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Home My WebLink About012 Appendix K - Preliminary Sewer Engineering Report Preliminary Engineering Report Sewer Main Extension South University District Phase 3 - Block 2 Updated September 2022 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 9/14/2022 Preliminary 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 Figure 2. C3.0 - Sewer Plan Table 1. Block 2 Sewer Flow Estimates Table 2. Capacity of 8” SDR 35 PVC Sewer Main Table 3. Capacity of Estimated Sewer Flows Appendices SUD Phase 3 Sewer Engineering Report Sewer Mains 2 9/14/2022 11.1 Engineering Report – General Information The proposed South University District Phase 3 Block 2 Development is located northeast of the intersection of 19th Avenue and Stucky Road. This development will consist of 162 residential units on an 8.60-acre lot. With 4 different building types; two rowhouse-style apartments, motor court apartments, and flat style apartments. The intent of this project is to create 162 residential units over a variety of building types to create a diverse neighborhood in a growing part of town adjacent to the university. The project will be served by extension of City of Bozeman water and sewer mains throughout the project. Individual service lines will be provided to each block of units. City of Bozeman water and sewer mains currently exist along all four sides of the project site. The developer’s contact information is: Wentworth Property Company Ray Brown 802 North 3rd Avenue Phoenix, Arizona 85003 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 location of the Block 2 Development is shown on the Vicinity Map in Figure 1. Sewer Mains 3 9/14/2022 Figure 1. Vicinity Map, Bozeman, Montana 11.11 Problem Defined Sanitary sewer service will be provided by an existing 8” sewer main running in State Street along the northern portion of the site. The proposed sewer system improvements are shown on the Sewer Plan (Figure 2) at the end of this report. New sewer mains will be within combined water and sewer easements and street rights-of-way running through the property. The easements are 30’ wide. No trees are proposed within the water and sewer easements or within 10’ of water and sewer mains. Each building will have an individual sewer service from the building to the main through parking areas and internal streets. Sewer service sizes are anticipated to be 6” to the building but will be finalized during infrastructure design. To avoid conflicts with water and sewer services, power and communication lines are typically placed in easements on the backs or sides of the buildings. 11.12 Design Conditions Estimated wastewater generation for Block 2 has been determined utilizing values provided in City Design Standards and Specifications Policy and is shown in Table 1. However, for sewer main sizing, typical City design flows are utilized. Sewer Mains 4 9/14/2022 Site Area (acres)8.6 Residential Units 162 Residential Occupants (2.11/unit)341.82 Per Capita Use (gpd)89 Residential Flow (gpd)30,422 Commercial Area (sf)0 Commercial Use (gpd/1000 sf)0 Commercial Flow (gpd)0 Commercial and Residential Flows (gpd)30,422 Commercial and Residential Flows (gpm)21 Inflow & Infiltration (gal/ac/day)150 Inflow & Infiltration (gpd)1,290 Design Sewer Use + Inflow & Infiltration (gpd)31,712 Peaking Factor (PF)4.05 Design Sewer Peak Hour Flow 89.27 The estimated residential wastewater generation is based on the City of Bozeman Design Standards. Typical design flow is 89 gallons per day (gpd) per capita, with an average population of 2.11 people per home. In accordance with DEQ-2 a peaking factor of 4.05:1 was used to determine the peak hour flow rate. The estimated total project design flow is 30,422 gpd. In accordance with City design standards, infiltration and inflow of 150 gpd/acre is added to the domestic wastewater generation. The total design flow from the Block 2 development is approximately 31,712 gpd, with a peak hour flow of 89.27 gpm. Table 1 – SUD Ph3 – Block 2 Estimated Sewer Use An 8” sewer main at minimum slope (0.4%) has sufficient capacity to provide sewer service for Block 2 at the estimated flow of 89.27 gpm. Table 2 shows that an 8” main has a 75% flow depth capacity of 313 gpm installed at a minimum slope of 0.4%. Table 2 Capacity of 8" SDR 35 PVC Sewer Main Sewer Mains 5 9/14/2022 Sewer Main Location On-site Pipe Diameter (in)8 Minimum Pipe Slope (ft/ft)0.004 