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Home My WebLink AboutWATERGLEN SOLAR ARRAY - PDP190010 - SUBMITTAL DOCUMENTS - ROUND 4 - DRAINAGE REPORTJJ Addendum to Final Drainage and Erosion Control Report Waterglen Solar Array Fort Collins, Colorado May 5, 2020 Prepared for: Sandbox Solar 430 N College Ave, Suite 435 Fort Collins, Colorado 80524 Prepared by: 301 N. Howes Street Suite 100 Fort Collins, Colorado 80521 Phone: 970.221.4158 www.northernengineering.com Project Number: 1628-001 Drainage Addendum To Final Drainage and Erosion Control Report for Waterglen P.U.D. Date: May 5, 2020 Project: Waterglen Solar Array Project No. 1628-001 Preliminary Development Plan (PDP) Fort Collins, Colorado Attn: City of Fort Collins Stormwater Utility 700 Wood Street Fort Collins, Colorado 80521 To whom it may concern: This letter serves to address the stormwater impacts of the proposed project known as “Waterglen Solar Array”. The site is located in northeast Fort Collins near East Vine Drive and Interstate I-25. More specifically, the site is bounded to the north and west by the Larimar-Weld Canal and to the south and east by the Waterglen P.U.D. open space “Tract A” and Rabbit Brush Park. The project site is also within “Tract A” of Waterglen P.U.D., which was approved in 2001 and included in the drainage study by JR Engineering for that project. The proposed solar array site is approximately 37,569 square feet (0.74 acres) in area. In addition to the solar arrays, the project includes an emergency access road. Existing Conditions The site is within open space for the Waterglen PUD. The site is adjacent to the Larimer-Weld Canal and the existing gravel access road along the south side of the canal. The existing topography of the proposed site area slopes to the east at approximately 1% to 4%. The onsite soils are Nunn clay loam and Hydraulic Soil Group C, per the USGS soils survey maps. The existing vegetation within the site area is native grass at about 80% ground cover. The existing canal access road is gravel, and the outer edge of the access is native grass and weeds at about 40% ground cover. Original Drainage Patterns The original JR Engineering drainage design included “Tract A”, which encompasses the proposed solar array site, within Basin E-8. Basin E-8 is 7.19 acres and generally drains from the north to the south. The basin drains to two existing 3’x10’ box culverts designated as Design Point 34. The box culverts were sized for the 100-year flow of 601.9 cfs. Drainage flows from Basin E-8 continue to the south to Pond C of the Waterglen P.U.D development. Site runoff will then discharge from Pond C into the Coopers Slough drainage. A Drainage Exhibit, and excerpts from the original report, is provided at the back of this report for reference. Proposed Drainage Patterns The Waterglen Solar Array project has proposed no change to any drainage patterns or surface slopes. Site Imperviousness Changes A percent impervious exhibit was developed to calculate and compare the existing vs proposed site area conditions. Using this approach, we find the proposed site conditions have a negligible increase on the Drainage Report April 1, 2020 Sand Box Solar Page 2 of 3 composite percent impervious within Basin E-8. The total increase of impervious area, due to the site’s improvements, is 2,445 square feet. Runoff The approved JR Engineering report showed a 100-yr flow of 601.9 cfs at Design Point 34. According to the JR Engineering report, the 601.9 cfs includes overflow from the Larimer Weld Canal. The impervious area from the site’s improvements will provide an additional 0.06 cfs at Design Point 34. The additional 0.06 cfs results in a negligible rise in both Swale T-T and the box culvert at Design Point 34. Due to the minimal change, the existing storm drainage facilities