The URL can be used to link to this page

Home My WebLink About011 Stormwater Report Engineering Report Stormwater Design Hidden Creek Residential Development Lot 1 Hidden Creek Minor Subdivision Site Plan December 2025 Revised February 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 Lot 1 Stormwater Plan 1 1/14/2026 Engineering Report Table of Contents General Information and Design Criteria Existing Site Conditions Storm Drainage Plan Description Estimation of Runoff Retention Facilities Conveyance Capacity Figures and Tables Figure 1. Vicinity Map Table 1 – Storm Runoff Calculations for Retention and Conveyance Table 2 – Retention Basin Capacity Table 3 – Roof Drain Conveyance Calculations Table 4 – Parking Lot Sub-area and Inlet Calculations Table 5 – Curb and Gutter Conveyance Calculations Table 6 – Storm Pipe Conveyance Calculations Table 7 – Drainage Swale Conveyance Calculations Attachments/References Exhibit 1 – On-site Drainage Exhibit C1.3 – Overall Grading Plan C1.3.1 – Enlarged Grading Plan C1.3.2 – Enlarged Grading Plan C1.4 – Overall Drainage Plan C2.2 – Drainage Details C2.3 – Stormtech Details Hydraflow Express Inlet Calculations Lot 1 Stormwater Plan 2 1/14/2026 11.1 Engineering Report – General Information This storm drainage report is for the on-site storm drainage for the Hidden Creek Residential Development located on Lot 1 of the Hidden Creek Minor Subdivision. The subdivision storm drainage improvements serving Lots 1, 3, and 4 of the Hidden Creek Minor Subdivision are provided separately with the Hidden Creek Subdivision Infrastructure. The Hidden Creek Minor Subdivision consists of approximately 9.8 acres of County owned property located between Durston Road and Juniper Street, between the Walton Subdivision and the Legion Villa Apartments. The southern 4 acres of the property is currently occupied by the Gallatin County Rest Home. The northern 5 acres of the property is vacant land. The subdivision will create 4 lots. Lot 1, a large lot in the northern portion of the property will be designated for an 182-unit affordable housing project (Hidden Creek). Lot 2 in the southern portion of the property will contain the existing Gallatin County Rest Home. Lots 3 and 4 are pad sites surrounded by Lot 1, that the County will use for future projects. Preliminary plans are that Lot 3 could be used for a community daycare facility and Lot 4 could be gifted to a Community Land Trust for additional affordable housing. The Lot 1 site improvements consist of 5 apartment buildings, sidewalks, parking, and landscaping. The site improvements and storm drainage facilities installed with Lot 1 will be shared with future development on Lots 3 and 4, and provide storm water mitigation for Lots 1, 3, and 4. This is monumented by the Shared Stormwater Easement and Agreement that will be filed with the Hidden Creek Minor Subdivision final plat. Water and sewer service for the subdivision is be provided by services from the mains running along the eastern edge of the property in Oak Park Drive and through the rest home lot installed with the subdivision improvements. The developer’s contact information is: Gallatin County 311 West Main Street Bozeman, MT 59715 Water supply and wastewater treatment will be provided by the City of Bozeman’s municipal systems. The system owner’s contact information is: City of Bozeman Public Service Department PO Box 1230 Bozeman, MT 59771-1230 Lot 1 Stormwater Plan 3 1/14/2026 Figure 1 shows a vicinity map of the Hidden Creek Minor Subdivision. Figure 1. Vicinity Map, Bozeman, Montana Existing Site Conditions The southern 4 acres of the property is currently occupied by the Gallatin County Rest Home. The