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Home My WebLink About016_Appendix O - Storm Engineering Design ReportEngineering Report Stormwater Design South University District Phase 3 Preliminary Plat Updated May 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) 522-9526 Stormwater Plan 1 5/27/2026 Engineering Report Table of Contents General Information and Design Criteria Existing Site Conditions Storm Drainage Plan Description Estimation of Runoff Drainage Areas Retention Facilities Groundwater Monitoring Groundwater Impact Analysis Storm Event Analysis Conveyance Capacity Curb and Gutter Storm Drains Facilities Maintenance Figures and Tables Figure 1. Vicinity Map Figure 2. Stormwater Plan Table 1. Storm Runoff Calculations Table 2 - New Retention Facilities Calculations Table 3 Groundwater Monitoring Table 4 Bozeman Rainfall Analysis (1990-2020) Table 5 Curb and Gutter Conveyance Calculations Table 6 Storm Drain Conveyance Calculations Stormwater Plan 2 5/27/2026 General Information and Design Criteria 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 8.64 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. Stormwater mitigation is achieved by using a compact, efficient stormwater management design utilizing Low Impact Design (LID) methodology to mitigate the impacts of stormwater runoff. The LID stormwater mitigation has been designed in accordance with City of Bozeman Design Standards and Specifications Policy for retention/infiltration facilities. Survivability of LID systems through conventional development is difficult. Erosion control Best Management Practices (BMP’s) will be utilized during construction to reduce the impacts of construction site runoff, until the LID systems are constructed. Plans are provided showing proposed grading, drainage flow paths, and stormwater retention/infiltration details and locations. 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 Stormwater Plan 3 5/27/2026 The location of the South University District Phase 3, Block 3 Development is shown on the Vicinity Map in Figure 1. Figure 1. Vicinity Map, Bozeman, Montana Existing Site Conditions SUD Phase 3 Block 3 is a block parcel created by the South University District Phase 3 Major Subdivision. The SUD Block 3 development site was previously used agriculturally prior to its subdivision development. The existing ground slopes north at grades between 1-3%. Existing offsite drainage is minimal, if present it would flow west to an existing irrigation ditch on the east side of 19th Avenue Right-of-Way and Mandeville Creek, east to Mandeville Creek or north to an irrigation ditch on the south side of Kagy Boulevard and either infiltrates or makes its way to Mandeville Creek. Block 3 will utilize Mandeville Creek as an outfall on the storage basin in park 4 through a 15” outfall. The other storage basins will either infiltrate the storm water or flow into Mandeville Creek in the case of the storage basin overflowing. 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. The underlying gravelly sandy material will provide long-term infiltration of stormwater. Stormwater Plan 4 5/27/2026 Groundwater monitoring was conducted in 2022 during the high groundwater season for the SUD Phase 3 subdivision development. Groundwater monitoring found groundwater depths ranged from 5.5’ to 10.0’ below existing grade in the areas of the site proposed for development. The site is not located within a 100-year floodplain. Storm Drainage Plan Description The storm drainage plan for the SUD Phase 3 Block 3 Subdivision development consists of one existing storm basin and two new storm basins located on the south end of the lot on each side of Mandeville Creek. These basins will overflow into Mandeville Creek. The third detention basin which is the biggest of the three is located northeast of the intersection of Stucky Rd. and South 17th Ave. in City Park 4. This pond was constructed as a part of the subdivisions “Common Subdivision Improvements (CSIs)” and has a 15” outfall flows to Mandeville Creek. The overall storm drainage plan for Block 3 subdivision is depicted on attached Figure 2. This plan shows proposed drainage basin