HomeMy WebLinkAbout009 Water and Sewer Memo
714 Stoneridge Drive · Suite 3 · Bozeman, MT 59718 · (406) 586-5599 · www.alpinesurveying.net
Page 1
April 25, 2025
City of Bozeman Engineering Department
20 E. Olive Street
Bozeman, MT 59715
RE: Water & Sewer Demand Memo
Lot 1&2, Block 9 of Blackwood Groves Subdivision Phase 3A,
S25, T02S, R05E, Bozeman, MT
To Whom it May Concern,
The intent of this memo is to outline the anticipated water and sanitary sewer demands for the
proposed Multifamily project on Lots 1 & 2, Block 9 of Blackwood Groves Subdivision, Phase
3A. The 1.4-acre site is located along the Alley (Arbor Way) in Block 9 and is currently zoned
REMU. The project site is currently undeveloped and is bounded by city park to the west and
Arbor Way (alley) to the east and north.
An 8” sanitary sewer main runs through the Alley from Cambridge Drive and provides service to
these lots. 4” sewer service stubs were installed with the subdivision improvements into each lot
and these will be extended to provide service connections to each of the proposed buildings.
(2) 4” DIP water service stubs were extended into the southern lot from Victoria Street with the
subdivision improvements. These service stubs will be extended into the southern building as fire
and domestic water services. Similarly, an 8” water main was stubbed into the northern lot from
Cambridge Drive with the subdivision improvements. (2) 2” domestic water services and (2) 4”
DIP fire services will be tapped off of this existing water main stub to provide service to the
northern buildings. The fire marshal has also requested that a fire hydrant be installed on the alley
as part of the site improvements. Therefore, a hydrant is proposed in the open space tract on the
northeast corner of the alley.
714 Stoneridge Drive · Suite 3 · Bozeman, MT 59718 · (406) 586-5599 · www.alpinesurveying.net
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Sanitary Sewer Demand
There are several ways to estimate Sanitary Sewer flows, including standards set forth by the
Montana DEQ Circular 2, Montana DEQ Circular 4, and the COB DSSP. However, the projected
wastewater flows in this memo are calculated using the values provided in the City of Bozeman
Wastewater Collection Facilities Plan Update Final – 2015. This document provides an updated
estimate of 2.17 persons per dwelling unit within the City of Bozeman Planning Area, which
differs slightly from the 2.11 person/du estimate outlined in the COB DSSP. The facilities plan
update also provides an updated wastewater generation estimate of 64.4 gallons per day per capita
based on extensive research of the existing system instead of the 89 gpcd estimate outlined in the
COB DSSP. The peaking factor is based on the Harmon Formula and an infiltration rate of 150
gallons per acre per day was also applied to the project, both as outlined in the COB DSSP. Based
on these criteria, the estimated demands from the proposed project are included in the table below.
It should also be noted that the Sanitary Sewer Design Report submitted with the Blackwood
Groves preliminary plat estimated 49 total dwelling units for this block (Block 9). Aside from the
two lots making up the proposed project site, there are also 19 single-family and townhome lots in
the block. So, by removing the 19 single-family dwellings from the original 49 dwelling unit
estimate, it can be assumed that the two lots making up the project site were planned for 30
dwelling units (49-19 = 30). Therefore, our proposed sewer demand for 30 dwelling units was
accounted for in the subdivision infrastructure design.
Peak Sanitary Sewer Demand Calculation
Dwelling Units Population
Average Day
Flow (GPD)
Average Day
Flow (GPM)
Peaking
Factor
Peak Hour
Flow (GPM)
Infiltration
(GPM)
Design Flow
Rate (GPM)
Design
Flow
Rate (cfs)
Per Building 10 22 1,416.8 0.98 4.37 4.28 0.00 4.28 0.0095
Total 30 66 4,250.4 2.95 4.29 12.66 0.15 12.81 0.0285
1.4 Acres
150 Gal./Acre/Day
0.15 GPM
4" PVC Pipe Capacity
Pipe Size (in) Flow Depth - d (in) Slope (ft/ft) Manning's n Flow (CFS)
4 3 0.02 0.012 0.27
Total Lot Area
Infiltration Rate
Total Infiltration
714 Stoneridge Drive · Suite 3 · Bozeman, MT 59718 · (406) 586-5599 · www.alpinesurveying.net
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Water Demand
In order to be consistent with the sanitary sewer estimates, the water demand calculations for the
proposed project were computed using 2.17 people per dwelling unit, based on Section 2.3.5 of
the 2015 City of Bozeman Wastewater Collection Facilities Plan Update. Per the COB Design
Standards and Specifications Policy (COB DSSP), a water demand of 170 gallons per capita per
day is to be used when calculating water demand. As outlined in the COB DSSP, peaking factors
of 2.3 and 3.0 were used to calculate Maximum Day and Peak Hour Demand, respectively. The
below table provides the estimated water demands based on be above assumptions.
Please feel free to contact me with any questions at (406) 586-5599, or by email at
ecote@alpinesurveying.net.
Sincerely,
Ethan Cote, P.E 60593
Type Dwelling Units Population
Average Day
(GPM)
Max Day
(GPM)
Peak Hour
(GPM)
Residential 30 66 7.79 17.92 23.38
Peak Water Demand Calculations