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HomeMy WebLinkAboutHOMESTEAD AT CLARENDON HILLS - PDP190007 - DOCUMENT MARKUPS - ROUND 4 - DRAINAGE REPORTAugust 14, 2020 DRAINAGE LETTER AND LID REPORT HOMESTEAD AT CLARENDON HILLS Fort Collins, Colorado Prepared for: Kenneth R. Mitchell Mosaic Real Estate LLC 1021 Nightingale Drive Fort Collins, Colorado. 80525 Prepared by: 301 North Mason Street, Suite 100 Fort Collins, Colorado 80524 Phone: 970.221.4158 www.northernengineering.com Project Number: 1013-008 Drainage Letter and LID Report – Homestead at Clarendon Hills Date: August 14, 2020 Project: Homestead at Clarendon Hills Project No. 1013-008 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 “Homestead at Clarendon Hills”. The site is located in southwest Fort Collins bounded to the north by Front Range Community College, to the east by existing Clarendon Hills Subdivision, to the south by Clarendon Hills Drive and Langdale Drive, and to the west by South Shields Street. The project is part of the Drainage Report for Clarendon Hills by Stuart and Associates written in 1986. The proposed development consists of 8 low-density single-family lots and a detention area. The project site also contains an existing irrigation ditch along the north property line. The project was included in the drainage report for Clarendon Hills Subdivision. Therefore, this letter intends to show conformance with the approved drainage design, including general drainage patterns and imperviousness. This letter also documents how the project meets new Low Impact Development requirements implemented City of Fort Collins since the approval of the original project. Original Drainage Patterns The original Clarendon Hill drainage design divided the subdivision into 3 basins. The current project site is split by two of these basins - Drainage Basin A draining to the north, and Drainage Basin B draining to the south. Drainage Basin B flows into Clarendon Hills Drive and Langdale Drive and travels via gutter and pan flow south along Clarendon Hills Drive. The flow is then collected by inlets at Clarendon Hills Drive and Hinsdale Drive and conveyed to a series of existing detention ponds. Drainage Basin A generally flows to the northeast to an existing irrigation ditch on the property. A small undisturbed area north of the irrigation ditch will flow into the Front Range Community College drainage system. From there, it flows through existing drainage ways east to Mail Creek, and eventually to Fossil Creek. An Existing Drainage Exhibit and excerpts from the original report are provided in the appendices for reference. Page | 3 Proposed Drainage Patterns The Homestead at Clarendon Hills project was divided into seven proposed drainage basins that essentially follow the established drainage patterns, as well as the drainage patterns approved with the Drainage Report for Clarendon Hills. Basin A encompasses the rear of the lots and open space between the rear lot lines and the existing irrigation ditch. The only proposed improvements with Basin A are landscaping in the rear of the lots. Flows from the basin will drain north into the irrigation ditch. Basin A is consistent with Basin A from the Drainage Report for Clarendon Hills. Basin B will roughly match the existing area draining south to the existing Clarendon Hills ponds. Proposed improvements will include single-family homes, the front and side of the lots, driveways, walks, and detention area. All proposed roof drains will drain south to the