areas, flow paths, and retention/infiltration system locations. In general, grades will be sloped towards gutters which will convey storm runoff to storm drain inlets which will then be directed to one of the three stormwater basins. The subdivision will be divided into the three primary drainage areas, one drainage area for each of the new storm basins and one for the existing storm basin. The existing storm basin draining area will be further divided into eleven sub-drainage areas, for detailed pipe analysis as shown on attached Figure 2 and described below. The first primary drainage area consisting of sub basins 12-16C, is located at the northwest corner of the site. Storm water will flow north to south 18th and will go to an existing storm drain system on Stucky Road. It will then flow to the east where it will flow into the detention pond located in City Park 4. The second primary drainage area is located on the southwest of the site along Arnold Street and drains northeast to the storm basin located on the west side of Mandeville Creek in Block 3 Park. The third primary drainage area is located on the southeast side of the site along Arnold Street and drains northwest to the storm basin on the east side of Mandeville Creek in Block 3 Estimation of Runoff Runoff estimates were obtained for each sub-drainage area using City of Bozeman standards and expected land use runoff coefficients. The sites stormwater facilities have been sized to fully capture the 10-year, 2-hour storm event. The Bozeman IDF (Intensity, Duration, and Frequency) curves show a rainfall intensity of 0.41 in/hour for the 10-year, 2-hour storm, totaling a 0.82-inch storm event. Table 1 shows the storm runoff calculations for the 3 primary drainage areas. Specifically, the design uses the volume of the 10-year, 2-hour storm for retention calculations. Stormwater Plan 5 5/27/2026 Drainage Areas The site is divided into 3 primary drainage basins. The primary basins are further divided into sub-basins for detailed pipe analysis. The first primary drainage basin, consisting of sub basins 12-16C, will flow into an existing storm basin that is located in City Park 4. Park 4 is located northeast of the intersection between south 17th Ave and Stucky Rd. The next two primary basins, 17 and 18 will flow into the two storm basins that are in City Park, 3 north of the proposed Arnold Street between S. 17th Ave and S. 14th Ave. Table 1 – Storm Runoff Calculations Retention Facilities Existing The storm basin in City Park 4 is a detention basin with a 15” outfall. The outfall will drain into Mandeville Creek. This pond was installed with the SUD Phase 3 subdivision and accounted for the stormwater runoff from the entirety of the Block 3 area. The pond routing calculations can be found in table 7 of the SUD Phase 3 Subdivision Stormwater report. This pond will account for the majority of the block 3 subdivision stormwater runoff including drainage basins 12 through 16C. It accounts for 22.92 acres of 25.13 total acres in Block 3 Subdivisions drainage area. New The two new storm basins that are in City Park 3 along Arnold Street are retention ponds, and account for drainage area 17 and 18. These drainage areas account for 2.21 acres of runoff area. The basins were added due to grading constrains not due to lack of storage capacity in the existing storm pond The storm basins are designed to infiltrate the water though the bottom of the pond. If these ponds were to overflow, they will drain into Mandeville Creek. Table 2 - New Retention Facilities Calculations Stormwater Plan 6 5/27/2026 Groundwater Monitoring As part of the SUD Phase 3 subdivision, groundwater monitoring data was collected and is summarized in Table 2 below. Due to ongoing construction and dewatering activities associated with the subdivision, reliable groundwater monitoring has not been able to take place since the start of subdivision construction. Table 3 – Groundwater Monitoring (Measured from TOC to GW (feet) Groundwater Impact Analysis Ground water in the area ranges from 5 feet to 10 feet. The depth of the detention pond in City Park 4 is 1.5 feet. The depths of the two retention ponds in Block 3 