:narge from the on-site rettntion arca~. the runoff w111 be directed to DNR Wetland 201~ lo:ated east of lextngton 8