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HomeMy WebLinkAbout11 Stormwater Concept 08-13-19memo Page 1 TO: Matt Ekstrom FROM: James Nickelson DATE: August 13, 2019 JOB NO.: 5659.005 RE: Northwest Crossing Master Plan – Drainage Concept CC: Urgent For Review Please Comment Please Reply For Your Use Northwest Crossing consists of approximately 160 acres. The property is master planned to be developed at various densities that will generate varying amounts of stormwater runoff. The topography of the property naturally drains to the north and to the northwest. Post development runoff will need to be controlled in order to comply with the City of Bozeman Design Standards. Water quality provisions will also need to be incorporated into the site to comply with the Design Standards. As the master site plan does not fully define quantities or locations of impervious surfaces it is not possible to complete a full stormwater analysis of the property. A conceptual volumetric approach is utilized at this point to get a broad understanding of the drainage needs for the property. It is anticipated that a combination of stormwater facilities will be utilized for the project. Conveyance facilities will likely consist of drainage swales, natural drainage ways, and storm sewers. Treatment and stormwater basins will likely consists of open ponds, underground storage systems and be a combination of retention and detention facilities. In order to arrive at an approximate land area that will be needed for stormwater basins two approaches have been utilized. The first methodology uses the City of Bozeman Design Standard water quality requirement to address the first 0.5 inches of rainfall and the second utilizes a retention volume approach. Utilizing the 0.5 inch water quality guideline results in a storage volume of 4.4 acre-feet and considering a maximum depth of 1.5 feet and a factor that considers side slopes an approximate area of 5.9 acres would be needed for stormwater storage. Northwest Crossing Master Plan – Drainage Concept Page 2 Using a retention volume approach and an estimated average runoff coefficient of 0.5 (based on Dense Residential classification from the City of Bozeman Design Standards) for the entire 160 acres results in a storage volume of 5.4 acre-feet which may require 7.2 acres of land for ponds. Both of these methodologies provide an approximate area of required property within the 160 acre to address stormwater storage requirements. Depending on the type, location, and number of storage facilities the land area could be lower than the areas estimated by these methods. It is anticipated that stormwater storage facilities will be concentrated to the north and to the northwest portion of the property based on topography and other considerations. In addition, a number of smaller facilities will likely be located within the various development parcels in order to reduce the magnitude of stormwater conveyance facilities and address concerns regarding phasing of the development. In summary it is anticipated that the 160 acre development will require approximately 6 to 7 acres dedicated to stormwater storage. This value may change as the development is further defined and that time the drainage plan will be updated.