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HomeMy WebLinkAbout15-1 Stormwater Management Update 2019-09-241 Stormwater Management Update An update to the Preliminary Grading and Drainage Plan for Bozeman Gateway Development, shows changes to the boundaries of the minor basins as individual site plans were more defined. The original report provides hydrologic and hydraulic calculations for sizing storm water conveyance, detention storage facilities and other storm drainage structures for the entire Bozeman Gateway PUD. The drainage plan consists of two systems, the major drainage system and the minor drainage system. The major drainage system shall convey 100-year events without inundating building structures. The minor system is designed to provide public convenience, to accommodate moderate, relatively frequent flows and to convey runoff from the site. Phase 4 lies within scope of the previously approved drainage submittal and is split into 6 different minor basins (minor basin 4-9). Runoff from each basin is treated and detained with a “StormTech” pre-fabricated underground storage and infiltration facility. The City’s design standards require Low Impact Development (LID) practices that infiltrate, evapotranspire or capture runoff, to the extent feasible, for a specified design runoff event. This requirement is addressed using existing and proposed StormTech underground detention and storage facilities. These systems combine infiltration and runoff capture. The StormTech system is a widely accepted LID practice found to remove 80 percent of total suspended solids from municipal storm runoff. It meets LEED criteria for storm water treatment (SS6.2 water quality credit). The facilities for this project provide over 6 inches of capture depth for infiltration and runoff volume reduction, plus treatment of additional runoff by deposition as water slowly travels through the long, linear rows of chambers and gravel base. The system solves the problem of runoff “short-circuiting” from inlet to outlet that is inherent in surface detention ponds. Runoff from the first ½” of rainfall will percolate through and fill a gravel filter before finding its way to a perforated low-flow outlet pipe that discharges into the detention outlet structure. Additional runoff from larger storms can bypass this discharge route, but only after storage depth reaches 2 feet (this correlates to about 64 percent of the total available storage volume). Although the detention calculations assume zero infiltration, the existing alluvial (gravel/pit run) soils directly under the StormTech systems provide excellent infiltration, which further improves runoff treatment. We believe this type of treatment not only meets, but exceeds, the City’s current design standards. The underground storage is designed using a conservative post development coefficient of 0.8 for the entire basin. As the post development coefficient will be less than or equal to 0.8 for the lots in Phase 4, the previous calculations in the Preliminary Grading and Drainage Plan meets the required storm design criteria for Phase 4. Like the previous phases 1-3, the final grading and drainage plans will be developed once lots in Phase 4 reach the site plan stage.