HomeMy WebLinkAbout006 - Sewer ReportSanitary Sewer Design Report │ DEVCO Alder Ridge & Sage Peak Apartments
March 2024
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Alder Ridge & Sage Peak Apartments
Sanitary Sewer Design Report
PREPARED FOR:
Graf St. Apartments, LLC.
PO Box 4108, Bellevue, WA 98009
(425) 736-0580
MARCH 2024
PREPARED BY:
1201 Third Avenue, Suite 2800, Seattle, WA 98101
(206) 745-9976
3/19/2024
Sanitary Sewer Design Report │ DEVCO Alder Ridge & Sage Peak Apartments
March 2024
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TABLE OF CONTENTS
1. INTRODUCTION ............................................................................................................. 1
1.1. PROJECT BACKGROUND ................................................................................................ 1
1.2. PROJECT PURPOSE ......................................................................................................... 1
2. SITE CONDITIONS ......................................................................................................... 2
2.1. GENERAL EXISTING SITE CONDITIONS ........................................................................ 2
2.2. EXISTING WASTEWATER INFRASTRUCTURE .............................................................. 2
2.3. GENERAL PROPOSED SITE DESIGN.............................................................................. 2
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. SANITARY SEWER PIPE CAPACITIES ............................................................................ 7
4.4. DOWNSTREAM IMPACTS ................................................................................................. 7
5. SUMMARY ...................................................................................................................... 8
FIGURES
FIGURE 1-1: VICINITY MAP ...................................................................................................... 1
FIGURE 4-1: PROPOSED SANITARY SEWER IMPROVEMENTS ........................................... 6
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 PIPE CAPACITY SUMMARY ................................................. 7
Sanitary Sewer Design Report │ DEVCO Alder Ridge & Sage Peak Apartments
March 2024
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APPENDICES
APPENDIX A – FEMA FLOOD INSURANCE RATE MAP .......................................................... A
Sanitary Sewer Design Report │ DEVCO Alder Ridge & Sage Peak Apartments
March 2024
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1. INTRODUCTION
1.1. PROJECT BACKGROUND
The Alder Ridge & Sage Peak Apartments Project (Project) proposes to develop 9.26 acres of two vacant
lots (Parcel Nos. RGG7308 & RGG70606) into a multi-family apartment housing complex. The Project site
is currently zoned as residential single-household low density district (R-1), but will be rezoned into
residential emphasis mixed-use district. The project site is located east of the intersection of Graf St. and
S 19th Ave. in Bozeman, MT within Gallatin County. The project area is bounded by undeveloped parcels
in all directions. All 9.26 acres of the site will be disturbed. The Project includes one senior living facility,
eight multifamily garden style apartment buildings, and a clubhouse available for use by all residents. In
total, 341 units will be provided in the development. A vicinity map of the project site is provided in Figure
1-1.
1.2. PROJECT PURPOSE
The Project includes onsite sanitary sewer infrastructure 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.
Figure 1-1: Vicinity Map
PROJECT SITE
Sanitary Sewer Design Report │ DEVCO Alder Ridge & Sage Peak Apartments
March 2024
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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 slopes from north to south at 1.6 percent. All surrounding lots are
currently undeveloped but are included in the South Range Crossing master planned community
development (SRX-North) and will be developed in the near term.
A geotechnical investigation was performed on the site in October 2016 by Allied Engineering Services.
Nine test pits were excavated for investigation. The test pits all generally showed topsoil in the upper 1.0
to 1.5 feet, silt/clay to depths of 3.5 to 4.5 feet and sandy gravel with sand and cobbles from 6.5 feet to the
bottom of the test pit at depths of 8 to 9 feet. The Natural Resources Conservation Service (NRCS) Web
Soil Survey (WSS) identifies the soil near the project site as predominately 510B – Meadowcreek 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) Panels 30031C0808E and 30031C0816E, effective date
4/21/2021 for both panels. The site lies within an unshaded FEMA Zone X, outside the 0.2% annual chance
floodplain. See the FIRM Panel in Appendix A.
2.2. EXISTING WASTEWATER INFRASTRUCTURE
Existing wastewater infrastructure is located in Graf St. and S. 19th Ave. SRX-North will extend sewer mains
from Graf St. north to provide points of connection for the Project. Ultimately, the Project will be bordered
on its north, south, and west frontages by 8-inch diameter sewer mains. Building sewer laterals will connect
directly to these mains. Sewage from the Project will be conveyed in the City sanitary sewer collection
system to the intersection of S 19th Ave and Graf St by through an existing 8-inch diameter main in Graf.