proposed detention area. Basin B is further subdivided into Basins B1, B2, B3, and B4. Basins B1 and B3 consists of the two lots (Lots 1 and 8) at the west and east ends, and Basins B1 and B3 will discharge into the Clarendon Hills and Langdon Drives curb and gutter. Basin B2 consists of the Lots 2 through 7 and driveways and discharge into the detention pond. Basin B4 consists of the landscaping along the front of the pond that is incapable of draining into the detention pond. The release rate from the detention pond is reduced to accommodate the undetained releases from Basins B1, B3, and B4. An exhibit calculating the release rate is included in the appendices. Additionally, Basin OS1 and OS2 are on the east and the north sides of the site to quantify flows to the Front Range Community College campus. Basin OS1 consists of the side and rear of Lot 8 and adjacent open space. Basin OS2 is a small sliver of open space on the north side of the irrigation ditch that drains directly to the Front Range Community College campus. Development of Existing Runoff Coefficient The drainage for Clarendon Hills Subdivision was originally analyzed in the Drainage Report: Clarendon Hills: Fort Collins, Colorado prepared by Stuart and Associates in 1986 as part of the approval process for Clarendon Hills 1st Filing. Homestead for Clarendon Hills is in the northwest corner of the original drainage master plan for Clarendon Hills 1st Filing. In the approved original drainage report for Clarendon Hills (1st Filing), the project site was planned as office and multi-family with a developed runoff coefficient of C=0.50. According to the approved drainage report for Clarendon Hills 4th Filing, prepared by Land Development Services and dated April 9, 1991, a church site of 8.27 acres in the middle of the Clarendon Hills 4th Filing was assumed to be an offsite undeveloped drainage basin with a developed runoff coefficient of C=0.25. According to the approved drainage report for Clarendon Hills 6th Filing, prepared by Land Development Services and dated January 8, 1992, the original developer of Clarendon Hills swapped the northwest corner of Clarendon Hills (including Homestead for Clarendon Hills) for the church site in Clarendon Hills 4th Filing. The old church site was then developed as Clarendon Hills 6th Filing with runoff coefficients from 0.28 to 0.70. Page | 4 Because of the subsequent drainage studies and land swaps, we assumed an existing runoff coefficient of C=0.25 for Homestead at Clarendon Hills. A runoff coefficient of 0.25 is consistent with the following design considerations: • The Fort Collins Stormwater Criteria Manual for lawns with clayey soil and an average slope of 2% to 7% requires a runoff coefficient of C = 0.25. The existing slopes throughout the site is between 2% to 7% with clayey soil. • The drainage report for Clarendon Hills 4th Filing assumed a runoff coefficient C=0.25 for the church site, and the church site was subsequently part of a land swap for the northwest corner of the Clarendon Hills Subdivision. Detention Per the Clarendon Hills 1st Filing Drainage Report, “The ponds were designed using a mass diagram that required 10.46 acre-feet to detain the 100-year developed storm and release at a 2-year historic rate…”. Assuming a historic runoff coefficient of 0.25, detention from Basin B was provided as long as the overall flow at Design Point B does not exceed 1.9 cfs. Because of the increased impervious area within Basin B, detention is required within Basin B2 to