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Home My WebLink AboutLIVING OAKS - PDP - PDP170009 - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE RELATED DOCUMENT1218 W. ASH, STE. A, WINDSOR, COLORADO 80550 TEL. 970.674.3300 – FAX 970.674.3303 February 21, 2017 Mr. Shane Boyle City of Fort Collins Stormwater 700 Wood Street Fort Collins, CO 80522-0580 RE: Preliminary Drainage Memo for Living Oaks PDP Dear Shane, I am pleased to submit for your review and approval, this Preliminary Drainage Memo for the Living Oaks development. I certify that this memo for the drainage design was prepared in accordance with the criteria in the City of Fort Collins Storm Drainage Manual. Location and Description of Property The Living Oaks development is located in the Southwest Quarter of Section 12, Township 7 North, Range 69 West of the Sixth Principal Meridian, in the City of Fort Collins, Larimer County, Colorado. Please refer to the attached vicinity map. The project site is located at the southwest corner of E. Oak Street and Mathews Street. The property consists of 4,623 sf of land and will consist of a new building, one parking space for electric vehicle charging and a drive aisle / hardscape area behind and tucked under the second floor of the building. The site currently has two driveways, both will be removed and only one will be replaced (so the net impervious area in the right of way will be reduced). The site generally slopes in an easterly direction at approximately 0.5% to 1.0%. The land is currently vacant. According to FEMA Panel 08069C0979H there are no mapped FEMA Floodways on this property. The proposed development lies within the Old Town Master Drainage Plan. The outfall for this site is the existing storm sewer system in Oak Street. Development Criteria Reference and Constraints The runoff from this site has been routed to conform to the requirements of the City Stormwater Department. Because the increase in impervious area for this site is not greater than 5,000 sf, onsite detention is not required. Water quality treatment for 50% of the site is provided for in the 2 Udall Natural Area water treatment facility. City LID requirements are in place which will account for the other 50% water quality treatment. The onsite water quality will be accomplished through the use of a porous pavement system that includes 12” of storage in the voids below the pavers. Runoff reduction practices (LID techniques) are also required. No less than fifty percent of any newly added impervious area must be treated using one or a combination of LID techniques. The project adds 4,326 sf of new impervious area. Using the porous paver LID technique, 2,792 sf of new impervious area (65%) will be treated which exceeds the 50% requirement. No less than twenty five percent of any newly added pavement areas must be treated using a permeable pavement technology. The project adds 805 sf of new pavement area (which is not under the second floor roof). This project will incorporate 857 sf of porous pavers which is more than 100% of the newly added pavement which meets the required 25%. Please refer to the attachments for LID calculations and information. Directly Connected Impervious Area (DCIA) Discussion Urban Drainage and Flood Control District (UDFCD) recommends a Four Step Process for receiving water protection that focuses on reducing runoff volumes, treating the water quality capture volume (WQCV), stabilizing drainageways and implementing long-term source controls. The Four Step Process applies to the management of smaller, frequently occurring events. Step 1: Employ Runoff Reduction Practices To reduce runoff peaks, volumes, and pollutant loads from urbanizing areas, implement Low Impact Development (LID) strategies, including Minimizing Directly Connected Impervious Areas (MDCIA). Runoff from the drive aisle and south half of the roof shall drain directly into a porous pavement system thereby slowing runoff and also promoting infiltration Step 2: Implement BMPs that Provide a Water Quality Capture Volume with Slow Release Once runoff has been minimized, the remaining runoff shall be treated through the porous pavement system. The porous pavement system allows for settlement of sediments. Step 3: Stabilize Drainageways Natural Drainageways are subject to bed and bank erosion due to increases in frequency, duration, rate and volume of runoff during and following development. Because water quality treatment for 50% of the site is provided for in the Udall Natural Area water treatment