HomeMy WebLinkAbout20 - Design Report - Bridger View PUD - Sewer
Engineering Report
Sewer Main Extension
Bridger View PUD
October 2020
Submitted to:
City of Bozeman Engineering Department
20 East Olive
Bozeman, MT 59715
Prepared by:
Stahly Engineering and Associates
851 Bridger Drive, Suite 1
Bozeman, MT 59715
(406) 522-8594
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Engineering Report
Contents
Note: The Engineering Report format follows DEQ-2 organization. Additional
information required by the City of Bozeman Design Standards is added where
required.
Report Narrative
Chapter 10 – Engineering Reports & Facility Plans
11.1 Engineering Report - General Information
11.11 Problem Defined
11.12 Design Conditions
11.13 Impact on Existing Wastewater Facilities
11.14 Project Description
11.15 Drawings
11.16 Design Criteria
11.17 Site Information
11.18 Alternate Selection/Analysis
11.19 Environmental Impacts
Figures and Tables
Figure 1. Vicinity Map
Figure 2. Sewer Plan
Table 1. BVR Sewer Flow Estimates
Table 2. Capacity of 8” SDR 35 PVC Sewer Main
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11.1 Engineering Report – General Information
The proposed Bridger View Planned Unit Development (PUD) is located
southwest of the intersection of Bridger Drive (MT 86) and Story Mill Road.
Bridger View PUD is a 62-home neighborhood consisting of modest homes
with a compact, urban design. This project is located on approximately 8
acres which was formerly occupied by 40 mobile homes within the Bridger
View Trailer Park. The Bridger View Trailer Park was connected to the
City’s sewer system via private sewer mains, which have been
disconnected and abandoned.
The proposed individual lots are compact with small yards and common
open space. Houses will be constructed by the developer (HRDC) and
range in size from 1, 2, and 3 bedrooms roughly proportioned at 25%, 50%
and 25% respectively. The subdivision will create 52 residential single-
household lots and 5 duplex lots. A common house is also proposed for
community functions. Five common garages are proposed which will not
have water or sewer facilities.
The project will be served by extension of City of Bozeman water and sewer
mains throughout the project. Individual service lines will be provided to
each residential lot and the common house. City of Bozeman water and
sewer mains currently exist along Bridger Drive to the north of the project.
The previous water and sewer lines serving the trailer park do not meet City
standards and will be abandoned during construction. The trailer park water
supply wells in the northern portion of the site will be used for irrigation of
the project.
The developer’s contact information is:
HRDC
Heather Grenier
32 South Tracy Ave.
Bozeman, MT 59715
Water supply and wastewater treatment will be provided by the City of
Bozeman’s municipal systems. The system owner’s contact information is:
City of Bozeman Public Service Department
PO Box 1230
Bozeman, MT 59771-1230
The location of the Bridger View PUD site is shown on the Vicinity Map in
Figure 1.
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Figure 1. Vicinity Map, Bozeman, Montana
11.11 Problem Defined
Sanitary sewer service will be provided by an existing 15” sewer main
running in an easement through the northern portion of the site. The
easement was provided by the Trust for Public Lands, the previous
landowner, to allow the City to construct this master planned trunk sewer
which will serve future development to the south and east. The easement
was located within the property to protect the existing mature spruce trees
along Bridger Drive. Lots adjacent to this sewer main (10 homes) can be
served by new services installed on this main. Other lots will be served by
new 8” sewer main extensions from the 15” main. The proposed sewer
system improvements are shown on the Sewer Plan (Figure 2) at the end of
this report.
New sewer mains will be in sewer and/or water and sewer easements and
street rights-of-way running through the property. Where water and sewer
are running together, the easements are 30’ wide. Where sewer mains run
individually, a 20’ easement is proposed. These 20’ easements are
narrower than City standards but are located adjacent to common open
space with a public access easement, which can provide additional access
to the sewer main. A deviation from the City standard is requested for the
20’ wide sewer main easements. This deviation allows trees to be planted in
the common open space just outside of the easement. No trees are
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proposed within the water and sewer easements, or within 10’ of water and
sewer mains.
