HomeMy WebLinkAbout006 Sewer Report
North 3rd Apartments
Sanitary Sewer Design Report
PREPARED FOR:
DEVCO, LLC
PO Box 4108, Bellevue, WA 98009
(425) 736-0580
NOVEMBER 2022
PREPARED BY:
1000 2nd Avenue, Suite 3900, Seattle, WA 98104
(206) 607-2600
NORTH 3RD APARTMENTS, LLC.
Sanitary Sewer Design Report │ DEVCO North 3rd Apartments
November 2022
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TABLE OF CONTENTS
1. INTRODUCTION ............................................................................................................. 1
1.1. PROJECT BACKGROUND ............................................................................................ 1
1.2. PROJECT PURPOSE .................................................................................................... 1
2. SITE CONDITIONS ......................................................................................................... 3
2.1. GENERAL EXISTING SITE CONDITIONS ..................................................................... 3
2.2. EXISTING WASTEWATER INFRASTRUCTURE ............................................................ 3
2.3. GENERAL PROPOSED SITE DESIGN .......................................................................... 3
3. WASTEWATER FLOW PROJECTIONS ......................................................................... 4
4. PROPOSED WATER INFRASTRUCTURE IMPROVEMENTS ....................................... 5
4.1. BASIS OF DESIGN ........................................................................................................ 5
4.2. PROPOSED SANITARY SEWER IMPROVEMENTS ...................................................... 5
4.3. DOWNSTREAM IMPACTS ............................................................................................. 6
5. SUMMARY ...................................................................................................................... 8
FIGURES
FIGURE 1-1: VICINITY MAP .......................................................................................................2
FIGURE 4-1: PROPOSED SANITARY SEWER IMPROVEMENTS ............................................7
TABLES
TABLE 2-1: PROPOSED DEVELOPMENT SUMMARY ..............................................................3
TABLE 3-1: WASTEWATER FLOW PROJECTION SUMMARY .................................................4
TABLE 4-1: BASIS OF DESIGN- SANITARY SEWER IMPROVEMENTS ..................................5
TABLE 4-2: SANITARY SEWER DESIGN SUMMARY ...............................................................6
APPENDICES
APPENDIX A – FEMA FLOOD INSURANCE RATE MAP .......................................................... A
Sanitary Sewer Design Report │ DEVCO North 3rd Apartments
November 2022
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1. INTRODUCTION
1.1. PROJECT BACKGROUND
The North 3rd Apartments Project proposes to develop 5.86 acres of two existing vacant lots (Parcel Nos.
RGG6409 & RGG6410) zoned as Residential Mixed-Use High-Density District (R-5) into a multi-family
apartment housing complex. The project site is located near 701 N 3rd Ave, Bozeman, MT in Gallatin
County. The project area is bounded to the north by the Homestead Townhouses Development, to the east
by single family residential housing, to the south by high density residential housing, and to the west by
Westlake Park. All 5.86 acres of the site will be disturbed. The proposed development includes seven (7)
garden style apartment buildings and a clubhouse available for use by all residents. The apartment
buildings are 4-story structures with 24-32 units per building, 216 units total. A vicinity map of the project
site is provided in Figure 1-1.
1.2. PROJECT PURPOSE
The project includes onsite and offsite sanitary sewer improvements for the conveyance of anticipated
sewer flows to the downstream City of Bozeman (City) sanitary sewer collection system. The purpose of
this Sanitary Sewer Design Report is to provide a description of the proposed sanitary sewer improvements
and to demonstrate that the improvements meet the sanitary sewer design requirements included in the
City Design Standards and Specifications Policy (Design Standards) and City Modifications to Montana
Public Works Standard Specifications (MPWSS) Sixth Edition.
Sources: Esri, HERE, Garmin, USGS, Intermap, INCREMENT P, NRCan,Esri Japan, METI, Esri China (Hong Kong), Esri Korea, Esri (Thailand),NGCC, (c) OpenStreetMap contributors, and the GIS User Community
PROJECT SITE
FIGURE 1-1: VICINITY MAP
Sanitary Sewer Design Report │ DEVCO North 3rd Apartments
November 2022
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2. SITE CONDITIONS
2.1. GENERAL EXISTING SITE CONDITIONS
The existing site is undeveloped land comprised primarily of grass and shrubs. There are no significant
topography changes on the site. The site generally slopes towards the northeast with a total of 12-feet of
elevation change. The surrounding land use is a mix of commercial, single-family residential, and multi-
family residential.
