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Home My WebLink About038 Groundwater Influence Letter - 2022 UpdateCivil/Structural Engineering and Surveying 1091 Stoneridge Drive • Bozeman, Montana • Phone (406) 587-1115 • Fax (406) 587-9768 www.chengineers.com • E-Mail: info@chengineers.com April 25, 2022 Griffin Nielsen, PE City of Bozeman 20 East Olive Street P.O. Box 1230 Bozeman, Montana 59771-1230 Re: Groundwater Influence on Storm Sewers - Norton Ranch, Phase 3 & 5 (161141) Dear Mr. Nielsen: This letter and the enclosed monitoring results are provided in response to the Norton East Ranch Subdivision Phase 5 condition of approval #25, which states: The stormwater design assumed a flow rate from the infiltration of groundwater. No data has been provided to justify this flow rate. Prior to completion and acceptance of the stormwater design, the infiltration flow rate must be verified during the period of seasonal high groundwater. If the flow rate exceeds the assumed value, the system must be altered to ensure all City drainage standards are met. Verification may not take place while dewatering is in operation. The applicant must send verification to the Community Development office before final plat approval. Methodology In April and May of 2021, the existing storm sewers in Phase 3 of Norton Ranch were inspected by C&H Engineering and Surveying, Inc (Now IMEG). During this time, it was determined that the retaining pond north of West Babcock Street and four stormwater inlets on West Babcock Street were inundated with groundwater during seasonal high groundwater. The next three inlets up-gradient were observed to have groundwater seeping through the grout around the stormwater pipes, though no standing water was found in these structures. A groundwater infiltration rate of 300 gal/acre/day was assumed to account for potential degradation of the storm sewer system. These results were summarized in Ethan Cote’s (C&H) June 14th, 2021 to Griffin Nielsen (City of Bozeman). The majority of the approved stormwater infrastructure for Norton Ranch Phase 5 has since been installed during 2021 and 2022. This included the removal of the existing pond north of West Babcock Street and connection to the existing 30” PVC storm sewer pipe along South Eldorado Avenue, north of West Babcock Street. On April 13th, 2022 IMEG made observations of the water flow in the 30” PVC located at the intersection of Sandpiper Road (Formerly Boomtown Road) and South Eldorado Avenue. At this point in time, no dewatering activities were taking place up-gradient (to the south). The nearest dewatering was occurring more than 300’ down-gradient of the observations. Based on existing groundwater monitoring 03/03/23 22203 Norton East Ranch Ph. 5B FP Civil/Structural Engineering and Surveying 1091 Stoneridge Drive • Bozeman, Montana • Phone (406) 587-1115 • Fax (406) 587-9768 www.chengineers.com • E-Mail: info@chengineers.com data, (provided as an attachment to this letter and with the original stormwater design report for Norton Ranch Phase 5) the groundwater is expected to peak in April or May. It is expected that maximum groundwater infiltration into the existing storm sewer system will also peak at this time. The observed groundwater infiltration rate is included in the stormwater