HomeMy WebLinkAboutEngineering Report - Sewer
Engineering Report
Sewer Main Extension
Six Range Condominiums Development
July 2022
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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Sewer Main Extension 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. Wastewater Generation and Conveyance Capacity
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11.1 Engineering Report – General Information
The proposed Six Range Condominiums Development is located southwest of the
intersection of West Babcock Street and South Ferguson Avenue. The Six Range
Condominiums Development consists of 3 large apartment buildings,4 triplexes,
and a relatively small commercial building. This project is located on a currently
undeveloped 8.64-acre lot, Tract A1 of Minor Subdivision #338.
The proposed apartment buildings consist of units ranging from one to three-
bedrooms, and each triplex unit will be three bedrooms. The apartment buildings
will provide 63 one-bedroom units, 102 two-bedroom units, and 15 three-bedroom
units. The four triplexes will provide 12 additional three-bedroom units. The
commercial building will provide 2500 sq ft of office space and 1300 sf of retail
space.
The project will be served by extension of City of Bozeman sewer mains
throughout the project. Individual service lines will be provided to each apartment
building, triplex, and the single commercial building. City of Bozeman sewer mains
currently existing in W. Babcock St., and Slough Creek Dr. The 10” sewer main in
W. Babcock St. terminates just past Kimball Dr. The 8” main in Slough Creek Dr.
extends to a manhole located in the drive access on the site, it then conveys north
where it connects to the W. Babcock St. main.
The developer’s contact information is:
Paine Group
Matt Paine
P.O. Box 418
Bozeman, MT 59771
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 Six Range Condominiums Development 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 3 new 8” main extensions, each
extending off of the existing 10” main in Babcock Street. Services will
connect each individual structure to the new main extensions. The
easternmost extension extends the existing main 140 feet east, within the
Babcock ROW, and heads south into the proposed development. Service
for the commercial building (Building D) will be connected to this extension.
Both of the remaining extensions are 8” mains extended south from new
manholes on the existing main in Babcock. The easterly service of these
two extensions runs approximately 190 ft south underneath the proposed
drive aisle and parking lot, terminating in a new manhole. Two of the three
large apartment buildings will be served by this extension. The western
main extension extends 151 feet south, before tee-ing off into a 320 foot
east-west length of pipe that serves each of the triplex buildings.. Finally,
from the westernmost end of this length of pipe, a final 185 length of main is
extended south. The two remaining apartment buildings will connect to this
main, with Building A featuring two connections.
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A total of 893 linear feet of 8” sewer main is proposed to serve the
development. Twelve new manholes are proposed, with 3 of the proposed
manholes installed on the existing main in Babcock. All proposed main
extensions will run within 30’ water/sewer easements or within street rights-
of-ways. A total of 9 service connections will be made to these extensions,
with Building B utilizing two service connections.
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 Six Range Condominiums
development has been determined utilizing values provided in City design
standards and Facility Plans and is shown in Table 1. For sewer main
sizing, typical City design flows are utilized.
Typical design flow for sewer use is 65 gallons per day (gpd) per capita,
with an average of 2.17 people per dwelling. Both office and retail space are
estimated to use 30 gpd/1000 sf of floor space. To estimate peak flows, a
peaking factor of 4.5 was used to determine the peak hour flow rate. The
estimated total project design flow is 27,042 gpd with a peak hour flow of
84.51 gpm.