will continue to operate as they did prior to the project’s proposed improvements. Detention The proposed solar array and emergency access road results in a negligible increase in the depth of Pond C. According to the approved JR Engineering report, the total depth of the 100-Year Storm Event in Pond C is 3.5 feet. The additional 0.06 cfs from the project will result in a total depth of the 100-Year Storm Event in Pond C of 3.5 feet. Water Quality & LID An existing wetland, 150’ grassland buffer, and a 277 ft wide grass swale is between the eastern edge of the solar array project and Design Point 34. The existing coverage consists of well-established native grasses with over 90% coverage. This large, stabilized area will provide water quality treatment as runoff passes over the vegetation. The project will not disturb the existing vegetation outside of the solar envelopes, security fence, and driveway platform. Erosion and Sediment Control During construction, the Contractor will follow the City of Fort Collins standards for erosion and sediment control. The total disturbance for the solar array project includes 32,695 square-feet of native topsoil and seeding, plus 4,858 square-feet CDOT class 5 or 6 road base for the emergency access road improvements. Since more than 10,000 square feet will be disturbed as a part of this project, an erosion control plan and escrow has been provided. Since the project area is less than an acre, a comprehensive Stormwater Management Plan is not required. Post construction water quality and erosion control will be achieved by a fully established final landscape to stabilize the site and existing downstream stormwater facilities. All areas disturbed during construction will receive permanent hardscape or landscape. Floodplains This project site is within the floodplain in zone AE on FEMA FIRM map 08069C0982F dated 12/19/2006 and updated by LOMR 17-08-1354P effective 2/21/2019. A floodplain use permit will be required for the project. An as-built survey is required to document the required flood elevation protection is satisfied. The solar array is located outside the 100-year floodway. The solar array is also outside the drainage conveyance channels and detention pond limits outlined in the Final Drainage & Erosion Control Report - Waterglen PUD, by JR Engineering, dated 11/6/1998. There is to be no grading, construction activity or storage of materials proposed in the floodway. All equipment is a minimum of 1-foot above the base flood elevation of 4975.49’. A post-construction elevation certificate must be approved before the solar array is operational. Drainage Report April 1, 2020 Sand Box Solar Page 2 of 3 Project Summary and Conclusions The proposed Waterglen Solar Array site maintains the original drainage paths and results in a negligible increase to downstream improvements. Because the increased flows are not significant no additional drainage improvements are required for this solar project. The existing drainage improvements provided by the original Final Drainage & Erosion Control Report - Waterglen PUD, BY JR Engineering, dated 11/6/1998 will not need any modifications or increase in detention. Therefore, Waterglen Solar Array will not have any negative impact on the downstream drainage infrastructure. Please do not hesitate to contact me if you have questions or require additional information. Sincerely, Northern Engineering Services, Inc. Frederick S. Wegert, PE Project Engineer ATTACHMENT 1 PREVIOUS REPORT – FINAL DRAINAGE AND EROSION CONTROL REPORT – WATERGLEN, P.U.D. By JR Engineering, Ltd. Dated August 11, 1997 (revised November 6, 1998) ATTACHMENT 2 DRAINAGE CALCULATIONS & DRAINAGE EXHIBITS Project: Calc. By: Date: Description Surface Area (ft2) Percent Impervious Impervious Area ( ft2) Solar Array - 0% - Concrete - 