reduce the post-developed flow rates at Design Point B to equal or less than 1.9 cfs. Since Basins B1, B3, and B4 will be released into the surrounding curb and gutter undetained, the detention pond in Basin B2 is designed to release the 100-year event at 0.3 cfs. A release rate of 0.3 cfs is required to maintain a 100-year flow rate of 1.9 cfs at Design Point B. (See the included flowrate calculations included in the appendices.) Inlets By limiting the 100-year flowrate at Design Point B, the flowrate at the existing inlets will match or decrease from the projected flowrates in the previously approved Clarendon Hills drainage reports. This results in matching or decreasing runoff to the existing inlets that were designed and approved with the earlier project, so no additional inlet calculations have been provided as a part of this report. Water Quality/LID Conformance City LID requirements specify that 50% of all single-family areas and 75% of all new impervious area receive water quality treatment from a LID facility. This project proposes to treat 79.8% of the new impervious areas through two rain gardens and a grass buffer. The rain gardens will also provide water quality treatment for the project site. A vegetated buffer will be located between the rear lot lines and the irrigation ditch. The ditch access road limits the available native land to be used as a vegetated buffer. An exhibit showing the area of LID treatment is included in the appendices. Although exact impervious area is unavailable at this preliminary stage, the majority of the residential area will drain towards the rain gardens or across the vegetated buffer. The front 2/3rds of the lot will be graded to drain south towards the rain gardens. The rear 1/3rd of the lot will be graded to drain towards the vegetated buffer. The high point along each lot line is set to maximize as much of the lot that drains towards the rain gardens as possible. Space limitations on the detention pond and rain gardens, natural buffer offsets from the ditch, and fire accessibility requirements restrict the drainage towards the front. Even with the grading restrictions, the proposed flowrate into the Mail Creek Basin is 0.8 cfs less than the existing flowrate. Page | 5 Erosion and Sediment Control During construction, the Contractor will follow the appropriate and applicable City of Fort Collins standards for erosion and sediment control. Since more than 10,000 sf will be disturbed as a part of this project, a comprehensive Stormwater Management Plan will be prepared for this project at final design. Post construction water quality and erosion control will be achieved by a fully established and stabilized site. All areas disturbed during construction will receive permanent hardscape, landscape, or building structure. Floodplains There are no regulatory floodplains associated with the project. Conclusions The proposed grading concept matches the original drainage patterns. The site matches the designed imperviousness draining to the existing Clarendon Hills curb and gutter. Onsite detention will be provided for the increased imperviousness of the area draining to towards the existing Clarendon Hills ponds, and onsite detention will be required to maintain historic flow rates within Clarendon Hills. Stormwater quality has been provided and meets the city requirements for Low Impact Development treatment. Therefore, it is my professional opinion that Homestead at Clarendon Hills satisfies all applicable stormwater criteria and will