Agenda - Environmental Policy Board - 10/15/2018

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limited runoff has diminished its overall hydrologic function and subsequent farming activities have taken advantage of the swale as tillable land. Reference to Figure 10.1, however, shows that Wetlands A and C occur within this swale, while Wetland B lies adjacent to it. The Wetland E, Type 1 acreage also occurs within this historic drainage swale. General concept possibilities for the drainage corridor and how it fits into the current design and the overall site stormwater mitigation plan are contained in Figures 17.2a, -b, and -c. The collection of features that will be incorporated into the new drainage swale includes a channel to convey baseflow, ponds to store water and promote infiltration, created and restored wetlands, and open space areas where excess water can temporarily be stored. The specifics of these features will not be fully known until the design phase proceeds prior to construction, but Figures 17.2a, -b, and -c contain schematics of how these features will generally appear. The presence of an historic drainage swale on the site presents an excellent opportunity to incorporate the feature into the site drainage system as an amenity. Although the actual drainage swale alignment will change, incorporating the vegetative and hydrologic character of this historic drainageway can provide both functional and recreational value to the feature. As shown in the preferred design in Figure 6.1, the corridor extends beyond the current terminus, reaching into the Mississippi Regional Park, creating a natural drainageway that could extend from Lake Itasca to the Mississippi River. The introduction of a drainage connection to the northern wetlands (see Figure 6.6) provides two more corridor connections that could establish greenways to connect to northern Ramsey natural areas. The surface water system can be modeled with these changes/additions in mind, and various optional innovative/natural surface water management assumptions can be used to maximize storage, infiltration, and water quality treatment within it. Surface water as an Amenity. Water can be treated as a nuisance that must be moved away quickly, or as an amenity that can enhance the natural features of a site. Ramsey Town Center will use water as an amenity. The large events will be drained to prevent flooding, and smaller events will be stored and infiltrated to the extent possible. The primary drainage -related consideration for the City is to assure the movement of the 100-year runoff event through the site without damage due to flooding. The proposed stormwater management system accomplishes this. However, with the sandy nature of the soils on site (see Figure 12.2), there is also an opportunity to soak water from smaller events into the ground to retain some of the recharge function that will otherwise be lost with development. This combination of safely routing the 100-year event and trying to reduce overall runoff from the site will form the basis for stormwater management developed in Item 17. Under this approach, provision will be made to route, store and treat the 100-year runoff event safely in a series of storm sewers, drainage swales, floodways and ponds. The system will take maximum advantage of the central drainage swale and its corridor to store water as it meanders through the site. Major storage will occur in lined detention ponds, with open areas also available to detain smaller volumes of water and allow it to soak into the ground after settling and vegetative filtering. The 17-2