northern 5 acres of the property is vacant land. The site slopes generally to the north and west. Storm runoff from the rest home is currently un-mitigated and flows to the vacant land to the north. A private irrigation ditch runs along the eastern property line which serves agricultural property to the east. A geotechnical investigation was conducted, and the report is provided separately with the subdivision submittal. In summary, the geotechnical investigation found a variable depth of fine-grained soils overlying gravel sediments. The depth to gravel varies between 6 to 7 feet. Groundwater monitoring was conducted throughout the 2024 season. Groundwater was not encountered in any monitoring well to depths of approximately 12’. There are no wetlands on the property. Lot 1 Stormwater Plan 4 1/14/2026 Storm Drainage Plan Description The Storm Drainage Plan for the Hidden Creek Subdivision consists of separate mitigation systems for subdivision streets (public) and lots (private). This report analyzes the on-site private storm drainage system. The public system was submitted for review with the Hidden Creek Subdivision Infrastructure application. Lots 1, 3, and 4 will utilize shared on-site sub-surface retention/infiltration systems located on Lot 1 that will serve these three lots. The overall site is divided into three drainage areas: North, Central, and South. Each of these three drainage areas will have a dedicated subsurface retention system. Lot 2 will be served by a new retention pond to capture and infiltrate existing runoff from the rest home on-site and is fully separate from the Lot 1 systems. The on-site stormwater improvements for Lot 1 will also consist of landscape area drains to collect and infiltrate surface runoff from landscaped areas. No runoff from parking lots or buildings is directed into landscape areas, so this area is no longer used for storm drainage mitigation. The landscape drains will provide improved drainage of landscaped areas immediately following construction until re-establishment of natural infiltration as vegetation matures and soil drainage naturally recovers. A drainage swale is provided along the west side of the site to prevent drainage from landscape areas from running onto adjacent property. This swale also serves as an overflow if the capacity of the South and Central on-site systems is exceeded by large rainfall events. The swale has the capacity to carry the 100-year runoff from these areas without flooding adjacent property. Maintenance for the on-site shared storm drainage systems will be the responsibility of the respective Lot owner(s). Full monitoring and maintenance requirements are described in the Shared Stormwater Easement & Agreement which is provided with the Hidden Creek Minor Subdivision Final Plat application. The runoff to the on-site shared storm drainage systems is from three drainage areas and is analyzed below for conveyance and retention requirements and sizing. The drainage systems are depicted on the On-site Drainage Exhibit attached to this report and depicts grading, drainage areas, and retention basins. More detailed information is included in the civil plans on the Grading and Drainage Plans (C1.3, C1.3.1, C1.3.2, C1.4) as well as the Drainage and Retention Detail sheets (C2.2 & C2.3). The site stormwater retention systems are sized to contain the storm runoff from the 25- year, 2-hour storm, in accordance with City of Bozeman Design Standards. The retention basins will contain 98% of all rainfall events and significantly reduce discharges from larger events. The retention basins are in areas that facilitate overflow to natural water ways to prevent property damage during