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HomeMy WebLinkAboutMAX FLATS - PDP - PDP120034 - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORTPage 1 of 2 D:\Projects\860-001\Drainage\860-001_Drainage-Letter_2012-12-12.docx Drainage Letter Report Date: December 12, 2012 Project: MAX Flats Project No. 860-001 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 MAX Flats mixed-use redevelopment located at 203 West Mulberry Street. The existing site includes a two-level concrete block building with a footprint of approximately 3050 square-feet. It is located along the western edge of the property and roughly centered north-to-south. The remainder of the site is almost entirely paved as a parking lot. Existing landscaping is minimal and generally limited to narrow strips of shrubs along the north and east edges of the property. The site currently surface drains from southwest to northeast along flat grades of less than one percent. Two existing off-site curb inlets are located at the southwest west corner of Mulberry and Mason Streets along the south and west flowlines of each, respectively. These inlets collect existing developed site runoff and discharge it to the public storm sewer system. The proposed condition will generally preserve historic drainage patterns. The total impervious area of the existing site is approximately 28,588 square-feet. The proposed redevelopment will completely remove all existing above-grade structures and adjacent hardscape. The project will consist of four stories of residential units over a ground level containing a commercial flex space and parking. The total impervious area of the proposed redevelopment is approximately 27,403 square-feet. The site is located in the Old Town master drainage basin, and therefore, on-site detention is not required so long as the increase in impervious area is less than 5,000 square-feet. The total impervious area will decrease with this development by approximately 1,185 square-feet (28,588 - 27,403). A detailed breakdown of the existing and proposed impervious areas is attached to this letter. Page 2 of 2 D:\Projects\860-001\Drainage\860-001_Drainage-Letter_2012-12-12.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 appropriate and applicable City of Fort Collins (City) standards for erosion and sediment control. Post construction erosion control will be achieved by a fully established and stabilized site. The areas disturbed during construction will be permanently stabilized with hardscape (i.e., concrete), landscape or building structure. This portion of the Old Town Basin does receive regional water quality treatment in the Udall Natural Area; therefore, permanent on-site water quality mitigation is not required. This project is located in the Old Town Basin floodplain. This is a City regulatory floodplain. Since a small portion of the development falls within a City regulatory floodplain, a Floodplain Use Permit (FUP) shall be obtained prior to commencing with any construction within the floodplain. The improvements within the flood fringe of the City basin floodplain consist of mixed-use redevelopment with the residential use on a floor completely above the regulatory flood protection elevation. The mixed-use redevelopment shall comply with requirements for floodproofing in lieu of elevating the structure. It is important to note that the floodway of the City basin floodplain currently follows the midpoint of the existing sidewalk along the project's north Mulberry Street frontage. The proposed improvements within the floodway of the City basin floodplain consist of nonstructural development (i.e., driveway and streetscape modifications). A No-Rise Certification is required for all work within the floodway. Further details and supporting documentation for the No-Rise Certification shall accompany the FUP application. The proposed grading concept closely matches existing elevations and drainage patterns, and only varies in an effort to improve the existing condition. On-site detention is proven to be unnecessary. On-site stormwater quality is not required as runoff is treated in the Udall Natural Area water treatment facility. The project is designed in accordance to the governing floodplain regulations contained within Chapter 10 of the City Code, and the Floodplain Use Permit and necessary certifications shall ensure that construction remains compliant with Chapter 10. Therefore, it is my professional opinion that the MAX Flats mixed-use redevelopment satisfies all applicable stormwater criteria. Please do not hesitate to contact me if you have questions or require additional information. Sincerely, Herman H. Feissner, PE enc.