effectively limit potential damage associated with its stormwater runoff. Please do not hesitate to contact me if you have questions or require additional information. Sincerely, Frederick S. Wegert, PE Project Engineer Project: Calculations By: Date: C2*Cf C10*Cf C100*Cf Length (ft) Elev Up Elev Down Slope (%) Ti 2-Yr (min) Ti 10-Yr (min) Ti 100-Yr (min) Length (ft) Elev Up Elev Down Slope (%) Velocity (ft/s) Tt (min) Length (ft) Elev Up Elev Down Slope (%) Velocity (ft/s) Tt (min) Tc 2-Yr (min) Tc 10-Yr (min) Tc 100-Yr (min) ex a EX A 0.25 0.25 0.31 65 93.83 88.55 8.12% 6.4 6.4 5.9 N/A N/A N/A 665 88.55 85.49 0.46% 0.10 108.93 14.06 14.06 14.06 ex b EX B 0.25 0.25 0.31 30 93.00 91.75 4.17% 5.4 5.4 5.0 340 91.75 84.93 2.01 2.83 2.00 N/A N/A N/A 7.41 7.41 7.01 ex os1 EX OS1 0.25 0.25 0.31 195 93.00 86.13 3.52% 14.6 14.6 13.5 N/A N/A N/A N/A N/A N/A 11.08 11.08 11.08 ex os2 EX OS2 0.25 0.25 0.31 45 92.23 88.81 7.60% 5.4 5.4 5.0 N/A N/A N/A N/A N/A N/A 5.42 5.42 5.02 ex a North 0.25 0.25 0.31 65 93.83 88.55 8.12% 6.4 6.4 5.9 N/A N/A N/A 665 88.55 85.49 0.46% 0.10 108.93 14.06 14.06 14.06 Gutter/Swale Flow, Time of Concentration: Tc2 Tc10 Tc100 C2 C10 C100 I2 (in/hr) I10 (in/hr) I100 (in/hr) Q2 (cfs) Q10 (cfs) Q100 (cfs) ex a EX A 2.382 14.06 14.06 14.06 0.25 0.25 0.31 1.92 3.29 6.71 1.1 2.0 5.0 ex b EX B 0.684 7.41 7.41 7.01 0.25 0.25 0.31 2.52 4.31 8.80 0.4 0.7 1.9 ex os1 EX OS1 0.147 11.08 11.08 11.08 0.25 0.25 0.31 2.13 3.63 7.42 0.1 0.1 0.3 ex os2 EX OS2 0.064 5.42 5.42 5.02 0.25 0.25 0.31 2.85 4.87 9.95 0.0 0.1 0.2 ex a North 2.594 14.06 14.06 14.06 0.25 0.25 0.31 1.92 3.29 6.71 1.2 2.1 5.4 Homestead at Clarendon F. Wegert August 5, 2020 Project: Gutter/Swale Flow, Time of Concentration: Calculations By: Date: Combined and Routed Basins HISTORIC DIRECT RUNOFF COMPUTATIONS Intensity Flow Overland Flow, Time of Concentration: Intensity, I from Fig. RA-2 Rational Equation: Q = CiA (Equation 6-1 per MHFD) Design Point Basin Area (acres) Tc (Min) Runoff C Tt = L / 60V (Equation 6-4 per MHFD) Tc = Ti + Tt (Equation 6-2 per MHFD) Velocity (Gutter Flow), V = 20·S½ Velocity (Swale Flow), V = 15·S½ (Equation 6-4 per MHFD) (Equation 6-3 per MHFD) } 𝑇𝑖 = 1.87 1.1 − 𝐶 ∗ 𝐶𝑓 𝐿 𝑆 ൗ 1 3 Page 3 of 3 Character of Surface Runoff Coefficient1 Percent Impervious1 0.95 100% 0.95 90% 0.50 40% 0.55 50% 0.20 2% 0.25 2% Basin ID Basin Area (sq.ft.) Basin Area (acres) Asphalt, Concrete (acres) Rooftop (acres) Gravel (acres) Residential: Low Density (acres) Undeveloped: Greenbelts, Agriculture (acres) Lawns, Clayey Soil, Avg Slope 2- 7% (acres) Percent Impervious C2*Cf Cf = 1.00 C5*Cf Cf = 1.00 C10*Cf Cf = 1.00 C100*Cf Cf = 1.25 A 67,789 1.556 0.000 0.000 0.176 0.338 1.043 0.000 17% 0.31 0.31 0.31 0.39 B1 7,111 0.163 0.000 0.000 0.012 0.123 0.000 0.029 41% 0.49 0.49 0.49 0.62 B2 52,848 1.213 0.149 0.000 0.000 0.902 0.000 0.162 50% 0.56 0.56 0.56 0.70 B3 3,897 0.089 0.000 0.000 0.000 0.082 0.000 0.007 46% 0.53 0.53 0.53 0.66 B4 3,034 0.070 2% 0.25 0.25 0.25 0.31 OS1 5,313 0.122 0.000 0.000 0.028 0.044 0.050 0.000 28% 0.40 0.40 0.40 0.49 OS2 2,806 0.064 2% 0.25 0.25 0.25 0.31 North 75,907 1.743 0.000 0.000 0.203 0.382 1.157 0.000 17% 0.31 0.31 0.31 0.39 South 66,891 1.536 0.149 0.000 0.012 1.107 0.000 0.268 46% 0.54 0.54 0.54 0.67 DEVELOPED RUNOFF COEFFICIENT CALCULATIONS Asphalt, Concrete Rooftop Gravel Residential: Low Density Homestead at Clarendon F. Wegert August 5, 2020 Project: Calculations By: Date: Undeveloped: Greenbelts, Agriculture Composite Runoff Coefficient2 1) Runoff coefficients per