St. Sewage will flow north from the intersection in S. 19th Ave through an 18-inch diameter sewer main.
2.3. GENERAL PROPOSED SITE DESIGN
The proposed site will consist of one senior multifamily residential building (Building 1), one clubhouse
(Building 2) with laundry facilities, fitness equipment and offices, and eight (8) multifamily residential
buildings (Buildings 3-10). 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.
Sanitary Sewer Design Report │ DEVCO Alder Ridge & Sage Peak Apartments
March 2024
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Table 2-1: Proposed Development Summary
Building Name No. Housing Units Footprint (sq. ft) Estimated Population1
Building 1 101 23,100 220
Building 2 - 2,300 -
Building 3 30 8,700 66
Building 4 30 7,100 66
Building 5 30 8,400 66
Building 6 30 7,100 66
Building 7 30 8,700 66
Building 8 30 8,700 66
Building 9 30 7,100 66
Building 10 30 8,400 66
Total 341 N/A 748
Note:
1. Based on Occupancy Rate of 2.17 people per unit per 2017 Water Facility Plan update.
Sanitary Sewer Design Report │ DEVCO Alder Ridge & Sage Peak Apartments
March 2024
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3. WASTEWATER FLOW PROJECTIONS
Wastewater flow projections for the Project were calculated to size 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. Peak hour flow
(PHF) 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 +√748/1000
4 +√748/1000 ~ 3.9
Wastewater flow projections for Building 2 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. The total square footage of the senior living facility and
apartment buildings is approximately 326,000 ft2 with 341 apartments (dwelling units); therefore, each
dwelling unit is approximately 960 ft2. Based on this approximation and a safety factor of 1.5, Building 2 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 ADF PHF
gpd gpm gpd gpm
Building 1 101 14,170 10 55,270 39
Building 2 4 560 1 2,190 2
Building 3 30 4,260 3 16,620 12
Building 4 30 4,260 3 16,620 12
Building 5 30 4,260 3 16,620 12
Building 6 30 4,260 3 16,620 12
Building 7 30 4,260 3 16,620 12
Building 8 30 4,260 3 16,620 12
Building 9 30 4,260 3 16,620 12
Building 10 30 4,260 3 16,620 12
Infiltration1 - 1,389 1 1,390 1
Total 345 50,199 36 195,780 138
Notes:
1. A steady state infiltration flow is added to the total flow. Infiltration is calculated as 150 gpd per acre of development,
1,390 gpd. The peaking factor is not applied to the infiltration flow.
Sanitary Sewer Design Report │ DEVCO Alder Ridge & Sage Peak Apartments
March 2024
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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 PHF, 138 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 sewer lateral) 2.0%
Minimum Slope (6-inch sewer lateral) 1.0%
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 improvements include constructing one onsite sewer main and sewer laterals for all buildings
to convey sewage to the City sewer collection system. The onsite sewer main will be constructed to convey
flows from Buildings 4, 5, and 6 to an 8-inch main located along the northern frontage of the Project,
constructed as part of SRX-North. Sewer laterals from buildings 4, 5, and 6 will be 6-inch diameter and
connect to the onsite sewer main. Buildings 1, 3, 8, 9, and 10 will have a 6-inch diameter sewer lateral
connected to future 8-inch sewer mains constructed as part of the SRX-North. Buildings 7 and 8 will have
a 6-inch diameter sewer lateral connected to the existing 8-inch diameter sewer main in Graf St. Building 2
will have a 4-inch diameter sewer lateral connected to the future 8-inch sewer main constructed west of the
Project as part of the SRX-North. The proposed sanitary sewer improvements are shown schematically in
Figure 4-1.
