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2131 GRAF MULTI-UNIT RESIDENTIAL DEVELOPMENT
WATER SYSTEM DESIGN REPORT
A. General
This design report will give an overview of the proposed water main extension for the development
of the “2131 Graf” Multi-Unit Residential Development Infrastructure Improvements. The
proposed Phase 1 improvements are anticipated to be constructed during the fall of 2020. Once the
water main is constructed and approved, the main will become a part of the City of Bozeman
system.
The following references were used in the preparation of this report:
a. COB Design Standards and Specifications Policy, 2017. Addendum #7
b. COB Modifications to Montana Public Works Standard Specifications (MPWSS).
c. MDEQ Circular No. 1
d. City of Bozeman Zoning Map
e. City of Bozeman Water Facility Plan
f. 2015 International Fire Code
Existing System
The proposed water main extension will be served by the existing City of Bozeman water
distribution system, which will be looped. The two connection locations are at Lantern Drive and
at the intersection of Graf Street and S 21st Avenue. There will also be a connection on S 21st Ave
with the Nexus Point project to the north. The following information was obtained by the City of
Bozeman at the intersection of Lantern Lane and Enterprise Boulevard.
Fire Hydrant: 2077
Static Pressure: 65 psi
Pitot Pressure: 56 psi
Residual pressure: 57 psi
Hydrant discharge: 1,250 gpm from 2.5” port
Distribution Main Capacity – Fire flow at minimum residual pressure
Qr = Qt[(Ps – Pm)/(Ps – Pt)]0.54
Qr = 1,250 [(65 –20)/(65 – 56]0.54
Qr = 2,981 gpm at 20 psi
Qt = Hydrant Discharge during Test
Qr = Fire Flow at Residual Pressure
Ps = Static Pressure
Pm = Minimum Residual Pressure = 20 psi
Pr = Residual Pressure
Pt = Pitot Pressure
B. Proposed Water System
The proposed water main extension for the project consists of approximately 4,060 linear feet of
8” ductile iron pipe that will connect to the existing water main in Graf St at S 21st Ave and the
existing main at Lantern Drive. The layout of the proposed water main meets the COB design
criteria and will be located within the easements/right of way and separated from the proposed
sanitary sewer main by a minimum of 10 feet horizontally. Fire hydrants will be installed within
COB design criteria and will not exceed the recommended distance.
C. Capacity Analysis
The water main extension is proposed for the development of the 18 buildings comprised of 460-
units, which sits on an estimated 25.99 acres of R-4 (Residential High Density) zoned land. An
estimation of the water flow rate was based on the City of Bozeman’s design standards and
specification policy, which suggests using an average daily flow rate of 2.11 people per dwelling
unit and 170-gallons per capita per day. Applied to the 460 units, the required water flow rate is
calculated:
൬457 units ∗ 2.11 residents
unit ∗ 170 gallons
capita day ൰ = 163,926 gpd
Based on these parameters, the 457-unit development at full build out will have an estimated water
usage of 163,926 gpd (113.84 gpm). The main will connect to the existing City of Bozeman system
and will serve the future Graf Street apartment buildings.
In addition to the average daily demand, the water main extension must account for peak periods
of daily use. Peaking factors are applied to the demand and account for these peak periods in daily
use. The peaking factors used are maximum daily use and peak hour and are obtained from the
City of Bozeman Design Standards. Table 1 shows the peaking factors and the resultant flows
from those factors when applied to the average daily demand of 113.84 gpm.
TABLE 1
PEAKING FACTORS
Flow Designation Peaking Factor Flow (gpm)
Average Day Demand 1 113.8
Maximum Day Demand 2.3 261.8
Maximum Hour
Demand
3 341.5
Based on communication with the architect, there will be three types of buildings that will consist
of four (4)-3 plex (Type A), nine (9)-32 plex (Type B), and five (5)-32 plex (Type C) for a total of
18 individual buildings. The square footage for each type of building are as follows: Type A =
5,072sf, Type B = 32,647sf and Type C = 38,647sf. The units will be V-A and V-B type
construction with fire sprinklers. A Type V-A 38,227sf building requires 3,500 gpm for a 3-hour
flow duration per the 2015 International Fire Code, Table B105.1(2). The flow capacity for the
largest apartment building was chosen to be more conservative. Based on the 2015 International
Fire Code’s Table B105.2, the 3,500 gpm fire flow can be reduced but shall not be less than the
minimum of 1,500 gpm.
D. Results
Based on the analysis outlined above, the required minimum fire flow is 1,500 gpm and with the
maximum hour demand of 341.5 gpm, a maximum hour flow of 1,841.5 gpm is required. The flow
analysis attached states a required fire flow of 1,500 gpm, which includes an additional max hour
demand of 341.5 gpm is analyzed throughout the system. The flow analysis shows that an 8-inch
DI pipe can provide the development with the required flow of 1,841.5 gpm and remain above the
minimum pressure of 20 psi. See attached flow analysis for calculations.
E. Conclusion
Based on this analysis, the proposed 8-inch ductile iron water main serving the Graf residential
development extension provides adequate service to meet the 457-unit peak hour demand and fire
flows requirements for the development. The water main was designed on conservative premise
for future development. The water main design is consistent with the City of Bozeman Water
Facility Plan.
This project will require the approval of the City of Bozeman Engineer and MDEQ.
Appendix:
Capacity Calculations
WaterCAD Fire Flow Report