Coefficient of Roughness (n)0.013 Depth in Pipe at 75% Full (in)6 Depth of Flow (ft)0.5 Pipe Radius (in)4 Pipe Radius (ft)0.33 Wetted Perimeter 1.40 Area of Flow (ft2)0.28 Hydraulic Radius 0.20 Velocity (ft/sec)2.48 Maximum Pipe Flowrate (gpm)312.8 Peak Hour Flow (gpm)89.3 Peak Flow % of Max Capacity 29% 11.13 Impact on Existing Wastewater Facilities Off-site sewer impacts were evaluated as part of the phase 3 Subdivision infrastructure review process. The sewer flow contributions for the proposed development were determined using the City of Bozeman Design Standards. The proposed sanitary sewer main extensions that will serve the proposed development will connect to the existing 8” sewer main in State street at the north end of the site and then conveyed by the 8” sewer running in S 17th Avenue to the 10” main in Kagy Boulevard and Willow Way, constructed during SUD Phase 2. During the Subdivision planning phase of Phase 3, it was determined that the 10” main has adequate capacity to serve future development of SUD Phase 3, annexation of county properties along Willow Way, future development along Kagy Boulevard. Sewer flows generated in Phase 3 of the SUD development will be collected in the ‘Farmer’s Canal Drainage Basin’ as identified by the City. The new mains within the SUD Phase 3 subdivision were 8” and 10” and connect to the 10” sewer main in Willow Way that continues north to the 15” main in Lincoln Street west through 19th Avenue eventually ending up in the 18” main in Garfield Street near the Bozeman Gateway Subdivision. Flows generated by the SUD Phase 3 development were combined with the estimated flows from currently undeveloped parcels, as well as existing developments (see Fig 3 in Appendix) to determine the impact of capacity within the downstream mains. The estimates for SUD Phase 3 Block 1, 2, Sewer Mains 6 9/14/2022 (in)(%)(gpd)(unitless)(gpm)(gpm)(gpm)(in)(gpm)(%) Lincoln 15 0.21%380,379 3.18 929 22 951 11.25 1,212 79% Willow 10 0.97%218,931 3.40 516 10 526 7.5 883 60% Kagy 10 0.28%106,824 3.65 271 10 281 7.5 475 59% Pipe Size Slope1 Avg Daily Flow Peaking Factor Peak Flow Infiltration2 Peak Flow + Infiltration Depth of Flow 75% Depth Capacity Flowrate Capacity Q/Q₇₅Sewer Main 3, and 4 have been updated based on current proposed development programming and included in the calculations below in Table 3. Table 3 shows the combined flows of the Willow Way and Tai Lane subbasins, -- in addition to anticipated development of un-incorporated properties in the area -- and evaluates the capacity within the adjacent sewer collection system. The combined peak flows that eventually end up in the 15” Lincoln Street main are estimated to be 951 gpm; this includes a 90 gpm peak for MSU home football games (established from monitoring), plus an additional 22 gpm for infiltration within the sewer drainage sub- basins. The anticipated peak flow of 951 gpm is under the capacity of a 15” main at minimum slope (0.21%) flowing 75% full (1,212 gpm). The approximate 850 LF of sewer main installed within Kagy Blvd (extending west from the intersection of Willow Way) was conservatively sized as a 10” main flowing at only 60% Q/Q75, because an 8” main neared 75% capacity with estimated peak flows. The Block 2 sewer flows conform to the previous flows estimated with the subdivision infrastructure design and do not exceed design constraints identified in the subdivision infrastructure report. Table 3: Capacity of Estimated Sewer Flows 1 Slopes either proposed or as-built 2 150 gal/ac/day from COB Design Standards 11.14 Project Description The project description is provided at the beginning of the Engineering Report. 11.15 Drawings Please see sheet C3.0 Sewer Plan, at the end of this report which shows the proposed sewer improvements. Plan and profile drawings will be submitted with the infrastructure design submittals. Sewer Mains 7 9/14/2022 11.16 Design Criteria The Block 2 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 SUD Phase 3 Block 2 site is typical of Bozeman with fine-grained soils overlying gravel deposits. Geotechnical borings have determined the depth to gravel ranges from 3.5 to 5 feet. Estimated depth to groundwater obtained from the geotechnical borings is 9 to 12 feet. If mains are installed during periods of high groundwater, dewatering may be required. 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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