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HomeMy WebLinkAboutDrainage Reports - 12/22/2015City of Fort Co ' s Appro edYlans Approved by. L& Date; la ad1.2 - I @ j This Drainage Report is consciously provided as a PDF. Please consider the environment before printing this document in Its entirety. When a hard copy is absolutely necessary, we recommend double -sided printing. December 18, 2015 FINAL DRAINAGE LETTER FOR MUSIC DISTRICT 619 S. COLLEGE AVENUE Fort Collins, Colorado Prepared for: City of Fort Collins Prepared by: NORTHERN ENGINEERING 301 N. Howes Street, Suite 100 Fort Collins, Colorado 80521 Phone: 970.221.4158 Fax: 970.221.4159 www. northemen®neeringcom Project Number: 1052-002 1 NorthernEnaineerina.com // 970.221.4158 11 I u 1 FJ 1 1 NORTHERN ENGINEERING December 18, 2015 Shane Boyle City of Fort Collins Stormwater Utility 700 Wood Street Fort Collins, Colorado 80524 RE: The Music District — 619 College Avenue, Drainage Analysis Fort Collins, Colorado Dear Shane: The site improvements for The Music District consist of renovating the existing building at 619 S. College Avenue, surface replacement of the existing alley between 619 and 621 S. College Avenue, and improvements to the surrounding parking lots and landscaped areas. The site was modeled as a single drainage basin that discharges into the gutter along the west side of College Avenue. Drainage Basin: The existing drainage patterns for the project sheet flows across the rear parking lot towards the northeast; along an existing concrete valley pan between 619 and 621 S. College Ave; and into the curb and gutter along College Avenue. The proposed drainage patterns will remain unchanged and sheet flow across the rear parking lot; into a relocated proposed concrete valley pan between 619 and 621 S. College Ave; and into a proposed rain garden (water quality pond). The proposed 0.5-ft deep rain garden (water quality pond) will discharge into the flowline along College Avenue at Design Point 1. The rain garden at Design Point 1 will intercept the proposed drainage to provide water quality treatment/extended detention prior to exiting the site. Within the project drainage basin, the developer proposes to replace the existing concrete plaza, asphalt alley, and landscaping, and renovate 619 S. College Ave. The proposed improvements will result in a minor decrease in the basin's fowrate (see Table 1). The proposed 100-Year Storm Event is 3.04 cfs (see Table 1). The aforementioned rain garden (water quality pond) provides water quality treatment/extended detention which is an improvement from existing conditions. Table 1: Basin Drainage Characteristics Existing Proposed Basin Area (sq. ft.) 20,255 20,255 Impervious Area (sq. ft.) 17,163 16,807 Percent Impervious 84.74% 82.98% 10-Year Flowrate (cfs) 1.40 1.33 100-Year Flowrate (cfs) 3. 99 3.04 1 301 N. Howes Street, Suite 100, Fort Collins, CO 80521 1 970.221.4158 1 www.northernengineering.com ' NORTHERN ENGINEERING ' Impervious Area: For the entire project, there is a net decrease of 356 SF of impermeable area (see Table 1). City of Fort Collins Stormwater Requirements does not apply since the change in impermeable area is less than the allowable increase of 350 SF. Minor storm water quality treatment/extended detention will be provided within a total of 33 SF of proposed rain gardens/water quality ponds. ' Impacts to Adjacent Streets and Properties: ' The stormwater impact on the adjacent streets and properties from this project is negligible. The existing and proposed drainage outfall locations continue to drain east towards College Avenue. The small reduction of impervious area throughout the project reduces the overall flow rate on College Avenue by 0.15 cfs (see Table 1), and the proposed rain gardens/water quality pond will provide water quality treatment. JIf you should have any questions or comments, please feel free to contact me at your convenience. Sincerely, NORTHERN ENGINEERING SERVICES, INC. Frederick S. Wegert, PE Project Engineer 1 UAUJArZQ L NI �PpO score LP ' 45018 �'. i2/I 8/15:: 4. 4 � o� NAI. 1 301 N. Howes Street, Suite 100, Fort Collins, CO 80521 1 970.221.4158 1 www.northernengineering.com LI I L I M C d o ! 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