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BOZEMAN HEALTH EMERGENCY DEPARTMENT
EXPANSION AND RENOVATION ENABLING
Project # 17095.16
Bozeman, Montana
JUNE 2026
DRAINAGE REPORT FOR BOZEMAN HEALTH EMERGENCY DEPARTMENT EXPANSION AND RENOVATION BUILDING CERTIFICATION I hereby state that this Final Drainage Report has been prepared by me or under my supervision and meets the standard of care and expertise which is usual and customary in this community of professional engineers. The analysis has been prepared utilizing procedures and practices specified by the City of Bozeman and within the standard accepted practices. Dylan Eve, P.E. Date
June 2026 Project No. 17095.13 DRAINAGE REPORT BOZEMAN HEALTH EMERGENCY DEPARTMENT EXPANSION AND RENOVATION ENABLING BOZEMAN, MONTANA 59715 SITE NARRATIVE The purpose of this drainage report is to present a summary of calculations to quantify the stormwater runoff for the Bozeman Health Emergency Department Expansion and Renovation Enabling project (referred to in this document as “the project”). All design criteria and calculations are in accordance with the City of Bozeman Design and Construction Standards, dated October 2024. The site stormwater improvements have been designed with the intent of meeting the current drainage requirements for the City of Bozeman. LOCATION The development site is located on the Bozeman Health Campus at 915 Highland Boulevard in Bozeman, Montana. The project is in Sections 8 and 17, Township 2 South, Range 6 East in Gallatin County. EXISTING SITE CONDITIONS The existing site is currently developed with asphalt parking lots, access drives, curb and gutter, storm drain infrastructure, and utilities associated with the Bozeman Health campus. The access aisle to parking lot B generally slopes to the east at approximately 2.5%. Within parking lot B, the southern portion of the lot slopes east at approximately 2%, while the northern portion slopes eastward at approximately 4%. Parking lot C generally slopes to the east with pavement grades ranging from approximately 0.5% to 3.5%. The access aisle located between parking lots B and C exhibits varying drainage patterns. North of the ambulance bay access, the drive aisle
Drainage Report Page 2 of 11 June 2026 slopes to the west at approximately 3%. South of the ambulance bay access, the drive aisle slopes to the southeast with grades between 1.5% to 2.5%. Existing stormwater runoff from parking lot C and the drive aisle north of the ambulance bay access is collected by inlets and is conveyed through the existing storm drain infrastructure to an underground stormwater chamber facility located beneath parking lot A. Runoff within parking lot B is also collected in the forementioned chamber facility. Stormwater runoff from parking lot C and the drive aisle south of the ambulance bay access is collected by inlets and conveyed through existing storm drain infrastructure to an outfall located southeast of Highland Boulevard. In addition to existing stormwater infrastructure, the project area contains several utility facilities that support the hospital campus. These utilities include propane, oxygen, electrical, telecommunications, and natural gas. The campus currently contains a single oxygen tank within parking lot G, there is a second oxygen tank proposed as part of the project improvements. PROPOSED PROJECT The proposed project will realign the site access located north of Highland Boulevard between parking lots B and C. The project also includes a temporary reconfiguration of the existing ambulance bay loop that is currently connecting parking lots B and C. The temporary reconfiguration will include the construction of a temporary structure and an access loop located east of parking lot C. A new oxygen tank is proposed in parking lot G with a new oxygen supply line. There will also be temporary construction of a helipad in the southeast corner of parking lot C and a mobile PET/CT facility located in parking lot D. The mobile PET/CT facility will require new underground electrical and telecommunication service connections. Temporary improvements are required to maintain hospital operations during the planned expansion of the Emergency Department. As part of the Emergency Department expansion, the existing helipad and adjacent propane tank area will be removed. The propane tanks will be removed, and new propane tanks will be installed to the south of parking lot B. The new propane tanks will require rerouting of the associated gas utility line. There will also be a final phase of the project. Within the final project we will add additional detention drainage facilities to maintain the additional runoff from proposed buildings and hardscape. This will be discussed in the final drainage report site plan submittal. Site improvements associated with the temporary facilities and utility relocations will include new asphalt pavement, curb and gutter, grading modifications, and drainage improvements. Drainage improvements are proposed within the vicinity of parking
Drainage Report Page 3 of 11 June 2026 lots B and C and will include installation of new inlets, manholes, and storm drain piping. The proposed improvements and temporary modifications are intended to support the continued hospital operations during construction of the Emergency Department expansion. Below is an image of the parking map for Bozeman Health. 1: Parking Map
Drainage Report Page 4 of 11 June 2026 HYDROLOGY The rational method was used to determine peak runoff rates and volumes since all basins within the project are less than 5 acres. The rational formula provided in the City of Bozeman Design and Construction Standards was used to calculate the peak runoff rates on-site, time of concentration, rainfall intensities, etc. To be conservative a time of concentration of 5 minutes was used. For asphalt concrete surfaces, a runoff coefficient of 0.95 was assumed, for landscape surfaces, a runoff coefficient of 0.15 was assumed, and for gravel surfaces, a runoff coefficient of 0.80 was assumed. The proposed project improvements primarily consist of site modifications, utility relocations, and localized grading changes. The runoff management design was developed to maintain existing drainage patterns and direct runoff to the same stormwater collection system that currently serves the site. Where grading modifications are proposed, drainage basin boundaries were designed to replicate existing drainage conditions and preserve the distribution of runoff between the existing stormwater facilities. Because the project does not significantly increase impervious area or alter the overall stormwater management approach for the site, additional stormwater detention or retention facilities are not proposed for most of the site. There will be one dry well designed to facilitate storm in one minor basin where a storm drain is being abandoned in place. The dry well is temporary and will be removed in the final phase of construction when the ambulance bay is expanded. The following sections summarize the hydrologic analysis and demonstrate that the proposed drainage improvements maintain runoff rates and storage requirements consistent within the existing site conditions. A. Pre-Development Basins For the following sections, please refer to Appendix A – Exhibit A, Exhibit B, and Exhibit C of this report which graphically shows and labels the existing drainage basins for the project. Existing basins were evaluated where grading was actively being changed within the scope of this project and included nearby run on areas to fully understand the total storm flow to existing storm infrastructure. On-site there were 22 pre-development basins. These basins were split into three areas; Mobile Pet Site, Oxygen Tank Site, and Emergency Department Site. Mobile Pet Site (Refer to Exhibit A) Pre-Development Basin A and B are located within the vicinity of the Mobile Pet Site. The two basins have only been analyzed as a pre-development basin and not a post-
Drainage Report Page 5 of 11 June 2026 development basin. Post-development adds no additional hardscape in this area and maintains the original intent of the pre-development storm infrastructure. Each basin flows to separate curb inlets within the Bozeman Health Parking lot D storm infrastructure. The table below summarizes the hydrology findings for the basins for the 10-year 2-hour storm event. Basin Imp. Area (AC) Perm. Area (AC) Weighted Runoff Coefficient C Rainfall Intensity (in/hr) Runoff Q (cfs) Volume V (cf) Water Quality Volume (cf) A 0.14 0.16 0.53 0.41 0.62 470 262 B 0.30 0.15 0.68 0.41 1.18 897 527 Oxygen Tank Site (Refer to Exhibit B) Pre-Development Basins C, D, E, and F are located within the vicinity of the proposed oxygen tank site. The four basins have only been analyzed as a pre-development basin and not a post-development basin. Post-development adds no additional hardscape and increases the landscape area of pre-development basin E by 132 sf. Landscape area increased by removing 2 existing parking stalls and pouring a concrete pad for the proposed oxygen tank. All new asphalt and concrete will be poured to maintain the original intent of the pre-development storm infrastructure within the area. The table below summarizes the hydrology findings for the basins for the 10-year 2-hour storm event. Basin Imp. Area (AC) Perm. Area (AC) Weighted Runoff Coefficient C Rainfall Intensity (in/hr) Runoff Q (cfs) Volume V (cf) Water Quality Volume (cf) C 0.19 0.04 0.82 0.41 0.70 533 323 D 0.01 0.02 0.32 0.41 0.03 26 12 E 0.13 0.01 0.88 0.41 0.49 374 229 F 0.17 0.12 0.61 0.41 0.68 518 298 Emergency Department Site (Refer to Exhibit C) Pre-Development Basins G through W are located within the vicinity of the proposed Emergency Department Site. Basins within the vicinity of the Emergency Department Site have been evaluated for both pre-development and post-development. There have been no detention or retention sites proposed within this phase of the project. Storm runoff will maintain its original intent of the pre-developed site once proposed work is completed. Inlets and storm drainpipes will be realigned where necessary.