large rainfall events. Lot 1 Stormwater Plan 5 1/14/2026 Storm drainage curbs, inlets, and pipes and overflows are sized to convey the peak flow from a 25-year storm in accordance with City of Bozeman Design Standards. Estimation of Runoff Runoff estimates were obtained using City of Bozeman Standards. The amount of pervious and impervious areas was quantified for each basin to calculate a composite runoff coefficient. The runoff coefficient used for impervious areas was 0.95 and for pervious areas was 0.20. Additionally, for the Central and South basins, the area of the basin planned for future development – those areas in Lot 3 and Lot 4 – were quantified and assigned a runoff coefficient of 0.70 based on the anticipated development plan. If the eventual development of these Lots results in a higher runoff coefficient than 0.70, additional on-site mitigation will be required of the Lot owner(s). The design storm event is a 25-year, 2-hour storm. The Bozeman IDF (Intensity, Duration, and Frequency) curves show a rainfall intensity of 0.49 in/hour for the 25-year, 2-hour storm, totaling a 0.98-inch storm event. The rainfall event is the 98th percentile event, meaning 98% of rainstorms are equal to or less than this amount. Table 1 – Storm Runoff Calculations for Retention and Conveyance Basin #North Central South Area (ac) 1.250 1.589 2.361 Area (sf) 54,442 69,207 102,845 Roof Area (sf) 23,712 17,115 16,348 Hardscape Area (sf) 16,916 29,368 47,614 Total Impervious (sf) (C = 0.95)40,628 46,483 63,962 Pervious (sf) (C = 0.20)13,814 14,716 20,286 Future Site (sf) (C=0.70)0 8,008 18,597 Composite C 0.76 0.76 0.76 Runoff Calculations 25-yr Time of Concentration (min) 10 10 10 Intensity at Tc (in/hr) 3.48 3.48 3.48 Peak Flow (cfs) (25-yr)3.30 4.21 6.22 Storm Depth (in) (25yr-2hr)0.98 0.98 0.98 Runoff Volume (cf) 3,378 4,304 6,357 Runoff Calculations 100-yr (post retention) Time of Concentration (min) 20 20 20 Intensity at Tc (in/hr) 2.89 2.89 2.89 Peak Flow (cfs) (25-yr)2.74 3.50 5.16 Retained Volume 3293 4196 6197 Lot 1 Stormwater Plan 6 1/14/2026 Table 1 shows the 25-year storm runoff calculations for each drainage area. This table also shows the 100-year storm overflow runoff resulting after retention systems are full. Retention Facilities Stormwater from these basins will be retained and infiltrated by the three on-site subsurface stormwater retention systems each of which is located in parts of Lot 1. The systems are comprised of buried Stormtech chambers which are buried modular polymer arch shaped structures capable of supporting appropriate traffic loading. The Stormtech chambers are surrounded on all sides by clean crushed stone reservoir rock with 30% porosity. The Stormtech chambers and the gravel section contribute to the overall retention volume capacity as shown in the calculations below in Table 2. Table 2 – Retention Basin Capacity Construction details of the retention systems are shown on Plan Sheet C2.3. Conveyance Capacity For the on-site storm drainage systems, runoff is collected separately from the parking lots and the building roof drains and combined before entering the subsurface retention systems. For conveyance capacity calculations, the overall drainage areas were further subdivided into drainage sub-areas for each building and parking lot inlet. This results in 5 building sub-areas and 6 parking lot sub-areas, which are shown on the attached On- site Drainage Exhibit. The north retention system collects runoff from Buildings 2 and 3 Basin #North Central South Required Storage (cf) 3,378 4,304 6,357 Chambers Storage Per Chamber (cf) 109.9 109.9 109.9 Number of Rows 3 3 4 Chambers per Row 6 8 8 Number of Chambers 18 24 