The URL can be used to link to this page

Home My WebLink AboutLIVING OAKS - FDP - FDP170020 - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE RELATED DOCUMENT1218 W. ASH, STE. A, WINDSOR, COLORADO 80550 TEL. 970.674.3300 – FAX 970.674.3303 July 17, 2017 Mr. Dan Mogen City of Fort Collins Stormwater 700 Wood Street Fort Collins, CO 80522-0580 RE: Final Drainage Memo for Living Oaks Dear Dan, I am pleased to submit for your review and approval, this Final 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 upper floors 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 drains 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 off site flows are minimal and the grading plan for Living Oaks accounts for these flows onto the site in the spillway calculation at the driveway to Mathews. This small area of imperviousness was also included in the 3:1 ratio check of run-on area to the proposed pavers. If and when either these properties redevelop, they will need to capture their water and this off site flow will be eliminated. 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 pervious pavement system that includes 12” of storage in the voids below the pavers. Runoff reduction practices (LID techniques) are 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 4,280 sf of new impervious area (including the pervious pavers). Using the pervious paver LID technique, all of new impervious area in Basin A will be treated (75% of the overall site) 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 1,640 sf of new pavement area in the on side driveway. This project will incorporate 1,000 sf of pervious pavers which is 61% of the newly added pavement. This 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 pervious pavement system thereby slowing runoff and also promoting filtration Step 2: Implement BMPs that Provide a Water Quality Capture Volume with Slow Release 3 Once runoff has been minimized, the remaining runoff shall be treated through the pervious pavement system. The pervious 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. It will be the responsibility of the contractor to develop a procedural best management practice for the site during construction. After construction is completed, the only likely pollutant source would be vehicles using the driveway. This potential pollutant would be treated with the pervious pavers. Drainage Facility Design The study has been divided into three drainage basins, A, B and OS. Basin A consists of the south side of the building and drive aisle. The roof in this area will drain south to downspouts which will drain over the top of the pervious pavement system. The system has an underdrain and will outfall into the inlet at the corner of the intersection. There is a ridgeline on the roof and the north 13’ of the roof will drain to the north (Basin B). There are two roof drains that will come out of the north wall. The project will build sidewalk chases under the Oak Street walk and these flows will enter Oak Street through two new curb openings. Basin OS (although very small) is accounted for with the run-on ratio design of the pavers. Basin A and OS 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. The lowpoint of the driveway is at elevation 4979.76 and the finished floor and lowest opening of the building is 4980.50. This elevation difference is only 9” and the standard from a spillway to a finished floor is 12”. Calculations are included showing that the overtopping depth at the lowpoint is 1-1/2” and a variance has been requested for the lack of freeboard (attached). 