Drainage Report Page 6 of 11 June 2026 Pre-Development Basins I, K, L, M, N, O, and R flow into a system of storm inlets that ultimately release into a 12-inch flared end section just south of the existing access between parking lots B and C on Highland Boulevard. The area of release is property also owned by Bozeman Health. The total runoff, storage volume, and water quality volume associated with these basins for the 10-year 2-hour storm event are: 3.72 cfs, 2,819 cf, and 1,647 cf. Pre-Development Basins P and Q also release onto the Bozeman Health property located southeast of Highland Boulevard via 8-inch flared end section located further north adjacent to the access along Highland Boulevard between parking lots A and B. The total runoff, storage volume, and water quality volume associated with these basins for the 10-year 2-hour storm event are: 3.52 cfs, 2,673 cf, and 1,555 cf. Pre-Development Basins G, H, J, S, T, U, V, and W flow into a system of storm inlets that ultimately release into the newly expanded detention pond located north of the newly expanded parking lot A. The total runoff, storage volume, and water quality volume associated with these basins for the 10-year 2-hour storm event are: 5.78 cfs, 4,390 cf, and 2,582 cf. The table below summarizes the hydrology findings for the basins for the 10-year 2-hour storm event. Basin Imp. Area (AC) Perm. Area (AC) Weighted Runoff Coefficient C Rainfall Intensity (in/hr) Runoff Q (cfs) Volume V (cf) Water Quality Volume (cf) G 0.21 0.02 0.89 0.41 0.80 604 370 H 0.14 0.03 0.83 0.41 0.54 411 249 I 0.19 0.06 0.76 0.41 0.74 559 334 J 0.03 0.01 0.75 0.41 0.13 96 57 K 0.17 0.00 0.9 0.41 0.63 475 294 L 0.08 0.06 0.59 0.41 0.32 243 139 M 0.00 0.31 0.15 0.41 0.18 138 28 N 0.12 0.02 0.83 0.41 0.46 349 212 O 0.25 0.02 0.88 0.41 0.92 699 428 P 0.42 0.06 0.85 0.41 1.56 1186 723 Q 0.46 0.48 0.54 0.41 1.96 1487 832 R 0.12 0.04 0.75 0.41 0.47 356 212 S 0.56 0.29 0.68 0.41 2.22 1684 989 T 0.25 0.30 0.51 0.41 1.07 814 449 U 0.07 0.06 0.58 0.41 0.28 215 122 V 0.09 0.00 0.95 0.41 0.32 240 148 W 0.11 0.03 0.77 0.41 0.43 326 196
Drainage Report Page 7 of 11 June 2026 B. Post-Development Basins For the following sections, please refer to Appendix A – Exhibit D of this report, which graphically shows and labels the on-site watersheds. No percolation rates have been included in the required detention and retention storage volume calculations to be conservative. Storage volume calculations used the 10-year, 2-hour design storm frequency for rainfall data. The 100-year storm was also evaluated for each drainage basin. The overall intention of the post-development storm was to mimic the pre-development site and only realign storm drain inlets and pipes where necessary. Therefore, most basins have very similar total areas, impermeable areas, and permeable areas for both pre-development and post-development except for pre-development basin J and pre-development basin M. Pre-development basin J was combined basin K and pre-development basin M was split between basin K and N. Post-Development Basins I, K, L, N, and O flow into a system of storm inlets that ultimately release into the existing 12-inch flared end section just south of the exiting access located between parking lots B and C on Highland Boulevard mentioned in the previous section. The total runoff, storage volume, and water quality volume associated with these basins for the 10-year 2-hour storm event are: 3.31 cfs, 2,514 cf, and 1,499 cf. This is a reduction of 0.41 cfs, 305 cf, and 148 cf from the related pre-development area. Post-Development Basins P and Q also release onto the Bozeman Health property located southeast of Highland Boulevard via the existing 8-inch flared end section located further north adjacent to the access along Highland Boulevard between parking lots A and B. The total runoff, storage volume, and water quality volume associated with these basins for the 10-year 2-hour storm event are: 3.46 cfs, 2,626 cf, and 1,521 cf. This is a reduction of 0.06 cfs, 47 cf, and 34 cf from the related pre-development area. Post-Development Basins G, H, R, S, T, U, and W flow into a system of storm inlets that ultimately release into the newly expanded detention pond located north of the newly expanded parking lot A. The total runoff, storage volume, and water quality volume associated with these basins for the 10-year 2-hour storm event are: 5.86 cfs, 4,446 cf, and 2,616 cf. This is an increase of 0.08 cfs, 57 cf, and 35 cf. From the expanded storm detention pond C project, the pond needed a water quality volume of 3,112 cf but was built to have a water quality volume of 4,128 cf at an elevation of 4,863.50 ft. This leaves the pond with an excess water quality volume storage of 1,016 cf. With an additional 57 cf the pond will still have 959 cf of capacity.