A geotechnical investigation was performed on the site in June 2022 by C & H Engineering and Surveying
Inc. Seven test pits were excavated for investigation. The test pits all generally showed organic silty clay
in the upper 1.5 to 2 feet, sandy lean clay to depths of 3.5 to 5 feet and poorly graded gravel with sand and
cobbles from 6.5 feet to the bottom of the test pit. The Natural Resources Conservation Service (NRCS)
Web Soil Survey (WSS) identifies the soil near the project site as 457A – Turner Loam.
The project site is not located within a FEMA Special Flood Hazard Area (SFHA). The site is mapped on
FEMA Flood Insurance Rate Map (FIRM) Panel 3000280816E, effective date April 21, 2021. The site lies
within an unshaded FEMA Zone X, area of minimal flood hazard. See the FIRM Panel in Appendix A.
2.2. EXISTING WASTEWATER INFRASTRUCTURE
Existing wastewater infrastructure in the vicinity of the project includes gravity sewer main ranging in size
from 8-inch to 15-inch diameter. The most feasible connection points to the City’s sewer are located north
of the project at existing manholes that convey sewage from the Homestead Townhouses development.
Ultimately, all sewage from the site will flow to the east to the Rouse Interceptor where it will be conveyed
to the Bozeman Water Reclamation Facility.
2.3. GENERAL PROPOSED SITE DESIGN
The proposed site will consist of seven (7) multifamily residential buildings and one clubhouse with laundry
facilities, fitness equipment and offices. Characteristics of each building are identified in Table 2-1.
Associated passenger vehicular parking and utility infrastructure will be constructed in conjunction with the
buildings.
Table 2-1: Proposed Development Summary
Building Name No. Apartments Footprint (ft2) Estimated
Population
Building A 32 8,930 70
Building B 32 8,920 70
Building C 32 8,920 70
Building D 32 8,930 70
Building E 24 8,520 53
Building F 32 7,260 70
Building G 32 7,260 70
Clubhouse - 2,530 -
Total 216 - 473
Sanitary Sewer Design Report │ DEVCO North 3rd Apartments
November 2022
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3. WASTEWATER FLOW PROJECTIONS
Wastewater flow projection for the proposed development were calculated to size onsite and offsite sanitary
sewer infrastructure improvements. In accordance with City Design Standards Section V Subsection B –
Sanitary Sewer System Design Criteria, wastewater flow projections are calculated based on an average
day flow (ADF) and dwelling unit occupancy factor. City engineering staff recommended using an updated
ADF of 64.4 gallons per capita per day (gpcd) and occupancy rate of 2.17 people per unit and as identified
in the 2015 Wastewater Collection Facilities Plan Update. Additionally, an infiltration rate of 150 gallons
per developed acre is added to all projected flows to account for inflow and infiltration occurring onsite.
Maximum day flow (MDF) is calculated using a peaking factor. The Design Standards provide the following
equation for calculation of the peaking factor:
𝑃𝑒𝑎𝑘𝑖𝑛𝑔 𝐹𝑎𝑐𝑡𝑛𝑟= 18 +√𝑃𝑛𝑛𝑡𝑘𝑎𝑡𝑖𝑛𝑛/1000
4 +√𝑃𝑛𝑛𝑡𝑘𝑎𝑡𝑖𝑛𝑛/1000 =18 +√460/1000
4 +√460/1000 ~ 4.0
Wastewater flow projections for the clubhouse were calculated by assuming an equivalent dwelling unit
(EDU) flow based on an equivalent square footage and safety factor of 1.5 to account for additional flows
produced by the proposed laundry facilities at the clubhouse. There are 216 apartments with a total living
area of 235,000 ft2; therefore, each dwelling unit is approximately 1,100 ft2. Based on this approximation
and a safety factor of 1.5 the clubhouse is equivalent to 4 EDUs.
The See Table 3-1 for a summary of the calculated wastewater flows.
Table 3-1: Wastewater Flow Projection Summary
Building Name EDUs ADF1 MDF1
gpd gpm gpd gpm
Building A 32 4,510 4 17,970 13
Building B 32 4,510 4 17,970 13
Building C 32 4,510 4 17,970 13
Building D 32 4,510 4 17,970 13
Building E 24 3,420 3 13,620 10
Building F 32 4,510 4 17,970 13
Building G 32 4,510 4 17,970 13
Clubhouse 4 580 1 2,310 2
Total 220 31,940 29 127,200 91
Notes:
1. A steady state infiltration flow is added to the total flow. Infiltration is calculated as 150 gpd per acre of
development, 880 gpd. The peaking factor is not applied to the infiltration flow.