park are 1.5 feet in depth. These detention and retention ponds have ideal separation between the bottom of the pond and the ground water to ensure long-term survivability and performance. Storm Event Analysis A storm event analysis was previously conducted to determine the percentile storm events in the City of Bozeman. The exercise analyzed 30 years of recorded 24-hour rainfall events using precipitation data collected at MSU rain gauge Station USC00241044. The analysis excluded snow fall events and 24-hour rainfall events that accumulated less than 0.1-inches. The table below summarizes the percentile storm events produced by the analysis. As indicated by the table, the site stormwater facilities as design would completely retain the 99th percentile storm event. Table 4 – Bozeman Rainfall Analysis (1990-2020) Percentile Storm Event Volume (in) 80th 0.41 85th 0.49 90th 0.57 95th 0.74 99th 1.2 100th 2.29 Conveyance Capacity Stormwater Plan 7 5/27/2026 As designed, each retention and detention system have the capacity to completely capture the 10-year 2-hour (0.82 inch) storm event. The distributed retention and detention systems throughout the site reduce the likelihood of overloading any single conveyance system. Each system has a primary overflow into Mandeville Creek. The site conveyance systems were each individually analyzed to ensure adequate capacity exists in the event there is a failure within the system (i.e., clogging). Curb and Gutter Curb and gutter are used on the site to direct runoff into the underground storm pipe network. Curb and gutter is primarily used along S. 17th Ave, Student Drive, and on Arnold Street. To ensure the standard curb design will provide adequate conveyance capacity for peak flows, curb and gutter with the greatest flows and least amount of redundant or secondary storm conveyance options was identified and analyzed for conveyance capacity. The critical curb required to convey the maximum flow was identified as the approximately 548’ length of curb running from south to north along S. 17th Ave, serving a catchment area of approximately 35,719 sf. This catchment area produces a peak flow of 1.91 cfs for the 25-year design storm. At the minimum slope of 0.5%, the curb has a conveyance capacity of 5.03 cfs. Table 5 – Curb and Gutter Conveyance Calculations Curb and Gutter Capacity Calcs Max Curb Conveyance Req'd (ft^3/sec) 1.71 Right-side Slope X:1 0.06 Left-side Slope X:1 33.00 Channel Bottom Width (ft) 0 Flow Depth (ft) 0.350 Flow Area (ft^2) 2.025 Wetted Perimeter (ft) 11.906 Width 0.044 Hydraulic Radius (ft) 0.170 Manning's Roughness 0.013 Slope (ft/ft) 0.005 Average Velocity (ft/sec) 2.49 Flow (ft^3/sec) 5.03 Storm Drains The hydraulic capacity of the storm drains were analyzed to determine the required pipe size to convey the 25-year, at the time to concentration storm event. The maximum 25- year event for any of the basins draining to a 24” storm pipe produces approximately 15.55 cfs of runoff. The capacity of a 24” storm drain at 1% slope is 24.11 cfs at 90% full pipe flow. Therefore, 24” storm drains have the capacity to convey the 25-year peak runoff of any onsite defined drainage areas. The maximum 25-year event for any of the Stormwater Plan 8 5/27/2026 basins draining to a 30” storm pipe produces approximately 16.17 cfs of runoff. The capacity of a 30” storm drain at 0.46% slope is 29.65 cfs at 90% full pipe flow. Therefore, 30” storm drains have the capacity to convey the 25-year peak runoff of any drainage basin to the retention systems. The maximum 25-year event for any of the basins draining to a 36” storm pipe, where the 24” and 30” storm pipes connect to go to the detention pond, produces approximately 29.58 cfs of runoff. The capacity of a 36” storm drain at 0.22% slope is 33.34 cfs at 90% full pipe flow. Therefore, 36” storm drains have the capacity to convey the 25-year peak runoff of any drainage basin to the retention systems. Table 6 – Storm Drain Conveyance Calculations Facilities Maintenance The proposed stormwater mitigation systems are located within private rights-of-ways and will be operated and maintained by the owners’ association. A detailed operations and