Drainage Report Page 8 of 11 June 2026 Post-Development Basin V will flow into a dry well. The dry well was sized to store the 10-yr 2-hour storm event. The dry well is temporary and will be removed during the construction of the expanded ambulance bay in the final phase. The total runoff, storage volume, and water quality volume for Post-Development Basin V is: 0.32 cfs, 240 cf, and 148 cf. The table below summarizes the hydrology findings for the basins for the 10-year 2-hour storm event. Basin Imp. Area (AC) Perm. Area (AC) Weighted Runoff Coefficient C Rainfall Intensity (in/hr) Runoff Q (cfs) Volume V (cf) Water Quality Volume (cf) G 0.21 0..02 0.88 0.41 0.78 593 363 H 0.20 0.01 0.93 0.41 0.73 553 341 I 0.13 0.03 0.82 0.41 0.49 370 224 K 0.29 0.10 0.75 0.41 1.13 857 512 L 0.09 0.04 0.70 0.41 0.37 280 166 N 0.09 0.11 0.51 0.41 0.41 308 170 O 0.25 0.02 0.88 0.41 0.92 699 428 P 0.42 0.06 0.85 0.41 1.56 1,186 723 Q 0.44 0.51 0.2 0.41 1.90 1,440 798 R 0.10 0.03 0.77 0.41 0.37 278 167 S 0.57 0.29 0.68 0.41 2.25 1,707 1,003 T 0.25 0.30 0.51 0.41 1.07 814 449 U 0.07 0.06 0.58 0.41 0.28 215 122 V 0.09 0.00 0.95 0.41 0.32 240 148 W 0.10 0.03 0.76 0.41 0.38 287 172 REQUIRED STORAGE VOLUME CALCULATIONS Dry Well A Post-Development Basin V is completely detained by the proposed dry well A for the 10-year 2-hour storm event. The required storage volume for the dry well is 240 cf. The storage of the dry well is 308 cf, making the storm system adequate to meet the storage requirements. V = 7,200 X C X I x A Where: C=0.95; i=0.41; A=0.09 acres Required Volume=240 ft3
Drainage Report Page 9 of 11 June 2026 Expanded Storm Pond C See Appendix E – BOZEMAN HEALTH CAMPUS PHASE 1 DESIGN, for information about the expanded storm pond C. HYDRAULICS Storm Inlets and Storm Drains All storm drainage pipes were sized to handle peak flow resulting from the minor storm event with a minimum slope to maintain a full-flow velocity of at least 2.5 feet per second while limiting the full flow velocity to a maximum of 12 feet per second. The Federal Highway Administration (FHWA) Hydraulic Toolbox Software Version 5.3.0 and Bentley FlowMaster softwares were used to determine pipe sizing for the full flow capacity of the pipes. Inlets were sized to handle the peak flow resulting from the minor storm event. Flow intercepted by drainage inlets were determined using Federal Highway Administration (FHWA) Hydraulic Toolbox Software Version 5.3.0 and Bentley FlowMaster. All proposed inlets located in sag conditions were sized assuming a 50% clogging factor, while inlets on-grade assumed a 25% clogging factor. For further information on storm drain and inlet capacity calculation, see Appendix C. Water Quality The City of Bozeman has a requirement to capture or reuse the runoff generated from the first 0.5 inches of rainfall from a 24-hour storm. We meet this requirement by detaining or retaining all runoff on-site in the two above ground detention ponds and the underground retention facility. In addition to the storage provided in the detention and retention ponds, sumps will be installed in the storm drain inlets prior to outflow, providing treatment before water infiltrates into the ground. Stormwater detention systems need to be maintained, per the recommendations in the Operations and Maintenance Manual, see Appendix D. A. Calculations WQV = ((P x Rv x A) / 12) x 43,560 Where: WQV=Water Quality volume P=Water Quality Rainfall Depth, inches (0.5-inches)
Drainage Report Page 10 of 11 June 2026 Rv=The Unitless Runoff Coefficient, Rv=0.5+0.9(I) I=the percent impervious cover draining to the facility A=total site area draining to the structure Dry Well A Water Quality Volume = 0.5in x 0.95 x 0.09ac x (1ft/12in) x 43,560ft/ac = 148 cf Drawdown time = 240 cf / 78.5 sf storage area / .28 ft/hr = 10.9 hours The storage area of the dry well is 78.5 sf which will draw down the 240 cf of water quality volume in 10.9 hours using the infiltration rate of 0.28 ft/hr. The dry well is to be tied into native gravels which are assumed to be poorly graded gravel. From USCS the minimum infiltration rate is 6.8 inches/hour for this soil type, to be conservative a factor of safety of 2 was applied. OUTLET STRUCTURES There is no outlet structures proposed for this project. There is an existing weir outlet structure located in the northeast corner of storm pond C. The weir controls flow from Bozeman Health into the City of Bozeman storm drain located within Ellis Street. See Appendix E – BOZEMAN HEALTH CAMPUS PHASE 1 DESIGN, for information about the outlet structure. CONCLUSION The proposed stormwater improvements for the Bozeman Health Emergency Department Expansion and Renovation Enabling Project have been designed in accordance with the City of Bozeman Design and Construction Standards (October 2024). The drainage analysis demonstrates that the proposed temporary facilities, utility relocations, access modification, and associated site improvements will maintain the overall intent and function of the existing stormwater management plan for the hospital, while also supporting continued hospital operations during construction of the Emergency Department Expansion. Hydrologic evaluations were performed using the Rational Method for the 10-year, 2-hour storm event. Proposed grading and drainage improvements and modifications have been designed to maintain pre-development drainage patterns and minimize impacts to downstream infrastructure. The evaluation shows that post-development runoff rates, volumes, and water quality volumes remain generally consistent with pre-development conditions. Where minor increases occur, sufficient downstream capacity exists to accommodate the additional storage requirements.
Drainage Report Page 11 of 11 June 2026 A new dry well facility has been incorporated to provide detention for Post-Development Basin V, where existing storm drain infrastructure will be abandoned. The dry well provides storage volume exceeding the required 10-year storm event storage and satisfies the City of Bozeman water quality treatment requirements. Based on the analyses presented above, the proposed stormwater management plan is adequate to convey runoff generated by the project. The proposed improvements will not negatively impact downstream drainage systems and are consistent with the City of Bozeman’s stormwater management requirements. Therefore, the project is considered compliant with applicable drainage standards. APPENDICES Appendix A – Stormwater Basins Appendix B – Hydrology Calculations Appendix C – Hydraulic Calculations Appendix D – O&M Plan Appendix E – Bozeman Health Campus Phase 1 Design
