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Home My WebLink About015_Appendix N - Sewer Engineering Design ReportEngineering Report Sewer Main Extension South University District Phase 3 - Block 3 Preliminary Plat Updated August 2026 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 8/5/2026 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, Bozeman, Montana Figure 2. C3.0 - Sewer Plan Figure 3. Sewer Capacity Analysis Table 1A. Block 3 Lot 1A Estimated Sewer Flows Table 1B. Block 3 Lot 1B Estimated Sewer Flows Table 2. Capacity of 8” SDR 35 PVC Sewer Main Table 3A. Impact on Capacity of Existing Sewer Mains Table 4. Sewer Flows and Infiltration by Basin/Bub-basin Appendices SUD Phase 3 Sewer Engineering Report City of Bozeman Memorandum Re: South University District Downstream Sanitary Sewer Capacity Summary Sewer Mains 2 8/5/2026 11.1 Engineering Report – General Information The proposed South University District Phase 3 Block 3 Subdivision consists of 17.51 Acres located southeast of the intersection of 19th Avenue and Stucky Road. The Block 3 subdivision will consist of 2 lots with the north lot being approximately 8.84 acres and the south lot being 8.65 acres. The total subdivision area including required offsite improvements is 23.26 acres. This Block 3 Subdivision is designed with the intention of there being 256 residential units between the two lots to create a diversified neighborhood in a growing part of town, adjacent to the Montana State University Campus. The project will be served by extension of City of Bozeman water and sewer mains throughout the project. City of Bozeman water and sewer mains currently exist along the west side of the project site. The developer’s contact information is: RTR Holdings II, LLC Randy Hecht 22 Turtle Rock Court Tiburon, CA 94920 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 Sewer Mains 3 8/5/2026 The location of the South University Block 3 Development is shown on the Vicinity Map in Figure 1. Figure 1. Vicinity Map, Bozeman, Montana 11.11 Problem Defined Sanitary sewer will be provided through existing and proposed sewer mains in adjacent City streets. There is an existing 18” sewer main running along S 19th Ave and an existing 8” sewer main in Stucky Rd. There are proposed 8” mains in S 17th Ave, Arnold St and Student Dr. This project will provide 3218 linear feet of new sewer main and 17 new manholes. The proposed sewer system improvements are shown on the Sewer Plan (Figure 2: C3.0 – Sewer Plan) at the end of this report. It is anticipated that future lot development will require on-site main extensions. Sewer main stubs are provided to lot lines where the on-site main extensions are envisioned. Sewer Mains 4 8/5/2026 11.12 Design Conditions Estimated wastewater generation for Block 3 has been determined utilizing values provided in City Design Standards and Specifications Policy and is shown in Table 1 and Table 2. However, for sewer main sizing, typical City design flows are utilized. The estimated residential wastewater generation is based on the City of Bozeman Design Standards. Typical design flow is 64.4 gallons per day (gpd) per capita, with an average population of 2.17 people per home. In accordance with DEQ-2 a peaking factor of 4.09 was used to determine the peak hour flow rate. 