design to show that the installed infrastructure has adequate capacity to handle this flow rate. Results The depth of the flow on April 13th, 2022 in the 30” PVC pipe into ST INLET-MH COMBO 3 was measured be 1.25”. Manning’s equation for open channel flow was used to determine the flow rate within the pipe. Calculations are attached for reference. The flow rate in the pipe was found to be approximately 0.11 CFS given the 30” PVC flowing at a slope of 0.68% per the approved infrastructure plans. This flowrate is included in the calculations for the inflow rate of Detention Pond #1, located in the northwest corner of the site. Detention Calculations are included here as an attachment. Conclusion The groundwater flow rate calculated in the 30” PVC pipe was added to the flow rate generated by the development of Phase 5 and Phase 3. The required volume of Detention Pond #1 given the initial assumptions of 300 gal/acre/day was 36,291 cubic feet. The required volume given the groundwater inflow rate of 0.11 CFS was determined to be 36,471 cubic feet. The provided volume of Detention Pond #1 is 46,463 cubic feet. The largest flow in the portion of the storm sewer is located in Pipe 5L- a 30” PVC pipe flowing from a ST MH 5L to Detention Pond #1. The required flow rate for this pipe is 30.79 CFS. The capacity of this pipe is 45.64 CFS, therefore the pipe should have adequate capacity for the additional groundwater flow. The proposed sealing of the storm inlets along Babcock and in Norton Ranch Phase 3 has not yet been completed at this time. SIME Construction has been scheduled to do so in the Spring of 2022. IMEG will continue to monitor the 30” PVC along South Eldorado through out the Spring of 2022 and update the necessary calculations if noticeable increases in the groundwater infiltration rate occur. If you require any further information, please give me a call. Sincerely, Drew Kirsch, E.I. MANNING'S EQUATION FOR PIPE FLOW Pipe: Pipe 5A Project: Norton East Ranch Subdivision, Phase 5 INPUT D= 30 inches d= 1.25 inches Mannings Formula n= 0.013 mannings 47.1 degrees Q=(1.486/n)ARh 2/3S1/2 S= 0.0068 slope in/in R=A/P A=cross sectional area P=wetted perimeter S=slope of channel V=(1.49/n)Rh 2/3S1/2 n=Manning's roughness coefficient Q=V x A Solution to Mannings Equation Area,ft2 Wetted Perimeter, ft Hydraulic Radius, ft velocity ft/s flow, cfs PVC 0.013 0.07 1.03 0.07 1.57 0.11 PE (<9"dia) 0.015 PE (>12"dia) 0.02 PE(9-12"dia) 0.017 CMP 0.025 ADS N12 0.012 HCMP 0.023 Conc 0.013 Manning's n-values d D DETENTION POND #1 REQUIRED VOLUME 1. Calculate Weighted C Factor for Right-of-Way Component Width C ROW Hardscape 43 0.95 ROW Landscape 17 0.2 Weighted C Factor =0.74 2. Calculate Area and Weighted C Factor (Post-Development) Contributing Area C Area (ft2 ) C * Area Composite ROW 0.74 337675 249035 Hardscape 0.95 0 0 OS 0.2 194595 38919 Low-Med Residential 0.35 693635 242772 Dense Residential 0.5 0 0 Norton Phase 3 0.49 1474393 722453 *From existing Norton Ranch Ph 3 Total 2700298 1253179 (see Appendix F) A = Area (acres)61.9903 C = Weighted C Factor 0.46 3. Calculate Tc (Pre-Development) Tc Overland Flow Tc = 1.87 (1.1-CC f)D1/2/S1/3 Storm S = Slope of Basin (%)1.49%Return (yrs) Cf C = Rational Method Runoff Coefficient 0.2 2 to 10 1 Cf = Frequency Adjustment