100% - Gravel - 50% - Greenbelt/Landscaping 39,870 0% - Total 39,870 - 0.0% Description Surface Area (ft2) Percent Impervious Impervious Area ( ft2) Solar Array 34,149 0% - Concrete 16 100% 16 Gravel 5,705 50% 2,853 Greenbelt/Landscaping - 0% - Total 39,870 2,869 7.2% Description Surface Area (ft2) Percent Impervious Impervious Area ( ft2) Existing 0 50% - Developed 5,721 50% 2,869 2,869 Description C2 C10 I2 in/hr I10 in/hr Q2 (cfs) Q100 (cfs) Existing 0.50 0.63 1.95 6.82 - - Developed 0.50 0.63 1.95 6.82 0.06 0.28 0.06 0.28 Impervious Area & Flowrate Waterglen Solar Array F. Wegert May 6, 2020 Weighted Percent Impervious Weighted Percent Impervious Change in Flowrate (cfs) Flowrate from Impervious Area Developed Impervious Areas Existing Impervious Areas Change in Impervious Area Net Change in Impervious Area Existing Q2 (cfs) Depth at Culvert (ft) Design Q2 (cfs) Depth at Culvert (ft) Existing Q2 (cfs) Depth at Culvert (ft) Design Q2 (cfs) Depth at Culvert (ft) 3.90 0.11 3.96 0.11 601.9 3.00 602.18 3.00 3.90 0.19 3.96 0.19 601.9 1.38 602.18 1.38 601.9 3.50 41,758 602.18 3.51 Water Surface Area (ft2) Design Q100 (cfs) Depth of Pond (ft) Q100 w/ Road (cfs) Depth of Pond (ft) Pond C 100-Year Storm Event 2 3 ft x 10 ft Box Culvert at DP 34 Hydraulic Impacts to Waterglen Subdivison Swale T-T Upstream from DP 34 Existing 2-Year Storm Event 100-Year Storm Event Q2 (cfs) Depth in Swale(ft) Q2 (cfs) Depth in Swale (ft) Existing Q2 (cfs) Depth in Swale(ft) Design Q2 (cfs) Depth in Swale (ft) Existing Developed Existing Developed Developed Existing Developed 2-Year Storm Event 100-Year Storm Event D S S S / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / Channel Report Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc. Friday, Mar 20 2020 2 3-ft x 10-ft Ex Box Culvert (Historic - 2-Year) Rectangular Bottom Width (ft) = 20.00 Total Depth (ft) = 3.00 Invert Elev (ft) = 100.00 Slope (%) = 0.41 N-Value = 0.013 Calculations Compute by: Known Q Known Q (cfs) = 3.90 Highlighted Depth (ft) = 0.12 Q (cfs) = 3.900 Area (sqft) = 2.40 Velocity (ft/s) = 1.63 Wetted Perim (ft) = 20.24 Crit Depth, Yc (ft) = 0.11 Top Width (ft) = 20.00 EGL (ft) = 0.16 0 2 4 6 8 10 12 14 16 18 20 22 24 Elev (ft) Depth (ft) Section 99.00 -1.00 100.00 0.00 101.00 1.00 102.00 2.00 103.00 3.00 104.00 4.00 Reach (ft) Channel Report Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc. Wednesday, May 6 2020 2 3-ft x 10-ft Ex Box Culvert (Developed - 2-Year) Rectangular Bottom Width (ft) = 20.00 Total Depth (ft) = 3.00 Invert Elev (ft) = 100.00 Slope (%) = 0.41 N-Value = 0.013 Calculations Compute by: Known Q Known Q (cfs) = 3.96 Highlighted Depth (ft) = 0.12 Q (cfs) = 3.960 Area (sqft) = 2.40 Velocity (ft/s) = 1.65 Wetted Perim (ft) = 20.24 Crit Depth, Yc (ft) = 0.11 Top Width (ft) = 20.00 EGL (ft) = 0.16 0 2 4 6 8 10 12 14 16 18 20 22 24 Elev (ft) Depth (ft) Section 99.00 -1.00 100.00 0.00 101.00 1.00 102.00 2.00 103.00 3.00 104.00 4.00 Reach (ft) Channel Report Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc. Friday, Mar 20 2020 2 3-ft x 10-ft Ex Box Culvert (Historic - 100-Year) Rectangular Bottom Width (ft) = 20.00 Total Depth (ft) = 3.00 Invert Elev (ft) = 100.00 Slope (%) = 0.41 N-Value = 0.013 Calculations Compute by: Known Q Known Q (cfs) = 601.90 Highlighted Depth (ft) = 2.56 Q (cfs) = 601.90 Area (sqft) = 51.20 Velocity (ft/s) = 11.76 Wetted Perim (ft) = 25.12 Crit Depth, Yc (ft) = 3.00 Top Width (ft) = 20.00 EGL (ft) = 4.71 0 2 4 6 8 10 12 14 16 18 20 22 24 Elev (ft) Depth (ft) Section 99.00 -1.00 100.00 0.00 101.00 1.00 102.00 2.00 103.00 3.00 104.00 4.00 Reach (ft) Channel Report Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc. Wednesday, May 6 2020 2 3-ft x 10-ft Ex Box Culvert (Developed - 100-Year) Rectangular Bottom Width (ft) = 20.00 Total Depth (ft) = 3.00 Invert Elev (ft) = 100.00 