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. 4 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: 7/17/2017 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 0.09 3,740 2,410 0 650 140 540 0.76 69 350 sf Pavers Under Roof - Total Paver Area=1,000 sf B 0.02 1,080 1,080 0 0 0 0 0.95 90 OS 0.02 827 51 170 0 160 446 0.55 45 A+B (SITE) 0.11 4,820 3,490 0 650 140 540 0.81 73 AREAS FOR LID CALCS A+OS 0.10 4,567 2,461 170 650 300 986 0.72 64 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 7-17-17 Final FC Flow-221 Oak.xls Interwest Consulting Group STANDARD FORM SF-2 TIME OF CONCENTRATION - 2 and 10 YR LOCATION: Living Oaks PROJECT NO: COMPUTATIONS BY: es DATE: 7/17/2017 2 and 10-yr storm Cf = 1.00 from Table RO-12 of City of Fort Collins Stormwater Code, Volume I SUB-BASIN INITIAL /OVERLAND TRAVEL TIME / GUTTER OR CHANNEL FLOW tc CHECK FINAL REMARKS DATA TIME (ti) (tt) (URBANIZED BASIN) tc DESIGN SUBBASIN(s) Area C Length Slope ti Length Slope n Vel. tt tc = Total L tc=(l/180)+10 PONIT (ac) (ft) (%) (min) (ft) (%) Manning (ft/s) (min) ti + tt (ft) (min) (min) (1) (2) (3) (4) (5) (6) (7) (8) rough. (9) (10) (11) (12) (13) (14) A 0.09 0.76 27 2.0 2.6 16 3.0 0.016 3.5 0.1 2.7 43 10.2 5.0 B 0.02 0.95 13 2.0 0.8 16 3.0 0.016 3.5 0.1 0.9 29 10.2 5.0 OS 0.02 0.55 30 2.0 4.5 16 3.0 0.016 3.5 0.1 4.6 46 10.3 5.0 A+OS 0.10 0.72 40 2.0 3.5 16 3.0 0.02 2.8 0.1 3.6 56 10.3 5.0 EQUATIONS: tc = ti + tt ti = [1.87 (1.1 - CCf ) L0.5 ] / S 1/3 tt = L/Vel. Velocity from Manning's Equation with R=0.1 (corresponds to Figure 3-3 of City of Fort Collins Design Manual) final tc = minimum of ti + tt and urbanized basin check min. tc = 5 min. due to limits of IDF curves 7-17-17 Final FC Flow-221 Oak.xls Interwest Consulting Group STANDARD FORM SF-2 TIME OF CONCENTRATION - 100 YR LOCATION: Living Oaks PROJECT NO: COMPUTATIONS BY: es DATE: 7/17/2017 100-yr storm Cf = 1.25 from Table RO-12 of City of Fort Collins Stormwater Code, Volume I SUB-BASIN INITIAL /OVERLAND TRAVEL TIME / GUTTER OR CHANNEL FLOW tc CHECK FINAL REMARKS DATA TIME (ti) (tt) (URBANIZED BASIN) tc DESIGN SUBBASIN(s) Area C C*Cf Length Slope ti Length Slope n Vel. tt tc = Total L tc=(l/180)+10 PONIT (ac) (ft) (%) (min) (ft) (%) Manning (ft/s) (min) ti + tt (ft) (min) (min) (1) (2) (3) (4) (5) (6) (7) (8) rough. (9) (10) (11) (12) (13) (14) A 0.09 0.76 0.95 27 2.0 1.1 16 3.0 0.016 3.5 0.1 1.2 43 10.2 5.0 B 0.02 0.95 1.00 13 2.0 0.5 16 3.0 0.016 3.5 0.1 0.6 29 10.2 5.0 OS 0.02 0.55 0.68 30 2.0 3.4 16 3.0 0.016 3.5 0.1 3.5 46 10.3 5.0 A+OS 0.10 0.72 0.91 40 2.0 1.8 16 3.0 0.02 2.8 0.1 1.9 56 10.3 5.0 EQUATIONS: tc = ti + tt ti = [1.87 (1.1 - CCf ) L0.5 ] / S 1/3 tt = L/Vel. Velocity from Manning's Equation with R=0.1 (corresponds to Figure 3-3 of City of Fort Collins Design Manual) final tc = minimum of ti + tt and urbanized basin check min. tc = 5 min. due to limits of IDF curves 7-17-17 Final FC Flow-221 Oak.xls Interwest Consulting Group RATIONAL METHOD PEAK RUNOFF (City of Fort Collins, 2-Yr Storm) LOCATION: Living Oaks PROJECT NO: COMPUTATIONS BY: es DATE: 