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)GRAF ST. APARTMENTSNORTHBUILDING 1 - SENIOR APARTMENTSBUILDING 2 -CLUBHOUSEBUILDING 3BUILDING 7BUILDING 8BUILDING 4BUILDING 5BUILDING 6
BUILDING 9 BUILDING 10
6" SEWERLATERAL6" SEWERLATERAL6" SEWERLATERAL6" SEWERLATERAL6" SEWERLATERAL6" SEWERLATERAL4" SEWER LATERAL6" SEWERLATERAL6" SEWERLATERALGRAF ST.TO INTERSECTIONOF S. 19TH AVEAND GRAF ST.6" SEWER LATERAL8" ONSITE SEWER MAINFIGURE 4-1: PROPOSED SANITARY SEWER IMPROVEMENTS
Sanitary Sewer Design Report │ DEVCO Alder Ridge & Sage Peak Apartments
March 2024
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4.3. SANITARY SEWER PIPE CAPACITIES
During PHF events the onsite sewer laterals and mains convey 2 to 45 percent of the total pipe capacity,
respectively, as calculated by Manning’s equation assuming a pipe roughness coefficient of 0.013 and
minimum pipe slopes as identified in the Montana Department of Environmental Quality Circular DEQ-2.
Table 4-2 provides a summary of critical sanitary sewer design parameters.
Table 4-2: Sanitary Sewer Pipe Capacity Summary
Pipe Description Minimum Slope Pipe Capacity @ Flow
Depth1 (gpm) Peak Hour Flow (gpm)
4" Sewer Lateral 2% 108 2
6" Sewer Lateral (Building 1) 1% 221 39
6" Sewer Lateral (Buildings 3-10 ) 1% 221 12
8" Sewer Onsite Sewer Main 0.4% 304 36
Notes:
1. Maximum allowed flow depth is 75% of pipe diameter, per City Design Standards
4.4. DOWNSTREAM IMPACTS
The wastewater flows produced by the 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.
Sanitary Sewer Design Report │ DEVCO Alder Ridge & Sage Peak Apartments
March 2024
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5. SUMMARY
The Project is anticipated to produce 36 and 138 gpm for ADF and PHF, respectively. The proposed
sanitary sewer improvements include approximately 400-lineal feet of 8-inch diameter sewer main, 6-inch
diameter sewer laterals for Buildings 1, 3, 4, 5, 6, 7, 8, 9, and 10 and a 4-inch sewer lateral for Building 2.
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 Project 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 Alder Ridge & Sage Peak Apartments
March 2024
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APPENDIX A – FEMA FLOOD INSURANCE RATE MAP
National Flood Hazard Layer FIRMette
0 500 1,000 1,500 2,000250
Feet
Ü
SEE FIS REPORT FOR DETAILED LEGEND AND INDEX MAP FOR FIRM PANEL LAYOUT
SPECIAL FLOODHAZARD AREAS
Without Base Flood Elevation (BFE)Zone A, V, A99With BFE or DepthZone AE, AO, AH, VE, AR
Regulatory Floodway
0.2% Annual Chance Flood Hazard, Areasof 1% annual chance flood with averagedepth less than one foot or with drainageareas of less than one square mileZone X
Future Conditions 1% Annual
Chance Flood HazardZone X
Area with Reduced Flood Risk due to
Levee. See Notes.Zone X
Area with Flood Risk due to LeveeZone D
NO SCREENArea of Minimal Flood Hazard Zone X
Area of Undetermined Flood HazardZone D
Channel, Culvert, or Storm Sewer
Levee, Dike, or Floodwall
Cross Sections with 1% Annual Chance
17.5 Water Surface Elevation
Coastal Transect
Coastal Transect Baseline
Profile Baseline
Hydrographic Feature
Base Flood Elevation Line (BFE)
Effective LOMRs
Limit of Study
Jurisdiction Boundary
Digital Data Available
No Digital Data Available
Unmapped
This map complies with FEMA's standards for the use of
digital flood maps if it is not void as described below.The basemap shown complies with FEMA's basemapaccuracy standards
The flood hazard information is derived directly from theauthoritative NFHL web services provided by FEMA. This mapwas exported on 2/29/2024 at 12:44 PM and does notreflect changes or amendments subsequent to this date andtime. The NFHL and effective information may change orbecome superseded by new data over time.
This map image is void if the one or more of the following map
elements do not appear: basemap imagery, flood zone labels,
legend, scale bar, map creation date, community identifiers,
FIRM panel number, and FIRM effective date. Map images for
unmapped and unmodernized areas cannot be used for
regulatory purposes.
Legend
OTHER AREAS OF
FLOOD HAZARD
OTHER AREAS
GENERAL
STRUCTURES
OTHER
FEATURES
MAP PANELS
8
B 20.2
The pin displayed on the map is an approximatepoint selected by the user and does not representan authoritative property location.
1:6,000
111°3'53"W 45°39'12"N
111°3'15"W 45°38'47"N
Basemap Imagery Source: USGS National Map 2023