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Home My WebLink AboutDrainage Reports - 12/21/2009August 31, 2009 Wes Lamarque City of Fort Collins Stormwater Utility 700 Wood Street Fort Collins, CO 80521 City of Ft.Collins proved Plans Approved By. Date 1�.��—o�_ RE: Sonic Drive In Riverside and Lemay Minor Amendilor ment ent — Drainage1) �L�ettter �/^ Dear Wes, We are pleased to submit for your review, this letter, which outlines the revised drainage I design for the proposed Sonic Restaurant at Riverside and Lemay that will replace the existing Wok -and -Roll. This letter references the reports "Final Storm Drainage Report for J.L. West Minor Subdivision" and "Addendum to Final Storm Drainage Report for i- J.L. West Minor Subdivision" by Stewart and Associates originally approved in 1995 and amended in 2000. All computations in this report have been completed in compliance with the City of Fort Collins Storm Drainage Criteria, dated May 1984, revised April 1997. The Sonic project proposes to modify the existing parking area and associated building in order to convert the facilities to suit the needs of a Sonic Restaurant. The storm water improvements proposed with the current renovations include the addition of two water quality ponds in each of the new parking islands. These water quality features will provide a higher level of water treatment than is currently provided. The previously approved parking lot detention concept will remain in the design because the site does not support the modification of this feature due to its limited size and shallow storm water infrastructure. Two variances are respectfully requested to cover two non-standard conditions that already exist at the current site. • A variance from the requirement that the water quality capture volume (WQCV) is added to the 100-year detention volume. • A variance from requirement to provide one -foot of free board. The site was originally designed with a drive entrance serving as the emergency spillway. An additional 1' around the site perimeter would negatively impact the site grading and would raise the emergency high water elevation to within 0.6' of the existing buildings finished floor elevation. Support for the requested variances includes the following: • The site was originally designed using pre-1999 rainfall data. Usually, this condition would be grandfathered in and the revised design would use the same intensities. Under the proposed design, the detention ponds and orifices were designed with the City's current IDF-curves. • Neither the site's 100-year flow-restrictor plate, nor the WQCV orifice plate from the previous design is installed in the current condition, resulting in the site releasing stormwater undetained. The proposed design would correct this condition. • At less than 1-acre in total size, the peak flow rates leaving the site are very small in comparison with the watershed. Since the WQCV was never provided by previous designs or constructed infrastructure, the variances would not result in an increased peak -discharge. Basin Summary The previous drainage designs identified two sub -basins, Basin -A and Basin-B, shown on the JL West Minor Subdivision Grading, Drainage and Erosion Control Plan. The proposed design of the Sonic project modifies portions of the existing parking infrastructure within Basin-B and diverts a small area of Basin-B to Basin A. Basin A During design, it was found that an additional curb cut located on the north side of the drive-thru lane would better drain the drive thru lane and north patio area due to the existing grades. For this reason, the basin boundary between A and B was modified as shown on the proposed Final Drainage Plan. The small amount of runoff from the drive thru lane and patio will be detained in a small pond in Basin A and released at a rate consistent with the previous designs (0.13cfs). Basin B Basin-B represents the remainder of the site and generally skirts the property boundary. No off site runoff appears to enter this basin and the basin drains to the center of the parking are consistent with the previous design. The report, "Addendum to The Final Storm Drainage Report for JL West" indicates the C-value for Basin B was 0.79. Under the proposed design, the landscaped area was slightly increased resulting in a lower C- value of 0.75. Thus, the peak runoff rates generated by the site are expected to remain similar, but slightly decreased, when