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NORTHWEST CROSSING SUBDIVISION
PHASE 1
SANITARY SEWER SYSTEM IMPROVEMENTS
SANITARY SEWER EXTENSION BASIS OF DESIGN REPORT
February, 2020
PREPARED BY:
MMI #: 5659.005
Table of Contents
1 PROBLEM DEFINED (DEQ 11.11) ...................................................................................................... 1
2 DESIGN CONDITIONS (DEQ 11.12) ................................................................................................... 1
3 IMPACT ON EXISTING WASTEWATER FACILITIES (11.13) ............................................................ 2
4 PROJECT DESCRIPTION (11.14) ....................................................................................................... 3
5 DRAWINGS (11.15) .............................................................................................................................. 3
6 DESIGN CRITERIA (11.16) .................................................................................................................. 3
7 SITE INFORMATION (11.17) ............................................................................................................... 3
8 ALTERNATIVE SELECTION/ANALYSIS (11.18) ................................................................................ 3
9 ENVIRONMENTAL IMPACTS (11.19) ................................................................................................. 3
EXHIBITS
Exhibit A Vicinity Map
Exhibit B Framework Plan – by Norris Design
Exhibit C C310- Sewer Concept Plan
Exhibit D Northwest Crossing Subdivision Preliminary Plat- Sheet 4
APPENDICES
Appendix A NRCS Soils Report
Prepared by:
Morrison-Maierle, Inc.
2880 Technology Blvd. W.
Bozeman, Montana 59771
Phone: (406) 587-0721
Fax: (406) 922-6702
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Northwest Crossing Subdivision: Phase 1
Sanitary Sewer Design Report
EXECUTIVE SUMMARY
The proposed Northwest Crossing Subdivision encompasses a total of 160.5 acres and is
generally located at the northwest corner of West Oak Street and Cottonwood Road. The
property is within the boundaries of the City of Bozeman, Montana and was annexed and zoned
REMU (130.994 acres) and B2-M (29.513 acres) in January of 2019. Phase 1 consists of
development tracts A-E for a total of 32.9 acres as detailed in the Master Plan Framework
developed by Norris Design (Exhibit B). See the vicinity map on Exhibit A for details.
The following is the legal description for the property.
Tract 5 of Certificate of Survey No. 2552, located in the NE1/4 of Section 4, Township 2 South,
Range 5 East, Principal Meridian, Gallatin County, Montana.
Sewer collection for the project will consist of sewer mains located in the right-of-way or
easements with individual services stubbed to each property or building location. Sewer mains
will generally flow to the north and east and connect to an existing 21” PVC main in Cottonwood
Road, which drains to a 27” interceptor in Baxter Lane.
This design report provides a basis of design for the sewer collection system for the Northwest
Crossing Subdivision. The sewer collection will be designed and installed in accordance with
the Montana Department of Environmental Quality (MDEQ) Circular No. 2; Montana Public
Works Standard Specifications (MPWSS); The City of Bozeman Modifications to MPWSS; City
of Bozeman Design Standards and Specifications Policy March of 2004, and all Addendum’s;
and the City of Bozeman Wastewater Facility Plan.
1 PROBLEM DEFINED (DEQ 11.11)
The purpose of this design report is to quantify the anticipated sewer flows from the Northwest
Crossing Subdivision and provide preliminary sizing for the proposed sewer infrastructure. The
flows from Phase 1 of the Northwest Crossing Subdivision will discharge into the existing 21”
sewer main in the Cottonwood Road right of way and the 27” sewer interceptor in Baxter Lane.
The following design report follows the section numbering of the Circular DEQ-2 Standards for
Public Sewage Systems, 2018.
2 DESIGN CONDITIONS (DEQ 11.12)
Northwest Crossing Subdivision Phase 1 consists of 16 tracts. 14 tracts are zoned B2-M and 2
tracts are zoned REMU. See sheet of the preliminary plat (Exhibit D) for details (the 16 tracts
exclude all of the restricted lots which are subject to further subdivision review. For the purpose
of this report, the service areas are defined by the Framework Plan (Exhibit B) as developed by
Norris Design for the Master Site Plan. In this plan, Phase 1 is divided into five parcels. Parcels
A and B will be commercial, while C, D and E will be multi-family residential units.
The wastewater generation for the proposed development includes sewer services for B2-M and
REMU Zoning designation includes uses outlined in the UDO Sec. 38.300.110. Design
flowrates for this development are based on the “City of Bozeman Wastewater Collection
Facilities Plan Update” prepared my Morrison-Maierle, Inc. in 2015. Based on Table 2-13, the
REMU zoned parcels use a flowrate of 1,456 gal/acre/day which is developed from an estimate
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of 2.17 people per dwelling and a maximum of 10.4 units per acre. The B-2M parcels are
designated a flowrate of 2,000 gal/acre/day. The peak hour factor is calculated based on the
City Design Standards and Specification Policy). The final addition to projected wastewater
generation is infiltration/inflow (I/I) which is 150 gpd/acre per the City Design Standards and
Specifications Policy (Section V.B.4 - Page 48).