facility, bank stabilization is unnecessary with this project. Step 4: Implement Site Specific and Other Source Control BMPs Proactively controlling pollutants at their source by preventing pollution rather than removing contaminants once they have entered the stormwater system or receiving waters is important when protecting storm systems and receiving waters. This can be accomplished through site 3 specific needs such as construction site runoff control, post-construction runoff control and pollution prevention / good housekeeping. It will be the responsibility of the contractor to develop a procedural best management practice for the site. Drainage Facility Design The development has been divided into two drainage basins, A and B. Basin A consists of the south half of the site and will be collected in the porous pavement system where it will then be conveyed to the existing storm drain system in Oak Street. The remainder of the development, Basin B, will connect roof drains to the drainage system from the pavers. Any overflow from Basin B will sheet flow to Oak Street where it will then enter the same system. Please refer to the attachments for the drainage plan. Conclusions All computations that have been completed within this report are in compliance with the City of Fort Collins Storm Drainage Design Criteria Manual. The proposed drainage concepts presented in this report and on the construction plans adequately provides for stormwater quantity and quality treatment of proposed impervious areas. Conveyance elements have been designed to pass required flows and to minimize future maintenance. If, at the time of construction, groundwater is encountered, a Colorado Department of Health Construction Dewatering Permit will be required. I appreciate your time and consideration in reviewing this submittal. Please call if you have any questions. Sincerely, Reviewed By: Erika Schneider, P.E. Michael Oberlander, P.E., LEED AP Colorado Professional Colorado Professional Engineer No. 41777 Engineer No. 34288 Interwest Consulting Group RUNOFF COEFFICIENTS & % IMPERVIOUS LOCATION: Living Oaks PROJECT NO: COMPUTATIONS BY: es DATE: 2/17/2017 Recommended Runoff Coefficients from Table RO-11 of City of Fort Collins Stormwater Code, Volume I Recommended % Impervious from Table RO-3 Urban Storm Drainage Criteria Manual, Volume I Type B Soils Runoff % coefficient Impervious C Streets, parking lots (asphalt): 0.95 100 Sidewalks (concrete): 0.95 96 Roofs: 0.95 90 Gravel or Pavers: 0.50 40 Landscape Areas (Flat, heavy) : 0.20 0 Landscape Areas (Steep, heavy) : 0.35 0 SUBBASIN TOTAL TOTAL ROOF PAVED PAVERS SIDEWALK LANDSCAPE RUNOFF % DESIGNATION AREA AREA AREA AREA AREA AREA AREA COEFF. Impervious REMARKS (ac.) (sq.ft) (sq.ft) (sq.ft) (sq.ft) (sq.ft) (sq.ft) (C) A 0.06 2,786 1,704 0 623 224 235 0.79 72 234 sf of pavers under roof and not included in this calc. B 0.04 1,837 1,704 0 0 71 62 0.92 87 TOTAL 0.11 4,623 3,408 - 623 295 297 0.84 78 Total Paver Area=857 sf Equations - Calculated C coefficients & % Impervious are area weighted C = Σ (Ci Ai) / At Ci = runoff coefficient for specific area, Ai Ai = areas of surface with runoff coefficient of Ci n = number of different surfaces to consider At = total area over which C is applicable; the sum of all Ai's 2-21-17 Final FC Flow-221 Oak.xls New Impervious Area 4,326 sf Required Minimum Impervious Area to be Treated 2,163 sf Treatment #1 - Pervious Pavers 857 sf Impervious Area to Treatment #1 (Basin A) 1,935 sf Total Area Treated 2,792 sf Actual % On-Site Treated by LID 65 % New Driveway Pavement Area (not under roof) 805 sf Required Minimum Area of Pervious Pavement 201 sf Area of Pervious Pavers Provided 857 sf Actual % of Pervious Pavement Provided 106 % Impervious Area to Pervious Pavers 1,935 sf Total Pervious Pavement Area 857 sf Ratio 2.3 50% On-Site Treatment by LID Requirement 3:1 Ratio Requirement LID Table 25% Porous Pavement Requirement New Impervious Area 4,326 sf Required Minimum Impervious Area to be Treated 2,163 sf Treatment #1 - Pervious Pavers 857 sf Impervious Area to Treatment #1 (Basin A) 1,935 sf Total Area Treated 2,792 sf Actual % On-Site Treated by LID 65 % New Driveway Pavement Area (not under roof) 805 sf Required Minimum Area of Pervious Pavement 201 sf Area of Pervious Pavers Provided 857 sf Actual % of Pervious Pavement Provided 106 % Impervious Area to Pervious Pavers 1,935 sf Total Pervious Pavement Area 857 sf Ratio 2.3 50% On-Site Treatment by LID Requirement 3:1 Ratio Requirement LID Table 25% Porous Pavement Requirement