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HomeMy WebLinkAbout1 Historic Flows - NarrativeHistorical Drainage Flows Narrative Master Site Plan Application Gardner Simmental Lot 1A, Tract 1 Project Name 24345 Bridger Point CONR This narrative outlines the historical drainage patterns on and around the project site. Please reference the “Exhibit: Historic Drainage Pathways” with this narrative as well as the old exhibit “L.4”, which was the original grading plan for the existing site. This is included for you to see all existing culverts and swales on site that are to be removed during this development. The proposed detailed grading and drainage plans are not discussed in this narrative but are included in this application. The purpose of this narrative is only to show the historical drainage pathways. The stormwater that flows onto and around the site is generally topographically divided into east and west drainage basins, which ultimately discharge to two separate culverts. The site is at the downstream end of a subdivision, which has flows from the east side of N 19th Ave and Simmental Ave. Western Drainage Patterns N 19th Ave has a roadside ditch that runs along eastern side of the road, which ultimately runs along the western portion of the site and discharges into a 24” culvert (“Culvert 1” on the Exhibit: Historic Drainage Pathways) at the SE corner of the N 19th Ave and Hwy 90 On-Ramp Intersection. “Culvert 1” discharges the flows to the west of N 19th Ave, and then finally into another culvert that flows under the N 19th Ave On-Ramp into a large MDT retention basin. The proposed grading of the site does not drain any of its stormwater into this N 19th Ave roadside ditch or “Culvert 1”. Eastern Drainage Patterns The north and north-eastern boundary of the site has a ditch that captures water from Hwy 90 and from a small portion of the northern portion of Simmental Road and discharges it into a 24” culvert (“Culvert 2” on the Exhibit: Historic Drainage Pathways). As projects are developed in the City of Bozeman on Simmental Road, they are required to capture, manage, and store stormwater on their own site. Much of the stormwater on Simmental Road is already captured and managed on developed sites, and as Simmental Rd becomes fully developed, no stormwater will discharge to the Hwy 90 roadside ditch. Therefore, the roadside ditch that borders the property on the east and north-east side will primarily capture and route stormwater from Hwy 90. The proposed grading on this site will route any stormwater flow from the adjacent southern property to either the eastern or western sides of the property and into either western or eastern ditches discussed above. This will prevent run-on to the site and maintain historical flow patterns. MDT Permit This site has an approved MDT permit (Permit #8566, included in this application) that permits discharge to “Culvert 2” at the pre-development rate of 2.32 CFS. This discharge rate will be split between Phase 1 (CFA Site Plan) and the future Phase 2. More detailed information regarding the proposed stormwater plan is included in the Stormwater Report and Plans in the application.