BOZEMAN HEALTH EMERGENCY DEPARTMENT
EXPANSION AND RENOVATION ENABLING
Project # 17095.16 APPENDIX A STORMWATER BASINS
BOZEMAN HEALTH EMERGENCY DEPARTMENT
EXPANSION AND RENOVATION ENABLING
Project # 17095.16 APPENDIX B HYDROLOGY CALCULATIONS
Design Storm
Frequency
(years)
Included Basins Area (ft2)Area
(AC)
Impermeable
Area (ft2)Impermeable
Area (AC)
Runoff
Coefficient C
Permeable
Area (ft2)Permeable
Area (AC)
Runoff
Coefficient C
Weighted
Runoff
Coefficient C
Frequency
Factor Cf Rainfall
Intensity
(in/hr)
Runoff
Q
(ft3 /s)
Volume
V (ft3)Rv
Water
Quality
Volume
(ft3)10A 13091.3 0.301 6266 0.144 0.95 6825 0.157 0.15 0.531 0.41 0.62 470 0.48 262
10B 19629.9 0.451 12953 0.297 0.95 6677 0.153 0.15 0.681 0.41 1.18 897 0.64 527
10C 9616.1 0.221 8072 0.185 0.95 1544 0.035 0.15 0.821 0.41 0.70 533 0.81 323
10D 1192.25 0.027 254 0.006 0.95 939 0.022 0.15 0.321 0.41 0.03 26 0.24 12
10E 6308.36 0.145 5758 0.132 0.95 551 0.013 0.15 0.881 0.41 0.49 374 0.87 229
10F 12578.8 0.289 7242 0.166 0.95 5337 0.123 0.15 0.611 0.41 0.68 518 0.57 298
10G 10074.5 0.231 9303 0.214 0.95 772 0.018 0.15 0.891 0.41 0.80 604 0.88 370
10H 7331.13 0.168 6245 0.143 0.95 1087 0.025 0.15 0.831 0.41 0.54 411 0.82 249
10I 10960.3 0.252 8307 0.191 0.95 2653 0.061 0.15 0.761 0.41 0.74 559 0.73 334
10J 1903.59 0.044 1422 0.033 0.95 481 0.011 0.15 0.751 0.41 0.13 96 0.72 57
10K 7420.66 0.170 7421 0.170 0.950 0.000 0.15 0.951 0.41 0.63 475 0.95 294
10L 6128.85 0.141 3358 0.077 0.95 2771 0.064 0.15 0.591 0.41 0.32 243 0.54 139
10M 13666.9 0.3140 0.000 0.95 13667 0.314 0.15 0.151 0.41 0.18 138 0.05 28
10N 6207.08 0.142 5306 0.122 0.95 901 0.021 0.15 0.831 0.41 0.46 349 0.82 212
10O 11817.5 0.271 10753 0.247 0.95 1065 0.024 0.15 0.881 0.41 0.92 699 0.87 428
10P 20578.3 0.472 18134 0.416 0.95 2444 0.056 0.15 0.851 0.41 1.56 1,186 0.84 723
10Q 40788.5 0.936 19918 0.457 0.95 20870 0.479 0.15 0.541 0.41 1.96 1,487 0.49 832
10R 7079.29 0.163 5266 0.121 0.95 1813 0.042 0.15 0.751 0.41 0.47 356 0.72 212
10S 36740.9 0.843 24340 0.559 0.95 12401 0.285 0.15 0.681 0.41 2.22 1,684 0.65 989
10T 23553.1 0.541 10670 0.245 0.95 12883 0.296 0.15 0.511 0.41 1.07 814 0.46 449
10U 5469.91 0.126 2956 0.068 0.95 2514 0.058 0.15 0.581 0.41 0.28 215 0.54 122
10V 3747.36 0.086 3747 0.086 0.950 0.000 0.15 0.951 0.41 0.32 240 0.95 148
10W 6315.67 0.145 4869 0.112 0.95 1447 0.033 0.15 0.771 0.41 0.43 326 0.74 196
Rational Method for Runoff Calculations Pre-Development 10-yr
Design Storm
Frequency
(years)
Included Basins Area (ft2)Area
(AC)
Impermeable
Area (ft2)Impermeable
Area (AC)
Runoff
Coefficient C
Permeable
Area (ft2)Permeable
Area (AC)
Runoff
Coefficient C
Weighted
Runoff
Coefficient C
Frequency
Factor Cf Rainfall
Intensity
(in/hr)
Runoff
Q
(ft3 /s)
Volume
V (ft3)Rv
Water
Quality
Volume
(ft3)100A 13091.3 0.301 6266 0.144 0.95 6825 0.157 0.15 0.67 1.25 0.61 0.77 879 0.48 262
100B 19629.9 0.451 12953 0.297 0.95 6677 0.153 0.15 0.85 1.25 0.61 1.48 1,677 0.64 527
100C 9616.1 0.221 8072 0.185 0.95 1544 0.035 0.15 1.03 1.25 0.61 0.88 996 0.81 323
100D 1192.25 0.027 254 0.006 0.95 939 0.022 0.15 0.40 1.25 0.61 0.04 48 0.24 12
100E 6308.36 0.145 5758 0.132 0.95 551 0.013 0.15 1.10 1.25 0.61 0.62 700 0.87 229
100F 12578.8 0.289 7242 0.166 0.95 5337 0.123 0.15 0.76 1.25 0.61 0.85 968 0.57 298
100G 10074.5 0.231 9303 0.214 0.95 772 0.018 0.15 1.11 1.25 0.61 0.99 1,128 0.88 370
100H 7331.13 0.168 6245 0.143 0.95 1087 0.025 0.15 1.04 1.25 0.61 0.68 768 0.82 249
100I 10960.3 0.252 8307 0.191 0.95 2653 0.061 0.15 0.95 1.25 0.61 0.92 1,045 0.73 334
100J 1903.59 0.044 1422 0.033 0.95 481 0.011 0.15 0.93 1.25 0.61 0.16 179 0.72 57
100K 7420.66 0.170 7421 0.170 0.950 0.000 0.15 1.19 1.25 0.61 0.78 888 0.95 294
100L 6128.85 0.141 3358 0.077 0.95 2771 0.064 0.15 0.74 1.25 0.61 0.40 454 0.54 139
100M 13666.9 0.3140 0.000 0.95 13667 0.314 0.15 0.19 1.25 0.61 0.23 258 0.05 28
100N 6207.08 0.142 5306 0.122 0.95 901 0.021 0.15 1.04 1.25 0.61 0.57 652 0.82 212
100O 11817.5 0.271 10753 0.247 0.95 1065 0.024 0.15 1.10 1.25 0.61 1.15 1,308 0.87 428
100P 20578.3 0.472 18134 0.416 0.95 2444 0.056 0.15 1.07 1.25 0.61 1.95 2,217 0.84 723
100Q 40788.5 0.936 19918 0.457 0.95 20870 0.479 0.15 0.68 1.25 0.61 2.45 2,779 0.49 832
100R 7079.29 0.163 5266 0.121 0.95 1813 0.042 0.15 0.93 1.25 0.61 0.59 665 0.72 212
100S 36740.9 0.843 24340 0.559 0.95 12401 0.285 0.15 0.85 1.25 0.61 2.77 3,149 0.65 989
100T 23553.1 0.541 10670 0.245 0.95 12883 0.296 0.15 0.64 1.25 0.61 1.34 1,521 0.46 449
100U 5469.91 0.126 2956 0.068 0.95 2514 0.058 0.15 0.73 1.25 0.61 0.35 401 0.54 122
100V 3747.36 0.086 3747 0.086 0.950 0.000 0.15 1.19 1.25 0.61 0.40 449 0.95 148
100W 6315.67 0.145 4869 0.112 0.95 1447 0.033 0.15 0.96 1.25 0.61 0.54 610 0.74 196
Rational Method for Runoff Calculations Pre-Development 100-yr
Design Storm
Frequency
(years)
Included Basins Area
(ft2)Area
(AC)
Impermeable
Area (ft2)Impermeable
Area (AC)
Runoff
Coefficient C
Permeable
Area (ft2)Permeable
Area (AC)
Runoff
Coefficient C
Weighted
Runoff
Coefficient C
Frequency
Factor Cf Rainfall
Intensity
(in/hr)
Runoff
Q
(ft3 /s)
Volume
V (ft3)Rv
Water
Quality
Volume
(ft3)10G 10000 0.230 9112 0.209 0.95 888 0.020 0.15 0.881 0.41 0.78 593 0.87 363
10H 8842 0.203 8591 0.197 0.95 250 0.006 0.15 0.931 0.41 0.73 553 0.92 341
10I 6703 0.154 5596 0.128 0.95 1107 0.025 0.15 0.821 0.41 0.49 370 0.80 224
10K 17026 0.391 12701 0.292 0.95 4325 0.099 0.15 0.751 0.41 1.13 857 0.72 512
10L 5892 0.135 4087 0.094 0.95 1805 0.041 0.15 0.701 0.41 0.37 280 0.67 166
10N 8916 0.205 4033 0.093 0.95 4883 0.112 0.15 0.511 0.41 0.41 308 0.46 170
10O 11817 0.271 10753 0.247 0.95 1065 0.024 0.15 0.881 0.41 0.92 699 0.87 428
10P 20578 0.472 18134 0.416 0.95 2444 0.056 0.15 0.851 0.41 1.56 1,186 0.84 723
10Q 41155 0.945 18988 0.436 0.95 22166 0.509 0.15 0.521 0.41 1.90 1,440 0.47 798
10R 5374 0.123 4147 0.095 0.95 1227 0.028 0.15 0.771 0.41 0.37 278 0.74 167
10S 37182 0.854 24679 0.567 0.95 12503 0.287 0.15 0.681 0.41 2.25 1,707 0.65 1003
10T 23553 0.541 10670 0.245 0.95 12883 0.296 0.15 0.511 0.41 1.07 814 0.46 449