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 3 Lot 1A development is approximately 19,217 gpd, with a peak hour flow of 54.61 gpm. The total design flow from the Block 3 Lot 1B development is approximately 19,185 gpd, with a peak hour flow of 54.53 gpm. Table 1A – Block 3 Lot 1A Estimated Sewer Flows Site Area (acres)8.86 Residential Units 128 Residential Occupants (2.17/unit)277.76 Per Capita Use (gpd)64.4 Residential Flow (gpd)17,888 Commercial Area (sf)0 Commercial Use (gpd/1000 sf)0 Commercial Flow (gpd)0 Commercial and Residential Flows (gpd)17,888 Commercial and Residential Flows (gpm)12 Inflow & Infiltration (gal/ac/day)150 Inflow & Infiltration (gpd)1329 Design Sewer Use + Inflow & Infiltration (gpd)19217 Peaking Factor (PF)4.09 Design Sewer Peak Hour Flow 54.61 Sewer Mains 5 8/5/2026 Table 1B – Block 3 Lot 1B Estimated Sewer Flows An 8” sewer main at minimum slope (0.4%) has sufficient capacity to provide sewer service for Block 3 Lot 1A and Lot 1B at the estimated flow of 54.6 gpm and 54.5 gpm respectively. 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 Site Area (acres)8.65 Residential Units 128 Residential Occupants (2.17/unit)277.76 Per Capita Use (gpd)64.4 Residential Flow (gpd)17,888 Commercial Area (sf)0 Commercial Use (gpd/1000 sf)0 Commercial Flow (gpd)0 Commercial and Residential Flows (gpd)17,888 Commercial and Residential Flows (gpm)12 Inflow & Infiltration (gal/ac/day)150 Inflow & Infiltration (gpd)1298 Design Sewer Use + Inflow & Infiltration (gpd)19185 Peaking Factor (PF)4.09 Design Sewer Peak Hour Flow 54.53 Sewer Main Location Lot 1A Lot 1B Pipe Diameter (in)8 8 Minimum Pipe Slope (ft/ft)0.004 0.004 Coefficient of Roughness (n)0.013 0.013 Depth in Pipe at 75% Full (in)6 6 Depth of Flow (ft)0.5 0.5 Pipe Radius (in)4 4 Pipe Radius (ft)0.33 0.33 Wetted Perimeter 1.40 1.40 Area of Flow (ft2)0.28 0.28 Hydraulic Radius 0.20 0.20 Velocity (ft/sec)2.48 2.48 Maximum Pipe Flowrate (gpm)312.8 312.8 Peak Hour Flow (gpm)54.6 54.5 Peak Flow % of Max Capacity 17%17% Sewer Mains 6 8/5/2026 11.13 Impact on Existing Wastewater Facilities Off-site sewer impacts were evaluated as part of the SUD Phase 3 Subdivision infrastructure review process. The sewer flow contributions for the proposed development were determined using the City of Bozeman Design Standards. The sanitary sewer main extensions that will serve the proposed development will connect to the existing sewer mains in Stucky Rd and South 19th Ave. Block 3 Lot 1A will be served by the proposed sewer mains in S17th Ave and Stucky road. Block 3 Lot 1B will be served via the proposed sewer mains in Arnold St, S 17th Ave and Student Dr. The sewer mains in Stucky Rd and South 17th Ave drain north via S 17th Ave to the 10” main in Kagy Boulevard and Willow Way, constructed during SUD Phase 2. The proposed sewer in Arnold St. and Student Dr. will flow into the 18” Sewer in S 19th Ave. The existing sewer in S. 19th Ave turns to the west at Stucky Road. 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 analysis of the existing system has determined that these sewer flows feed or ‘jump’ into the Cattail Creek Drainage Basin at the Lincoln Street & South 19th Avenue connection. It has been verified that the east-west Lincoln sewer main crosses underneath the north-south 19th avenue sewer main to enter the Cattail Creek Drainage Basin. This configuration was established in 2017 during the “South University Area Sewer Main Interceptor Project”. Block 3 Lot 1B is the only exception with its sewer flows collecting directly into the Cattail Creek Drainage Basin. Flows generated by the SUD Phase 3 development were combined with the estimated flows from zoned parcels, as well as existing developments (see Figure 3: Sewer Capacity Analysis) to determine the impact of capacity within the downstream mains. The estimated residential wastewater generation is based on the City of Bozeman