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The Guthrie at 5th and Villard
Site, Water, Sewer, Storm Water Engineering Report
December 1, 2023, Site Plan, Revised January 26, 2024
Prepared for: HomeBase Montana
Prepared by: Stahly Engineering and Associates
Engineer of Record: Cordell D. Pool, PE
Quality Control Reviewer: Zach Lowe, PE
Introduction
The Guthrie is a site redevelopment project located on Lots 1A, Block 6, of the Amended Karp’s
Addition to the City of Bozeman [Plat E-3-B], located at the southwest corner of North 5th Avenue
and West Villard Street (figure 1). The lots are currently occupied by a building and a parking lot. The
Guthrie site is bounded by West Villard Street to the north, North 5th Avenue to the east, and
occupied lots to the south and west.
The Guthrie site development is shown in Figure 2 and features 1 apartment building, 37 parking
spaces, indoor/outdoor bike parking, and outdoor porch. The 5-story apartment building has a
12,000 sf footprint with 111 single and double units. 50% of the apartment units are designated as
affordable.
Figure 1 – The Guthrie Project Vicinity
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Civil Specifications and Design Standards
The civil specifications for the project are the Montana Public Works Standard Specifications
(MPWSS) and the City of Bozeman Modifications to MPWSS (COB Mods). Construction plans are
developed in accordance with the City of Bozeman Design Standards.
Site Demolition
Demolition and replacement of existing site improvements on the site will be required as part of The
Guthrie construction. The existing building and parking lot will be demolished. The existing overhead
power line will be relocated to the western edge of the property. The existing 4 water services and 2
sewer services serving the property will be removed and capped at the main. The existing curb and
sidewalks along N. 5th and Villard will be removed and replaced during construction.
Site Conditions
The site is generally flat, gently sloping to the north and east. Bozeman Creek exists far to the east
of the site, crossing Villard Street near N. Bozeman Ave. Currently no wetlands are located on the
Guthrie property. There is no FEMA mapped floodplain near this site.
A geotechnical investigation was conducted in 2023 by TD&H Engineering. The investigation found
conditions typical of mid-town Bozeman, with 13.3 to 16 feet of fine-grained soil overlying gravel, and
a deep groundwater table. Groundwater was observed during the investigation and found depth to
groundwater was approximately 15.9’-16.4’ below existing ground.
Proposed Site Improvements
The site development proposes the building to line the perimeter streets along with an interior
parking lot. Generally, the site and buildings will be elevated above existing grade to improve
drainage and provide privacy to ground level residential units.
Site improvements consist of new water and sewer services, storm drainage mitigation, franchise
utilities, an overhead power reroute, parking, hardscape, landscape, and site lighting improvements.
Civil site improvements are shown in Figure 2 and in detail on the Civil Plans.
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Figure 2 – The Guthrie Civil Layout
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Water
Existing 6” Cast Iron City of Bozeman water mains run adjacent to the site in North 5th Avenue and
West Villard. Water pressure is approximately 135 psi at this location and the City of Bozeman
Water Facility Plan does not identify any system limitations in this area. A nearby hydrant flow test
(Test #48) shows a fire flow of 3,478 gpm with an 11.3 psi pressure reduction in the system.
Water and fire services will be provided by new services installed on the existing main in North 5th
Avenue. A new fire hydrant is proposed along North 5th Avenue to supply the building fire
department connection. The fire flow from the new hydrant is anticipated to be similar to the nearby
hydrants of approximately 2500 gpm.
The proposed services are a 4” domestic water service and a 6” fire service. The water service
sizing is based on the fixture counts in accordance with the Uniform Plumbing Code. The water
fixture unit count for The Guthrie is 1123.5 units, corresponding to a peak instantaneous flow of 212
gpm and which requires a 4” service and 3” meter.
The estimated water use for The Guthrie is provided below in Table 1. Estimated water use per
COB standards is shown in Appendix A at the end of this report. Due to the small unit size, and high
efficiency fixtures, domestic water use is estimated to be 65 gpd/capita for single and double
occupancy units. Irrigation water supply will be provided through the domestic water service.
Preliminary irrigation demands are provided by the landscape architect. Irrigation is anticipated to
occur over an approximately 100-day season resulting in a total annual water use of 94,214 gallons,
or 0.28 ac-ft. The average daily irrigation flow is 942 gpd, in other words, this is equivalent to only 5
gallons per day per resident during summer, much less than typical single-family residential use.
