Agenda - Environmental Policy Board - 10/15/2018

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exact character of the corridor and the stormwater management system has not yet been determined, but a range of runoff management effectiveness is discussed in the mitigation section of this Item. The open space value of the corridor will be enhanced with pathways that will parallel the corridor. Runoff from the areas draining to the central drainage corridor or elsewhere off of the site also could be managed to reduce overall runoff volumes. During the design phase, each major parking area within the Town Center will be evaluated to see whether a system that will pre -treat runoff prior to its introduction into the central drainage system is feasible. Under the ideal scenario, runoff would be routed to the pre-treatment BMP (small-scale detention or filtration) prior to entry into a vegetated flow system that will encourage further filtration and infiltration. Excess flows from these connector drainage features should only occur with substantial precipitation events. Most routinely occurring, small- scale events would soak into the sandy soils. This conveyance system will likely be a connected system of pervious drainage swales, wetlands and vegetated drainageways, but could also include sub -grade settling and filtration treatment trains. The exact character of this system will be determined as part of the final design prior to building construction. During the detailed design process, the City has the option of incorporating additional volume control features into the drainage system. The specifics of these features can not be defined until the design phase, but they could be used to minimize runoff on a parcel or block scale within the Town Center to hold down the amount of water that will eventually reach the surface water drainage system. Impervious area reduction BMPs that will be used to do this are numerous and will be pursued at the proper design stage. These alternative design features are not intended to replace standard engineering practices of assuring the movement of large storm -related water volumes, but rather supplement the drainage system by reducing overall runoff volumes and peaks. In addition to the flow reduction benefits, there are water quality benefits. Pre -treating runoff from potentially high loading areas, such as parking lots and roadways, reduces the amount of pollution moving to the regional collection system. Routing pre -settled runoff through wetlands and vegetated swales furthers treatment through vegetative filtration. It is anticipated that these two BMP suites (pre -settling and filtration) will properly prepare water for infiltration into the soil, where additional physical and biological treatment will cleanse the water on its way toward the regional groundwater system. Using the natural cleansing ability of settling areas, vegetative and soil filtration, microbes in the soil and vegetative uptake of nutrients will make the RTC site compatible with the needs of a wellhead protection area. Additional discussion occurs in the groundwater protection section of Item 13 and a recommendation in the Mitigation Plan (Item 33) is made to assure that an ordinance is adopted to assure that incompatible land uses are not allowed within the DWSMA. 17-3