The anticipated sewer flows are calculated as follows:
Table 1 – Project Anticipated Demand
Development
Parcel Area (acres)Average Demand 1(2,400
gpd/ac)
Average Demand
(gpm)Equivalent Population Peaking Factor
Peak Hour Flow
(gpm)
Infiltration
(gpm)
Design Flow
(gpm)
A 9.0 21672 15.1 337 4.06 61.05 0.94 61.99
B 8.5 20400 14.2 317 4.07 57.63 0.89 58.52
TOTAL 17.53 42072.0 29.2 653.3 -118.7 1.8 120.5
Development
Parcel Area (acres)Total Allowable Units /
Designed Numer of Units
Population
2(2.17 ppl/DU)
Average Demand
2(64.4gpd/person)Average Demand
(gpm)
Peaking Factor Peak Hour Flow
(gpm)
Infiltration
(gpm/acre)Design Flow (gpm)
C 7.58 79 171 11016.61 7.65 4.17 31.92 0.79 32.71
D 3.36 35 76 4883.35 3.39 4.27 14.50 0.35 14.85
E 4.42 46 100 6423.94 4.46 4.24 18.93 0.46 19.39
TOTAL 15.36 159.7 346.6 22323.9 15.5 -65.3 1.6 66.9
Residential (REMU)
Community Commercial Mixed Use -(B2-M)
1 2015 City of Bozeman Wastewater Facility Plan (Table 2-14, Community Commercial)
2 2015 City of Bozeman Wastewater Facility Plan
Phase 1: Total Average Daily Flow
Total Average Daily Flow = 29.2 + 15.5 gpm
= 44.7 gpm
= 64,368 gpd
Phase 1: Total Peak flow
Total Peak Flow = 120.5 + 66.9 gpm
= 187.4 gpm
Sewer Capacity Calculations
3 IMPACT ON EXISTING WASTEWATER FACILITIES (11.13)
The property is within the wastewater planning boundary for the City of Bozeman as described
in the 2015 City of Bozeman Wastewater Collection System Facilities Plan. The property is
planned to be served by the proposed Davis Lift Station. This lift station and associated piping
to serve the drainage basin is currently under design by HDR Engineering. We have contacted
HDR and they have confirmed that infrastructure is currently being designed and approved for
bid in the spring of 2020.
All of Phase 1 will flow into the existing Cottonwood main in two locations. New manholes will
be placed on the main approximately 1200 ft and 1800 ft north of the Oak Street intersection.
See exhibit C310 for the proposed sewer layout. Capacity for the entire Northwest Crossing
subdivision is addressed in the Master Site Plan Sewer Design Report included in Appendix B.
From a treatment perspective, the City of Bozeman’s Water Reclamation Facility has adequate
capacity to serve the development.
3
4 PROJECT DESCRIPTION (11.14)
The purpose of this design report is to quantity the anticipated sewer flows from Phase 1of the
Northwest Crossing Subdivision and provide preliminary sizing for the proposed sewer
infrastructure. The flows from the Northwest Crossing Subdivision will discharge into the
existing 21” sewer main in the Cottonwood Road right of way and continue on to the 27” sewer
main in Baxter Lane.
5 DRAWINGS (11.15)
Drawings identifying the site of the project, including the location and alignment of proposed
facilities are included in this submittal. See Exhibit C.
6 DESIGN CRITERIA (11.16)
Design criteria including average and peak flows were provided in previous sections. The
proposed sewer mains will conform to the State’s minimum vertical and horizontal separation
criteria from water mains.
The proposed conventional gravity sewer collection system is to be constructed to City of
Bozeman and the 6th Edition of Montana Public Works (MPW) standard specifications. The
sewer mains shall be SDR 35 PVC pipe. All manholes shall be standard concrete manholes
spaced no more than 400 feet apart. An 8 inch pipe at minimum slope and 75% full has the
capacity of about 313 gpm, which is greater than the estimated peak flow of 187.4 gpm.
7 SITE INFORMATION (11.17)
The development is bounded by Cottonwood Street (principal arterial) to the east, Baxter Lane
(minor arterial) to the north, Laurel Parkway (collector) to the west and West Oak Street
(principal arterial) to the south. The proposed development is currently undeveloped agricultural
land which slopes gently to the north with existing grades of less than 4% and mostly
Meadowcreek, Turner and Amsterdam silt loam soil deposits (USDA, Natural Resources
Conservation Service, Web Soil Survey). See the soils report located in Appendix A for details.
8 ALTERNATIVE SELECTION/ANALYSIS (11.18)
No proposed alternatives were considered
9 ENVIRONMENTAL IMPACTS (11.19)
There are no expected environmental impacts from this sewer main extension, as the City of
Bozeman Wastewater Treatment Plant has more than adequate capacity for this extension.