Facility Plan. Typical design flow is 64.4 gallons per day (gpd) per capita, with an average population of 2.17 people per home. The estimates for SUD Phase 3 Block 1, 2, 3, and 4 have been updated based on current or proposed development and included in Table 3A numbers. The estimated pipe capacities and their respective flow allocations based on existing developments, zoned parcels and SUD phase 3, are shown in Table 3A. Sewer Mains 7 8/5/2026 Table 3A: Impact on Capacity of Existing Sewer Mains 1 Slopes either proposed or as-built 2 150 gal/ac/day per COB Design Standards 3 Lincoln sewer main contributing area - Map # Designations: 1-17 4 Willow sewer main contributing area - Map # Designations: 11-17 5 Kagy sewer main contributing area - Map # Designations: 14-17 6 19th main contributing area - Map # Designations: 18 7 19th flow conditions only represent SUD flows The flows from Willow Way, Mandeville and Tai Lane subbasins, are allocated based on Table 4: Sewer Flows and Infiltration by Basin/Sub- basin. The combined peak flows and infiltration that eventually end up in the 15” Lincoln Street main are estimated to be 994 gpm; this includes a 90 gpm peak for MSU home football games (established from monitoring), plus an additional 22.2 gpm for infiltration within the sewer drainage sub-basins. The anticipated peak flow and infiltration of 994 gpm is under the capacity of a 15” main at minimum slope (0.19%) flowing 75% full (1,152 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 44% Q/Q75. (in)(%)(gpd)(unitless)(gpm)(gpm)(gpm)(in)(gpm)(%) Lincoln 15 0.19%402,863 3.15 972 22.2 994 11.25 1,152 86% Willow 10 0.97%193,426 3.44 462 8.0 470 7.5 883 53% Kagy 10 0.28%79,651 3.74 207 4.2 211 7.5 475 44% 19th 18 1.85%17,882 4.09 51 1.0 52 13.5 5,847 1% Infiltration2 Peak Flow + Infiltration Depth of Flow 75% Depth Capacity Flowrate Capacity Q/Q₇₅Pipe Size Slope1 Avg Daily Flow Peaking Factor Peak FlowSewer Main Sewer Mains 8 8/5/2026 Table 4: Sewer Flows and Infiltration by Basin/Sub-basin 1 Numbers Correlate with Figure 3 2 Rates taken from Table V-1 City of Bozeman Design Standards and Specifications Policy (p. 49) 3 Infiltration calculated at 150 gal/ac/day per City of Bozeman Design Standards and Specifications Policy 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. 11.16 Design Criteria The Block 3 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. Map # Designation1 Description Sub Basin Subbasin Subtotal Subbasin Area (sf)(ac) #Units or Beds (gal/ac/day)(gpd)(gpd)(ac) (gpm)(gpd)(gpd) 1 RC Mandeville 824,613 18.93 -2800 53,005 2 RD Mandeville 158,037 3.63 -2800 10,158 66,548 3 County A Mandeville - 0.00 -0 0 4 County B Tai Lane - 0.00 -0 0 5 B1 Tai Lane 460,873 10.58 -1100 11,638 6 R4 Tai Lane 463,928 10.65 -2800 29,821 7 B-2M Tai Lane 676,957 15.54 -2800 43,514 157,333 8 MSU Stadium Tai Lane 799,299 18.35 -500 9,175 9 SUD PH 1B Tai Lane 252,977 5.81 -2800 16,261 10 SUD PH 1 Tai Lane 557,933 12.81 -2800 35,863 11 R4 Willow Way 225,119 5.17 -2800 14,470 12 SUD PH 2 Willow Way 843,741 19.37 887.00 -57,123 13 SUD PH 3 - Block 4 Willow Way 612,385 14.06 655.00 -42,182 14 REMU Willow Way - - --0 204,133 15 SUD PH 3 - Block 1 Willow Way 458,500 10.53 273.00 -39,138 16 SUD PH 3 - Block 2 Willow Way 495,062 11.37 162.00 -22,631 17 SUD PH 3 - Block 1A Willow Way 556,700 10.88 128.00 -17,882 Totals 402,863 167.67 17.47 25,150.92 428,014 18 SUD PH 3 - Block 1B 19th Sewer 372,865 8.56 128.00 -17,882 17,882 8.56 0.89 1,284.00 19,166 193,426 71.38 7.44 10,706.65 Area/ Population 146,273 73.74 7.68 11,060.49 63,164 22.56 2.35 3,383.78 Average Daily Water