10U 5470 0.126 2956 0.068 0.95 2514 0.058 0.15 0.581 0.41 0.28 215 0.54 122
10V 3747 0.086 3747 0.086 0.950 0.000 0.15 0.951 0.41 0.32 240 0.95 148
10W 5595 0.128 4280 0.098 0.95 1315 0.030 0.15 0.761 0.41 0.38 287 0.74 172
Rational Method for Runoff Calculations Post-Development 10-yr
Design Storm
Frequency
(years)
Included Basins Area
(ft2)Area
(AC)
Impermeable
Area (ft2)Impermeable
Area (AC)
Runoff
Coefficient C
Permeable
Area (ft2)Permeable
Area (AC)
Runoff
Coefficient C
Weighted
Runoff
Coefficient C
Frequency
Factor Cf Rainfall
Intensity
(in/hr)
Runoff
Q
(ft3 /s)
Volume
V (ft3)Rv
Water
Quality
Volume
(ft3)100G 10000 0.230 9112 0.209 0.95 888 0.020 0.15 1.10 1.25 0.61 0.98 1,108 0.87 363
100H 8842 0.203 8591 0.197 0.95 250 0.006 0.15 1.16 1.25 0.61 0.91 1,033 0.92 341
100I 6703 0.154 5596 0.128 0.95 1107 0.025 0.15 1.02 1.25 0.61 0.61 691 0.80 224
100K 17026 0.391 12701 0.292 0.95 4325 0.099 0.15 0.93 1.25 0.61 1.41 1,602 0.72 512
100L 5892 0.135 4087 0.094 0.95 1805 0.041 0.15 0.88 1.25 0.61 0.46 523 0.67 166
100N 8916 0.205 4033 0.093 0.95 4883 0.112 0.15 0.64 1.25 0.61 0.51 575 0.46 170
100O 11817 0.271 10753 0.247 0.95 1065 0.024 0.15 1.10 1.25 0.61 1.15 1,308 0.87 428
100P 20578 0.472 18134 0.416 0.95 2444 0.056 0.15 1.07 1.25 0.61 1.95 2,217 0.84 723
100Q 41155 0.945 18988 0.436 0.95 22166 0.509 0.15 0.65 1.25 0.61 2.37 2,693 0.47 798
100R 5374 0.123 4147 0.095 0.95 1227 0.028 0.15 0.96 1.25 0.61 0.46 520 0.74 167
100S 37182 0.854 24679 0.567 0.95 12503 0.287 0.15 0.85 1.25 0.61 2.81 3,191 0.65 1003
100T 23553 0.541 10670 0.245 0.95 12883 0.296 0.15 0.64 1.25 0.61 1.34 1,521 0.46 449
100U 5470 0.126 2956 0.068 0.95 2514 0.058 0.15 0.73 1.25 0.61 0.35 401 0.54 122
100V 3747 0.086 3747 0.086 0.950 0.000 0.15 1.19 1.25 0.61 0.40 449 0.95 148
100W 5595 0.128 4280 0.098 0.95 1315 0.030 0.15 0.95 1.25 0.61 0.47 537 0.74 172
Rational Method for Runoff Calculations Post-Development 100-yr
BOZEMAN HEALTH EMERGENCY DEPARTMENT
EXPANSION AND RENOVATION ENABLING
Project # 17095.13 APPENDIX C HYDRAULIC CALCULATIONS
Hydraulic Analysis Report Project Data Project Title: Designer: Project Date: Tuesday, June 9, 2026 Project Units: U.S. Customary Units Notes: Channel Analysis: Full Flow 12" 0.83% Notes: Input Parameters Channel Type: Circular Pipe Diameter 1.00 ft Longitudinal Slope: 0.0081 ft/ft Manning's n: 0.0130 Depth 1.0000 ft Result Parameters Flow 3.2065 cfs Area of Flow 0.7854 ft^2 Wetted Perimeter 3.1416 ft Hydraulic Radius 0.2500 ft Average Velocity 4.0827 ft/s Top Width 0.0000 ft Froude Number: 0.0000 Critical Depth 0.7666 ft Critical Velocity 4.9632 ft/s
Critical Slope: 0.0093 ft/ft Critical Top Width 0.85 ft Calculated Max Shear Stress 0.5054 lb/ft^2 Calculated Avg Shear Stress 0.1264 lb/ft^2 Channel Analysis: Full Flow 12" 0.9% Notes: Input Parameters Channel Type: Circular Pipe Diameter 1.00 ft Longitudinal Slope: 0.0090 ft/ft Manning's n: 0.0130 Depth 1.0000 ft Result Parameters Flow 3.3800 cfs Area of Flow 0.7854 ft^2 Wetted Perimeter 3.1416 ft Hydraulic Radius 0.2500 ft Average Velocity 4.3035 ft/s Top Width 0.0000 ft Froude Number: 0.0000 Critical Depth 0.7861 ft Critical Velocity 5.1031 ft/s Critical Slope: 0.0098 ft/ft Critical Top Width 0.82 ft Calculated Max Shear Stress 0.5616 lb/ft^2 Calculated Avg Shear Stress 0.1404 lb/ft^2
BOZEMAN HEALTH EMERGENCY DEPARTMENT
EXPANSION AND RENOVATION ENABLING
Project # 17095.13 APPENDIX D O&M PLAN
June 2026 Project No. 17095.16 STORM DRAINAGE FACILITY MAINTENANCE PLAN BOZEMAN HEALTH EMERGENCY DEPARTMENT EXPANSION AND RENOVATION ENABLING BOZEMAN, MONTANA 59715 OVERVIEW NARRATIVE The purpose of this maintenance plan is to outline the necessary details related to ownership, responsibility, and cleaning schedule for the storm drainage facilities for Bozeman Health Emergency Department Expansion and Renovation Enabling project. This plan has been completed in accordance with City of Bozeman Design and Construction Standards, dated October 2024. The site stormwater improvements have been designed with the intent to meet the current City of Bozeman drainage regulations for the entire site to the extent feasible. Specific site information and criteria are described below: OWNERSHIP OF FACILITIES Bozeman Health Bozeman Health will own all stormwater facilities which includes the dry well, manholes, outlet structures, and piping within the site boundary. The City of Bozeman will have physical access to the existing surface ponds from the public right-of-way and within the existing drainage easements surrounding the surface ponds.
INSPECTION THRESHOLDS FOR CLEANING Dry Well/Manholes/Inlets If sediment in the dry well exceeds 60 percent of the sump or comes or inlet grate is more than 25 percent clogged with debris, clean grate and/or structure and vacuum sump. CLEANING Dry Well/Manholes/Inlets To clean grate of structure, remove and dispose of debris clogging the grate. To clean the structure, use catch basin vacuum to remove sediment and debris. INSPECTION, MAINTENANCE, AND REPLACEMENT SCHEDULE Dry Well/Manholes/Inlets • Inspection: Every 6 months and after storm events larger than 0.5 inches of precipitation • Maintenance: Clean grate of structure and vacuum sediment and debris out of the sump every 5 years or as needed based on inspection • Design Life/Replacement Schedule: 50 years RESPONSIBLE PARTY Bozeman Health Bozeman Health will be responsible for the inspection, maintenance, and replacement of all stormwater facilities located within the project limits. I agree to the above inspection, maintenance, and replacement schedule detailed above. Signature: __________________________________________ Bozeman Health Representative
BOZEMAN HEALTH EMERGENCY DEPARTMENT
EXPANSION AND RENOVATION ENABLING
Project # 17095.13 APPENDIX E BOZEMAN HEALTH CAMPUS PHASE 1 DESIGN
BOZEMAN HEALTH CAMPUS PHASE 1 DESIGN
Project # 17095.13
Bozeman, Montana
JULY 2025
DRAINAGE REPORT
FOR
BOZEMAN HEALTH CAMPUS PHASE 1 DESIGN
CERTIFICATION
I hereby state that this Final Drainage Report has been prepared by me or under my
supervision and meets the standard of care and expertise which is usual and
customary in this community of professional engineers. The analysis has been
prepared utilizing procedures and practices specified by the City of Bozeman and
within the standard accepted practices.