32 Total Chamber Storage (cf)1,978 2,638 3,517 Gravel Bed Length (ft) 48.75 63 63 Width (ft) 28 28 38 Depth (ft) 5.5 5.5 5.5 Total Volume (cf) 7,508 9702 13167 Less Chamber Volume (cf) 1,978 2,638 3,517 Net Gravel Volume (cf) 5,529 7,064 9,650 Porosity (%) 30% 30% 30% Gravel Storage (cf) 1,659 2,119 2,895 Total System Storage (cf) 3,637 4,757 6,412 Lot 1 Stormwater Plan 7 1/14/2026 and two parking lot sub-areas. The central retention system collects runoff from Buildings 4 and 1 and two parking lot sub-areas. The south retention system collects runoff from Building 5 and two parking lot sub-areas. Because of the smaller size of these sub-areas the time of concentration is 5 minutes. The 5-minute intensity of the 25-year event is 4.76 inches/hour. The building roof drains are analyzed for conveyance capacity from each building to its associated retention system. Generally each building has two 6” drains running along each side of the building conveying half of the building’s runoff. These 6” pipes come together into 8” pipes and join with drains from other buildings. All of the critical pipe segments were identified and are shown below in Table 3 along with their respective 25-year runoff peak flow and capacity calculations. Table 3 – Roof Drain Conveyance Calculations The parking lot sub-areas were analyzed to determine curb and inlet capacity. The 25- year storm runoff peak flow and inlet capacity calculations for each parking lot sub-area are shown in Table 4. The two largest parking lot sub-areas (sub-areas 3 and 5) require two inlets to capture the sub-area flow without overtopping the curb. Curb inlet capacity calculations were done with Hydraflow Express and calculation results for each inlet are provided at the end of the report. All curb inlets have the capacity to carry the 25-year peak flow without overtopping the curb. Note that the northern curb inlet for the Central system (curb inlets 3A&3B) is located near a 3” drive over curb, but the 25-year flow for depth for these two inlets is only 2.20 inches. Inlet Subarea Basin #Bldg 2 Half Bldg 3 Half Bldg 3 Bldg 2 & 3 Bldg 1 Bldg 4 Half Bldg 4 Bldg 5 Half Bldg 5 Drainage Area (acres) 0.13 0.14 0.28 0.54 0.14 0.12 0.24 0.18 0.37 Drainage Area (sf)5,800 6,000 12,000 23,400 6,303 5,236 10,470 8,034 16,068 Slope (%) Runoff Coefficients 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.95 Frequency Adjustment Factor, Cf 1.1 1.1 1.1 1.1 1.1 1.1 1.1 1.1 1.1 Basin Length (ft) Time of Concentration (min) 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 Peak Flow Calculations Design Storm 25yr 25yr 25yr 25yr 25yr 25yr 25yr 25yr 25yr Time of Concentration (min) 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 Intensity at Tc (in/hr) 4.76 4.76 4.76 4.76 4.76 4.76 4.76 4.76 4.76 Peak Runoff Rate at Tc (Q = CIA) (cfs)0.60 0.62 1.25 2.43 0.65 0.54 1.09 0.83 1.67 Storm Drain Pipe Capacity Calculations Drainage Subarea #Bldg 2 Half Bldg 3 Half Bldg 3 Bldg 2 & 3 Bldg 1 Bldg 4 Half Bldg 4 Bldg 5 Half Bldg 5 Pipe Material PVC PVC PVC PVC PVC PVC PVC PVC PVC Pipe Size (in)6.00 6.00 8.00 10.00 6.00 6.00 8.00 8.00 8.00 Manning's "n" (PVC) 0.013 0.013 0.013 0.013 0.013 0.013 0.013 0.013 0.013 Area (ft2)0.20 0.20 0.35 0.55 0.20 0.20 0.35 0.35 0.35 Wetted Perimeter (ft) 1.57 1.57 2.09 2.62 1.57 1.57 2.09 2.09 2.09 Hydraulic Radius (ft) 0.13 0.13 0.17 0.21 0.13 0.13 0.17 0.17 0.17 Slope (ft/ft)0.0150 0.0150 0.0150 0.0150 0.0150 0.0150 0.0150 0.0150 0.0200 Full Flow Capacity (cfs) 0.69 0.69 1.48 2.69 0.69 0.69 1.48 1.48 1.71 Velocity (ft/sec) 3.51 3.51 4.25 4.93 3.51 3.51 4.25 4.25 4.91 Contributing Inlet Basin 1 1 1 1 1 1 1 1 1 Stormwater Flow (cfs) 0.60 0.62 1.25 2.43 0.65 0.54 1.09 0.83 1.67 % Capacity 87% 90% 84% 90% 95% 79% 73% 56% 97% Lot 1 Stormwater Plan 8 1/14/2026 Table 4 Parking Lot Sub-area and Inlet Calculations. Curb capacity was also analyzed for each sub-area. Table 5 shows the capacity of the most limiting curb, just to the south of Building 1. This curb coincides with the parking lot sub-area (sub-area 5) having the largest flow, 4.72 cfs. The curb and gutter has a conveyance capacity of 12.63 cfs which is more than twice the largest sub-area 25-year peak flow of 4.72 cfs. Table 5 – Curb and Gutter Conveyance Calculations Inlet Sub-areas 1 2 3 3 4 5 5 6 Basin-Inlet# North-1 North-2 Central-3A&3B Central-4 South-5A&5B South-6 Drainage Area (acres) 0.195 0.224 0.542 0.526 1.273 0.454 Drainage Area (sf) 8494 9741 23602 22894 55473 19761 Slope (%) 2.1 2.1 1.9 1.9 1.85 1.85 Total Impervious (sf) (C = 0.95)8040 9230 21950 14750 29740 16780 Pervious (sf) (C = 0.20)454 511 1652 1280 6093 2981 Future Site (sf) (C=0.70)0 0 0 6864 19640 0 Composite C 0.910 0.911 0.90 0.83 0.78 0.84 Peak Flow Calculations Design Storm 25yr 25yr 25yr 25yr 25yr 25yr Frequency Adjustment Factor, Cf 1.1 1.1 1.1 1.1 1.1 1.1 Basin Length (ft) 140 140 290 330 355 255 Time of Concentration (min)* 5.0 5.0 5.0 5.0 5.0 5.0 Intensity at Tc (Figure I-2 pg. 29) (in/hr) 4.76 4.76 4.76 4.76 4.76 4.76 Peak Runoff Rate at Tc (Q = CIA) (cfs) 0.84 0.97 2.31 2.08 4.72 1.81 Inlet Capacity Calculations Inlet Type Combination Combination Combination Grate Combination Combination Combination Combination Location Sag Sag Sag Sag Sag Sag Sag Sag Length (ft) 3 3 3 3 3 3 3 3 Throat ht (in) 5.00 5.00 5.00 5.00 5.00 5.00 5.00 5.00 Open Area (sft) 2.10 2.10 2.10 1.00 2.10 2.10 2.10 2.10 Grate Width (ft) 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 Grate Length (ft) 3 3 3 1.5 3 3 3 3 Local Depression (in) 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Slope, Sw (ft/ft) 0.04 0.04 0.04 0.04 0.04 0.04 0.04 0.04 Slope, Sw (ft/ft) 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 Gutter Depression (in) 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 Width (ft) 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 Slope(%)0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 n-value 0.013 0.013 0.013 0.013 0.013 0.013 0.013 0.013 Captured Flow (cfs) 0.84 0.97 1.25 1.50 2.08 2.36 2.36 1.81 Height Over Inlet (in) 1.92 2.07 2.29 2.29 3.21 3.46 3.46 2.96 *Inlet capacity calculations were performed using Hydraflow Express. Output reports of these calculations are attached Right-side Slope X:1 0.06 Left-side Slope X:1 67.00 Channel Bottom Width (ft) 0 Flow Depth (ft) 0.350 Flow Area (ft^2) 4.108 Wetted Perimeter (ft) 23.803 Width (ft) 23.472 Hydraulic Radius (ft) 0.173 Manning's Roughness 0.013 Slope (ft/ft) 0.008 Average Velocity (ft/sec) 3.07 Flow (ft^3/sec)12.63 Curb and Gutter Capacity Calcs Lot 1 Stormwater Plan 9 1/14/2026 After determining the sub-area flow rates from the building rooftops and parking lot inlets, the pipe capacity downstream of the curb inlets to the subsurface retention systems was analyzed. Some pipes receive flow from more than one inlet and a couple of pipes also convey building roof drain runoff. Table 6 shows the pipe flow downstream of each inlet by adding the applicable runoff data from Table 3 (roof drains) and Table 4 (parking lots). All pipes have the capacity to transmit the 25-year peak runoff flow. Table 6 – Storm Pipe Conveyance Calculations A drainage swale is provided along the west side of the site to prevent drainage from landscape areas from running onto adjacent property. This swale also serves as an overflow if the capacity of the South and Central on-site systems is exceeded by large (>25-year) rainfall events. The ditch