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Home My WebLink AboutSTONER SUBDIVISION - FDP - FDP130028 - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORTPage 1 of 2 D:\Projects\358-002\Drainage\Reports\Drainage Report\358-002_Drainage-Letter.docx Drainage Letter Report Date: July 3, 2013 Project: Stoner Subdivision Project No. 358-002 Project Development Plan Fort Collins, Colorado Attn: Mr. Glen Schlueter City of Fort Collins Stormwater Utility 700 Wood Street Fort Collins, Colorado 80521 Dear Glen: This letter serves to address the stormwater impacts of the Stoner Subdivision Project Development Plan. The existing site contains an existing residence that is situated towards the western side of the existing lot, as well as an existing garage located along the alley. In addition to the rooftops of those two structures, there are also existing concrete walks, turf, trees, and other vegetation. It presently functions as a ‘B’ lot, with historic drainage being split towards Wayne Street to the west and towards the alley to the east. The total impervious area of the existing site is approximately 4,189 square-feet. The existing percent impervious for the entire site is 23.6%. The proposed project will subdivide the existing lot to create a new lot on the eastern half of the property. The project intends to leave the existing building footprints unchanged, while adding a single family home on the newly created lot. For the most part, existing paving will remain unchanged, although there will be small areas of concrete removed. Also, there will be small areas of concrete walk added to serve the new home. While minor grading changes will occur, existing drainage patterns will be maintained. The total impervious area of the proposed redevelopment is roughly 5,513 square-feet. This is a slight increase from the existing condition. The proposed percent impervious for the entire site is 31.0%. 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 increase in impervious area proposed with this development is approximately 1324 square-feet (5,513 – 4,189). A detailed breakdown of the existing and proposed impervious areas is attached to this letter. This proposed building is located within a City regulatory moderate risk floodplain in the Old Town Basin. There are no elevation requirements associated with this type of floodplain, however efforts will be made to elevate the proposed home 12” above the BFE to provide additional protection from flooding. Page 2 of 2 D:\Projects\358-002\Drainage\Reports\Drainage Report\358-002_Drainage-Letter.docx Although regional stormwater quantity detention is provided by the Udall Natural Area for this property, onsite stormwater quality mitigation will still 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 6,467 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 increasing only slightly 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 earth lined swales to collect runoff from the new residence, which allows for the filtering and 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. 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 Stoner 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. Effective Impervious Area (%) Project: Stoner Subdivsion Calculations By: A. Reese 100 Date: July 3, 2013 100 40 100 22 0 Basin Area (s.f.) Basin Area (ac) Area of Asphalt (ac) Area of Concrete Pavers (ac) Area of Concrete (ac) Area of Roofs (ac) Area of Gravel (ac) Area of Lawn and Landscape (ac) Effective Impervious Area (ac) Effective Impervious Area (sf) Net Change Existing 15987 0.37 0.00 0.00 0.03 0.07 0.00 0.27 0.10 4188.82 Proposed 15987 0.37 0.00 0.00 0.03 0.10 0.00 0.24 0.13 5512.83 1324.01 Pavers………………………..…………………………………………….. Landscaping ….….……….…………………………………………………. Streets, Parking Lots, RoofS, Alleys, and Drives: Character of Surface IMPERVIOUS AREA CALCULATIONS (PROPOSED VS. EXISTING) Asphalt ……....……...…...…...………………………………………….. Concrete …….......…….……………….………………………………… Gravel ……….………….………………………………………………….. Roofs …….…….………………………………………………………….. CHARACTER OF SURFACE: Runoff Coefficient Percentage Impervious Project: Stoner Subdivision Streets, Parking Lots, Roofs, Alleys, and Drives: Calculations By: A. Reese Asphalt ……....……………...……….....…...……………….………………………………….0.95 . 100 Date: Concrete …….......……………….….……….………………..….………………………………… 0.95 90 Gravel ……….…………………….….…………………………..……………………………….0.50 . 40 Roofs …….…….………………..……………….…………………………………………….0..95 90 Concrete Pavers…………………………...………………..…………………………………………….0.40 . 22 Lawns and Landscaping Sandy Soil ……..……………..……………….…………………………………………….. 0.15 0 Clayey Soil ….….………….…….…………..………………………………………………. 0.25 0 2-year Cf = 1.00 100-year Cf = 1.25 Basin ID Basin Area (s.f.) Basin Area (ac) Area of Asphalt (ac) Area of Concrete (ac) Area of Roofs (ac) Area of Gravel (ac) Area of Concrete Pavers (ac) Area of Lawns and Landscaping (ac) 2-year Composite Runoff Coefficient 10-year Composite Runoff Coefficient 100-year Composite Runoff Coefficient Composite % Imperv. SITE 15,987 0.37 0.00 0.03 0.07 0.00 0.00 0.27 0.36 0.36 0.45 23.6 EXISTING COMPOSITE % IMPERVIOUSNESS AND RUNOFF COEFFICIENT CALCULATIONS Runoff Coefficients are taken from the City of Fort Collins Storm Drainage Design Criteria and Construction Standards, Table 3-3. % Impervious taken from UDFCD USDCM, Volume I. 10-year Cf = 1.00 July 3, 2013 D:\Projects\358-002\Drainage\Hydrology\358-002_Rational-Calcs_Existing.xlsx\C-Values CHARACTER OF SURFACE: Runoff Coefficient Percentage Impervious Project: Stoner Subdivision Streets, Parking Lots, Roofs, Alleys, and Drives: Calculations By: A. Reese Asphalt ……....……………...……….....…...……………….………………………………….0.95 . 100 Date: Concrete …….......……………….….……….………………..….………………………………… 0.95 90 Gravel ……….…………………….….…………………………..……………………………….0.50 . 40 Roofs …….…….………………..……………….…………………………………………….0..95 90 Concrete Pavers…………………………...………………..…………………………………………….0.40 . 22 Lawns and Landscaping Sandy Soil ……..……………..……………….…………………………………………….. 0.15 0 Clayey Soil ….….………….…….…………..………………………………………………. 0.25 0 2-year Cf = 1.00 100-year Cf = 1.25 Basin ID Basin Area (s.f.) Basin Area (ac) Area of Asphalt (ac) Area of Concrete (ac) Area of Roofs (ac) Area of Gravel (ac) Area of Concrete Pavers (ac) Area of Lawns and Landscaping (ac) 2-year Composite Runoff Coefficient 10-year Composite Runoff Coefficient 100-year Composite Runoff Coefficient Composite % Imperv. A 15,987 0.37 0.00 0.03 0.10 0.00 0.00 0.24 0.43 0.43 0.53 31.0 DEVELOPED COMPOSITE % IMPERVIOUSNESS AND RUNOFF COEFFICIENT CALCULATIONS Runoff Coefficients are taken from the City of Fort Collins Storm Drainage Design Criteria and Construction Standards, Table 3-3. % Impervious taken from UDFCD USDCM, Volume I. 10-year Cf = 1.00 July 3, 2013 D:\Projects\358-002\Drainage\Hydrology\358-002_Rational-Calcs_Proposed.xlsx\C-Values