The total estimated water use is 12,138 gallons per day (gpd), with a peak hour demand of 35.2
gallons per minute (gpm). Considering the termination of existing building’s water use, the new
demand on the City’s system will be less than the values above.
Table 1. The Guthrie Estimated Water Use
Cash-in-lieu of water rights are assessed within the City of Bozeman based on estimated annual
average water use for a project. Currently, a $6,000 fee is assessed per acre-ft/year. Cash-in-lieu
amounts will be determined by the City during the Site Plan review.
The Guthrie Estimated Water Use
Unit Type Units Capita/Unit Population GPD/Capita Water Use (gpd)
Single 44 1.25 55 65 3,575
Double 67 1.75 117 65 7,621
Total 111 172 11,196
Site Area - Lot (ac)0.84
Average Day Water Use [Equal to Wastewater Flow](gpd) 11,196
Irrigation (gpd)942
Max Day Water Use [Average Day Use + Irrigation] (gpd)12,138
;P = Pop. ÷ 1,000 Peaking Factor (PF)4.17
Water Use Peak Hour Flow (gpm)35.2
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Sewer
An existing 8” Vitrified Clay City of Bozeman sewer main runs adjacent to the site in North 5th
Avenue. The City of Bozeman Wastewater Facility Plan does not identify any system limitations in
this area. A new manhole and 8” sewer service is proposed to be connected to this main.
The estimated wastewater flow for The Guthrie is provided below in Table 2. Estimated wastewater
flow per COB standards is shown in Appendix A at the end of this report. Daily wastewater
generation was determined utilizing the same residential unit estimates as described in the water
use estimate. Infiltration and Inflow (I&I) is 150 gpd/acre. Applying these values results in an average
daily flow for the project of 11,323 gpd. Using a peaking factor of 4.17, based on City Design
Standards, the peak hour flow is estimated at 32.8 gpm. These values and demands are
summarized and tabulated below. Considering the termination of existing building’s sewer use, the
new demand on the system will be less than the values above.
Table 2. The Guthrie Estimated Wastewater Flow
Storm Water
The existing site development has significant impervious area (existing building, paved parking lot,
sidewalks) without any stormwater mitigation. Currently, runoff from the site is directed north and
east to adjacent City streets, where it is collected in existing City storm drains. Storm water
eventually outfalls at N. 5th Avenue and W. Tamarack Street.
Existing and post development stormwater runoff volumes were determined by the Rational Method
with a weighted coefficient determined for the building’s rooftop, pavement area, sidewalks, patios,
and landscaped areas. It is notable that the Guthrie site has slightly less impervious area than the
existing development. Storm runoff calculations are shown in Table 3. Runoff flow rates are
provided for the highest intensity (short duration) events for a range of storm frequencies. Runoff
volumes are provided for the different storm frequencies and precipitation depths. Note runoff flow
rate and volume are not related. Peak runoff flow rate only depends on rainfall intensity at the time
of concentration and the frequency of the event. Runoff volume depends only on the storm duration
The Guthrie Estimated Wastewater Flow
Units Capita/Unit Population GPD/Capita Wastewater
Flow (gpd)
Single 44 1.25 55 65 3,575
Double 67 1.75 117 65 7,621
Total 111 172 11,196
Site Area (ac)0.84
Inflow & Infiltration (gal/ac/day)150
Inflow & Infiltration (gpd)127
Wastewater Flow + Inflow & Infiltration (gpd)11,323
;P = Pop. ÷ 1,000 Peaking Factor (PF)4.17
Wastewater Peak Hour Flow (gpm)32.8
Page | 6
and precipitation depth. For example the 10-year 2hr, and 10-year 24hr storms have the same peak
flow, but different runoff volumes due to the different precipitation depths.
The Guthrie will have an on-site subsurface retention system beneath the parking lot that captures
and retains site and building storm runoff. The retention system consists of a gravel bed with a
single row of Stormtech chambers with an inlet/access manhole at each end. The parking lot is
graded to the retention system inlet and to a parking lot area drain inlet. Runoff from the building
rooftop is piped underground to the retention system. A small portion of the site consisting of the
front yard will continue to drain to the street. Since this is predominantly landscaping, this runoff is
minimal.