Robert Egeberg, P.E. Date
7/8/2025
July 2025 Project No. 17095.13 DRAINAGE REPORT BOZEMAN HEALTH CAMPUS PHASE 1 DESIGN BOZEMAN, MONTANA 59715 SITE NARRATIVE The purpose of this drainage report is to present a summary of calculations to quantify the stormwater runoff for the Bozeman Health Campus Phase 1 project (referred to in this document as “the project”). All design criteria and calculations are in accordance with the City of Bozeman Design and Construction Standards, dated October 2024. The site stormwater improvements have been designed with the intent of meeting the current drainage regulations for the City of Bozeman. LOCATION The development site is located on the Bozeman Health Campus at 915 Highland Boulevard in Bozeman, Montana. The project is in Sections 8, 17, and 18, Township 2 South, Range 6 East in Gallatin County. Parking Lot E is west of the Bozeman Health Complex, Gravel Lot I is south, and Lot A is east. EXISTING SITE CONDITIONS The Lot A addition location is currently an undeveloped lot with native grasses. The native grasses generally slope to the northeast at grades of 10-20% towards an existing detention pond in the northeast corner of the undeveloped area. Lot A is bordered by Highland Boulevard to the east, Ellis Street to the north, a one-way access to the west, and the existing Lot A to the south. The Lot E addition is currently an undeveloped lot with native grasses. The native grasses slope towards an existing detention pond directly to the west of the existing parking Lot E. There is an access road that loops around the existing stormwater pond and provides westerly access to the maintenance yard. The Maintenance yard is gravel-surfaced and slopes to the north at grades of 2-5%. Lot I is a gravel-surfaced parking lot that slopes to the north at grades of 5-10%. There are concrete jersey barriers that act as the northern boundary of the parking lot. Old
Drainage Report Page 2 of 12 July 2025 Highland Boulevard runs to the west of the parking lot and Highland Boulevard is to the east. The parking lot is accessed by two approaches from Old Highland Boulevard. There is no existing storm infrastructure within the parking lot. PROPOSED PROJECT The proposed project includes the expansion of parking Lot A and Lot E. Parking Lot A will have an addition of 192 parking stalls. The expansion will connect the existing Lot A to the one-way access road that connects to Ellis Street. The addition to Lot A will require the construction of four retaining walls and an expansion to the existing stormwater detention pond. A series of inlets and manholes are proposed to transport stormwater to the expanded detention pond. The addition of Lot E will add 168 parking stalls. The expansion will connect Lot E to the maintenance yard, and the gravel maintenance yard will be expanded to the east paved with asphalt. A new stormwater detention pond will be developed to the west to manage stormwater from the east face of Peets Hill. The existing stormwater detention pond between the maintenance yard and Old Highland Boulevard will be expanded to manage stormwater from the paved maintenance yard. An underground ADS chamber system will be constructed for the purpose of retaining stormwater within the paved parking lot. Rock mulch swales are to be constructed between the parking aisles to treat and infiltrate water prior to discharging into the proposed ADS chamber system. Lot I will be reclaimed as a landscape area within the project and no longer be available for staff or patient parking. The gravel surfacing will be transported to the westerly expansion of Parking Lot E and used as base course material. HYDROLOGY The rational method was used to determine peak runoff rates and volumes since all basins within the project are less than 5 acres. The rational formula provided in the City of Bozeman Design and Construction Standards was used to calculate the peak runoff rates on-site, time of concentration, rainfall intensities, etc. To be conservative we used a time of concentration of 5 minutes within the parking lot basins. For asphalt concrete surfaces, a runoff coefficient of 0.95 was assumed, for landscape surfaces, a runoff coefficient of 0.15 was assumed, and for gravel surfaces, a runoff coefficient of 0.80 was assumed. The project proposes the redevelopment of two existing detention ponds, and the construction of an underground chamber system to manage the on-site drainage. The required detention and retention volumes have been sized for the 10-year, 2-hour storm with an intensity of 0.41 in/hour. To be conservative with sizing an infiltration rate of 0 was assumed to size the detention and retention ponds.
Drainage Report Page 3 of 12 July 2025 A. Pre-Development Basins For the following sections, please refer to Appendix A – Exhibit A and Exhibit B of this report which graphically shows and labels the existing drainage basins for the project. Existing basins were only evaluated where grading was actively being changed within the scope of this project. Changes within the existing basins to the proposed basins were evaluated to properly resize the two existing detention basins within the project. Existing Basin A Existing Basin A has 11,706 ft2 of gravel area, 71 ft2 of asphalt/concrete area, and 100,237 ft2 of landscape area. Runoff generated in this basin sheet flows from southwest to northeast towards an existing culvert, located within existing storm pond B. The culvert flows under the existing gravel access road that connects to the maintenance yard. This culvert flows off-site to City of Bozeman property directly to the North of Parking Lot E. Existing Basin A has a peak flow of 2.17 cfs and a water quantity storage requirement of 1,658 cf for the 10-year storm event. Existing Basin B Existing Basin B has 22,942 ft2 of gravel area, 0 ft2 of asphalt/concrete area, and 25,823 ft2 of landscape area. Existing Basin B flows from southwest to northeast to existing detention storm pond A. Existing detention storm pond A has a storage capacity of 2,700 cf. Existing Basin B has a peak flow of 1.97 ft cfs and a water quality storage requirement of 102 cf for the 10-year storm event. Existing Basin C Existing Basin C has 1,320 ft2 of gravel area, 0 ft2 of asphalt/concrete area, and 77,625 ft2 of landscape area. Existing Basin B flows from southwest to northeast to existing storm pond C. Existing storm pond C has a storage capacity of 39,500 cf. There is an outlet structure located in the northeast corner of existing storm pond C. Existing Basin C has a peak flow of 1.13 cfs, a water quantity storage requirement of 0 cf, and a water quality storage of 164 cf for the 10-year storm event. The existing storm pond A has a concrete outlet structure that has an orifice limiting the pond discharge to 2.9 cfs. The existing storm pond C has a concrete outlet structure that has a slotted weir limiting the pond discharge to 4.90 cfs. An existing basin was not created for parking lot I. Parking lot I does not currently have any specific stormwater management facilities and is a gravel parking lot. The drainage from the gravel parking lot flows from south to north into the existing inlets in the asphalt parking lot north of the gravel lot and is conveyed west and north around
