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HomeMy WebLinkAbout008 Appendix G - Sewer Design Report Engineering Report Sewer Main Extension Stockyard Property October 2024 Revised February 2025 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 02/14/2025 Sewer Mains 1 2/14/2025 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 Table 1 – Stockyard Property Wastewater Generation Table 2 – Relative Water Use – Per Unit to Per Square Foot Table 3 – Sewer Main Conveyance Capacity Attachments and Appendices C3.0. Sewer Plan Sewer Mains 2 2/14/2025 11.1 Engineering Report – General Information The Stockyard Property is located west of the intersection of East Griffin Drive and Story Mill Road. The Stockyard Property is a total of 15.28 acres comprised of 6 tracts with REMU and B-2M zoning. Water and sewer service for the property will be provided by extensions from existing City of Bozeman water and sewer mains adjacent to the site. The developer’s contact information is: Wake Up, Inc. Christine & Kevin Huyser P.O. Box 23 Harrison, MT 59735 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 Stockyard Property is located as shown on the Vicinity Map in Figure 1. Figure 1: Vicinity Map, Bozeman, Montana Sewer Mains 3 2/14/2025 Space Unit Units/1,000 SF GPD/Unit GPD/1,000 SF Office Employee 4 15 60 Restaurant Meal 125 4 500 Employee 2 10 Customer 5 3Retail 35 11.11 Problem Defined Sanitary sewer service will be provided by new 8” main extensions from existing sewer mains in adjacent City streets, and by new service connections on existing sewer mains adjacent to the property. Proposed sewer mains will be in new City streets constructed with the subdivision. The project will provide approximately 1,061 linear feet of new sewer main and 9 manholes. The proposed sewer system improvements are shown on the Plan Sheet C3.0 Sewer Plan attached at the end of this report. Three 8” sewer main stubs will be installed for future use. In addition, one service will be installed for use by the existing sale barn building. 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. Sewer use per square foot of office, retail, and restaurant was estimated based on DEQ Circular 4 – Table 3.1. Conversion factors and usage rates used to derive per square foot rates from per unit rates are shown below in Table 1. Table 1. Relative Water Use – Per Unit to Per Square Foot Estimated wastewater generation for the Stockyard Property as shown below in Table 2 has been estimated utilizing values provided in City design standards, Facility Plans, and those values shown above in Table 1. Wastewater flows generated on-site are split into a north and south basin draining to an existing sewer on Hillside Lane and an existing sewer on E Griffin Dr. respectively. For sewer main sizing, typical City design flows are utilized. Sewer Mains 4 2/14/2025 Table 2 – Stockyard Property Wastewater Generation A small area of R-4 zoned property immediately to the south of the Stockyard may be served by future service connections to the sewer main installed in Story Mill Road. A standard value of 150 gpd/acre has been applied to the site to account for infiltration and inflow into the sewer collection system. Applying a peaking factor of 4.09, produces a peak hour flow of 79.46 gpm. Sewer mains are sized to convey peak hour flows at a maximum flow depth of 75% full. Stockyard Sewer Use Residential Commercial Area (sf)Area (ac)Units Capita/Unit GPD/Capita Residential Sewer Use (gpd) Office Space GPD/1,000 SF (Office) Restaurant Space GPD/1,000 SF (Restaurant) Retail Space GPD/1,000 SF (Retail) Combined Commercial Sewer Use (gpd) Residential & Commercial Sewer Use (gpd) Tract 1 (REMU)56,765 1.30 44 2.11 89.0 8,263 8,263 Tract 3 (REMU)25,053 0.58 4 2.11 89.0 751 