compared to the previous design and the existing site conditions. The 100-year historic release rate from the pond in Basin B that was previously established at 0.90 cfs by the approved designs will be maintained. An FAA analysis was completed that revealed a detention volume of approximately 3000 cubic feet of storm water is needed to reduce the developed peak flowrate as previously established. Consistent with the approved design and addendum, parking area storage, not exceeding 0.5 feet in depth, will be utilized to store excess storm water runoff. As previously noted, water quality treatment was not included in the originally approved design and only briefly mentioned in the approved addendum. Under the proposed design, a water quality capture of approximately 1050 cubic feet is required in accordance with USDCM Volume 3. Under the proposed design, two small ponds in the landscape islands will provide 1070 cubic feet of water quality storage space. A water quality outlet structure is proposed in the western pond that will serve to regulate the release of the WQCV over 40 hours in both ponds The outlet structure will connect to the existing storm system via a I RCP pipe tied in at the existing storm manhole south of the renovated building. ;A restrictor'plate will regulate the peak discharge to*the rate previously established resulting in an,anticipated high water elevation of 4957.0. During the. unlikely event of a.pipe clog, excess storm water will spill from the site via the existing spillway, the drive entrance off Lemay, consistent with the previously approved design..: The above -described design maintains the intent of the .previously approved drainage designs and is expected to improve the current stormwater system with the addition of both water quality and detention infrastructure that `furictions as designed. Since the design serves to maintain the'-voluine and-peaLflow=hate of iunoff-leaving the site; no negative impacts are anticipated downstream. Therefore, the design proposed in this letter and corresponding construction documents is assumed adequate to serve proposed renovations. We greatly appreciate your time and consideration in reviewing this project. If you have any questions or require additional information, please contact us at (970) 491-9888. Sincerely, RUNOFF COEFFICIENTS & % IMPERVIOUS LOCATION: Sonic at Riverside and Lemay PROJECT NO: 39649.00 COMPUTATIONS BY: JPF DATE: 811812009 Recommended Runoff Coefficients from Table 3-3 of City of Fort Collins Design Criteria Recommended % Impervious from Urban Storm Drainage Criteria Manual Streets, parking lots (asphalt): Sidewalks (concrete): Roofs: Lawns (flat <2%, sandy soil): Runoff % coefficient Impervious C 0.95 100 0.95 96 0.95 90 0.10 1 0 JR Engineering 2620 E. Prospect Rd., Ste. 190 Fort Collins, CO 8525 SUBBASIN DESIGNATION TOTAL AREA (ac.) TOTAL AREA (sq.ft) ROOF AREA (sq.ft) PAVED AREA (sq.ft) SIDEWALK AREA (sq.ft) LANDSCAPE AREA (sq.ft) RUNOFF COEFF. (C) % Impervious Basin A 0.10 4288.87 0 2236.87 503.29 2052 0.65 63 Basin 0.70 30,416 4,625 17,177 1,561 7,053 0.75 75 Equations - Calculated C coefficients & % Impervious are area weighted C=E(Ci Ai) /At Ci = runoff coefficient for specific area, Ai Ai = areas of surface with runoff coefficient of Ci In = number of different surfaces to consider At = total area over which C is applicable; the sum of all Ai's 3964900flow.xls JR Engineering DETENTION VOLUME CALCULATIONS Rational Volumetric (FAA) Method 100-Year Event LOCATION: SONIC RIVERSIDE AND LEMAY Pgakinll, W PROJECT NO: 39649.00 COMPUTATIONS BY: JPF DATE: 8/ 18/2009 Equations: Area trib. to pond = 0.7 acre Developed flow = Qo = CIA C (100) = 0.75 Vol. In = Vi = T C I A = T Qp Developed C A = 0.5 acre Vol. Out = Vo =K QPo T Release rate, Qp0 = 0.9 cfs storage = S = Vi - Vo K = 0.9 (from fig 2.1) Rainfall intensity from City of Fort Collins IDF Curve with updated (3.67) rainfall Storm Duration, T (min) Rainfall Intensity, I (in/hr) Qo (cfs) Vol. In Vi W) Vol. Out Vo (ft3) Storage S (ft3) Storage S (ac-ft) 5 9.95 5.2 1567 243 1324 0.03 10 7.77 4.1 2446 486 1960 0.05 20 1 5.62 3.0 3541 972 2569 0.06 30 4.47 2.3 4223 1458 2765 0.06 40 3.74 2.0 4712 1944 2768 0.06 50 3.23 1.7 5093 2430 2663 0.06 60 2.86 1.5 5405 2916 2489 0.06 70 2.57 1.3 5669 3402 2267 0.05 80 2.34 1.2 5898 3888 2010 0.05 90 2.15 1.1 6100 4374 1726 0.04 100 1.99 1.0 6282 4860 1422 0.03 110 1.86 1.0 6447 5346 1101 0.03 120 1.75 0.9 6598 5832 766 