Tables 3.2-1 & 3.2 of the FCSM. Percent impervious per Tables 4.1-2 & 4.1-3 of the FCSM. Project: Calculations By: Date: C2*Cf C10*Cf C100*Cf Length (ft) Elev Up Elev Down Slope (%) Ti 2-Yr (min) Ti 10-Yr (min) Ti 100-Yr (min) Length (ft) Elev Up Elev Down Slope (%) Velocity (ft/s) Tt (min) Length (ft) Elev Up Elev Down Slope (%) Velocity (ft/s) Tt (min) Tc 2-Yr (min) Tc 10-Yr (min) Tc 100-Yr (min) a A 0.31 0.31 0.39 60 92.89 89.01 6.47% 6.1 6.1 5.5 N/A N/A N/A 705 89.01 85.49 0.50% 0.11 110.86 14.25 14.25 14.25 b B1 0.49 0.49 0.62 40 93.97 92.33 4.10% 4.5 4.5 3.6 326 92.33 84.93 2.27 3.01 1.80 N/A N/A N/A 6.28 6.28 5.37 b B2 0.56 0.56 0.70 95 94.23 92.41 1.92% 7.9 7.9 5.9 350 92.41 84.93 2.14 2.92 2.00 N/A N/A N/A 9.93 9.93 7.88 b B3 0.53 0.53 0.66 60 93.10 91.71 2.32% 6.3 6.3 4.8 305 91.71 84.93 2.22 2.98 1.70 N/A N/A N/A 7.99 7.99 6.55 b B4 0.25 0.25 0.31 20 91.89 90.41 7.40% 3.6 3.6 3.4 167 90.41 84.93 3.28 3.62 0.77 N/A N/A N/A 5.00 5.00 5.00 os1 OS1 0.40 0.40 0.49 160 93.83 93.10 0.46% 21.7 21.7 18.6 N/A N/A N/A N/A N/A N/A 10.89 10.89 18.62 Tc2 Tc10 Tc100 C2 C10 C100 I2 (in/hr) I10 (in/hr) I100 (in/hr) Q2 (cfs) Q10 (cfs) Q100 (cfs) a A 1.556 14.25 14.25 14.25 0.31 0.31 0.39 1.92 3.29 6.71 0.9 1.6 4.0 b B1 0.163 6.28 6.28 5.37 0.49 0.49 0.62 2.67 4.56 9.31 0.2 0.4 0.9 b B2 1.213 9.93 9.93 7.88 0.56 0.56 0.70 2.26 3.86 7.88 1.5 2.6 6.7 b B3 0.089 7.99 7.99 6.55 0.53 0.53 0.66 2.46 4.21 8.59 0.1 0.2 0.5 b B4 0.070 5.00 5.00 5.00 0.25 0.25 0.31 2.85 4.87 9.95 0.0 0.1 0.2 os1 OS1 0.122 10.89 10.89 18.62 0.40 0.40 0.49 2.17 3.71 7.57 0.1 0.2 0.5 os2 OS2 0.064 5.00 5.00 5.00 0.25 0.25 0.31 2.85 4.87 9.95 0.0 0.1 0.2 a North 1.743 14.25 14.25 116.38 0.31 0.31 0.39 1.92 3.29 6.71 1.0 1.8 4.6 b South 1.536 10.27 10.27 8.30 0.54 0.54 0.67 2.21 3.78 7.72 1.8 3.1 7.9 Gutter/Swale Flow, Time of Concentration: Project: Calculations By: Date: Homestead at Clarendon F. Wegert August 5, 2020 Combined and Routed Basins DEVELOPED DIRECT RUNOFF COMPUTATIONS Intensity Flow Overland Flow, Time of Concentration: Intensity, I from Fig. RA-2 Rational Equation: Q = CiA (Equation 6-1 per MHFD) Design Point Basin Area (acres) Tc (Min) Runoff C Tt = L / 60V (Equation 6-4 per MHFD) Tc = Ti + Tt (Equation 6-2 per MHFD) Velocity (Gutter Flow), V = 20·S½ Velocity (Swale Flow), V = 15·S½ (Equation 6-4 per MHFD) (Equation 6-3 per MHFD) } 𝑇𝑖 = 1.87 1.1 − 𝐶 ∗ 𝐶𝑓 𝐿 𝑆 ൗ 1 3 Page 3 of 3 Project Number: Project Name: Project Location: Basin Notes Historic Basin EX A 5.0 cfs Historic Basin EX OS1 0.3 cfs Historic Basin EX OS2 0.2 cfs Total Q100 to the North 5.5 cfs Proposed Basin A 4.0 cfs Proposed Basin OS1 0.5 cfs Proposed Basin OS2 0.2 cfs Total Q100 to the North 4.7 cfs Change in Q100 to the North 0.8 cfs Results in a decrease for the 100-year storm. Basin Notes Historic Basin EX B 1.9 cfs Total Q100 to the South 1.9 cfs Proposed Basin B1 0.9 cfs Proposed Basin B3 0.5 cfs Proposed Basin B4 0.2 cfs Detention Pond Release Rate 0.3 cfs Pond release rate determined by subtracting Basin B1, B3, and B4 from Basin EX B. Total Q100 to the South 1.9 cfs Change in Q100 to the South 0.0 cfs No change for the 100-year storm. Q100 Historic vs. Developed Flowrates to the North Historic vs. Developed Flowrates to the South HISTORIC VS. DEVELOPED FLOWRATES 1013-008 Homestead at Clarendon Fort Collins, Colorado Q100 Project: Calc. By: Date: 1.213 <-- INPUT from impervious calcs 50.00 <-- INPUT from impervious calcs 0.5000 <-- CALCULATED 12 hours <-- from MHFD Vol. 3 Table 3-2 0.8 <-- from MHFD Vol. 3 Table 3-2 0.165 <-- MHFD