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HomeMy WebLinkAboutMITSUBISHI @ 2712 S. COLLEGE AVE. - MJA - MJA120007 - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORTPage 1 of 2 D:\Projects\874-001\Drainage\Reports\Drainage Report\874-001_Drainage-Letter.docx Drainage Letter Report Date: November 21, 2012 Project: Lot 8, Thunderbird Estates Filing 6th Subdivision Project No. 874-001 Major Amendment Fort Collins, Colorado Attn: Mr. Wes Lamarque City of Fort Collins Stormwater Utility 700 Wood Street Fort Collins, Colorado 80521 Dear Wes: This letter serves to address the stormwater impacts of the above referenced Major Amendment. The existing building is situated towards the western portion of the site. The site currently consists of rooftop, concrete walks, asphalt parking areas, some turf, trees, and other vegetation. It presently functions similar to a ‘B’ lot, with historic drainage being split towards the frontage road to the west and towards the private drive to the east. The total impervious area of the existing site is approximately 26,366 square-feet. The existing percent impervious for the entire site is 85.3%. The proposed redevelopment will leave the existing building footprint unchanged, while adding a vehicle display area on the west side of the building constructed of concrete pavers. Portions of existing paving along the north side of the building will be modified, and a bio-retention area will be added on the east side of the existing parking area. While very minor grading changes will occur, existing drainage patterns will be maintained. The total impervious area of the proposed redevelopment is roughly 27,400 square- feet. This is a slight increase from the existing condition. Due to the use of permeable concrete pavers and the additional landscape area, the proposed percent impervious for the entire site is 83.3%. The site is located in the Spring Creek Master Drainage Basin. On-site detention is not required so long as there is no increase in impervious area. Since the redevelopment proposes a slight decrease in percent impervious, detention storage is not required. A detailed breakdown of the existing and proposed impervious areas is attached to this letter, as is an exhibit depicting the areas. There are no regulatory floodplains associated with the project. Page 2 of 2 D:\Projects\874-001\Drainage\Reports\Drainage Report\874-001_Drainage-Letter.docx Although stormwater quantity detention is not required, stormwater quality mitigation will be addressed by both temporary and permanent Best Management Practices (BMPs). During construction, the Contractor will follow the appropriate and applicable City of Fort Collins standards for erosion and sediment control. Since the approximately 8,500 sf area of disturbance for the project is less than one acre, a comprehensive Stormwater Management Plan will not be prepared for this project. Post construction water quality and erosion control will be achieved by a fully established and stabilized site. All areas disturbed during construction will receive permanent hardscape, landscape, or building structure. There are no existing stormwater BMP’s currently on-site. Since the percent of site imperviousness is decreasing from the existing condition, formal water quality treatment is not required. While formal water quality treatment is not required for this site, an effort has been made to employ low-impact development strategies to treat the majority of stormwater runoff from the site. This treatment occurs through a variety of methods. The first method is the use of a bio-retention area along the eastern property line that intercepts runoff from the existing asphalt parking lot, which allows for the settling of particulates to occur prior to drainage leaving the site. Another method that is being utilized is to discharge roof drains into landscaped areas where available. The final method of treatment is the use of permeable pavers along the western portion of the site, which allows minor rain events to infiltrate into the soil rather than leaving the site. To summarize, the proposed grading concept very closely matches existing elevations and drainage patterns. On-site detention is proven to be unnecessary. Stormwater quality is not required, but has been provided anyways. Therefore, it is my professional opinion that the remodeled building proposed with Lot 8 of the Thunderbird Estates Sixth Subdivision satisfies all applicable stormwater criteria. Please do not hesitate to contact me if you have questions or require additional information. Sincerely, Andrew Reese Nicholas W. Haws, PE Project Engineer Project Manager enc.