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Home My WebLink About017 Appendix P - Sewer Engineers Report Engineering Report Sewer Main Extension Aaker Phase 1 Subdivision Preliminary Report for Preliminary Plat Review October 2022 Rev. March 2023 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 Sewer Mains 1 3/28/2023 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 – Aaker Phase 1 Subdivision Estimated Sewer Use Table 2 – Sewer Main Conveyance Capacity Attachments and Appendices C 3.0. Sewer Plan Sewer Mains 2 3/28/2023 11.1 Engineering Report – General Information The Aaker property consists of approximately 95 acres of vacant land located west of South 19th Avenue, north of Stucky Road, and extending west and north to the MSU BART Farm. Concept master planning and initial REMU and B-2M zoning of the Aaker Property envision a mixed-use development with commercial uses along South 19th Avenue grading to predominantly residential uses toward the north and west. The Phase 1 Subdivision includes the extension of Kagy Boulevard and the land to the north, comprising 31.73 acres, with a 63.68-acre restricted future development lot south of Kagy Boulevard. Phase 1 will create 7 lots for development with REMU Zoning. Planned land use includes moderate density residential with some commercial uses integrated into lots adjacent to Kagy Boulevard. Water and sewer service for the subdivision will be provided by extensions from existing City of Bozeman water and sewer mains adjacent to the site. The developer’s contact information is: West University, LLC 113 East Oak, Suite 4A Bozeman, MT 59715 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 Figure 1 shows a vicinity map of the Aaker property and Phase 1 Subdivision. Figure 1. Vicinity Map, Bozeman, Montana Sewer Mains 3 3/28/2023 11.11 Problem Defined Sanitary sewer service will be provided by new 8”, 10”, and 12” main extensions from existing sewer mains in adjacent City streets. Proposed sewer mains will be in new city streets, constructed with the subdivision. The project will provide approximately 4,630 linear feet of new sewer main and 15 new manholes. The proposed sewer system improvements are shown on the Plan Sheet C3.0 - Sewer Plan attached at the end of this report. Due to the large lot size, it is anticipated that future lot development will require on-site main extensions to serve future development. Sewer main stubs are provided to the lot lines where on-site main extensions are envisioned. 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. At the time of subdivision, the ultimate wastewater generation of the lots are unknown. For the purposes of preliminary design and sewer main sizing, the wastewater generation for the Aaker Phase 1 Subdivision has been estimated in Table 1. Estimated domestic water use each lot is based on planned use and density, utilizing standard City of Bozeman design values of 89 gpd per capita. A standard value of 150 gpd/acre has been applied to the site to account for infiltration and inflow into the sewer collection system. Future phases of the Aaker Property will be served by sewer mains installed in the Aaker Phase 1 Subdivision. This future development demand has been estimated and in included in Phase 1 wastewater generation for the purposes of sewer main sizing. Applying a peaking factor of 3.43, produces a peak hour flow of 747.1 gpm for the entire Aaker property (Phase 1 and future phases). Sewer mains are sized to convey peak hour flows at a maximum flow depth of 75% full. Phase 1 alone produces a peak hour flow of 290 gpm. Since the area surrounding the Aaker property is currently served by City sewers, it is unlikely that any sewer mains will be extended beyond the Aaker property. Sewer Mains 4 3/28/2023 Table 1 – Aaker Phase 1 Subdivision Estimated Sewer Use (with upstream buildout) Aaker Subdvision Ph. 1 Estimated Sewer Use Residential Commercial Units Capita/Unit Population GPD/Capita Residential Sewer Use (gpd) Commercial Space (sf) GPD/1000 SF Commercial Space Commercial Sewer Use (gpd) Block 1 Lot 1 0 2.11 0 89 - 20,000 100 2,000 Block 2 Lot 1 125 2.11 264 89 23,474 Lot 2 125 2.11 264 89 23,474 Block 3 Lot 1 52 2.11 110 89 9,765 Lot 2 52 2.11 110 89 9,765 Lot 3 68 2.11 143 89 12,770 10,000 100 1,000 Lot 4 68 2.11 143 89 12,770 10,000 100 1,000 Future Development 1000 2.11 2110 89 187,790 160,000 100 16,000 Total Units 1490 Total Population 3144 Residential Subtotal 279,807 Commercial Subtotal 20,000 Domestic Sewer Use (gpd)299,807 Site Area (ac)95.41 Inflow & Infiltration (gal/ac/day)150 Inflow & Infiltration (gpd)14,312 Design Sewer Use + Inflow & Infiltration (gpd)314,119 ;P = Pop. ÷ 1,000 Peaking Factor (PF)3.43 Design Sewer Peak Hour Flow 747.1 Table 2– Sewer Main Conveyance Capacities Sewer Main Location Ph. 1 Streets 25th Ave - N. of Kagy 25th Ave - S. of Kagy Pipe Diameter (in) 8 12 10 Minimum Pipe Slope (ft/ft) 0.004 0.01 0.01 Coefficient of Roughness (n) 0.013 0.013 0.013 Depth in Pipe at 75% Full (in) 6 9 7.5 Depth of Flow (ft) 0.5 0.75 0.625 Pipe Radius (in) 4 6 5 Pipe Radius (ft) 0.33 0.50 0.42 Wetted Perimeter 1.40 2.09 1.75 Area of Flow (ft2) 0.28 0.63 0.44 Hydraulic Radius 0.20 0.30 0.25 Velocity (ft/sec) 2.48 5.14 4.55 Maximum Pipe Flowrate (gpm) 312.8 1458.3 896.8 Peak Hour Flow (gpm) 290.0 746.0 456.0 Peak Flow % of Max Capacity 93% 51% 51% Sewer Mains 5 3/28/2023 Table 2, above, shows that an 8” sewer main has a 75% flow depth capacity of 312 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 Aaker Phase 1 Subdivision of 290 gpm peak hour flow. Based on similar analysis of the proposed system, two sections of main require greater capacity than an 8” pipe provides. These sections are both along the proposed 25th Avenue with the section south of Kagy Boulevard requiring a 10” sewer main and the section north of Kagy Boulevard requiring a 12” sewer main. 11.13 Impact on Existing Wastewater Facilities The Phase 1 Sewer Improvements are located within the Cattail Creek Drainage Basin. A review of the City of Bozeman 2015 Wastewater Facility Plan indicates that this basin should have sufficient capacity for service of the Aaker Phase 1 Subdivision. 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 will be submitted with the subdivision infrastructure design submittals, after Preliminary Plat. 11.16 Design Criteria The Aaker Phase 1 Subdivision 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. Generally, sewer mains will be installed at a depth of 9-10’ and cross below water mains. Sewer Mains 6 3/28/2023 11.17 Site Information The Aaker Phase 1 property was previously used for agriculture. The site slopes generally to the north and contains a stream and wetland in the western portion of the site. A geotechnical investigation was conducted, and the report is provided separately with the subdivision submittal. In summary, the geotechnical investigation found a variable depth of fine-grained soils overlying gravel sediments. The depth to gravel varies between 1 and 4 feet. Groundwater monitoring was conducted throughout the 2022 season. Depth to seasonal high groundwater varied from near the surface in the wetlands to about 3-4’ below ground at the proposed lots. Groundwater is generally flowing to the north. Proposed street and lot grading will raise the developed portions of the site approximately 2-3’ resulting in a typical depth to seasonal high groundwater in developed areas of about 5-7’. Groundwater dewatering will likely be required during sewer main installation, depending on groundwater depth at the time of installation. 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. 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