maintenance plan has been developed and included in the final covenants. At a minimum, the owners’ association is responsible for quarterly inspection and annual maintenance of all stormwater facilities. The inspection and maintenance responsibilities include, but are not limited to, general housekeeping responsibilities, visual inspection for performance, removal of sediment from mitigation facilities, vegetative maintenance, as well as any necessary repairs to the facilities themselves. WWSDSDSDSDSDSDSDSDSDSDSDSDSDSDWWWWW SDSDSDSDSDSDSDWWWWWWWWWWWSOUTH 14TH AVESDW WSDSDSDSDSDSDSDWWWWWSDSDWILLOW WAY SD SDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSD SDSDSDSDSDSDSDSDSOUTH 14TH AVESDSTUCKY RD(48' WIDTH - 90' ROW) W W W W W W W W W W W W W W W W W W W W W W W W WWWWWWWWWWWWWWWWW WWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWW W W W W W W W WSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSD WWWWWSTATE ST(35' WIDTH - 60' ROW)SDSDSDS 19TH AVE(47' WIDTH - 70' ROW) SOUTH 17TH AVE(EXISTING 70' ROW)SDSDKAGY BLVD(ROW VARIES) STUCKY RD(48' WIDTH - 90' ROW) KAGY BLVD(ROW VARIES) ARNOLD ST (36' WIDTH - 105' ROW) SD SD SD IRRIRRIRRIRRIRRIRRIRRIRRIRRIRRIRRIRRIRRIRRIRRIRRIRRIRRIRRIRRIRRIRR IRR IRR SOUTH 14TH AVESOUTH 17TH AVE(47' WIDTH - 70' ROW)(ROW WIDTH VARIES)S 19TH AVE(ROW WIDTH VARIES)S 19TH AVE(ROW WIDTH VARIES)SD EXISTINGTRAILS (TYP)STORM BASINS T O RM B A S I N (47' WIDTH - 70' ROW)STUDENT DRIVES 19TH AVE(ROW WIDTH VARIES)IRRIRRIRRIRRIRRIRR IRR IRRIRR IRRIRRIRR IRRIRR IRRIRR IRRIRRIRRIRRIRR IRRIRR IRRIRRIRR IRRIRR IRR IRRIRR IRREXISTINGSTORM BASIN CITY PARK 1 4.41 ACRES PLAT J-605 Tract 3ACOS 2661BTract 1ACOS 2661B CITY PARK 2 9.64 ACRES PLAT J-605 LOT 1, BLOCK 1"PHASE 3"8.20 ACRESPLAT J-695 CITY PARK 4 1.83 ACRES PLAT J-695 COMMON OPEN SPACELOT 1, BLOCK 1, 0.09 ACPLAT 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 ACRES ARNOLD ST STUDENT DRIVE SOUTH 17TH AVES 19TH AVESTUCKY RD YELLOWSTONE THEOLOGICAL INSTITUTE MINOR SUBDIVISIONYELLOWSTONE THEOLOGICAL 15A115A2 15A315A6 15A5 15A4STORM WATERBASIN 11.03 AC GROSSSTORM WATERBASIN 30.36 AC GROSS STORM WATERBASIN 20.36 AC GROSS STORM WATER BASIN 60.32 AC GROSSSTORM WATERBASIN 42.27 AC GROSS 9A 14A 12B 15A8A7A 11A12A13B 13A 15B10A15C3A5A2A1B1A6ASTORM WATERBASIN 50.32 AC GROSS 16A17A 16C16B 17B18AIRRIRRSDSDBLOCK 3, LOT 1B8.636 ACRES(376,169 SF)(36' WIDTH - 62' ROW)IRRIGATIONDITCH CULVERTIRRIGATIONDITCH CULVERTMODIFIED DITCHWATERCOURSENATURAL BOTTOMDRAINAGE CULVERTEXISTING DITCHWATERCOURSEWWWWWWWWWWWWW W W W W W W W W W W W W W WSDSDSDSDSDSDSDSDSD SD SD SD SDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSD SDSDSDSDSDSD SD SD SDSDSDSDSDSDSD SD SD W W W W W WSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSD SDSDSDSDSD SD SD SDSDSDSDSDSD SDSDSDSDSDSDSDSDSD SDSD SDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSD SDWWWWWWWWWWWWWWWWWWWWWWWWWWWWSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDW EXISTING TRAILS (TYP) EXISTING TRAILS (TYP) LOT 1, BLOCK 2 "PHASE 3" 8.60 ACRES PLAT J-695 STORM WATER BASIN 11 0.45 AC GROSS STORM WATER BASIN 12 0.49 AC STORM WATERBASIN 7 0.21 AC GROSS STORM WATER BASIN 8 0.21 AC GROSS STORM WATER BASIN 9 0.84 AC GROSS STORM WATER BASIN 10 0.94 AC GROSS STORM WATER DRAINAGE BASIN 14 0.37 ACRES STORM WATER DRAINAGE BASIN 16B 7.16 ACRES STORM WATER DRAINAGE BASIN 16A 1.05 ACRES STORM WATER DRAINAGE BASIN 13 2.94 ACRES STORM WATER DRAINAGE BASIN 16 C 8.47 ACRES STORM WATER DRAINAGE BASIN 17 1.21 ACRES STORM WATER DRAINAGE BASIN 18 1.00 ACRES STORM WATER DRAINAGE BASIN 15A1 0.48 ACRES STORM WATER DRAINAGE BASIN 15A2 0.67 ACRES STORM WATER DRAINAGE BASIN 15A4 0.47 ACRES STORM WATER DRAINAGE BASIN 15A5 0.82 ACRES WWSDSDSDSDSDSDSDSDSDSDSDSDSDWWWWWWWWW(27' WIDTH - 70' ROW+29.5' EASE)ARNOLD ST BASIN 24 BASIN 29 BASIN 15 BASIN 28 0 SCALE IN FEET 20050100 1 12/15/2025 PRELIMINARY PLAT ZWL CDP 2 5/26/2026 PRELIMINARY PLAT RC1 JWF CDP PROFESSIONALENGINEERS &SURVEYORSSTAHLYENGINEERING& ASSOCIATESSUD PH. 3 BLOCK 3 SUBDIVISION 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)604-4644FIGURE 2SUD PH3 BLOCK 3STORM DRAINAGEFIGUREOFF-SITE STORM FACILITIESBLOCK 3 STORM FACILITIESSITE/SUBD. FACILITIESPH. 3 SUBD. STORM FACILITIES