Drainage Report Page 4 of 12 July 2025 the existing buildings until it ultimately discharges into the existing detention pond on the southwest corner of Ellis Street and Highland Boulevard (pond C). Within our design we are going to remove the gravel within the lot and restore the area to its historical nature with a mixture of native grasses. This will greatly decrease the amount of runoff that flows north towards Bozeman Health from the gravel parking lot. However, we assume existing detention pond C was not increased in capacity when the gravel lot was developed since it was intended to be temporary. Therefore, we have not accounted for a reduction in detention pond C volume to be conservative B. Post-Development Basins For the following sections, please refer to Appendix A – Exhibit C and Exhibit D of this report, which graphically shows and labels the on-site watersheds. No percolation rates have been included in the required detention and retention storage volume calculations to be conservative. Storage volume calculations used the 10-year, 2-hour design storm frequency for rainfall data. The 100-year storm was also evaluated for each drainage basin, see Appendix B. Basin 1 Basin 1 has 2,743 ft2 of landscape area and 9,317 ft2 of asphalt/concrete area. Runoff within Basin 1 sheet flows in the proposed parking lot. Runoff then flows through curb cuts within the parking islands and then flows through a rock mulch swale. Runoff then flows into a landscape inlet before it is transported through a series of pipes and manholes to an underground chamber system located within the parking lot. Basin 1 has a peak flow of 0.82 cfs and a water quantity storage of 628 cf for the 10-year storm. Basin 2 Basin 2 has 17,048 ft2 of landscape area and 40,290 ft2 of asphalt/concrete area. Runoff within Basin 2 sheet flows in the proposed parking lot. Runoff then flows though curb cuts within the parking islands and then flows through a rock mulch swale. Runoff then flows into a landscape inlet before it is transported through a series of pipes and manholes to an underground chamber system located within the parking lot. Basin 2 has a peak flow of 3.63 cfs and a water quantity storage of 2,767 cf for the 10-year storm. Basin 3 Basin 3 has 8,567 ft2 of landscape area and 24,766 ft2 of asphalt/concrete area. Runoff within Basin 3 sheet flows in the proposed parking lot. Runoff then flows through curb cuts within the parking islands and then flows through a rock mulch swale. Runoff then flows into a landscape inlet before it is transported through a series of pipes and
Drainage Report Page 5 of 12 July 2025 manholes to an underground chamber system located within the parking lot. Basin 3 has a peak flow of 2.20 cfs and a water quantity storage of 1,682 cf for the 10-year storm. Basin 4 Basin 4 has 18,253 ft2 of landscape area and 41,006 ft2 of asphalt/concrete area. Runoff within Basin 4 sheet flows in the maintenance yard towards a sag inlet. Runoff is then transported through a storm pipe to the expanded storm pond A. Basin 4 has a peak flow of 3.70 cfs and a water quantity storage of 391 cf for the 10-year storm. Basin 5 Basin 5 has 461 ft2 of landscape area and 19,461 ft2 of asphalt/concrete area. Runoff within Basin 5 sheet flows from south to north in the parking lot. Runoff then flows in a gutter along the retaining wall where it is then captured by a sag inlet. Runoff then is transported by a series of pipes and manholes to the expanded storm pond C. Basin 5 has a peak flow of 1.65 cfs for the 10-year storm. Basin 6 Basin 6 has 1,200 ft2 of landscape area and 2,579 ft2 of asphalt/concrete area. Runoff within Basin 6 sheet flows out of the parking lot and then flows north along the one-way service drive that connects to Ellis Street. It then flows through an on-grade inlet that releases water into a bio-retention swale that leads to the expanded storm pond C. Basin 6 has a peak flow of 0.14 cfs for the 10-year storm. Basin 7 Basin 7 has 681 ft2 of landscape area and 16,370 ft2 of asphalt/concrete area. Runoff within Basin 7 sheet flows to the north across the expanded parking lot. Runoff then flows in a gutter where it is then captured by a sag inlet. Runoff then is transported by a series of pipes and manholes to the expanded storm pond C. Basin 7 has a peak flow of 1.39 cfs for the 10-year storm. Basin 8 Basin 8 has 882 ft2 of landscape area and 15,779 ft2 of asphalt/concrete area. Runoff within Basin 8 sheet flows to the north across the expanded parking lot. Runoff then flows in a gutter where it is then captured by a sag inlet. Runoff then is transported by a series of pipes and manholes to the expanded storm pond C. Basin 8 has a peak flow of 1.34 cfs for the 10-year storm.
Drainage Report Page 6 of 12 July 2025 Basin 9 Basin 9 has 823 ft2 of landscape area and 10,974 ft2 of asphalt/concrete area. Runoff within Basin 9 sheet flows to the north across the expanded parking lot. Runoff then flows in a gutter where it is then captured by a sag inlet. Runoff then is transported by a series of pipes and manholes to the expanded storm pond C. Basin 9 has a peak flow of 0.94 cfs and for the 10-year storm. Basin 10 Basin 10 has 1,025 ft2 of landscape area and 5,727 ft2 of asphalt/concrete area. Runoff within Basin 10 sheet flows to the north across the expanded parking lot. Runoff then flows in a gutter where it is then captured by a sag inlet. Runoff then is transported by a series of pipes and manholes to the expanded storm pond C. Basin 10 has a peak flow of 0.50 cfs for the 10-year storm. Basin 11 Basin 11 has 0 ft2 of landscape area and 8,791 ft2 of asphalt/concrete area. Runoff within Basin 11 sheet flows to the north across the expanded parking lot. Runoff then flows in a gutter where it is then captured by a sag inlet. Runoff then is transported by a series of pipes and manholes to the expanded storm pond C. Basin 11 has a peak flow of 0.74 cfs for the 10-year storm. REQUIRED STORAGE VOLUME CALCULATIONS ADS Chamber System Basins 1, 2 and 3 are completely retained by the proposed ADS Chamber system for the 10-year 2-hour storm event. The three basins have a combined total area of 102,731 ft2 (2.36 acres) and have a runoff coefficient of 0.73, and a required water quantity storage of 5,076 ft3. The ADS chamber system has a total retention volume of 5,125 ft3, making the storm system adequate to meet the storage requirements. V = 7,200 X C X I x A Where: C=0.73; i=0.41; A=2.36 acres Required Volume=5,076 ft3
Drainage Report Page 7 of 12 July 2025 Detention Storm Pond A Basin 4 is in the middle of Existing Basin B. Basin 4 changes part of Existing Basin B from gravel to hardscape. Storm pond A will keep its original grading. The required water quantity storage will be increased by removing the existing structure and replacing it with a new structure that has an increased rim elevation. Water quality volume will be used in design as it is greater than the water quantity volume. Basin 4 requires a water quality storage of 1,661 cf. Existing Basin B had a water quality storage of 102 cf, the site development has resulted in an increased need of 1,559 cf for pond A. Pond A will be expanded to have a storage capacity of at least 1,661 cf. The expanded storm pond A will remove the existing structure and have a new structure placed. The structure will have a limited discharge of 2.4 cfs through an orifice. The proposed orifice was raised from the existing structure orifice to maintain the water quality volume below the orifice and maintain a discharge rate that is less than the pre-development discharge rate. WQV = ((P)(Rv)(A))/12 Where: P=0.5; Rv=0.67; A=1.36 acres Required Volume=1,661 ft3 Expanded Storm Pond C Basins 5, 6, 7, 8, 9, 10, and 11 are all detained by the expanded storm pond C for the 10-year storm event. The existing storm pond C has an existing water quality storage capacity of 164 cf. Storm Pond C will be expanded by the required storage of the basins for the 10-year 2-hour storm event. Basins 5, 6, 7, 8, 9, 10, and 11 have a combined area of 84,314 ft2 (1.94 acres) and a runoff coefficient of 0.89 and a water quality storage of 3,112 cf. Existing Basin C had a water quality storage of 164 cf, the site development has resulted in an increased need of 2,948 cf. Pond C will be expanded by 2,948 cf to have a total storage capacity of at least 3,112 cf. Water quality storage was used as it was greater than the water quantity storage volume. The expanded storm pond C will keep the existing outlet structure and has a limited discharge of 2.5 cfs through the slotted weir. The slotted weir has its invert raised to an elevation of 4863.50 to keep the discharge less than the pre-development discharge and have storage capacity for the new water quality volume. WQV = ((P)(Rv)(A))/12 Where: P=0.5; Rv=0.89; A=1.94 acres