Factor 1 11 to 25 1.1 D = Length of Basin (ft)1168 26 to 50 1.2 51 to 100 1.25 Tc (Pre-Development) (minutes)50 4. Calculate Rainfall Intensity (Duration = Pre-Development Tc i = 0.64x-0.65 (10-yr Storm, Fig. I-3, COB Design Standards) x = storm duration (hrs)0.84 (Tc Pre-Development) i = rainfall intensity (in./hr.)0.72 5. Calculate Runoff Rate (Pre-Development) Q = CiA C = Rational Method Runoff Coefficient 0.2 (open land) i = rainfall intensity (in./hr.)0.72 (calculated above) A = Area (acres)61.99 (calculated above) Q = Runoff Rate (Pre-Development) (cfs)8.89 6. Calculate Required Pond Volume Total Area (acres) =61.99 acres Weighted C =0.46 Discharge Rate (cfs) =8.89 cfs (Equal to Pre-Development Runoff Rate) Groundwater Influence System Infiltration=0.11 cfs (6.6 cfm) Flow Rate into System Duration(min)Duration(hrs)Intensity (in/hr)Qin (cfs) Qin (cfs) (GW Inflow) Qin (cfs) (Total) Runoff Volume (cf) Release Volume (cf) Required Storage (ft3) 31 0.52 0.98 28.28 0.11 28.39 52,810 16,538 36,272 32 0.53 0.96 27.70 0.11 27.81 53,405 17,072 36,333 33 0.55 0.94 27.16 0.11 27.27 53,987 17,605 36,382 34 0.57 0.93 26.63 0.11 26.74 54,558 18,139 36,420 35 0.58 0.91 26.14 0.11 26.25 55,119 18,672 36,447 36 0.60 0.89 25.66 0.11 25.77 55,670 19,206 36,464 37 0.62 0.88 25.21 0.11 25.32 56,210 19,739 36,471 38 0.63 0.86 24.78 0.11 24.89 56,742 20,273 36,469 39 0.65 0.85 24.36 0.11 24.47 57,264 20,806 36,458 40 0.67 0.83 23.96 0.11 24.07 57,778 21,340 36,438 OUTLET STRUCTURE SLOT Q=CLH3/2 Q = Discharge (cfs)8.89 (calculated above) C = Weir Coefficient 3.33 (per COB Design Standards) H = Head (ft)0.9 L = Horizontal Length (ft)3.13 L = Slot Width (inches)37.5 Check the half inch requirement (per DSSP II.A.4) 1. Determine Area of Hardscape within Drainage Area #2 Contributing Area Area (ft2) Hardscape 1214260 2. Calculate 1/2" runoff volume over hardscape (aka Runoff Reduction Volume [RRV] as calculated in Montana Post- Construction Storwater BMP Manual - Equation 3-1) RRV = [P*Rv*A]/12 P = Water quality rainfall depth 0.50 inches Rv = Dimensionless runoff coefficient 0.47 0.05 + 0.9*I I = Percent impervious cover (decimal)0.46 decimal A = Entire drainage area 21.50 acres RRV = Runoff Reduction Volume 0.419 acre-ft RRV = Runoff Reduction Volume 18250 cubic feet Project Engineer: Project: Well Information: bgs = below ground surface ags = above ground surface MW-1 MW-2 MW-3 MW-4 MW-5 MW-6 MW-7 MW-8 MW-9 MW-10 MW-11 MW-12 MW-13 3.50 3.08 2.67 1.58 2.08 2.50 2.00 Groundwater Information: MW-1 MW-2 MW-3 MW-4 MW-5 MW-6 MW-7 MW-8 MW-9 MW-10 MW-11 MW-12 MW-13 02/13/18 1.97 1.32 frzn frzn frzn frzn 2.56 04/13/18 1.03 0.74 0.63 0.37 0.35 0.44 1.12 05/11/18 2.34 1.20 0.63 1.51 1.28 1.68 2.49 05/25/18 2.29 1.42 0.69 1.84 1.61 2.01 2.40 06/08/18 2.97 1.90 0.83 3.42 2.00 2.27 3.03 06/22/18 2.91 1.34 2.14 2.15 1.82 2.11 2.81 07/06/18 4.08 3.03 3.06 4.50 3.12 3.38 3.46 07/20/18 4.99 3.02 2.78 3.30 2.21 2.55 3.86 08/03/18 5.15 3.87 3.01 4.19 ~ 3.48 4.69 08/17/18 6.04 4.02 3.23 5.52 3.17 4.11 5.22 08/30/18 4.70 3.47 2.28 4.47 2.82 2.45 3.85 09/14/18 Dry 3.95 2.38 2.32 2.62 2.70 3.90 Well ID Well Depth (feet-bgs) Top of Well (feet-ags) Ground Elevation Date Depth to Ground Water (feet-bgs) Project Number: 161141 Norton Ranch Phase 5 Project Location: Bozeman Montana Monitor Well Data