2514 RAINIER AVE 2022-03-01 - Laserfiche WebLink

NORTH MIDDLE SCHOOL STORMWATER FLOW CONTROL PLAN . Minimum Requirement #7: Flow Control requires stormwater discharge to "match developed discharge durations to predeveloped durations for the range of predeveloped discharge rates from 50 percent of the 2-year recurrence interval peak flow up to the full 50-year peak flow." 1�i • As noted above, the continuous modeling software MGS Flood was used to determine the design durations and flow rates for the pre -developed and developed conditions. Calculated results are included in Appendix C. To comply with Minimum Requirement #7, detention is not required for this site. The peak flows for the post - developed are less than the peak flows of the pre -developed. This is because the landscaped area for the proposed site are greater than the existing landscaped area (See Figure 3A — Proposed Conditions). In the event that the athletic field alternative is chosen, a flow control system in the way of a CMP pipe system will be provided. This detention system will be located under the west parking lot within the main site with runoff from the field being routed to the system via an 8 inch pipe that collects runoff from the field underdrain system. Approximately 4.2 acres of impervious (synthetic) field and surrounding areas were input into MGS Flood to determine 10 connecting pipes at 194 ft each with a 4 ft diameter will be needed. (See Figure 3A — Proposed Conditions). The existing field was modeled with 3.609 acres of pervious field and 0.657 acres of impervious gravel and other field structures. The proposed field alternative was modeled as 100% impervious. The total volume of runoff being detained by this system is 24,400 cu-ft. Calculated results for the field alternative are included in Appendix C. MAINTENANCE AND OPERATION PLAN A Maintenance and Operations Plan is included in Appendix E which addresses Minimum Requirement #9: Operation and Maintenance. COUGHLINPORTERL.UNDEEN 12