Drainage Report Page 8 of 12 July 2025 Required Volume=3,112 ft3 HYDRAULICS Storm Inlets and Storm Drains All storm drainage pipes were sized to handle peak flow resulting from the minor storm event with a minimum slope to maintain a full-flow velocity of at least 2.5 feet per second while limiting the full flow velocity to a maximum of 12 feet per second. The Federal Highway Administration (FHWA) Hydraulic Toolbox Software Version 5.3.0 and Bentley FlowMaster softwares were used to determine pipe sizing for the full flow capacity of the pipes. Inlets were sized to handle the peak flow resulting from the minor storm event. Flow intercepted by drainage inlets were determined using Federal Highway Administration (FHWA) Hydraulic Toolbox Software Version 5.3.0 and Bentley FlowMaster. All proposed inlets located in sag conditions were sized assuming a 50% clogging factor, while inlets on-grade assumed a 25% clogging factor. For further information on storm drain and inlet capacity calculation, see Appendix C. Water Quality The City of Bozeman has a requirement to capture or reuse the runoff generated from the first 0.5 inches of rainfall from a 24-hour storm. We meet this requirement by detaining or retaining all runoff on-site in the two above ground detention ponds and the underground retention facility. In addition to the storage provided in the detention and retention ponds, sumps will be installed in the storm drain inlets prior to outflow, providing treatment before water infiltrates into the ground. Stormwater detention and retention systems need to be maintained, per the recommendations in the Operations and Maintenance Manual, see Appendix D. A. Calculations WQV = ((P x Rv x A) / 12) x 43,560 Where: WQV=Water Quality volume P=Water Quality Rainfall Depth, inches (0.5-inches) Rv=The Unitless Runoff Coefficient, Rv=0.5+0.9(I) I=the percent impervious cover draining to the facility A=total site area draining to the structure
Drainage Report Page 9 of 12 July 2025 ADS Chamber System Water Quality Volume = 0.5in x 0.70 x 2.36ac x (1ft/12in) x 43,560ft/ac = 3,003 cf Drawdown time = 3,003 cf / 2,255 sf storage area / .28 ft/hr = 4.8 hours The storage area of the ADS Chamber System is 2,255 sf which will draw down the 3,003 cf of water quality volume in 4.8 hours using the infiltration rate of 0.28 ft/hr. The ADS Chamber System is located within TP-2 from DOWL Geotechnical Report, see Appendix E. The chamber system will tie into poorly graded gravel. From USCS the minimum infiltration rate is 6.8 inches/hour for this soil type, to be conservative a factor of safety of 2 was applied. Pond A Water Quality Volume = 0.5in x 0.67 x 1.36ac x (1ft/12in) x 43,560ft/ac = 1,661 cf Drawdown time = 1,661 cf / 1,724 sf storage area / .02 ft/hr = 48.2 hours Pond A has an area of 1,724 sf which will draw down the 1,661 cf of water quality volume in 48.2 hours using the infiltration rate of 0.02 ft/hr. The soil under the pond was assumed to be poorly graded gravel. From USCS the minimum infiltration rate is 0.50 inches/hour for this soil type, to be conservative a factor of safety of 2 was applied. Expanded Pond C Water Quality Volume = 0.5in x 0.89 x 1.94ac x (1ft/12in) x 43,560ft/ac = 3,112 cf Drawdown time = 3,112 cf / 19,858 sf storage area / .02 ft/hr = 7.8 hours The expanded pond C has an area of 19,858 sf which will draw down the 3,112 cf quality volume in 7.8 hours using the infiltration rate of 0.02 ft/hr. The soil under the pond was assumed to be poorly graded gravel. From USCS the minimum infiltration rate is 0.50 inches/hour for this soil type, to be conservative a factor of safety of 2 was applied. OUTLET STRUCTURES There is an existing weir outlet structure located in the northeast corner of storm pond C. The weir controls flow from Bozeman Health into the City of Bozeman storm drain located within Ellis Street. The existing slotted weir was defined with the following equation: h = Head Water Depth ft Cw = Discharge Coefficient
Drainage Report Page 10 of 12 July 2025 L = Length of Weir Q = Flow cfs Q = Cw x L x h1.5 4.9 cfs = 3.33 x 0.33 ft x (2.7 ft)1.5 The proposed slotted weir was defined with the following equation: H= Head Water Depth ft Cw = Discharge Coefficient L = Length of Weir Q = Flow cfs Q = Cw x L x h1.5 2.5 cfs = 3.33 x .33 ft x (1.7)1.5 The proposed ADS Storm Chamber System will have an overflow pipe that connects to the existing Bozeman Health storm drain infrastructure. The pipe has been sized to limit the flow to the 100-year peak flow from Existing Basin A, 4.28 cfs. The cross-sectional average velocity was defined with the following equation: Q = (k/n) x Rh2/3 x S1/2 x A Q = volumetric flow rate cfs K = unit conversion factor 1.4859 n = Manning roughness coefficient Rh = hydraulic radius S = pipe slope A = cross-sectional flow area ft2 4.26 cfs = (1.4859/.015) x (0.363 ft)2/3 x .0051/2 x 1.194 ft2 There is an existing orifice outlet structure located in the north corner of the storm pond A. The outlet controls flow from Bozeman Health into its private storm drain system.
Drainage Report Page 11 of 12 July 2025 The existing orifice was defined with the following equation: Q = C x A x (2 x g x h)0.5 Q = flow cfs C = discharge coefficient A = open area ft2 g = gravity constant h = head water depth ft 2.9 cfs = 0.6 x .35 x (2 x 32.2 x 3).5 The proposed orifice was defined with the following equation: Q = C x A x (2 x g x h)0.5 Q = flow cfs C = discharge coefficient A = open area ft2 g = gravity constant h = head water depth ft 2.4 cfs = 0.6 x .35 x (2 x 32.2 x 2.1).5 CONCLUSION All runoff from the new hardscape and landscape areas being constructed will be captured and detained or retained on-site. There are two above ground detention ponds that will be expanded from existing conditions to capture the additional stormwater capacity from the new hardscape areas. There is one underground retention system designed for the development of the parking lot expansion E. The retention system has been designed to completely store the minor storm event with no outflow. There are two planned outlet structures, and one existing structure planned for this project. During the large storm event, the underground retention system will overflow. This will then cause runoff to flow into the existing Bozeman Health storm facilities via a 15” overflow pipe limited to the 100-year pre-development peak flow. In the case runoff overtops the existing facilities runoff will backflow into the rock mulch swales located within the parking islands of Lot E. There is a proposed outlet structure for storm pond A. There is an existing outlet structure located within storm pond C. The outlet structure will be modified to maintain the water quality volume within the project. The outlet structure located within pond A will control flow in the case of the large storm event and releases water into the private Bozeman Health storm drain
Drainage Report Page 12 of 12 July 2025 system. The pond C outlet will control flow in the case of the large storm event and releases runoff into the City of Bozeman storm system located within Ellis Street. In the case of the large storm event no runoff will impact the existing Bozeman Health Structures. APPENDICES Appendix A – Stormwater Basins Appendix B – Hydrology Calculations Appendix C – Hydraulic Calculations Appendix D – O&M Plan Appendix E – DOWL Geotechnical Report Appendix F – Groundwater Monitoring Data from 2024