Usage Rates2 Infiltration3 Total Sewer Mains 9 8/5/2026 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 3 site is typical of Bozeman with fine-grained soils overlying gravel deposits. Geotechnical borings have determined the depth to gravel ranges from 2.5 to 6.5 feet. Estimated depth to groundwater obtained from the geotechnical borings is variable from 4 to 9 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. R4 SUD PH 2FUTUREREMU PARK SUD PH 1 SUD PH1b RD B1 RC B-2M MSU STADIUMCOUNTY AWILLOW WAYRCCOUNTY BBLOCK 3 LOT 1A PH 3 BLOCK 2 PH 3 BLOCK 1 PH 3 BLOCK 4 PH 3 1 4 2 3 5 6 7 8 13 14 9 1016 17 15 11 12 ARNOLD ST17TH AVESTUCKY RD KAGY BLVD 19TH AVELINCOLN STREET 18 BLOCK 3 LOT 1B PH 3 PROFESSIONALENGINEERS &SURVEYORS3SEWER STAHLYENGINEERING& ASSOCIATESPHASE 3SOUTH UNIVERSITYDISTRICTRTR HOLDING II, LLC.CAPACITYANALYSISIN FEET LEGEND MANDEVILLE SUBBASIN TAI LANE SUBBASIN WILLOW WAY SUBBASIN 19TH SEWER SUBBASIN EXISTING SEWER PROPOSED SEWER SSSSSSSSSSSSSSW W W WSSSS SS SS WSDSDSDSDSDSD SDSDSDWWWWWSOUTH 14TH AVESS HC HC HC HC OHE OHE OHE OHE OHE OHE OHE OHE OHE OHE OHE OHE OHE OHESSSSSSSS SSSSSSSSW WSSSSSSSSSSSSSSOHEOHEOHESDSDSDWWWY3 Y3 Y3 Y3 Y3 P3 P3 P3 P3 SD SDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSTUCKY RD SOUTH 14TH AVESTUCKY RD(48' WIDTH - 90' ROW)W W W W W W W W W W W WOHEOHEOHE (48' WIDTH - 90' ROW)WWWWWWWWWWWWWWWWWWWWWWWWSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSWWSSSSSSSSSTATE ST(35' WIDTH - 60' ROW)SD(47' WIDTH - 70' ROW)SSSS SOUTH 17TH AVE(EXISTING 70' ROW) STUCKY RD(48' WIDTH - 90' ROW) ARNOLD ST(36' WIDTH - 105' ROW) SD IRRIRRIRRIRRIRRIRRIRRIRRIRRIRRIRRIRR SOUTH 14TH AVESOUTH 17TH AVE(47' WIDTH - 70' ROW)S 19TH AVE(ROW WIDTH VARIES)S 19TH AVE(ROW WIDTH VARIES)OHEOHEOHEOHEOHEOHEOHEOHEOHEOHEOHEEXISTINGTRAILS (TYP)SSSSSSSSSSSSSSSSSSWWWWWWWWWWSTORM BASINS T O RM BA S IN (47' WIDTH - 70' ROW)STUDENT DRIVES 19TH AVE(ROW WIDTH VARIES)IRRIRRIRRIRRIRRIRRIRRIRRIRRIRRIRRIRRIRRIRRIRREXISTINGSTORM BASIN CITY PARK 14.41 ACRESPLAT J-605Tract 3ACOS 2661B CITY PARK 2 9.64 ACRES PLAT J-605 CITY PARK 41.83 ACRESPLAT J-695COMMON OPEN SPACELOT 2, BLOCK 1, 0.07 ACPLAT J-695COMMON OPEN SPACELOT 1, BLOCK 2, 0.07 ACPLAT J-695 COMMON OPEN SPACELOT 2, BLOCK 2, 0.07 ACPLAT J-695 BLOCK 3 , LOT 1A8.840 ACRES(385,071 SF)CITY PARK 3 13.36 ACRES PLAT J-695 LOT 2 "PHASE 1"6.41 ACRESPLAT J-518 BLOCK 3 SUBDIVISIONINFRASTRUCTURE EXTENTSBLOCK 3 SUBDIVISIONINFRASTRUCTUREEXTENTSARNOLD ST STUDENT DRIVE SOUTH 17TH AVES 19TH AVESTUCKY RD YELLOWSTONE THEOLOGICAL INSTITUTE MINOR SUBDIVISION RESTRICTED LOT 4 10.519 ACRES YELLOWSTONE THEOLOGICAL INSTITUTE MINOR SUBDIVISION COMMON OPEN SPACE LOT 2 1.826 ACRES YELLOWSTONE THEOLOGICAL INSTITUTE MINOR SUBDIVISION LOT 3 7.644 ACRES YELLOWSTONE THEOLOGICAL INSTITUTE MINOR SUBDIVISION LOT 1 15.069 ACRES ALLISON SUBDIVISION, PH 4A BLOCK 1, LOT 1 W WWW SDBLOCK 3, LOT 1B8.636 ACRES(376,169 SF)WWWWW(36' WIDTH - 62' ROW)SSWWWWWW W SSSSSSSSSSSS SSWW SSSSW WSSSSSDSDSDSDSDSDSDSDSDSDSD SDSDSD SDSDSD SDWW WSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDWWWWW WWWWWSDSDSDSDSSSS EXISTING TRAILS (TYP) EXISTING TRAILS (TYP)SSSSSSSSLOT 1, BLOCK 2"PHASE 3"8.60 ACRESPLAT J-695 WWSDSDSDSDSDSDSDSSWWWWSSSSSSSS(27' WIDTH - 70' ROW+29.5' EASE)ARNOLD ST SCALE IN FEETC3.0SEWER PLAN1 12/15/2025 PRELIMINARY PLAT ZWL CDP 2 5/26/2026 PRELIMINARY PLAT RC1 JWF CDP PROFESSIONALENGINEERS &SURVEYORSSTAHLYENGINEERING& ASSOCIATESSUD PH. 3 BLOCK 3SUBDIVISION RTR HOLDINGS II, LLC HELENA Phone:(406)442-8594CODY Phone:(307)509-5541851 BRIDGER DR. STE 1BOZEMAN, MT 59715 Phone:(406)522-9526BILLINGS Phone:(406)601-4055GREAT FALLS Phone:(406)604-4644