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SEWER MAIN
DESIGN REPORT
FOR:
BOZEMAN COHOUSING
BOZEMAN, MT
Prepared By:
Madison Engineering
895 Technology Blvd Ste 203
Bozeman, MT 59718
(406) 586-0262
AUGUST 2021
Bozeman Cohousing
Sewer Main Design Report
Page 1 of 3
BOZEMAN COHOUSING
SEWER MAIN DESIGN REPORT
A. General
This report will give an overview of the proposed design and wastewater flow generation from
the new Bozeman CoHousing development located at 3120 Wagonwheel Rd. The site is situated
at the intersection of Wagonwheel Rd and Concord Dr. The proposed development is zoned R-3
and will consist of 43 dwelling units throughout multi-family buildings. No phasing is proposed
for the sewer system; however the units within the development will be phased.
This report provides a detailed description of the proposed wastewater collection system and
design parameters. Once the wastewater system is constructed and approved, the system will
become a part of the City of Bozeman system. The following references were used in the
wastewater facilities design and capacity analysis:
a. COB Design Standards and Specifications Policy, 2004, Addendum #6
b. COB Modifications to Montana Public Works Standard Specifications (MPWSS), Addendum #3
c. City adopted MPWSS and Adopted Addenda
d. COB Wastewater Facility Plan, 2015
e. DEQ Circular No. 2
f. City of Bozeman Zoning Map
g. City of Bozeman Infrastructure Viewer
The procedures and parameters outlined in the COB Design Standards and Specifications Guide
were used in the design of the sewer infrastructure for the proposed development.
B. Capacity Analysis
The proposed wastewater collection system consists of 490 linear feet of new sewer main that
will provide service to all units within the CoHousing development. A cut-in manhole with a
pre-cast base is proposed for connection to the City of Bozeman sewer system. The cut-in
manhole will be located in Wagonwheel Rd approximately thirteen feet to the north of existing
manhole F0905 as seen on the City’s infrastructure viewer. Effluent generated will flow west
from the site into the existing 8” sewer main located in Wagonwheel Rd.
The proposed sewer main has been sized to flow at no more than 75% of full capacity at peak
hour conditions at full buildout. An infiltration rate of 150-gallons/acre/day was added to the
design flow. A Manning coefficient of 0.013 was used for PVC pipe.
The proposed development totals 5.31 acres of R-3 zoned land. The capacity requirement of the
proposed sewer main was determined based on the proposed buildout of the entire development.
The City of Bozeman's Design Standards specify an average daily flow rate of 2.11 people per
dwelling unit and 89-gallons per capita per day.
Bozeman Cohousing
Sewer Main Design Report
Page 2 of 3
Table 1 shows the estimated wastewater demands of the development. Peaking factors were
calculated based on the following formula:
Qmax / Qave = (18 + P0.5) / (4 + P0.5) (P = Population/thousands)
The calculated peaking factors are included in the appendix of this report. The peak flow rate
anticipated from the development was calculated by multiplying the average flow rate by the
peaking factor. The peak flow rate was then converted from gpd to cfs.
Table 1
Peak Flow Rates
Sewer
System
Proposed
no. of
units
Density
(Persons
/unit)
Total
no. of
people
Gallons
per capita
per day
Avg. Daily
Flow Rate
(gpd)
Peaking
factor
Peak Flow
Rate (gph)
Peak Flow
Rate + I&I
(gph)
Peak Flow
Rate + I&I
(cfs)
Bozeman
Cohousing 43 2.11 91 89 8,099 4.25 1,434 1,467 0.053
Infiltration calculations are provided in Table 2, and added to the peak flow to determine the
total flow capacity required.
Table 2
Infiltration Rates
Sewer
System
I & I
(gal/acre/day)
Area
(acres)
Total I & I
(gal/day)
Total I & I
(gal/hr)
Bozeman
Cohousing 150 5.31 797
33.2
An 8-inch PVC sewer main at minimum slope of 0.4% and a Manning Coefficient of 0.013 has a
capacity of 0.64 cfs at 75% of full flow capacity. This is sufficient to accommodate the peak
flow of 0.053 cfs for the Cohousing development. The sewer main capacity calculations are
provided in the appendix of this report.
C. Proposed Collection System
PVC pipe meeting COB specifications will be used for all the sewer mains. The sewer main
proposed for this project will be SDR-35 PVC 8-inch diameter with a maximum manhole
spacing of 400 feet and a minimum manhole barrel size of 48-inches. Manholes will be
constructed according to COB specifications. Sewer services will be a minimum of 4-inch
diameter and sloped at a 2% minimum slope. A site plan showing the proposed sewer main is
included in this submittal. All sewer mains will be located within a 30-foot wide utility
easement and be located a minimum of 10-feet from the proposed public water main.
Bozeman Cohousing
Sewer Main Design Report
Page 3 of 3
D. Upstream and Downstream Capacity Analysis
The proposed sanitary sewer main has been designed in conformance with City of Bozeman
Design Standards and Specifications. It is anticipated that the City of Bozeman sanitary sewer
system has enough capacity to accommodate the main extension.
Based on the City of Bozeman Wastewater Improvement Plan, it is not anticipated that any other
areas will be served by the sanitary sewer main within the Bozeman CoHousing development.
E. Conclusion
As outlined above, the proposed 8-inch sewer main has the capacity for the effluent generated by
the proposed Bozeman Cohousing development. This project will require the approval of the
City Engineer and MDEQ.
Appendix
1. Sewer Main Capacity Calculations
Appendix
Wastewater Daily Flow Analysis
Bozeman CoHousing
Average Daily Flow
No of Units 43
Residents/unit 2.11
Total No. Residents 91
Gallons per Capita/day 89
Average Daily Flow 8,099 gpd
Peaking Factor Calculations
Zoning Density
(residents/unit)
Total
Popoulation
R3 2.11 91
=Q(max)= 18 + P
0.50 P = Population/thousands = 91/1000 = 0.091
Q(ave)4 + P
0.50
Peaking Factor =
Peak Hour Sewer Demand
Average Daily Flow 8,099 gpd
337.5 gph
5.6 gpm
Peaking Factor 4.25
Peak Hour Sewer Demand 1435.8 gph
23.9 gpm
191.96 cfh
3.20 cfm
0.053 cfs
4.25
Peaking
Factor
Units/AcreAcres
2.13 20.2