0.02 130 1.65 0.9 6738 6318 420 0.01 140 1.56 0.8 6867 6804 63 0.00 150 1.48 0.8 6989 7290 -301 -0.01 160 1.41 0.7 7103 7776 -673 -0.02 170 1.35 0.7 7211 8262 -1051 -0.02 180 1.29 0.7 7313 8748 -1435 -0.03 Required Storage Volume: 2768 ft3 0.06 acre-ft detpond.xls,FAA-100yr SOUTH JR Engineering DETENTION VOLUME CALCULATIONS Rational Volumetric (FAA) Method 100-Year Event LOCATION: SONIC RIVERSIDE AND LEMAY - NOArN RA/v PROJECT NO: 39649.00 COMPUTATIONS BY: JPF DATE: 8/ l 8/2009 Equations: Area trib. to pond = 0.1 acre Developed flow = Qp = CIA C (100) = 0.64 Vol. In = Vi = T C I A = T Qo Developed C A = 0.1 acre Vol. Out = Vo =K Qpo T Release rate, Qpo = 0.10 cfs storage = S = Vi - Vo K = 0.9 (from fig 2.1) Rainfall intensity from City of Fort Collins IDF Curve with updated (3.67") rainfall Storm Duration, T (min) Rainfall Intensity, I (in/hr) Qo (cfs) Vol. In Vi (ft31 Vol. Out Vo (ft') Storage S (ft) Storage S (ac-ft) 5 9.95 0.6 191 27 164 0.00 10 7.77 0.5 298 54 244 0.01 20 5.62 0.4 432 108 324 0.01 30 4.47 0.3 515 162 353 0.01 40 3.74 0.2 574 216 358 0.01 50 3.23 0.2 621 270 351 0.01 60 2.86 0.2 659 324 335 0.01 70 2.57 0.2 691 378 313 0.01 80 2.34 0.1 719 432 287 0.01 90 2.15 0.1 744 436 258 0.01 100 1.99 0.1 766 540 226 0.01 110 1.86 0.1 786 594 192 0.00 120 1.75 0.1 804 648 156 0.00 130 1.65 0.1 821 702 119 0.00 140 1.56 0.1 837 756 81 0.00 150 1.48 0.1 852 810 42 0.00 160 1.41 0.1 866 864 2 0.00 170 1.35 0.1 879 918 -39 0.00 180 1.29 0.1 891 972 -81 0.00 Required Storage Volume: 358 ft3 0.01 acre-ft detpond.xls.FAA-100yr NORTH Sonic PARKING and ISLANDS Proposed Detention Pond - Stage/Storage LOCATION: SONIC RIVERSIDE AND LEMAY PROJECT NO: 39649.00 COMPUTATIONS BY: JPF SUBMITTED BY: JR ENGINEERING DATE: 8/ l9/2008 WQCV 100 yr WSEL V = 1 /3 d (A + B + sgrt(A"B)) where V = volume between contours, ft3 d = depth between contours, ft A = surface area of contour PARKING and ISLANDS Stage (ft) Surface Area (t`) Incremental Storage (ft) Total Storage (ft°) 55.1 0 55.5 231 30.03 30.03 56.0 931 270.96 300.99 56.5 1340 564.66 865.64 56.6 2905 207.25 1072.89 57.0 9163 2334.51 3200.15 detpond.xls Sonic NORTH DRIVE THRU Proposed Detention Pond - Stage/Storage LOCATION: SONIC RIVERSIDE AND LEMAY PROJECT NO: 39649.00 COMPUTATIONS BY: JPF SUBMITTED BY: JR ENGINEERING DATE: 8/ 19/2008 V = 1/3 d (A + B + sgrt(A*B)) where V = volume between contours, ft3 d = depth between contours, ft A = surface area of contour NORTH DRIVE THRU Stage A Surface Area (ft') Incremental Storage (W) Total Storage (fe) 56.6 0 57.0 78 10.40 10.40 57.9 2156 793.22 803.62 detpond.xls Design Procedure Form: Extended Detention Basin (EDB) - Sedimentation Facility Sheet 1 of 3 Designer: JPF Company: JR Engineering Date: August 26, 2009 Project: Sonic -Riverside and Lemay Location: 1. Basin Storage Volume la = 73 00 % A) Tributary Area's Imperviousness Ratio (i = I,/ 100 ) i = 0.73 B) Contributing Watershed Area (Area) Area = 0.800 acres C) Water Quality Capture Volume (WQCV) WQCV = 0.29 watershed inches (WQCV =1.0'(0.91'13-1.19-IZ+0.78-1)) D) Design Volume: Vol = (WQCV / 12)' Area' 1.2 Vol = 0.0231 acre-feet 2. Outlet Works A) Outlet Type (Check One) X Orifice Plate Perforated Riser Pipe Other B) Depth at Outlet Above Lowest Perforation (H) H = 1.50 feet C) Recommended Maximum Outlet Area per Row, (A,) Ap = 0.1 square inches D) Perforation Dimensions: i) Circular Perforation Diameter or D = 0.375 inches ii) Width of 2" High Rectangular Perforations W = inches E) Number of Columns (nc, See Table 6a-1 For Maximum) nc = 1 number F) Actual Design Outlet Area per Row (A.) Aa = 0.1 square inches G) Number ofRows (nr) nr= 5 number H) Total Outlet Area (A,r) Am = 0.5 square inches 3. Trash Rack A) Needed Open Area. A, = 0.5 ' (Figure 7 Value)' A„ A, = 18 Squareinches B) Type of Outlet Opening (Check One) X < 2" Diameter Round 2" High Rectangular Other: C) For 2", or Smaller, Round Opening (Ref.: Figure 6a): i) Width of Trash Rack and Concrete Opening (Wwnc) from Table 6a-1 Wca,c = 3 inches ii) Height of Trash Rack Screen (HTR) H, = 48 inches UD-BMP_v2.08.x1s, EDB 8/26/2009, 11:05 AM go M 1,✓.47-Cle aUr4G1n/ FGocJ e0AIM404- - j7FGr0 i�-rnP 7/_l1TG -BPS. I MEAT, 67A L VA �1 � �. IFFL pC.ATr::� TbP = 5&, &O a 115 u..4c�,e 5/ 19 Lq uoN Gram2 55 jo = sue./ v ZESrPIGiDi2 pG14t� loG-A •CZ jT37-.L�cf•9` wsEc= s•7-o 4 C) i23 F=rTz 55. o a(t+ A segn)e rr = Asecaoiz - Air, o jle 7� SeGMPnf = O, A we= Iir2x F} r= 6.5, B = ►J0, t� i� Zz' cl XL 'Z r 110 c