Table 1 – Wastewater Generation
Estimated Sewer Use and Demands
On-site Residential #Units Capita/Unit Gpd/Capita Gpd/Unit Gallons/Day
Building A 48 2.17 65 141 6,770
Building B 75 2.17 65 141 10,579
Building C 57 2.17 65 141 8,040
Triplexes 12 2.17 65 141 1,693
Non-Residential Area (Sf) Gpd/1,000 Sf Gallons/Day
Office Space 2,500 30 75
Retail Space 600 30 18
Total Domestic Use 27,175
Sewer Infiltration Area (Ac) Infiltration (Gal/Ac/Day) Gallons/Day
Onsite 8.64 150 1,296
Total Sewer Design Flow (Gpd) 28,453
Average Day Demand (Gpm) 19.76
Peaking Factor 4.5
Peak Hour (Gpm) 88.91
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Capacity of 8" SDR 35 PVC Sewer Pipe
Status Proposed
Location 6 Range
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) 312.8
Peak Hour Flow (gpm) 88.91
Peak Flow % of Max Capacity 28.42%
Table 2, above, shows that an 8” sewer main has a 75% flow depth capacity
of 313 gpm installed at a minimum slope of 0.4%. An 8” sewer main at
minimum slope (0.4%) has sufficient capacity to provide sewer service for
the Six Range Condominiums Development.
11.13 Impact on Existing Wastewater Facilities
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 Bozeman’s collections system to
rectify these limitations have been identified as “future improvements”, the
designation for limitations that are not urgent.
The capacity within the Valley West Sewer trunk main was evaluated after
possible capacity issues were raised by the City of Bozeman Engineering
Department. By reviewing the “Valley West Sewer Trunk Line” design
report (Morrison-Maierle, 1998), we determined that the original design of
the trunk main allocated less demand from the proposed Six Range site, but
quite a bit more demand from the adjacent Cottonwood Condominiums.
The “Design Report Water, Sewer and Stormwater Management
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Cottonwood Condominiums” (C&H Engineering, 2004) estimated 4,389 gpd
on an average day (excluding infiltration), and the Morrisson-Maeirle study
allocated 44,697 gpd from the Cottonwood Condominiums. This 40,000
gpd discrepancy more than makes up for the discrepancy of the Six Ranges
site, where the Morrison-Maeirle report estimated 13,445 gpd and we have
calculated 27,175 gpd for the developed site.
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 Six Range Condominiums Development 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 Six Range Condominiums Development project is located on the
southwest quadrant of Babcock Ave. and Ferguson Ave. and is currently
undeveloped. Adjacent development consists primarily of residential
subdivisions. The lot is bounded by West Babcock Street, South Ferguson
Avenue, South Hanley Drive to the west, and an asphalt path to the south.
Due to the adjacent residential development, water, sewer, and shallow
utilities are readily available for connection in the area. The Vicinity Map,
seen in Figure 1 in this report, shows the existing conditions and
surrounding development.
A geotechnical evaluation was conducted by Allied Engineering and is
provided in a separate appendix of this submittal. The soil conditions across
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the site are consistent. The geotechnical investigation revealed that the site
consists of predominantly native soils including 12 to 18 inches of topsoil
overlaying sandy gravel. Full geotechnical evaluation results are available in
the attached appendix.
Groundwater monitoring was conducted throughout the 2021 groundwater
season. During the initial site evaluation and monitoring well installation,
groundwater depths ranged from 5-6’ below existing grade. Based on these
results, seasonally high groundwater is assumed to be at least 5’ below
existing grade. 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. Other configurations of
main extensions were considered during design. The selected design was
chosen as the best fit with the site’s constraints and requirements.
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.
W. BABCOCK ST. (90' ROW)N. FERGUSON AVE. (50' ROW)
S. HANLEY AVE.
KIMBALL AVE.
HANLEY AVE.SLOUGH CREEK DR.S Hanley Ave.Palisade Dr.Mill Creek Dr.SCALE:DRAWN:PROJECT NO:3081-001204270 W Babcock Street, Bozeman MT
Contractor Note: All site conditions and dimensions to be
varified on-site. Inform Architect of design discrepancies.PO box 1001 Bozeman, MT 5977112/13/2021DATE:PROFESSIONALENGINEERS &SURVEYORSSTAHLYENGINEERING& ASSOCIATESSix Range Condominiums
Site Plan Submittal
Paine Group
DRC Site Plan Resubmittal 2021.12.08FIGURE21" = 40'Sewer Plan