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Home My WebLink AboutMASON STREET SUSTAINABLE DEVELOPMENT - PDP - PDP130038 - REPORTS - DRAINAGE REPORTUNITED CIVIL Design Group .� Drainage Summary 401 South Mason Street Fort Collins, CO Low Impact Development (LID) Requirements The developed site (Basin Al) of 2,830 sf will be entirely covered by the proposed building footprint. Therefore, with no paving being provided, the site is not providing pervious paved areas. In order to meet LID requirements, the rooftop of the building is being proposed with approximately 970 sf of green roof resulting in a lower percent - imperviousness and less overall runoff from the site compared to existing conditions. Details of the green roof will be provided with subsequent submittals; however, for this submittal calculations have been provided for the green roof assuming characteristics of a rain garden with an underdrain system. Refer to Appendix B for rain garden calculations. Conclusions In conclusion, the drainage system proposed within this drainage summary provides adequate conveyance of the developed stormwater runoff from the proposed development to existing discharge locations and drainage systems. It should also be noted that with this summary and the corresponding design, every attempt has been made to minimize any negative impacts on the downstream receiving waters. With the incorporation of the green roof, the site will discharge a lower sediment load and less runoff than existing conditions. In addition, the proposed design and ultimate construction shall comply with Chapter 10 of the City of Fort Collins code in regards to Flood Prevention and Protection. We understand that review by the City of Fort Collins is to assure general compliance with standardized criteria contained in the Fort Collins Stormwater Criteria Manual (FCSCM). This letter was prepared in compliance with technical criteria set forth in both the Urban Drainage and Flood Control District (UDFCD) Urban Storm Drainage Criteria Manual and the FCSCM. If you should have any questions or comments as you review this drainage summary, please feel free to contact us at your convenience. Sincerely, United Civil Design Group, LLC Kevin Brazelton, PE Engineering Manager 4 U13008-DrainageSummary-Preliminary.doc UNITED CIVIL Design Group .< Drainage Summary 401 South Mason Street Fort Collins, CO • Improvements being made within the floodway will be constructed at or slightly below existing grades. The following requirements shall be met regarding the design and construction of the proposed mixed -use building: • All flood proofing measures shall adhere to Section 10-38 of the City Code and the requirements set forth within the City's "Floodproofing Guidelines". A Post - Construction Flood Proofing Certificate will be required prior to the issuance of a certificate of occupancy. • All development within the city regulatory floodplain must be preceded by an approved floodplain use permit and applicable fees. • All development within the city regulatory floodway must be preceded by an approved no -rise certification followed by re -certification of no -rise in the as - built condition. The following table summarizes the floodproofing elevation requirements to be met during construction: Description Elevation (NAVD-88) City Base Flood Elevation (BFE)* 4996.93 Regulatory Flood Protection Elevation (RFPE) 4998.43 Lowest Floor Elevation 4985.78 Flood Proofing Elevation 4998.78 HVAC Equipment Elevation (In Floodproofed Basement) 4985.78 *Information provided in the above table is based upon the interpolated BFE located at the northwest corner of the proposed building. Detention Requirements Per communication with City personnel, the site has historically paid fees for 100% imperviousness. As the existing site currently has an approximate imperviousness of approximately 88%, City personnel have indicated that the development may request a variance to the detention requirement for the site. This variance request is being requested with this PDP Submittal and the site currently does not provide onsite detention. Water Quality Requirements The site is located within the Udall Natural Area which provides water quality for the site; and therefore, additional water quality is not required. However, the development is currently proposing to provide approximately 970 sf of green roof which will provide additional water quality for the site. 3 U13008-Drainage5ummaryPreliminary.doc A�UNITED CIVIL Design Group uc Existing Conditions Drainage Summary 401 South Mason Street Fort Collins, CO Hydrology The site (Basin Al), which is located within the Old Town Basin, currently consists of an existing one-story building, parking area, and a small landscape area with an existing percent -imperviousness of approximately 88%. Storm runoff from the site currently drains towards Magnolia and Mason Streets where runoff is intercepted by existing storm sewer inlets and piping located within Mason Street. Refer to Appendix A for additional information. Floodplain The site is located within the City -regulatory Old Town Basin 100-year floodway, flood fringe, and moderate risk flood fringe. The Old Town Master Plan was approved July 15, 2003. The site is not located within a FEMA floodplain or floodway. Refer to Appendix C for the Drainage Exhibit and FEMA FIRM panel. Developed Conditions Hydrology The developed site of 2,830 sf will be entirely covered by the proposed building footprint. The rooftop is currently being proposed to provide approximately 970 sf of green roof which results in a proposed percent -imperviousness for the site (Basin Al) of approximately 60%. Existing drainage patterns are being preserved with the proposed development and water from the site will be directed from the rooftop to the existing storm sewer system located with Mason Street. Refer to Appendix A for additional information. Floodplain The site is located within the City -regulatory Old Town Basin 100-year floodway, flood fringe, and moderate risk flood fringe. The following design elements are being proposed with the development: • The proposed non-residential building (having no residential component) will be located outside of the floodway and is more specifically located approximately 6-feet south and 4.5-feet west of the floodway boundaries. • The development team is currently proposing to flood proof the proposed building in lieu of raising the finished floor 18-inches.above the Base Flood Elevation (BFE). Refer to the Drainage Exhibit and architectural plans for additional information regarding the building flood proof design. • The building construction will include a basement with concrete caisson foundations. Refer to the Drainage Exhibit for a typical cross section detail of the footing design. 2 U33008-Dralnage5ummary-7reliminery.doc UNITED CIVIL Design Group , April 22, 2014 City of Fort Collins Stormwater Utility 700 Wood Street Fort Collins, Colorado 80521 RE: 401 South Mason Street Fort Collins, Colorado Project Number: U13008 Dear Staff: Drainage Summary 401 South Mason Street Fort Collins, CO United Civil Design Group, LLC. is pleased to submit this Preliminary Drainage Summary for the 401 South Mason Street site in Fort Collins, Colorado. In general, this summary serves to document the stormwater impacts associated with the proposed 401 South Mason Street project. Site Location and Project Description The development site (referred herein as "the site") is located at 401 South Mason Street in the City of Fort Collins, Colorado. The site consists of approximately 2,830 square -feet (sf) and is bounded by Magnolia Street on the north, Mason Street on the East, an existing restaurant on the south, and a residential property to the west. See the attached Drainage Exhibit for additional information. The planned development for the site will consist of the following: • The removal of the existing one-story building and parking areas on the site. • The construction of multi -story, mixed -use building with a proposed footprint of approximately 2,530 sf. • The construction of utilities to support the proposed building including sanitary, water and storm drainage. • The construction of right-of-way improvements including curb and gutter, public sidewalk and landscaping. U 13008-Dralnage5ummary-Preliminary.doc L AUNITED CIVIL Design Group uc Preliminary Drainage Summary 401 South Mason Street Fort Collins, CO Prepared for: Coin Street Mason; LLC. 5425 Tradewind Drive Windsor, CO 80528 April 22, 2014 1501 Academy Ct. Ste. 203 Fort Collins, Colorado 90524 (970) S30-4044