Each lot will have an individual 4” sewer service from the main to the front of
the structure, except for the homes served by new services on the existing
main which are served from the rear. Due to the small lot size, a deviation is
requested to have 5’ separation between water and sewer services and
between services and trees. This deviation will allow for the trees to be
planted in the front yards of the lots and in common open spaces. Without
this deviation, very few trees could be planted on lots and in common open
spaces.
To avoid conflicts with water and sewer services, power and communication
lines are typically run in easements at the rear of the lots.
11.12 Design Conditions
Sanitary sewer service will be provided by the City of Bozeman Municipal
system. This project is located within the planned service area of the City’s
system.
Estimated wastewater generation for the Bridger View PUD has been
determined utilizing values provided in City design standards and Facility
Plans and is shown in Table 1. Due to the small, efficiently designed homes,
and the irrigation by wells, the water use is anticipated to be lower than
typical. However, for sewer main sizing, typical City design flows are
utilized.
The estimated residential wastewater generation is based on the City of
Bozeman design standards. Typical design flow is 89 gallons per day (gpd)
per capita, with an average population of 2.11 people per home. In
accordance with DEQ-2 a peaking factor of 4.2:1 was used to determine the
peak hour flow rate. For design purposes the common house was
estimated to have the same wastewater generation as a typical home, for a
total of 63 users. The estimated total project design flow is 11,831 gpd with
a peak flow of 35 gallons per minute (gpm).
It is likely that adjacent upgradient property to the south will be served by
extension of the Bridger View PUD sewer mains. This adjacent property is
within the City but continues to use its original septic systems that pre-dated
annexation. It is estimated that approximately 3.1 acres of additional area
will be served by sewers running through the Bridger View PUD. Of this,
approximately 1.2 acres is zoned B-1, and approximately 1.9 acres is zoned
R4. Using City of Bozeman design standard allocation rates, it is estimated
that this adjacent property will contribute approximately 5,000 gpd to Bridger
View sewers.
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In accordance with City design standards, infiltration and inflow of 150
gpd/acre is added to the domestic wastewater generation. The total design
flow from the Bridger View PUD and adjacent property wastewater
generations with infiltration and inflow is approximately 18,467 gpd, with a
peak hour flow of 53.5 gpm.
Table 1 Bridger View PUD Sewer Flow Estimates
On-site Residential #Units Capita / Unit Gpd/Capita Gallons/day
Housing Unit 63* 2.11 89.0 11,831
Offsite added areas Area (acres) Gal / Acre / Day
Offsite Zone B1 1.20 1000.00 1,200
Offsite Zone R4 1.93 1950.00 3,764
Total Domestic Use (gpd) 16,794
Sewer Infiltration & Inflow Area (acres) Infiltration
(gal/ac) Gallons/day
On-site 8.02 150.00 1,203
Off-site 3.13 150.00 4,670
Total Sewer Design Flow (gpm) 18,467
Average Day Demand (gpm) 12.8
Peaking Factor 4.2
Peak Hour Flow (gpm) 53.5
*Includes Common House
An 8” sewer main at minimum slope (0.4%) has sufficient capacity to
provide sewer service for Bridger View PUD and adjacent property at the
estimated flow of 53.5 gpm. Table 2 shows that an 8” main has a 75% flow
depth capacity of 313 gpm installed at a minimum slope of 0.4%.
Table 2 Capacity of 8" SDR 35 PVC Sewer Main
Status Proposed
Location Bridger View
Pipe Material 8” SDR 35 PVC
Pipe Diameter (in) 8
Minimum Pipe Slope (ft/ft) 0.004
Coefficient of Roughness (n) 0.013
Depth in Pipe at 75% Full (in) 6
Depth of Flow (ft) 0.5
Pipe Radius (in) 4
Pipe Radius (ft) 0.33
Wetted Perimeter 1.40
Area of Flow (ft2) 0.28
Hydraulic Radius 0.20
Velocity (ft/sec) 2.48
Maximum Pipe Flowrate (gpm) 313
Peak Flow % of 75% Max Capacity 17%
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11.13 Impact on Existing Wastewater Facilities
The existing 15” sewer providing service to Bridger View PUD was recently
installed in an easement provided by the previous landowner, the Trust for
Public Lands, and has no capacity limitations. This sewer is planned to
provide service to future development along Bridger Drive well into the
future.