Slope (%) = 0.41 N-Value = 0.013 Calculations Compute by: Known Q Known Q (cfs) = 602.18 Highlighted Depth (ft) = 2.56 Q (cfs) = 602.18 Area (sqft) = 51.20 Velocity (ft/s) = 11.76 Wetted Perim (ft) = 25.12 Crit Depth, Yc (ft) = 3.00 Top Width (ft) = 20.00 EGL (ft) = 4.71 0 2 4 6 8 10 12 14 16 18 20 22 24 Elev (ft) Depth (ft) Section 99.00 -1.00 100.00 0.00 101.00 1.00 102.00 2.00 103.00 3.00 104.00 4.00 Reach (ft) Channel Report Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc. Friday, Mar 20 2020 Swale T-T (Historic - 2-Year) Triangular Side Slopes (z:1) = 85.00, 50.00 Total Depth (ft) = 2.00 Invert Elev (ft) = 100.00 Slope (%) = 1.49 N-Value = 0.030 Calculations Compute by: Known Q Known Q (cfs) = 3.90 Highlighted Depth (ft) = 0.21 Q (cfs) = 3.900 Area (sqft) = 2.98 Velocity (ft/s) = 1.31 Wetted Perim (ft) = 28.35 Crit Depth, Yc (ft) = 0.19 Top Width (ft) = 28.35 EGL (ft) = 0.24 0 50 100 150 200 250 300 350 400 Elev (ft) Depth (ft) Section 99.50 -0.50 100.00 0.00 100.50 0.50 101.00 1.00 101.50 1.50 102.00 2.00 102.50 2.50 103.00 3.00 Reach (ft) Channel Report Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc. Wednesday, May 6 2020 Swale T-T (Developed - 2-Year) Triangular Side Slopes (z:1) = 85.00, 50.00 Total Depth (ft) = 2.00 Invert Elev (ft) = 100.00 Slope (%) = 1.49 N-Value = 0.030 Calculations Compute by: Known Q Known Q (cfs) = 3.96 Highlighted Depth (ft) = 0.21 Q (cfs) = 3.960 Area (sqft) = 2.98 Velocity (ft/s) = 1.33 Wetted Perim (ft) = 28.35 Crit Depth, Yc (ft) = 0.19 Top Width (ft) = 28.35 EGL (ft) = 0.24 0 50 100 150 200 250 300 350 400 Elev (ft) Depth (ft) Section 99.50 -0.50 100.00 0.00 100.50 0.50 101.00 1.00 101.50 1.50 102.00 2.00 102.50 2.50 103.00 3.00 Reach (ft) Channel Report Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc. Friday, Mar 20 2020 Swale T-T (Historic - 100-Year) Triangular Side Slopes (z:1) = 85.00, 50.00 Total Depth (ft) = 2.00 Invert Elev (ft) = 100.00 Slope (%) = 1.49 N-Value = 0.030 Calculations Compute by: Known Q Known Q (cfs) = 601.90 Highlighted Depth (ft) = 1.38 Q (cfs) = 601.90 Area (sqft) = 128.55 Velocity (ft/s) = 4.68 Wetted Perim (ft) = 186.32 Crit Depth, Yc (ft) = 1.38 Top Width (ft) = 186.30 EGL (ft) = 1.72 0 50 100 150 200 250 300 350 400 Elev (ft) Depth (ft) Section 99.50 -0.50 100.00 0.00 100.50 0.50 101.00 1.00 101.50 1.50 102.00 2.00 102.50 2.50 103.00 3.00 Reach (ft) Channel Report Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc. Wednesday, May 6 2020 Swale T-T (Developed - 100-Year) Triangular Side Slopes (z:1) = 85.00, 50.00 Total Depth (ft) = 2.00 Invert Elev (ft) = 100.00 Slope (%) = 1.49 N-Value = 0.030 Calculations Compute by: Known Q Known Q (cfs) = 602.18 Highlighted Depth (ft) = 1.38 Q (cfs) = 602.18 Area (sqft) = 128.55 Velocity (ft/s) = 4.68 Wetted Perim (ft) = 186.32 Crit Depth, Yc (ft) = 1.38 Top Width (ft) = 186.30 EGL (ft) = 1.72 0 50 100 150 200 250 300 350 400 Elev (ft) Depth (ft) Section 99.50 -0.50 100.00 0.00 100.50 0.50 101.00 1.00 101.50 1.50 102.00 2.00 102.50 2.50 103.00 3.00 Reach (ft) / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / D S S S / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / X GREENBELT LANDSCAPE CONCRETE GRAVEL SURFACE AREA (SF) % IMPERV. IMPERV. AREA (SF) 39,870 0 0 0% 100% 50% TOTALS 39,870 ASPHALT CONCRETE GRAVEL SURFACE AREA (SF) % IMPERV. IMPERV. AREA (SF) 0 16 5,705 TOTALS 39,870 EXISTING PROPOSED 34,149 100% 100% 50% 0 0% 0 0 0 16 0 2,853 0 2,869 DRAWN BY: SCALE: ISSUED: EXISTING VS. PROPOSED IMPERVIOUS AREA EXHIBIT SHEET NO: FORT COLLINS: 301 North Howes Street, Suite 100, 80521 GREELEY: 820 8th Street, 80631 ENGINEER ING N O R T H E RN 970.221.4158 northernengineering.com P:\1628-001\DWG\DRNG\1628-001_IMPERVIOUS.DWG WATERGLEN SOLAR ARRAY FORT COLLINS, COLORADO F. Wegert 1" = 100' May 6, 2020 1 OF 1 ASPHALT 0 100% 0 GREENBELT LANDSCAPE