The retention system is sized to contain runoff from the 10-year, 2-hour storm event which is a 0.82”
storm event. This storm event represents the 96th percentile storm event in Bozeman and produces
approximately 1864 cubic feet (cf) of storm runoff. The retention system also reduces stormwater
flow leaving the site from larger storm events to below existing flows. Table 3 shows that stormwater
from the design event (10-year, 2-hour storm) is completely retained on site and the estimated
reduction of runoff from larger storm events.
Table 3. The Guthrie Storm Water Calculations
Land Classification Post Dev Area (sf)
Rooftops
Pavement
Sidewalk
Landscape
Total
Weighted Runoff Coeff. (C )
Design Storm 0.5-Inch 10y 2hr 10-Year 25-Year 50-Year 100-Year
Drainage Area (acres)0.844 0.844 0.844 0.844 0.844 0.844
Drainage Area (sf)36,752 36,752 36,752 36,752 36,752 36,752
Slope (%)2 2 2 2 2 2
Time of Concentration (min)5.0 5.0 5.0 5.0 5.0 5.0
24 Hour Precipitation Volumes (in)0.50 0.82 1.84 2.16 2.42 2.67
Design Storm 0.5-Inch 10y 2hr 10-Year 25-Year 50-Year 100-Year
Intensity at Tc (Figure I-2 pg. 29) (in/hr)NA 0.41 3.22 3.83 4.74 5.34
Peak Runoff Rate at Tc (Q = CIA) (cfs)NA 0.26 2.03 2.41 2.99 3.37
Runoff Volume (cf)1,144 1,876 4,211 4,943 5,538 6,110
Design Storm 0.5-Inch 10y 2hr 10-Year 25-Year 50-Year 100-Year
Intensity at Tc (Figure I-2 pg. 29) (in/hr)NA 0.41 3.22 3.83 4.74 5.34
Peak Runoff Rate at Tc (Q = CIA) (cfs)NA 0.26 2.02 2.40 2.97 3.34
Runoff Volume (cf)1,137 1,864 4,183 4,911 5,502 6,070
Design Storm 0.5-Inch 10y 2hr 10-Year 25-Year 50-Year 100-Year
Runoff Volume Increase (cf)-7 -12 -27 -32 -36 -40
Retention Volume (cf)2,557 2,557 2,557 2,557 2,557 2,557
Net Runoff Volume Post Mitigation (cf)-1,420 -693 1,626 2,354 2,945 3,513
% Decrease in Runoff from Existing 100.0 100.0 52.3 57.5 51.8 42.5
0.9
0.9
0.9
0.2
Design Storm Information
Existing Peak Flow Calculations
Post Dev Peak Flow Calculations
Mitigation Calculations
Existing Area (sf)
15,702
7,148
5,879
13,027
9,666
36,752
0.75
8,023
5,782
Site Statistics
8,277
36,752
0.74
C
Page | 7
The retention system storage calculations are shown in Table 4. The proposed storage volume
exceeds the runoff from the 10-yr 2-hr storm event. The retention system is sized without
consideration of infiltration, which provides significant additional capacity and a comfortable margin
of safety.
Table 4. The Guthrie Retention System Calculations
Stormwater conveyance capacity has been considered but on a project of this small scale is not
worthy of detailed reporting. The site has 5 separate sub-basins described as: Building Rooftop and
Canopy, West Parking Lot, East Parking Lot, West Side Landscaping, and Front Yard. The
proportional runoff flow rate for each of the sub-basins is. All storm runoff conveyances have the
capacity to convey the peak flow from the 100-year event. Note that the drainage leaving the site
(from the front yards) is only 8% of the runoff that left the site from the previous development.
Table 5. The Guthrie Sub-basin Summary
Drainage Sub-basin % of Total Runoff 100-year Peak Flow (cfs)
Building Rooftop 43% 1.44
West Parking Lot 38% 1.28
East Parking Lot 9% 0.29
West Side Landscape 2% 0.08
Front Yard 8% 0.26
Subsurface Chamber System
Chambers Length (ft)63
Number of chambers 9
Storage per chamber 45.9
Chambers Storage (cf)413
Gravel Bed Length (ft)70
Gravel Bed Width (ft)12
Gravel Bed Depth (ft)9.0
Cube Volume 7560.0
Net Gravel Volume 7146.9
Gravel porosity 0.30
Total Gravel Storage 2144
Total 2,557
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Storm Water Maintenance
It is the property owner’s responsibility to monitor and maintain their stormwater mitigation system.