Vol. 3 Equation 3-1 0.017 <-- MHFD Vol. 3 Equation 3-3 727 <-- MHFD Vol. 3 Equation 3-3 ** <-- INPUT from stage-storage table ** <-- CALCULATED from Figure EDB-3 dia (in) = ** <-- INPUT from Figure 5 n = ** <-- INPUT from Figure 5 t (in) = ** <-- INPUT from Figure 5 number of rows = ** <-- from WQ Depth and row spacing August 5, 2020 WQCV (ft3) = WATER QUALITY POND DESIGN CALCULATIONS Primary Pond Required Storage & Outlet Works Homestead at Clarendon F. Wegert Circular Perforation Sizing **To be completed at final design Basin Area = Basin Percent Imperviousness = Basin Imperviousness Ratio = WQCV (watershed inches) = WQCV (ac-ft) = WQ Depth (ft) = Area Required Per Row, a (in2) = Drain Time Coefficient = Drain Time = Project: Date: Bottom Elevation: 5088.6 364 ft 3 Overflow Elevation: 5089.6 316 ft 3 Crest of Pond Elev.: 5090.0 5089.70 Maximum Elevation Minimum Elevation cu. ft. acre ft cu. ft. acre ft 5088.6 N/A 0.0 0 0.0 0.00 0.0 0.00 5089.0 5088.6 358.8 0.4 71.8 0.00 71.8 0.00 5089.5 5089.0 431.8 0.5 197.7 0.00 269.4 0.01 5090.0 5089.5 509.8 0.5 235.4 0.01 504.8 0.01 Volume at Overflow: Project Number: Project Location: Calculations By: Pond No.: West Rain Garden Stage Storage Curve Contour Contour Surface Area (ft 2 ) Depth Incremental Volume Cummalitive Volume Pond Stage Storage Curve 1013-008 Fort Collins, Colorado F. Wegert West Rain Garden Elev at Design Volume: Homestead at Clarendon 8/5/2020 Pond Outlet and Volume Data Design Volume: 8/4/2020 5:50 PM P:\1013-008\Drainage\WatQual\ 1013-008_Stage-Storage West.xlsx\Pond Project: Homestead at Clarendon Calc. By: Location: Fort Collins, Colorado Date: 42,675 sf 75% 32,006 sf 26,137 sf 7,897 sf 34,034 sf 79.8% Total Treatment Area Percent Total Project Area Treated 08/05/20 Onsite LID Treatment F. Wegert Vegetated Buffers Total New Impervious Area Project Summary Target Treatment Percentage Minimum Area to be Treated by LID measures LID Treatment Rain Gardens VAULT ELEC E ELEC E VAULT ELEC CONTROL IRR CONTROL IRR CONTROL IRCRONTROL IRR CONTROL IRR CONTROL IRR CONTROL IRR CONTROL IRR X X / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / Project: Calc. By: Date: 2.0% per City of Fort Collins SWCM 5.0% per City of Fort Collins SWCM 10.0% per City of Fort Collins SWCM 2.5% 8.3% 14 ft per City of Fort Collins SWCM 14 ft to 48 ft 25 ft 14 ft to 52 ft 42 ft 15,794 ft2 Entire Residential Area 24,698 ft2 1.56 Min. Allowable Cross-Slope Max. Allowable Cross-Slope for LID Max. Allowable Cross-Slope VEGETATED BUFFER Basin A Design Data & Analysis Vegetated Buffer Area Run-on Ratio Homestead at Clarendon F. Wegert August 5, 2020 Design Width (S <= 10.0%) Average Design Width (S <= 10.0%) Min. Design Cross-Slope (Smin) Max. Design Cross-Slope (Smax) Min. Allowable Width Design Width (S <= 5.0%) Average Design Width (S <= 5.0%) Treatment Area Note: Existing coverage consists of well established native grasses with over 95% coverage. As much existing vegetation and grading will be preserved as feasibly possible. Disturbed areas within vegetated buffer will occur along rear lot lines, and this work is necessary to daylight lot grades into existing topography. Any disturbed areas will be replaced with a native grass seed mixture and landscaping in accordance with the erosion control and landscaping plans. Average slopes are around 2.5% towards the south and east and gradually get steeper towards the north and west. An existing irrigation ditch along the north edge of the grass buffer impacts the slopes along the north edge of