7/17/2017 2 yr storm, Cf = 1.00 DIRECT RUNOFF CARRY OVER TOTAL REMARKS Design Tributary A C Cf tc i Q (2) from Q (2) Q(2)tot Sub-basin Design Point (ac) (min) (in/hr) (cfs) Point (cfs) (cfs) A 0.09 0.76 5.0 2.85 0.2 0.2 B 0.02 0.95 5.0 2.85 0.1 0.1 OS 0.02 0.55 5.0 2.85 0.0 0.0 A+OS 0.10 0.72 5.0 2.85 0.2 0.2 Q = Cf C iA Q = peak discharge (cfs) C = runoff coefficient Cf = frequency adjustment factor i = rainfall intensity (in/hr) from City of Fort Collins IDF curve (4/16/99) A = drainage area (acres) i = 24.221 / (10+ tc)0.7968 7-17-17 Final FC Flow-221 Oak.xls Interwest Consulting Group RATIONAL METHOD PEAK RUNOFF (City of Fort Collins, 10-Yr Storm) LOCATION: Living Oaks PROJECT NO: COMPUTATIONS BY: es DATE: 7/17/2017 10 yr storm, Cf = 1.00 DIRECT RUNOFF CARRY OVER TOTAL REMARKS Design Tributary A C Cf tc i Q (10) from Q (10) Q(10)tot Sub-basin Design Point (ac) (min) (in/hr) (cfs) Point (cfs) (cfs) A 0.09 0.76 5.0 4.87 0.3 0.3 B 0.02 0.95 5.0 4.87 0.1 0.1 OS 0.02 0.55 5.0 4.87 0.1 0.1 A+OS 0.10 0.72 5.0 4.87 0.4 0.4 Q = Cf C iA Q = peak discharge (cfs) C = runoff coefficient Cf = frequency adjustment factor i = rainfall intensity (in/hr) from City of Fort Collins IDF curve (4/16/99) A = drainage area (acres) i = 41.44 / (10+ tc)0.7974 7-17-17 Final FC Flow-221 Oak.xls Interwest Consulting Group RATIONAL METHOD PEAK RUNOFF (City of Fort Collins, 100-Yr Storm) LOCATION: Living Oaks PROJECT NO: COMPUTATIONS BY: es DATE: 7/17/2017 100 yr storm, Cf = 1.25 DIRECT RUNOFF CARRY OVER TOTAL REMARKS Des. Area A C Cf tc i Q (100) from Q (100) Q(100)tot Design Point Design. (ac) (min) (in/hr) (cfs) Point (cfs) (cfs) A 0.09 0.95 5.0 9.95 0.8 0.8 B 0.02 1.00 5.0 9.95 0.2 0.2 OS 0.02 0.68 5.0 9.95 0.1 0.1 A+OS 0.10 0.91 5.0 9.95 0.9 0.9 Q = C iA Q = peak discharge (cfs) C = runoff coefficient i = rainfall intensity (in/hr) from City of Fort Collins IDF curve (4/16/99) A = drainage area (acres) i = 84.682 / (10+ tc) 0.7975 7-17-17 Final FC Flow-221 Oak.xls New Impervious Area 4,280 sf Required Minimum Impervious Area to be Treated 2,140 sf Treatment #1 - Pervious Pavers 1,000 sf Impervious Area to Treatment #1 (Basin A) 3,200 sf Total Area Treated 3,200 sf Actual % On-Site Treated by LID 75 % New Driveway Pavement Area (including area under roof) 1,640 sf Required Minimum Area of Pervious Pavement 410 sf Area of Pervious Pavers Provided 1,000 sf Actual % of Pervious Pavement Provided 61 % Impervious Area to Pervious Pavers - INCLUDING BASIN OS EXISTING IMPERVIOUS 2,931 sf Total Pervious Pavement Area 1,000 sf Ratio 2.9 50% On-Site Treatment by LID Requirement 3:1 Ratio Requirement LID Table 25% Porous Pavement Requirement New Impervious Area 4,280 sf Required Minimum Impervious Area to be Treated 2,140 sf Treatment #1 - Pervious Pavers 1,000 sf Impervious Area to Treatment #1 (Basin A) 3,200 sf Total Area Treated 3,200 sf Actual % On-Site Treated by LID 75 % New Driveway Pavement Area (including area under roof) 1,640 sf Required Minimum Area of Pervious Pavement 410 sf Area of Pervious Pavers Provided 1,000 sf Actual % of Pervious Pavement Provided 61 % Impervious Area to Pervious Pavers - INCLUDING BASIN OS EXISTING IMPERVIOUS 2,931 sf Total Pervious Pavement Area 1,000 sf Ratio 2.9 50% On-Site Treatment by LID Requirement 3:1 Ratio Requirement LID Table 25% Porous Pavement Requirement