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New Brighton/St. Anthony Basic Water Management Project – Phase 1 Rice Creek Watershed District 5555-221 Revised Final 17 June 10, 2014 4.1 CONVEYANCE MODIFICATION One method to lower flood elevations on a drainage system is to simply increase the downstream conveyance capacity. Although this is frequently the least expensive method of decreasing flood elevations at a given location, a project solely consisting of increased conveyance will result in higher peak flow rates and potentially greater flood risk downstream. In some locations where development has occurred with little or no stormwater rate management (such as the contributing drainage to RCD 3), increasing conveyance capacity may be the only feasible alternative for decreasing flood elevations since locations for other stormwater management practices are limited. An increase in discharge to the public drainage system open channels may require mitigation caused for flow changes, which can increase erosion rates and require more frequent maintenance. There is no water quality benefit from this project type. 4.2 DETENTION Temporarily storing runoff, known as Detention Storage, is an effective way of controlling discharge rate thereby reducing flood peaks and elevations downstream. There are several natural water bodies that currently provide detention storage in the RCD 2, 3, and 5 watershed, including Jones Lake, Poplar Lake, Silver Lake, Langton Lake, Wilson Lake, Hart Lake, Pike Lake, and Hansen Park along with several other smaller water bodies. Preserving these volumes and examining opportunities to enhance their detention capacity is critical in reducing downstream flooding. The opportunities to create additional detention storage by constructing new stormwater ponds is likely limited because they typically require a sizeable land footprint which may not be readily available in the RCD 2, 3, and 5 watershed due to its fully developed condition. Thus, future project siting will likely focus on modifications to existing regional basins rather than identifying new detention locations to increase the watershed’s detention storage. Properly designed stormwater detention ponds can provide a significant water quality benefit in addition to providing rate control. 4.3 VOLUME CONTROL Volume control (i.e., reduction) is any practice that decreases runoff through infiltration, evapotranspiration (ET), or reusing it for some other purpose. Volume control can be implemented through a variety of practices summarized below. Each of these practice types improves downstream water quality in addition to decreasing runoff rates. Infiltration is providing an opportunity for runoff to infiltrate into the ground mimicking the natural process that occurred prior to development. Common practices include infiltration basins and trenches and curbside rain gardens. Evapotranspiration is promoting the natural processes of evaporation and transpiration for stormwater runoff. This can occur through combinations of standing water surfaces and vegetative root exposure that yield volume reductions. Common practices that utilize this process include rain gardens and bioretention basins. 79