the grass buffer. The steeper slopes, greater than 5.0%, are generally along the irrigation ditch. Some minor grading will be required in the grass buffer to daylight proposed lawn grades into existing grades. However, any disturbed areas within the grass buffer will be revegetated per the erosion control / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / CLARENDON HILLS DRIVE LANGDALE DRIVE VEGETATED BUFFER RAIN GARDEN RAIN GARDEN HOMESTEAD AT CLARENDON HILLS SHEET NO: P:\1013-008\DWG\DRNG\1013-008_LID.DWG 301 N. Howes Street, Suite 100 Fort Collins, Colorado 80521 E NGINEER ING N O R T H E RN PHONE: 970.221.4158 www.northernengineering.com DRAWING REFERENCE: LID TREATMENT EXHIBIT F. Wegert 1 in = 60 ft August 5, 2020 LID-1 DRAWN BY: SCALE: ISSUED: ( IN FEET ) 0 1 INCH = 60 FEET 60 60 LEGEND RAIN GARDEN TREATMENT AREA VEGETATED BUFFER VEGETATED BUFFER TREATMENT AREA UNTREATED AREA NOTES: 1. EXCEPT FOR LOTS 1 AND 8, UNTREATED AREA CONSISTS OF LANDSCAPING AND LAWNS. 2. RAIN GARDEN TREATMENT AREA CONSISTS OF APPROXIMATELY SOUTH 2 3 OF LOT, ROOFS, DRIVEWAYS, AND SHARED DRIVEWAY FOR LOTS 2 - 7. 3. APPROXIMATELY NORTH 1 3 OF LOTS WILL DRAIN TOWARDS VEGETATED BUFFER AREA BETWEEN REAR LOT LINES AND IRRIGATION DITCH. Block 23 / Morningstar LID Summary Project Summary Total New Impervious Area 42,675 sf Target Treatment Percentage 75% Minimum Area to be Treated by LID measures 32,006 sf Rain Gardens Total Rain Garden Treatment Area 26,137 sf Vegetated Buffers Total Vegetated Buffer Treatment Area 7,897 sf Total Treatment Area 34,034 sf Percent Total Project Area Treated 79.8% os2 OS2 0.25 0.25 0.31 25 92.23 90.55 6.72% 4.2 4.2 3.9 N/A N/A N/A N/A N/A N/A 5.00 5.00 5.00 a North 0.31 0.31 0.39 60 92.89 89.01 6.47% 6.1 6.1 5.5 N/A N/A N/A 705 89.01 85.49 0.50% 0.11 110.86 14.25 14.25 116.38 b South 0.54 0.54 0.67 95 94.23 92.41 1.92% 8.3 8.3 6.3 350 92.41 84.93 2.14 2.92 2.00 N/A N/A N/A 10.27 10.27 8.30 Overland Flow, Time of Concentration: DEVELOPED TIME OF CONCENTRATION COMPUTATIONS Channelized Flow Swale Flow Design Point Basin(s) Tt = L / 60V (Equation 6-4 per MHFD) Tc = Ti + Tt (Equation 6-2 per MHFD) Overland Flow Time of Concentration Homestead at Clarendon F. Wegert August 5, 2020 Gutter/Swale Flow, Time of Concentration: Combined and Routed Basins Intensity, i (per Table 3.4-1 of the Fort Collins Stormwater Manual) Velocity (Gutter Flow), V = 20·S½ Velocity (Swale Flow), V = 15·S½ Rational Equation: Q = CiA (Equation 6-1 per MHFD) (Equation 6-4 per MHFD) (Equation 3.3-2 per Fort Collins Stormwater Manual) } 𝑇𝑖 = 1.87 1.1 − 𝐶 ∗ 𝐶𝑓 𝐿 𝑆 ൗ 1 3 Page 2 of 3 Lawns, Clayey Soil > 7%: C = 0.35, Percent Impervious = 2% Combined and Routed Basins 2) Composite Runoff Coefficient adjusted per Table 3.2-3 of the Fort Collins Stormwater Manual (FCSM). Lawns, Clayey Soil, Avg Slope 2-7% Open Space (undeveloped): C = 0.25, Percent Impervious = 2% USDA SOIL TYPE: B Page 1 of 3 Combined and Routed Basins Intensity, i (per Table 3.4-1 of the Fort Collins Stormwater Manual) Velocity (Gutter Flow), V = 20·S½ Velocity (Swale Flow), V = 15·S½ Rational Equation: Q = CiA (Equation 6-1 per MHFD) Overland Flow, Time of Concentration: HISTORIC TIME OF CONCENTRATION COMPUTATIONS Channelized Flow Swale Flow Design Point Basin(s) Tt = L / 60V (Equation 6-4 per MHFD) Tc = Ti + Tt (Equation 6-2 per MHFD) Overland Flow Time of Concentration Homestead at Clarendon F. Wegert August 5, 2020 (Equation 6-4 per MHFD) (Equation 3.3-2 per Fort Collins Stormwater Manual) } 𝑇𝑖 = 1.87 1.1 − 𝐶 ∗ 𝐶𝑓 𝐿 𝑆 ൗ 1 3 Page 2 of 3