Sanitary Sewer Design Report │ DEVCO North 3rd Apartments
November 2022
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4. PROPOSED WATER INFRASTRUCTURE IMPROVEMENTS
4.1. BASIS OF DESIGN
The proposed sanitary sewer improvements were sized according to the Design Standards based on the
wastewater flow projections included in Section 3. The design flow for the project is MDF, 91 gpm. See
Table 4-1 for a summary the proposed sanitary sewer improvements basis of design.
Table 4-1: Basis of Design- Sanitary Sewer Improvements
Design Parameter1 Value
Sewer Main Pipe Material (4-, 6-, and 8-inch diameter pipe) Polyvinyl Chloride (PVC) SDR 35
Pipe Roughness (Manning’s Friction Factor) 0.013
Minimum Slope (4-inch side sewer) 2.0%
Minimum Slope (8-inch sewer)2 0.4%
Maximum Flow Depth 75% of Pipe Diameter
Maximum Manhole Spacing (8-inch to 15-inch Sewer) 400-feet
Notes:
1. In accordance with the Design Standards unless otherwise identified.
2. In accordance with Montana Department of Environmental Quality Circular DEQ-2: Design Standards for Public
Sewage Systems.
4.2. PROPOSED SANITARY SEWER IMPROVEMENTS
The proposed sanitary sewer improvements are shown schematically in Figure 4-1. Two 8-inch diameter
sewer mains are provided, one onsite main to collect wastewater flows from Buildings A, C, E, and F and
one offsite main, located in North 3rd Ave, to collect wastewater flows from the clubhouse and Buildings B,
D, and G. The sewer mains connect to the existing system at downstream manholes located near the
northern boundary of the project site as shown in Figure 4-1. In total there is approximately 1,360-lineal
feet of 8-inch diameter sewer main included in the proposed improvements. Side sewers will be installed
at each building to convey wastewater flows to one of the two 8-inch sewer mains. The side sewers will be
4-inch diameter PVC, SDR 35 installed at a minimum slope of 2.0 percent, per City Design Standards. See
Figure 4-1 for an approximate location of the proposed side sewers.
During ADF and MDF events the onsite sewer mains convey approximately 5 to 16 percent of the total
pipe capacity, respectively, as calculated by Manning’s equation assuming a pipe roughness coefficient of
0.013 and minimum pipe slope of 0.4 percent. Table 4-2 provides a summary of critical sanitary sewer
design parameters.
Sanitary Sewer Design Report │ DEVCO North 3rd Apartments
November 2022
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Table 4-2: Sanitary Sewer Design Summary
Parameter Onsite Sewer Main Offsite Sewer Main
(North 3rd Ave)
Nominal Diameter 8-inch 8-inch
Material PVC SDR 35 PVC SDR 35
Pipe Capacity @ Minimum Slope (0.4%) 304 gpm 304 gpm
ADF1 15.5 gpm 13.5 gpm
MDF2 49.5 gpm 41.5 gpm
Notes:
1. Total ADF = 29 gpm
2. Total MDF = 91 gpm
4.3. DOWNSTREAM IMPACTS
The wastewater flows produced by the North 3rd Apartments Project increase flow in the downstream piping.
Per discussion with City of Bozeman Engineering Staff, the flows included in this report will be input into
the City’s sewer model and the model will be ran to evaluate the downstream impact of the wastewater
flows produced by the development. If downstream pipe capacities are exceeded, additional offsite sanitary
sewer improvements will be required as part of this project to accommodate the projected wastewater flows.
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Source: Esri, Maxar, Earthstar Geographics, and the GIS User Community
Legend
!.Existing Sewer Manhole
Existing Sanitary Sewer MainProposed Sanitary Sewer Improvements
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FIGURE 4-1: Proposed Sanitary Sewer ImprovementsBUILDING ABUILDING BBUILDING CBUILDING DBUILDING EBUILDING FBUILDING GCLUBHOUSEPROPOSED SEWER
CONNECTION POINT
PROPOSED SEWER
CONNECTION POINT
Sanitary Sewer Design Report │ DEVCO North 3rd Apartments
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5. SUMMARY
The proposed development is anticipated to produce 29 and 91 gpm for ADF and MDF, respectively. The
proposed sanitary sewer improvements include approximately 1,360-lineal feet of 8-inch diameter sewer
main, and 4-inch diameter side sewers for all proposed buildings. All improvements have been sized in
accordance with City Design Standards to accommodate projected wastewater flows. City of Bozeman
Engineering Staff will conduct hydraulic modelling to understand the impact of the development on the
downstream sanitary sewer collection system. If downstream pipe capacities are exceeded additional
offsite sanitary sewer improvements will be required and this Design Report will be updated.
Sanitary Sewer Design Report │ DEVCO North 3rd Apartments
November 2022
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APPENDIX A – FEMA FLOOD INSURANCE RATE MAP