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Home My WebLink AboutRIGDEN FARM FILING NINETEEN - PDP/FDP - FDP150035 - REPORTS - DRAINAGE REPORTFebruary 26, 2016 Mr. Wes Lamarque City of Fort Collins Storm Water Utility 700 Wood Street Fort Collins, CO 80522 Re: Rigden Farm Filing Nineteen (Formerly known as Filing Six Lots M8 & M9 Minor Amendment) ICG Project No. 1233-136-00 Dear Wes, Please accept the following letter report on behalf of Schuman Companies, Inc. to demonstrate the site’s compliance with the previously approved Final Drainage Report for Rigden Farm Filing Six, as prepared by JR Engineering, dated February 8, 2002. INTRODUCTION This proposed development is legally known as Lots M8 & M9 of Rigden Farm Filing Nineteen and contains 10,995 square feet or 0.25 acres and 12,569 square feet or 0.29 acres, respectively. Lot M8 is bounded on the north by William Neal Parkway, the west by Tract BD, the south by Lot 319 and the east by Bryce Drive. Lot M9 is bounded on the north by William Neal Parkway, on the west by Bryce Drive, on the south by Lot 232 and on the east by Tract AS. Lot M8 generally drains to the east where flows are intercepted by existing curb and gutter and conveyed to a storm sewer system that discharges into existing detention pond 269. Lot M9 generally drains to the east and sheet flows directly into existing detention pond 269. The design for this pond was included as part of the Filing Six Drainage Report and was constructed with that Filing. The original runoff coefficient calculations and stage-storage table are attached for reference and all detention and water quality requirements for this site have been met by the overall Ridgen Farm development. EXISTING CONDITIONS The site is currently vacant and contains a stand of grasses. The Overall Drainage Basin Map that was included in the previously approved Final Drainage and Erosion Control Report shows Lot M8 draining into sub-basin 146, the western 30 feet of Lot M9 draining into sub-basin 142, and the remainder of Lot M9 draining into sub-basin 171, as shown on the enclosed map. Mr. Wes Lamarque February 26, 2016 Page 2 of 3 PROPOSED DEVELOPMENT The proposed development continues to direct runoff to the perimeter streets and ultimately to Pond 269 as anticipated in the Filing Six Final Drainage Report. Pond 269 releases into the Fossil Creek Reservoir Inlet Ditch (FCRID). The southern half of Lot M8 will drain south to the proposed flowline of the access drive and then flow east into the flowline of Bryce Drive. The majority of the runoff will be pulled off a series of curb cuts that will direct the runoff to a shallow grass lined swale and then into the flowline of Bryce Drive via a new 2 foot sidewalk chase. This location has been identified as design point #1 on the proposed drainage basin map. The northern half of Lot M8 will drain north toward William Neal Parkway and sheet flow into the flowline of the street. Flows will then be conveyed east to design point 42 of the Rigden Farm Filing Six report where it will be collected by an existing sump inlet and conveyed via storm pipe to Pond 269. The majority of Lot M9 will follow current conditions and drain east toward Tract AS. The western most 25 feet of the development will drain west toward Bryce Drive and sheet flow into the flowline of the street (as designated in the Filing Six drainage basin map). The northern half of Lot M9 will drain north toward William Neal Parkway and sheet flow into the flowline of the street. Similarly to the flows from Lot M8, these flows will be conveyed east to design point 42. The southern half of Lot M9 will flow south to the proposed flowline of the access drive and then flow east into a curb cut in the southeast corner of the site. This location has been identified as design point #2 on the proposed drainage basin map. The concentrated flow at this point will be dissipated and spread out through the use of a cobble dissipater and a section of ScourStop Mat that will prevent erosion immediately downstream of the curb cut. The water will then flow over approximately 50 feet of established turf and finally sheet flow over the existing concrete trail and into Pond 269. Please refer to the attached Drainage and Erosion Control Plan for flow direction. The Filing Six design utilized an “Ultimate Development” condition for the basin that makes up the site. Per the original report, the area within M8 (sub-basin 146) was assumed to be Multi- Family