A review of the City of Bozeman 2015 Wastewater Facility Plan identifies
future capacity limitations in the collection system downstream of this
project impacting a large portion of Bozeman. This limitation is not directly
caused by this project and upgrades to this interceptor sewer are currently
planned for FY 2023 in the City’s Capital Improvements Plan.
11.14 Project Description
The project description is provided at the beginning of the Engineering
Report.
11.15 Drawings
Please see Figure 2, the Sewer Plan, at the end of this report which shows
the proposed sewer improvements. Plan and profile drawings will be
submitted with the infrastructure design submittals.
11.16 Design Criteria
The Bridger View PUD sewer main extensions will be designed in
accordance with Montana Department of Environmental Quality Circular 2,
and the City of Bozeman Design Standards and Specifications Policy.
Construction specifications will be Montana Public Works Standards and
Specifications (MPWSS), and the City of Bozeman Modifications to
MPWSS.
11.17 Site Information
The Bridger View PUD site is located on the former site of the Bridger View
Trailer Park. The trailer park was served by private sewer mains connected
to the City system and with a privately-owned, on-site water supply. The
existing sewer mains and water lines will be abandoned with site
construction.
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The trailer park had some minor cut/fill grading to establish level trailer sites;
but previous site work did not significantly impact the soils since the mobile
homes did not have foundations. Beyond the minor site grading, the soils
are typical of the Bozeman area. A geotechnical evaluation was conducted
and is provided in a separate appendix of the submittal. The soils can
generally be described as a deep topsoil overlaying a fine-grained subsoil,
with gravelly sand material at a depth of approximately 8 feet.
Because basements are proposed for the new homes, groundwater
monitoring was conducted throughout the summer of 2019. Monitoring
results are provided with the geotechnical report. Groundwater depths were
generally greater than 10 feet and should not impact homes or utilities. The
underlying gravelly sand material will provide a suitable foundation material
for pipes and basements.
The site is not located within a 100-year floodplain.
11.18 Alternative Selection/ Analysis
Due to the project’s location within the City of Bozeman, extension of public
mains was the only sewer collection alternative.
11.19 Environmental Impacts
Continued use and new extensions of City of Bozeman municipal sewer
mains has the least environmental impact. No new environmental impacts
will be caused by these sewer main extensions.
PHASE 1PHASE 3PHASE 1PHASE 2PHASE 1PHASE 2PHASE 2PHASE 3STORY MILLSPUR TRAILSTORY MILL COMMUNITY PARKCITY OFBOZEMANPROPERTYSTORY MILL ROAD (60' ROW & 30' ESMT)BRIDGER DRIVE (Aka Hwy 86) - (110' ROW)HILLSIDE LANE(60' ESMT)HILLSIDE LANE (64' ROW)FLOURHOUSECOURTMILLWORKSCOURTLOT 56LOT 53LOT 49LOT 46LOT 21LOT 40LOT 35LOT 23LOT 13LOT 26LOT 27LOT 4LOT 16LOT 19LOT 47LOT 12LOT 44LOT 43LOT 39LOT 22LOT 38LOT 31LOT 33LOT 42LOT 2LOT 41LOT 37LOT 10LOT 9LOT 32LOT 29LOT 30LOT 5LOT 7LOT 8LOT 52LOT 55LOT 57LOT 54LOT 51LOT 50LOT 48LOT 45LOT 17LOT 25LOT 14LOT 15LOT 11LOT 20LOT 24LOT 36LOT 34LOT 1LOT 6LOT 3LOT 28HILLSIDE GREENHILLSIDE GROVEPHASE 2PHASE 3BLOCK 1BLOCK 2BLOCK 3BLOCK 4PATH TO THE "M"PARKSIDE WALKMILLWORKS ROW
FLOURHOUSE WAY (30' ROW)BLUE SILOS WAY (30' ROW)BLUE SILOSCOURTMILLWORKS WAY (30' ROW)ADDITIONAL ROW 1976 SFLOT 18LOTC-1LOTC-2LOTC-4LOTC-5LOTC-3LOTC-6OS 1OS 5OS 6OS 3OS 4OS 7OS 7OS 8OS 7C3.0SEWER PLANPROFESSIONALENGINEERS &SURVEYORSBRIDGER VIEW STAHLYENGINEERING& ASSOCIATESHRDC BOZEMAN MONTANA