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Truman Flats
AKA Lots 1 and 2 of Block 4 of Canyon Gate Subdivision
Site, Water and Sewer, and Storm Water Engineering Report
May 19th, 2023, Site Plan Application
Prepared for: Canyon Gate Investors, LLC
Prepared by: Stahly Engineering and Associates
Engineer of Record: Cordell D. Pool, PE
Quality Control Reviewer: Zach W. Lowe, PE
Introduction
Truman Flats is a multi-household development on Lots 1 and 2, Block 4, of Canyon Gate
Subdivision. The current zoning of the two lots is R-5. Canyon Gate Subdivision was recently
awarded Preliminary Plat approval and is currently undergoing infrastructure review and is planned
to be platted later this summer. Truman Flats will be constructed concurrently with the subdivision
infrastructure improvements.
The proposed development is shown in Figure 1 and consists of identical buildings and parking lots
on each lot, only mirrored. Each building will be a 4-story apartment building with 74 units. Units will
be a mix of 1, 2 and 3 bedrooms and at least 60 units will be workforce housing units per the zoning
and annexation agreement.
Figure 1 – Truman Flats Site Plan
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The infrastructure Engineering Reports and Traffic Impact Study for these two lots were included in
the Canyon Gate Subdivision reports. The adjacent City Streets and Public Park will be completed
with the subdivision improvements. Mass grading of the lot will also be completed as part of the
subdivision drainage and grading improvements.
A geotechnical investigation was conducted with the Canyon Gate Subdivision. The geotechnical
report determined elevations of target bearing gravel and groundwater. After site grading is
complete the depth to target bearing gravel is estimated to be approximately 8 feet and the depth to
seasonal high groundwater is estimated to be approximately 9 feet.
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 Improvements
Site improvements consist of site parking and hardscape improvements, franchise utility services,
building water and sewer services, and on-site stormwater mitigation. As stated previously the site
development proposed for each lot is identical, only mirrored. Each parking lot includes 86 parking
spaces, which includes 4 accessible spaces and 18 compact spaces. Lot 1 has 371’ of on-street
parking, reducing the required on-site parking by 15 spaces. Lot 2 has a total of 363’ of on-street
parking, reducing the required on-site parking by 14 spaces. Each parking lot has 4 additional
spaces, which will be used seasonally to provide additional room for snow storage.
Fire protection is provided from adjacent City streets and fire hydrants. Trash will be collected from
on-site trash enclosures located at the north edge of the parking lots. A vehicle turning analysis
included on the plans show that a City of Bozeman Trash Truck can turn around in the parking lot
and have straight access to the trash enclosure. Additionally, a turning movement analysis shows
that a typical ambulance can turn around in the parking lot to access the rear building entry if
required.
Figure 2 – Turning Analysis (see plan sheet for additional details)
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Franchise utilities (power and communication) will be served from mains located in 10’ utility
easements along the streets and installed with the subdivision improvements. All utilities are
underground and building meters are proposed within a semi-enclosed alcove within the building
footprint and screened from view.
Water and Sewer
Water and sewer mains will be installed in the streets as part of the Canyon Gate subdivision
improvements. Engineering analysis conducted with the subdivision show that adequate capacity
exists to supply Truman Flats with water supply, fire protection and sewer service. The water
pressure is estimated to be approximately 120 psi. Fire flow analysis conducted with the subdivision
show fire hydrants capable of providing over 3000 gallons per minute (gpm).
Water and sewer service to Truman Flats will be by extension of services installed with the
subdivision improvements. Services are sized in accordance with plumbing and fire code and fixture
counts. Water fixture counts are estimated to be 1091.5 fixture units, equating to a peak
instantaneous flow of 205 gpm. This will be provided by a 4” domestic service with a 3” meter. The
fire sprinkler water service will be 6”. Wastewater fixture counts are estimated to be 984 fixture
units, requiring an 8” service. The sewer service has a manhole at the main, installed with
subdivision improvements.
Cash-in-lieu of water rights for this lot were not paid with the subdivision and will be required for
domestic use and on-site irrigation.
Storm Water
Canyon Gate Subdivision improvements provide stormwater collection and treatment for City Streets
and Public Parks. This area also includes front yard setbacks. All lots within Canyon Gate
Subdivision are required to provide on-site stormwater mitigation to manage stormwater originating
from the lot development.
Both Truman Flats lots (Lots 1 and 2 of Block 4) will have on-site subsurface retention systems
beneath the parking lots that capture and retain site and building storm runoff. The retention system
consists of a gravel bed with a single row of Stormtech chambers and inlet/access manholes at each
end. The parking lots are graded to the retention system inlets. Runoff from building rooftops is
piped underground to the retention system. The roof drain piping is 8” diameter in accordance with
building code requirements.
The retention system is sized to contain runoff from the 10-year, 2-hour storm event which is a 0.81”
storm event. This storm event represents the 96th percentile storm event in Bozeman. The system
is sized without consideration of infiltration, which provides significant additional capacity and a
comfortable margin of safety.
Stormwater runoff volumes were determined by the Rational Method with a weighted coefficient
determined for each site’s building rooftops, pavement area, sidewalks and patios, and landscaped
areas. Storm runoff calculations and required retention volume are shown in Table 1. This table
also shows the retention system sizing calculations. The retention volume provided exceeds the
storm runoff, without considering any infiltration that occurs during the storm event.
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Groundwater monitoring wells were installed as part of the geotechnical investigation for the Canyon
Gate Subdivision. The depth to seasonal high groundwater beneath the retention systems is
approximately 8’ below finished grade, or approximately 3’ below the bottom of the chambers.
Table 1. Truman Flats Storm Water Calculations
Stormwater system maintenance. Subdivision covenants identify lot owner’s responsibility to
monitor and maintain their stormwater mitigation system. Each lot owner shall monitor and maintain
their on-site Stormwater Retention System. The systems are designed to capture sediment and
debris and require 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 systems consist 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 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.
RETENTION VOLUME CALCUATIONS
Drainage Area Runoff Coeff. C Block 4, Lot 1 Block 4, Lot 2
Rooftop area (sf)0.9 18,437 18,437
Pavement Area (sf)0.9 29,245 29,245
Sidewalk and Patio Area (sf)0.9 5,482 5,482
Landscape Area (sf)0.2 20,323 21,405
Total Area (sf)73,487 74,569
Weighted Runoff Coeff. (C )0.71 0.70
10-yr 2-hr Rainfall (in)0.82 0.82
Volume Req'd (cf)3547 3562
RETENTION SYSTEM SIZING CALCULATIONS
Gravel Bed Length (ft)160 160
Gravel Bed Width (ft)15 15
Gravel Bed Depth (ft)4.00 4.00
Cube Volume (cf))9600 9600
Chambers Length (ft)154 154
Number of chambers 22 22
Storage per chamber (cf)45.90 45.90
Chambers Storage (cf)993 993
Net Gravel Volume (cf)8607 8607
Gravel porosity 0.30 0.30
Total Gravel Storage (cf)2582 2582
Total Retention Volume (cf)3,575 3,575