2000 60 2000 35 190 941 Tract 2 (REMU)47,233 1.08 40 2.11 89.0 7,512 7,512 Tract 4 (B-2M)46,262 1.06 34 2.11 89.0 6,385 20000 60 2000 500 2000 35 2,270 8,655 Tract 5 (B-2M)168,336 3.86 4 2.11 89.0 751 24000 60 6000 500 2000 35 4,510 5,261 Tract 6 (B-2M)266,348 6.11 4 2.11 89.0 751 2000 60 10000 500 5,120 5,871 Total Units 130 Office Subtotal (gpd) 2,880 Restaurant Subtotal (gpd) 9,000 Retail Subtotal (gpd) 210 274 Residential Sewer Use Subtotal 24,413 Commercial Sewer Use Subtotal 12,090 36,503 Total Domestic Sewer Use (gpd)36,503 Residential Commercial Subtotal I&I (gpd)Peaking Factor Peak Hour Flow (gpm)Site Area 15.28 North Sewer Basin Subtotals (gpd)9,014 190 9,204 307 4.09 27 Inflow & Infiltration (gal/ac/day)150 South Sewer Basin Subtotals (gpd)15,399 11,900 27,299 1,985 4.09 83 Inflow & Infiltration (gpd)2,292 Total Domestic Sewer Use + Inflow & Infiltration (gpd)38,795 ;P = Pop. ÷ 1,000 Peaking Factor (PF)4.09 Design Sewer Peak Hour Flow (gpm)110 Total Population = (Total # of Units * 2.11 cap/Unit) Sewer Mains 5 2/14/2025 Table 3– Sewer Main Conveyance Capacities Sewer Main Location On-site North Basin South Basin Pipe Diameter (in)8 8 8 Minimum Pipe Slope (ft/ft)0.004 0.004 0.004 Coefficient of Roughness (n)0.013 0.013 0.013 Depth in Pipe at 75% Full (in)6 6 6 Depth of Flow (ft)0.5 0.5 0.5 Pipe Radius (in)4 4 4 Pipe Radius (ft)0.33 0.33 0.33 Wetted Perimeter 1.40 1.40 1.40 Area of Flow (ft2)0.28 0.28 0.28 Hydraulic Radius 0.20 0.20 0.20 Velocity (ft/sec)2.48 2.48 2.48 Maximum Pipe Flowrate (gpm)312.8 312.8 312.8 Peak Hour Flow (gpm)110.0 27.0 83.0 Peak Flow % of 0.75d Capacity 35%9%27% 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 0.4% has sufficient capacity to provide sewer service for the entire Stockyard Property, however some buildings will be served by connections to existing mains and not the new sewer mains. 11.13 Impact on Existing Wastewater Facilities A review of the City of Bozeman 2015 Wastewater Facility Plan identifies some long-term improvements may be required on downstream portions of the City’s sewer collection system. 11.14 Project Description The project description is provided at the beginning of the Engineering Report. 11.15 Drawings Please see Plan Sheet C3.0 Sewer Plan attached at the end of this report, which shows the proposed sewer improvements. Plan and profile drawings and sewer detail sheets are included with the subdivision infrastructure plan set. Sewer Mains 6 2/14/2025 11.16 Design Criteria The Stockyard Property 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 Stockyard project is located in the northwest and southwest quadrants at the intersection of East Griffin Dr and Story Mill Rd. The property was previously occupied by an auction house, restaurant, several single-family residences and associated outbuildings. The site is gently sloping to the west and south. A geotechnical investigation was conducted, and the report is provided as an attachment to the infrastructure submittal. In summary the geotechnical investigation found a variable depth of fine-grained soils overlying gravel sediments. The depth to gravel varies between 3 and 13 feet. Groundwater monitoring was conducted throughout the 2022 season and found seasonal high groundwater depths ranged from 2’ to greater than 13’ below existing grade. Groundwater is flowing to the west and is assumed to discharge at the East Gallatin River which is the western boundary of the southern part of this project. The range of groundwater depth encountered was largely due to the rolling topography of the site with some wells located at lower or higher relative locations. After site grading, high groundwater will more consistently be about 10’ below the surface. 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.