is 10-feet wide, 1-foot deep, grass channel with a triangular cross-section and a capacity of 13.15 cfs at a flow depth of 1-foot. The 100-year peak flow after on-site retention is shown for each drainage basin is shown on Table 1. This was calculated based on the on-site retention system capturing the short-duration high-intensity portion of the 100-year storm. The on-site retention system can capture events up to the 100-year, 20-minute storm. Table 1 shows that the volume of this storm is slightly less than the 25-year, 2-hour storm. Thus, the peak flow that overflows the on-site retention systems is based on 100-year storm durations of 20- minutes or larger. The combined overflow from the south and central basis is 8.66 cfs which is significantly less than the drainage swale capacity of 13.15 cfs. calculated in Table 7. Drainage Subarea #1 2 3 4 5 6 Downstream of Basin-Inlet# North-1 North-2 Central-3A&3B Central-4 South-5A&5B South-6 Pipe Flows Building Roof Drains N/A 2&3 N/A N/A N/A Bldg 5 Roof Drain Flow 0 2.43 0 0 0 1.67 Upstream Pipes N/A Inlet 1 N/A N/A N/A N/A Upstream Pipe Flow 0 0.84 0 0 0 0 Downstream pipe flow 0.84 4.24 2.31 2.08 4.72 3.48 Pipe Material PVC PVC PVC PVC PVC PVC Pipe Size (in)15.00 15.00 15.00 15.00 15.00 15.00 Manning's "n" (PVC) 0.013 0.013 0.013 0.013 0.013 0.013 Area (ft2)1.23 1.23 1.23 1.23 1.23 1.23 Wetted Perimeter (ft) 3.93 3.93 3.93 3.93 3.93 3.93 Hydraulic Radius (ft) 0.31 0.31 0.31 0.31 0.31 0.31 Slope (ft/ft)0.0100 0.0100 0.0100 0.0100 0.0100 0.0100 Full Flow Capacity (cfs) 6.48 6.48 6.48 6.48 6.48 6.48 Full Flow Velocity (ft/sec) 5.28 5.28 5.28 5.28 5.28 5.28 Stormwater Flow (cfs) 0.84 4.24 2.31 2.08 4.72 3.48 % Capacity 13% 66% 36% 32% 73% 54% Velocity (ft/sec) 0.69 3.46 1.89 1.70 3.85 2.83 Lot 1 Stormwater Plan 10 1/14/2026 Table 7 – Drainage Swale Conveyance Calculations Overflows from the north basin will flow over the plaza space located between Buildings 2 and 3. This plaza is approximately 20-feet wide and at least 15” below the lowest adjacent building floor. The 100-year post-retention flow of 2.74 cfs can be conveyed with a flow depth of less than 2 inches. Any large event (>25-year) overflow can safely pass through this space without damaging adjacent buildings. Storm Water System Maintenance Maintenance for the on-site shared storm drainage systems will be the responsibility of the respective Lot owners. Full monitoring and maintenance requirements are described in the Shared Stormwater Easement & Agreement which is provided with the Hidden Creek Minor Subdivision Final Plat application. Right-side Slope X:1 5.00 Left-side Slope X:1 5.00 Channel Bottom Width (ft) 0 Flow Depth (ft) 1.000 Flow Area (ft^2) 5.000 Wetted Perimeter (ft) 10.198 Width (ft) 10.000 Hydraulic Radius (ft) 0.490 Manning's Roughness 0.040 Slope (ft/ft) 0.013 Average Velocity (ft/sec) 2.63 Flow (ft^3/sec)13.15 West Side Swale Capacity Calc's SSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSW[][][][]Tract BCOS 24399.824 acNorth 14thAvenueLot 10Lot 11Lot 12Lot 13Lot 14Lot 15Lot 16Lot 17Lot 18Lot 19ALot 20ALot 21Lot 22Lot 9Lot 8Lot 7HomesteadSubdivisionLot 23JUNIPER STREETSSWWWW WWWWWWW WWW COMMCOMMCOMMCOMMCOMMCOMMW W W W W W W W W W W W W W W W W W W W W W W WW W CCCCCCCCCCCCCCCCCCCCCCCCCCCSD SDSDSDSDCCCCCCCCCCCCCCCSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSD SD SD SD SD SD SDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSD SD SD SD SD SD SD SDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSD SD SD SD SDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDCCCCCCCWWWWWWWWWW UGE UGE UGECOMMCOMMCOMM SSSSSSSSSSSSSSSSSSSSGAS GAS YD X X X X X X X X X X X X[][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][]OAK