with an imperviousness of 65% and a Runoff Coefficient of 0.65. It is noted that Table 3-3 from the City of Fort Collins Design Criteria Manual currently shows a runoff coefficient of 0.65 and percent impervious of 70% for multi-family area. Per the weighted average runoff calculations, the actual imperviousness of Lot M8 is 62%, with a minor storm Runoff Coefficient of 0.67 as shown on the attached spreadsheet. The small increase in the runoff coefficient of M8 will not have a substantial impact to the overall flow being conveyed to the existing pond. Per the original report, the area within M9 (sub-basin 171) was assumed to be single family development. The single family imperviousness is 55% with a minor storm Runoff Coefficient of 0.65. However, M9 is zoned multi-family and it is our belief that the assumption that all lots in sub-basin 171 were single family was a small oversight in the original drainage report. The actual imperviousness of Lot M9 is 56%, with a minor storm Runoff Coefficient of 0.61 as shown on the attached spreadsheet. Even though the percent impervious is 1% greater than the original assumption, the runoff coefficient is less than 0.65 and this additional impervious area will not have an adverse impact to the function of the existing pond. Mr. Wes Lamarque February 26, 2016 Page 3 of 3 It is requested that the site be deemed in substantial conformance with the previously approved Final Drainage Reports. Please see the attached excerpts from the Filing Six Final Drainage Report detailing the assumptions and original flow calculations for these two sub-basins. LOW IMPACT DEVELOPMENT REQUIREMENTS City Staff has stated that the use of porous pavers is not applicable on the residential driveways providing access to the units on M8 and M9; however, Low Impact Development (LID) criteria should be achieved where readily available. The sites are constrained in both size and relative to storm drain outfall options as there are no storm pipes adjacent to the property that could be used to serve a variety of LID options such as rain gardens. Therefore, he project is meeting the intent of runoff reduction practices through the use of two LID techniques: grass buffers and a grass swale. The southern half of the roof area of M8 in addition to the driveway runoff will be conveyed to a grass swale that runs along the south property line. Runoff from the northern half of the roof area and the access walks for both M8 and M9 will flow over a grass buffer prior to sheet flowing over the sidewalk adjacent to William Neal Parkway. As previously mentioned, runoff from the southern half of the roof area of M9 in addition to the driveway runoff will be conveyed to a curb cut. This concentrated runoff will be spread out through the use of a cobble dissipater and segment of ScourStop Mat and then flow through a previously established grass buffer located in the open space directly east of the site before ultimately entering the regional pond. Please refer to the attached LID Exhibit for a breakdown of the proposed impervious area and LID treatment areas. If you have any questions or comments please contact me at (970) 460-1091. Sincerely, Skylar Brower, P.E. Colorado Professional Engineer 44248 Attachments FF=99.93 GFF=98.60 BFF=90.09 FF=98.93 GFF=97.60 BFF=89.09 FF=97.93 GFF=96.60 BFF=88.09 FF=96.23 GFF=94.90 BFF=85.39 FF=95.73 GFF=94.40 BFF=86.89 FF=96.23 GFF=94.90 BFF=85.39 LOT M8 LOT M9 TRACT AS TRACT BD LOT 319 LOT 232 POND 269 PREPARED FOR PROJ. NO. DATE: SCALE (H): SCALE (V): CHECKED BY: DESIGNED BY: PROJECT NAME RIGDEN FARM FILING NINETEEN LOTS M8 & M9 DRAINAGE & EROSION CONTROL PLAN 2/26/16 1"=20' N/A SB RA 1233-133-00 6 of 8 SCHUMAN COMPANIES, INC. 605 S. COLLEGE AVE, SUITE 100 FORT COLLINS, COLORADO 80524 PHONE: 970.219.1493 0 SCALE: 1" = 20' 20 10 20 40 LEGEND BASIN COEFFICIENT (100-YR) BASIN DESIGNATION BASIN AREA DESIGN POINT DRAINAGE FLOW ARROW DRAINAGE BASIN BOUNDARY SILT FENCE INLET PROTECTION Interwest Consulting Group 1218 West Ash, Suite A Windsor, Colorado LOCATION: Rigden Farm M8 & M9 PROJECT NO: 1233-136-00 COMPUTATIONS BY: SB DATE: 2/26/2016 Recommended Runoff Coefficients from Table 3-3 of City of Fort Collins Design Criteria Recommended % Impervious from Urban Storm Drainage Criteria Manual Runoff % coefficient Impervious C Streets, parking lots (asphalt): 0.95 100 Concrete: 0.95 96 Roofs: 0.95 90 Permeable Pavers: 0.50 40 General multi-family area: 0.65 70 General single family area: 0.65 55 