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Home My WebLink AboutLIVING OAKS - PDP - PDP170009 - SUBMITTAL DOCUMENTS - ROUND 2 - DRAINAGE RELATED DOCUMENT1218 W. ASH, STE. A, WINDSOR, COLORADO 80550 TEL. 970.674.3300 – FAX 970.674.3303 March 29, 2017 Mr. Dan Mogen City of Fort Collins Stormwater 700 Wood Street Fort Collins, CO 80522-0580 RE: Preliminary Drainage Memo for Living Oaks PDP Dear Dan, 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. The project site is located at the southwest corner of East Oak Street and Mathews Street. The property consists of 4,620 sf of land and will consist of a new building 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. Off Site Flows The survey indicates that the portion of the sidewalk (not under the roof overhang) directly south of the property does drain into the site. The roof to the south appears to drain south (based on downspout locations). Additionally a portion of the property to the west also drains into the site, 2 although it appears to be extremely flat, so there is a chance that no runoff actually leaves that site. The existing impervious areas for the off site flows have been accounted for with the paver design on the site. This results in a 3.1 ratio of “run-on” water. We would request that this be allowed for two reasons. First, if and when these properties redevelop, they will need to capture their own water and reduce the flow onto this project site. Second, most of our onsite run-on water is the roof, which is cleaner than typical pavement. 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 Udall Natural Area water treatment facility. City LID requirements are in place which require 50% water quality treatment onsite. 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 3,541 sf of new impervious area (not including the pavers). Using the porous paver LID technique, all of new impervious area (100%) 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 900 sf of new pavement area (which is not under the second floor roof). This project will incorporate 1,265 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 roof drain directly into a porous pavement system thereby slowing runoff and also promoting filtration 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 3 pavement system. The porous pavement system allows for stormwater filtration. 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. Downstream drainageways have already been designed and built for the development of this parcel. 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 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 study has been divided into two drainage basins, A and OS. Basin A consists of the entire site (building roof and drive aisle). The roof will drain south to downspouts which will drain over the top of the porous pavement system. The system has an underdrain and will outfall into the inlet at the corner of the intersection. Basin OS is accounted for with the run-on ratio design of the pavers. The porous paver field size has been increased because of the new roof area draining to it. It is has been further increased for the existing impervious areas draining onto the site from the west and south. The ratio is slightly below City standard, but we would ask that a 3.1:1 ratio be allowed as the site is 100% treated with the pavers and the roof should not have the same sediment load as typical parking and drives. All stormwater in the study area will reach the paver system and underdrain. In the event that these are overwhelmed, stormwater will spill directly east through the driveway and into Mathews Street to the east. 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. 4 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: 3/29/2017 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 0.11 4,620 3,380 0 705 161 374 0.82 75 560 sf of pavers under roof and not included in this calc. OS 0.02 827 51 170 0 160 446 0.55 45 TOTAL 0.13 5,447 3,431 170 321 820 0.71 65 Total Paver Area=1265 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 3-29-17 Final FC Flow-221 Oak.xls New Impervious Area 3,541 sf Required Minimum Impervious Area to be Treated 1,771 sf Treatment #1 - Pervious Pavers 1,265 sf Impervious Area to Treatment #1 (Basin A) 3,541 sf Total Area Treated 3,541 sf Actual % On-Site Treated by LID 100 % New Driveway Pavement Area (not under roof) 900 sf Required Minimum Area of Pervious Pavement 225 sf Area of Pervious Pavers Provided 1,265 sf Actual % of Pervious Pavement Provided 141 % Impervious Area to Pervious Pavers - INCLUDING BASIN OS EXISTING IMPERVIOUS 3,922 sf Total Pervious Pavement Area 1,265 sf Ratio 3.1 50% On-Site Treatment by LID Requirement 3:1 Ratio Requirement LID Table 25% Porous Pavement Requirement New Impervious Area 3,541 sf Required Minimum Impervious Area to be Treated 1,771 sf Treatment #1 - Pervious Pavers 1,265 sf Impervious Area to Treatment #1 (Basin A) 3,541 sf Total Area Treated 3,541 sf Actual % On-Site Treated by LID 100 % New Driveway Pavement Area (not under roof) 900 sf Required Minimum Area of Pervious Pavement 225 sf Area of Pervious Pavers Provided 1,265 sf Actual % of Pervious Pavement Provided 141 % Impervious Area to Pervious Pavers - INCLUDING BASIN OS EXISTING IMPERVIOUS 3,922 sf Total Pervious Pavement Area 1,265 sf Ratio 3.1 50% On-Site Treatment by LID Requirement 3:1 Ratio Requirement LID Table 25% Porous Pavement Requirement