PARK DRIVEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGE COMMCOMMLOT 15.165 ac4.589 ac (net)LOT 30.184 acLOT 40.427 acBUILDING 5BUILDING 1BUILDING 4BUILDING 2BUILDING 3SD SD SD SD SD SDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDNEW 30' x<< <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<477247734774477547764777477847794780<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<4781477147824783CB#5B 24"X36" CURB INLETTBC = 4777.22RIM=4776.6415" INV IN (W)=4771.4615" INV OUT (S) = 4771.46SUMP=4770.71CB#6 24"X36" CURB INLETTBC=4777.39RIM=4776.8115" INV OUT=4771.918" INV IN=4772.49SUMP=4771.16LANDSCAPE DRAIN (TYP)LANDSCAPE DRAIN (TYP)UGEUGEUGEUGEUGEUGE UGE UGE UGE UGE UGE UGE UGE UGE4772COMM<SSSSSSSSSD SD SD SD SD SD SD SD SD SD SD SD SD SD SDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDooo***************************************************COMM COMM COMMCOMMCOMMCOMMCOMMCOMMCOMMCOMMCOMMCOMMCOMMCOMMCOMMCOMMCOMMCOMMCOMMCOMMCOMMCOMMCOMMCOMMCOMMCOMMCOMMCOMMCOMMCOMMCOMMCOMMCOMMCOMMCOMMCOMMCOMMCOMMCOMMCOMMCOMMCOMMCOMMCOMMCOMMCOMMCOMMCOMMCOMMCOMMCOMMCOMMCOMMCOMMCOMMCOMMCOMMCOMM4775477647774778477947804779478047814778477947754776477747744775477447724773 47714778477947774778 47734774 4775477647724773<<<<<<<<<<<<<<SDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSD SD SD SD SDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSD SD SD SD SD 478247814783<SDSDSDSDLANDSCAPE DRAIN (TYP)LANDSCAPE DRAIN (TYP)CB#4 24"X36" CURB INLETTBC=4774.35RIM = 4773.7715" INV OUT (E) = 4768.44SUMP=4767.69CB#3B 24"X36" CURB INLETTBC=4774.03RIM=4773.4515" INV IN (E)=4768.6615" INV OUT (S)=4768.66SUMP=4767.91SDMH-NORTHRIM=4772.9915" INV OUT (W)=4768.4915" INV IN (N)=4767.2224" INV OUT (W)=4766.72SUMP= 4764.72CB#1 24"x36" CURB INLETTBC=4773.43RIM=4772.8515" INV OUT (E)=4767.52SUMP=4766.77CB#2 24"X36" CURB INLETTBC=4773.26RIM=4772.6815" INV IN (W)=4767.3110" INV IN (NE)=4767.7315" INV OUT (S)=4767.31SUMP=4766.56SDMH-CENTRAL #2RIM=4774.926" INV IN (E)=4769.108" INV IN (SE)=4769.1024" INV. OUT (N)=4767.77SUMP=4765.77SDMH-SOUTHRIM = 4777.0415" INV IN (NW)=4771.2715" INV IN (SW)=4771.2715" INV OUT (S)=4772.5424" INV OUT (E)=4770.77SUMP= 4768.77SDMH-CENTRAL #1RIM = 4774.0415" INV IN (N) = 4768.2715" INV IN (W) = 4768.2715" INV OUT (E)=4769.5424" INV OUT (S) = 4767.77SUMP=4765.7715" HP PIPE @1.00% (TYP)15" HP PIPE @1.00% (TYP)15" HP PIPE @1.00% (TYP)8" SDR35 PVC ROOFDRAIN @ 2.00%6" SDR35 PVC ROOFDRAIN @ 1.50%8" SDR35 PVC ROOFDRAIN @ 1.50%10" SDR35 PVC ROOFDRAIN @ 1.50%15" HP PIPE @1.00% (TYP)8" SDR35 PVC ROOFDRAIN @ 1.50%6" SDR35 PVC ROOFDRAIN @ 1.50% ALONGSIDES OF BUILDING6" SDR35 PVC ROOFDRAIN @ 1.50% ALONGSIDES OF BUILDING6" SDR35 PVC ROOFDRAIN @ 1.50% ALONGSIDES OF BUILDING6" SDR35 PVC ROOFDRAIN @ 1.50% ALONGSIDES OF BUILDING8" SDR35 PVC ROOF DRAIN@ 1.50% DOWNSTREAMFROM HERESTREET STORMWATER RETENTION SYSTEMSEE INFRASTRUCTURE PLANSPATIO DRAINS (4)SD SDPATIO DRAINS (2)***<<<<<<<<<<<<<<<<<<<<<4782[][][][][][][][][][][][][][][][][][][][][][][][][][][][][][]CB#3A -24" DIA AREA INLETRIM=4773.4915" INV OUT (W)= 4768.72SUMP=4767.97CB#5A 24"X36" CURB INLETTBC = 4777.19RIM=4776.6115" INV OUT (E) = 4771.48SUMP=4770.736" SDR35 PVC ROOFDRAIN @ 1.50% ALONGSIDES OF BUILDING909 OAK PARK DRIVE905 OAK PARK DRIVE863 OAK PARK DRIVE847 OAK PARK DRIVE775 OAK PARK DRIVE841 OAK PARK DRIVE801 OAK PARK DRIVE805 OAK PARK DRIVEOFF-SITE IMPROVEMENTS ANALYZED SEPARATELY CENTRAL BASIN 1.589 ACRES COMPOSITE C = 0.76 SOUTH BASIN 2.361 ACRES COMPOSITE C = 0.76 NORTH BASIN 1.250 ACRES COMPOSITE C = 0.76 NORTH INLET 1 SUB-BASIN 0.195 ACRES COMPOSITE C = 0.91 NORTH INLET 2 SUB-BASIN 0.224 ACRES COMPOSITE C = 0.91 CENTRAL INLET 4 SUB-BASIN 0.526 ACRES