Lawns (flat <2%, sandy soil): 0.10 0 SUBBASIN TOTAL TOTAL PAVED CONCRETE ROOF PAVER 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) M8 0.25 10,995 2,036 322 4,991 0 3,646 0.67 62 M9 0.29 12,569 2,147 390 4,991 0 5,041 0.61 56 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 RUNOFF COEFFICIENTS & % IMPERVIOUS Flow-FINAL - No Pavers.xls Cval Interwest Consulting Group 1218 West Ash, Suite C Windsor, Colorado LOCATION: Rigden Farm M8 & M9 PROJECT NO: 1233-136-00 COMPUTATIONS BY: SB DATE: 2/26/2016 10-yr storm Cf = 1.00 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 POINT (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) M8 0.25 0.67 30 2.0 3.4 115 1.00 0.013 2.5 0.8 4.2 145 10.8 5.0 M9 0.29 0.61 30 2.0 3.8 98 1.00 0.013 2.5 0.7 4.5 128 10.7 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 STANDARD FORM SF-2 TIME OF CONCENTRATION - 10 YEAR Flow-FINAL - No Pavers.xls TOC-10 Interwest Consulting Group 1218 West Ash, Suite C Windsor, Colorado LOCATION: Rigden Farm M8 & M9 PROJECT NO: 1233-136-00 COMPUTATIONS BY: SB DATE: 2/26/2016 100-yr storm Cf = 1.25 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 POINT (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) 1 M8 0.25 0.67 0.84 30 2.0 2.1 115 1.00 0.013 2.5 0.8 2.8 145 10.8 5.0 2 M9 0.29 0.61 0.76 30 2.0 2.7 98 1.00 0.013 2.5 0.7 3.3 128 10.7 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 TIME OF CONCENTRATION - 100 YR STANDARD FORM SF-2 Flow-FINAL - No Pavers.xls TOC-100 Interwest Consulting Group 1218 West Ash, Suite C Windsor, Colorado RATIONAL METHOD PEAK RUNOFF (2-YEAR) LOCATION: Rigden Farm M8 & M9 PROJECT NO: 1233-136-00 COMPUTATIONS BY: SB DATE: 2/26/2016 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 (new) Design Point (ac) (min) (in/hr) (cfs) Point (cfs) (cfs) 1 M8 0.25 0.67 5.0 2.85 0.5 2 M9 0.29 0.61 5.0 2.85 0.5 Q = Cf C iA Q = peak discharge (cfs) C = runoff coefficient Cf = frequency adjustment factor I = rainfall intensity (in/hr) from IDF curve A = drainage area (acres) NOTE: Intensity based on Figure 3-1, 1999 (Regional Analysis) Flow-FINAL - No Pavers.xls Q2 Interwest Consulting Group 1218 West Ash, Suite C Windsor, Colorado RATIONAL METHOD PEAK RUNOFF (10-YEAR) LOCATION: Rigden Farm M8 & M9 PROJECT NO: 1233-136-00 COMPUTATIONS BY: SB DATE: 2/26/2016 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 (new) Design Point (ac) (min) (in/hr) (cfs) Point (cfs) (cfs) 1 M8 0.25 0.67 5.0 4.87 0.8 0.8 2 M9 0.29 0.61 5.0 4.87 0.9 0.9 Q = Cf C iA Q = peak discharge (cfs) C = runoff coefficient Cf = frequency adjustment factor I = rainfall intensity (in/hr) from IDF curve A = drainage area (acres) NOTE: Intensity based on Figure 3-1, 1999 (Regional Analysis) Flow-FINAL - No Pavers.xls Q10 Interwest Consulting Group 1218 West Ash, Suite C Windsor, Colorado RATIONAL METHOD PEAK RUNOFF (100-YEAR) LOCATION: Rigden Farm M8 & M9 PROJECT NO: 1233-136-00 COMPUTATIONS BY: SB DATE: 2/26/2016 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 (new) Design Point Design. (ac) (min) (in/hr) (cfs) Point (cfs) (cfs) 1 M8 0.25 0.84 5.0 9.95 2.1 2.1 2 M9 0.29 0.76 5.0 9.95 2.2 2.2 Q = C iA Q = peak discharge (cfs) C = runoff coefficient I = rainfall intensity (in/hr) from IDF curve A = drainage area (acres) NOTE: Intensity based on Figure 3-1, 1999 (Regional Analysis) Flow-FINAL - No Pavers.xls Q100 SUMMARY DRAINAGE SUMMARY TABLE Design Tributary Area C (2) C (10) C (100) tc (2) tc (10) tc (100) Q(2)tot Q(10)tot Q(100)tot DRAINAGE Sub-basin STRUCTURE Point (ac) (min) (min) (min) (cfs) (cfs) (cfs) /REMARKS M8 0.25 0.67 0.67 0.84 5.0 5.0 5.0 0.5 0.8 2.1 M9 0.29 0.61 0.61 0.76 5.0 5.0 5.0 0.5 0.9 2.2 Page 7 RIGDEN FARM M8 & M9 LID AREA EXHIBIT DATE: 02/26/16 CONSTRUCTION ENTRANCE W/ VEHICLE TRACKING CONTROL GRAVEL BAG CONCRETE WASHOUT AREA NOTES: 1. CONCRETE WASHOUT LOCATION TO BE DETERMINED IN THE FIELD. 2. CONTRACTOR TO ESTABLISH PERIMETER PROTECTION ALONG THE LIMITS OF DISTURBANCE TO LIMIT ACCESS TO THE ACTIVE CONSTRUCTION AREA. CONTRACTOR TO LIMIT SEDIMENT FROM ENTERING THE PUBLIC RIGHT OF WAY THROUGH THE USE OF A VEHICLE TRACKING PAD OR OTHER APPROVED METHOD. 3. THE DOWNSTREAM INLET ON THE SOUTH SIDE OF WILLIAM NEAL PARKWAY JUST WEST OF THE ZIEGLER INTERSECTION MAY BE REQUIRED TO RECEIVE INLET PROTECTION IF GRAVEL BAG AT PROPERTY LINE IS NOT ADEQUATELY MAINTAINED. 4. THE SITE CURRENTLY HAS EXISTING PERIMETER SILT FENCE INSTALLED. CONTRACTOR TO INSTALL NEW SILT FENCE AS NEEDED AND/OR MAINTAIN THE EXISTING SILT FENCE IN PLACE.