Agenda - Environmental Policy Board - 10/15/2018

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Runoff Under Developed Conditions. As development proceeds on the 300+ acres that are part of the RTC, runoff will markedly increase. Conversion of sandy open space and agricultural land to commercial and residential uses invariably leads to increased runoff from paved surfaces associated with that development. The translation of the preferred design in Figure 6.1 to a developed schematic for runoff routing was shown in Figure 6.6 as part of the site description. The essential elements of the drainage system proposed for the site are as follows: • It incorporates a 100-year design event with no infiltration considered, thus generating the —worst case" scenario upon which design can proceed. • It routes water locally into the central drainage corridor, using a system of smaller ponds, followed by an area of flood storage and infiltration. • It uses existing detention storage and develops increased storage for the highly impermeable retail center on City property between the railroad tracks and TH10. • It proposes a connection of the site to the Mississippi River via the County -owned swath of land. This piece of land would contain a detention facility on the upstream side to add storage, followed by an infiltration zone, then a stabilized channel (piped or series of landscaped drop -structures) over the bluff to the river. Of note here is the additional need of this outflow as an outlet for any future TH10 upgrade. Although the development of this corridor for the passage of water has not been approved yet by the County, discussions are under way. This corridor presents the best option for out -letting this closed basin for the RTC site and for future TH10 work. If reaching the River through this option is not approved, another option will need to be pursued, most likely to the southeast along TH10. However, outflow in that direction is also closed and prevented from out -letting to the River, so additional study would be needed to identify an ultimate connection. • It incorporates infiltration throughout the RTC site as an added benefit rather than as a design component for runoff management. The LRRWMO will not allow infiltration in design of the 100-year event. Rather, whatever other soaking -in that can be achieved in the central corridor will supplement water management. Infiltration can be used to cut peaks and volume, reduce major parts of small-scale events, maintain recharge and treat water quality. Each infiltration feature will need to be designed with an overflow/outlet to assure that water will not remain a permanent feature. • It develops on -site detention in the central corridor on the western -most of the two sets of available areas; that is, parcel #s 49 and 54. The eastern -most cells (#s 51 and 56) will then be areas with a meandering (baseflow) stream that will rise during runoff events and spill over into a floodplain/infiltration zone, where water can soak into native vegetation, grading upward to a more landscaped, green 17-6