Agenda - Council - 02/26/2019 - Laserfiche WebLink

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Summary of Environmental Impact. The incorporation of a stormwater management system into the RTC site as it develops raises the need for proper collection, routing and storage of runoff. The standard routing of the 100-year frequency event without consideration of any infiltration, in accordance with LRRWMO regulations, yields a volume of 113 ac-ft that must be accounted for in on -site or near -site storage. When infiltration is considered, the volume can be reduced to 105 ac-ft. For events with a return frequency less than 100-years, infiltration can be designed to reduce volume substantially and provide continued recharge to a certain degree. Mitigation element. The conversion of agricultural land to urban land ultimately increases the amount and rate of runoff leaving the land. Minimizing the impact of that increased runoff is the objective of this mitigation plan. It must be stressed that this portion of Ramsey does not have a natural outlet to the Mississippi River. The preliminary drainage system described within this AUAR assumes an outlet that takes advantage of publicly -owned, County land that extends from Highway 10 to the Mississippi River south of the RTC site. The County has agreed in concept on the use of its land for a drainage route, and preliminary discussions among the City, County and developer on the details have begun. Appendix J contains the results of an analysis conducted for the County to evaluate the hydrologic and structural character of an outlet on its land. The analysis excluded storage from any land within the Regional Park and included a series of anticipated County (Armstrong and Ramsey Blvds. and Co. Rd. 116) and State (TH 10 and its interchanges) roadway upgrades in the immediate vicinity of the RTC site. The details of using the County drainage route or any alternate route will be developed with City, County and State input at the time of design. Advantages of using the County land extend beyond the RTC site, to the entire sub - watershed, including the eventual upgrade of TH 10, which will also need a River outlet. If use of the County land encounters any obstacles, an outlet option will need to be pursued to the southeast, along TH 10 through the Rivenwick Development or in the vicinity of Sunfish Lake Blvd. The small amount of water that now leaves the site, runs southeast along the highway, but infiltrates within a short distance. Mitigation Approach The quantity and quality approach laid -out earlier in this section describes an approach driven by the need to comply with runoff control rules of the LRRWMO and water quality requirements that are described in the next section. The runoff management system proposed in Figure 17.2 is done in a preliminary sense to allow the City and the developer to proceed with some knowledge of what design specifics will be needed. The City will assure that the developer(s) will design and build the final drainage and runoff management system within this overall framework, in compliance with the mandates of the LRRWMO. 17-14