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SRF Consulting Group, Inc. <br />Proposed Conditions <br />Storm Water Discharge Rates <br />The existing discharge rates will be reduced under the proposed conditions utilizing Best <br />Management Practices (BMPs). Existing cropland will be replaced with building <br />development, parking areas, grass meadows, storm water ponding areas, wetland swales, <br />and bioretention areas. Based on drainage patterns, the rate control analysis has been <br />split between two areas, County Road 21 and the primary development site. The existing <br />County Road 21 runoff routes will be maintained in the proposed condition. The <br />roadway runoff will be routed to the wetland located to the north of the project site and to <br />a swale outlet south of the property. Due to space constraints, no rate control devices <br />area proposed for this portion of the project. <br />The runoff from the proposed building and parking portion of the site, as well as the areas <br />to the east of the proposed parking lot are routed through bioretention and swale systems -'` <br />to the proposed detention ponds. The ponds are sized to provide water quality treatment <br />for the 2.5" storm event per watershed district requirements, while also providing 1 -year <br />storm event skimming per watershed district requirements. As can be seen by the table <br />below, significant rate control from the site is also demonstrated. Overall, the proposed <br />development will significantly decrease the runoff rates from the property relative to <br />existing conditions. <br />Table 1: Storm Water Runoff Rates <br />At outlet of Lower Pond. <br />H: \Civil \047 \4443\sent\BWBR \10 -17 -03 \City submittal narrative.doc <br />2 - YR Storm Event <br />Discharge (cfs) <br />10 — YR Storm <br />Event <br />Discharge (cfs) <br />100 - YR Storm <br />Event Discharge <br />(cfs) <br />Existing Site <br />52.9 <br />113.9 <br />192.5 <br />Proposed Site* <br />3.7 <br />13.4 <br />18.6 <br />Existing CSAH 21 <br />5.8 <br />10.1 <br />14.5 <br />Proposed CSAH 21 <br />7.4 <br />13.0 <br />18.4 <br />TOTAL RUNOFF <br />RATE CHANGE <br />33.2 cfs decrease <br />97.6 cfs decrease <br />170 cfs decrease <br />At outlet of Lower Pond. <br />H: \Civil \047 \4443\sent\BWBR \10 -17 -03 \City submittal narrative.doc <br />