HomeMy WebLinkAbout034 Phase 3Groundwater Influence LetterCivil/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-CCf)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