The system is designed to capture sediment and debris and requires regular monitoring and periodic
maintenance to remove sediments and debris. If this sediment and debris is not periodically
removed, it can cause undesired ponding and clogging. The stormwater retention system consists
of manhole structures and underground chambers. These components need to be inspected
quarterly, except in winter, and cleaned if required.
• Inspect for sediment or debris in the manhole/inlet structures and remove if present.
• Inspect infiltration chambers through inspection ports for sediment accumulation. Sediment
depth less than 3” is acceptable. Sediment depth exceeding 3” requires removal by a cleaning
service.
• Check for damage, repair as needed.
• Maintain records of quarterly inspections and cleaning and provide records to the City upon
request.
Page | 9
Appendix A: City of Bozeman Design Flows
This appendix shows water and sewer flow rates per City of Bozeman DSSP Section V per City
comments received during the Site Plan review. It is the reviewer’s perogitive to select the design
flows they feel most accurately represent the design conditions.
Estimated Water Use
Per the DSSP the average day flow is 170 gallons per day per capita (gpd/capita). The unit
occupancy is assumed to be 2.17 people per unit. Maxium day flow is 2.3 times the average day
and peak hour is 3 times the average day. The results of this methodolgy are shown in Table A-1.
This results an average day water flow estimate of 40,948 gpd, a max day flow estimate of 94,180
gpd, and a peak hour flow of 85.3 gpm.
Table A-1. The Guthrie Estimated Water Use per COB Methodolgy
Estimated Wastewater Flow
Per the DSSP and comments received by the engineering department the average day flow is 64.4
gallons per day per capita (gpd/capita). Infiltration and inflow is 150 gallons per day per acre, and the
peak hour flow is per the peaking factor formula provided in the DSSP. The results of this
methodogy are shown in Table A-2. This results in a average day wastewater flow estimate of
15,639 gpd, and a peak hour flow of 44.7 gpm.
Table A-2. The Guthrie Estimated Wastewater Flow per COB Methodolgy
The Guthrie Estimated Water Use per COB Methodology
Unit Type Units Capita/Unit Population GPD/Capita Water Use (gpd)
Single 44 2.17 95 170 16,232
Double 67 2.17 145 170 24,716
Total 111 241 Average Day Use (gpd)40,948
Max Day Factor 2.30
Max Day Use (gpd)94,180
Peak Hour Factor 3
Water Use Peak Hour Flow (gpm)85.3
The Guthrie Estimated Wastewater Flow per COB Methodology
Unit Type Units Capita/Unit Population GPD/Capita Wastewater Flow (gpd)
Single 44 2.17 95 64.4 6,149
Double 67 2.17 145 64.4 9,363
Total 111 241 Average Day Domestic Use 15,512
Site Area (ac)0.84
Inflow & Infiltration (gal/ac/day)150
Inflow & Infiltration (gpd)127
Average Day Wastewater Flow (gpd)15,639
;P = Pop. ÷ 1,000 Peaking Factor (PF)4.12
Wastewater Peak Hour Flow (gpm)44.7
Bozeman Water DistributionSystem Model CalibrationField Test Data Sheet
Test Number: __________
Test Date: ______________
Start Time: ___________
End Time: ___________
Test By: _________
Zone: ___________________
48
316 18 210RESIDUAL HYDRANT
Hydrant No. ______
HPR No. ________
Static: __________
Residual: ________
FLOW HYDRANT #1
Hydrant No. ______
HPR No. ________
Flow: ______________
FLOW HYDRANT #2
Hydrant No. ______
HPR No. ________
Flow: ______________
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18
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7TH AVE9TH AVEBEALL ST10TH AVELAMME ST
VILLARD ST 6TH AVE8TH AVEPEACH ST
SHORT ST
7TH AVE8TH AVEG!!Flow Hydrants GFOther HydrantsG!!Test Hydrants
9/30/2015
7:50 AM
8:07 AM
JDH
1240
137.6 psi
126.3 psi
1241
1,817 gpm 1,661 gpm
201250
SOUTH