COMPOSITE C = 0.83 SOUTH INLETS 5A&5B SUB-BASIN 0.1.273 ACRES COMPOSITE C = 0.78 SOUTH INLET 6 SUB-BASIN 0.454 ACRES COMPOSITE C = 0.84CENTRAL INLETS 3A&3B SUB-BASIN0.542 ACRESCOMPOSITE C = 0.90BUILDING 2 SUB-BASIN 0.270 ACRES COMPOSITE C = 0.95 BUILDING 3 SUB-BASIN 0.316 ACRES COMPOSITE C = 0.95 BUILDING 4 SUB-BASIN0.245 ACRESCOMPOSITE C = 0.95BUILDING 1 SUB-BASIN0.148 ACRESCOMPOSITE C = 0.95BUILDING 5 SUB-BASIN0.382 ACRESCOMPOSITE C = 0.95EX. 1C:\Users\cpool\FileCloud\Vault\PROJECTS\3668-UHP_Design_LLC\-00325_Hidden_Creek_Civil\DWG\Exhibits\EX 1 - ON-SITE DRAINAGE EXHIBIT.dwg, C1.4 - OVERALL DRAINAGE PLAN, Plotted: Feb 09, 2026 - 2:23pm, cpool00 30'60' SCALE IN FEET 111/21/2025COB INF SUBMITTAL RC1CDPZWL212/23/2025COB SITE PLAN RC2CDPZWL312/23/2025COB INF SUBMITTAL RC2CDPZWL412/23/2025MDEQ SUBMITTALCDPZWL51/15/2026COB INF SUBMITTAL RC3CDPZWL61/22/2026COB INF SUBMITTAL RC4CDPZWLPROFESSIONALENGINEERS &SURVEYORS STAHLYENGINEERING& ASSOCIATES HIDDEN CREEKDEVELOPMENTUNITED HOUSINGPARTNERS, LLCBOZEMAN, MTGREAT FALLS Phone:(406)604-4644 CODY Phone:(307)509-5541 851 BRIDGER DR. STE 1BOZEMAN, MT 59715 Phone:(406)522-9526 BILLINGS Phone:(406)601-4055 HELENA Phone:(406)442-8594 72/04/2026BUILDING PERMITCDPZWLON-SITEDRAINAGEEXHIBIT Inlet ReportHydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc. Wednesday, Jan 14 2026<Name>Combination InletLocation = SagCurb Length (ft) = 3.00Throat Height (in) = 5.00Grate Area (sqft) = 2.10Grate Width (ft) = 1.50Grate Length (ft) = 3.00GutterSlope, Sw (ft/ft) = 0.040Slope, Sx (ft/ft) = 0.030Local Depr (in) = -0-Gutter Width (ft) = 1.50Gutter Slope (%) = -0-Gutter n-value = -0-CalculationsCompute by: Known QQ (cfs) = 0.84Highlighted Q Total (cfs) = 0.84 Q Capt (cfs) = 0.84 Q Bypass (cfs) = -0-Depth at Inlet (in) = 1.92Efficiency (%) = 100Gutter Spread (ft) = 4.82Gutter Vel (ft/s) = -0- Bypass Spread (ft) = -0- Bypass Depth (in) = -0- INLET - NORTH 1 Inlet ReportHydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc. Wednesday, Jan 14 2026<Name>Combination InletLocation = SagCurb Length (ft) = 3.00Throat Height (in) = 5.00Grate Area (sqft) = 2.10Grate Width (ft) = 1.50Grate Length (ft) = 3.00GutterSlope, Sw (ft/ft) = 0.040Slope, Sx (ft/ft) = 0.030Local Depr (in) = -0-Gutter Width (ft) = 1.50Gutter Slope (%) = -0-Gutter n-value = -0-CalculationsCompute by: Known QQ (cfs) = 0.97Highlighted Q Total (cfs) = 0.97 Q Capt (cfs) = 0.97 Q Bypass (cfs) = -0-Depth at Inlet (in) = 2.07Efficiency (%) = 100Gutter Spread (ft) = 5.26Gutter Vel (ft/s) = -0- Bypass Spread (ft) = -0- Bypass Depth (in) = -0- INLET - NORTH 2 Inlet ReportHydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc. Wednesday, Jan 14 2026<Name>Combination InletLocation = SagCurb Length (ft) = 3.00Throat Height (in) = 5.00Grate Area (sqft) = 2.10Grate Width (ft) = 1.50Grate Length (ft) = 3.00GutterSlope, Sw (ft/ft) = 0.030Slope, Sx (ft/ft) = 0.030Local Depr (in) = -0-Gutter Width (ft) = 1.50Gutter Slope (%) = -0-Gutter n-value = -0-CalculationsCompute by: Known QQ (cfs) = 1.25Highlighted Q Total (cfs) = 1.25 Q Capt (cfs) = 1.25 Q Bypass (cfs) = -0-Depth at Inlet (in) = 2.30Efficiency (%) = 100Gutter Spread (ft) = 6.38Gutter Vel (ft/s) = -0- Bypass Spread (ft) = -0- Bypass Depth (in) = -0- INLET - CENTRAL 3A Inlet ReportHydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc. Wednesday, Jan 14 2026<Name>Drop Grate InletLocation = SagCurb Length (ft) = -0-Throat Height (in) = -0-Grate Area (sqft) = 1.00Grate Width (ft) = 1.50Grate Length (ft) = 1.50GutterSlope, Sw (ft/ft) = 0.030Slope, Sx (ft/ft) = 0.030Local Depr (in) = -0-Gutter Width (ft) = 1.50Gutter Slope (%) = -0-Gutter n-value = -0-CalculationsCompute by: Known QQ (cfs) = 1.50Highlighted Q Total (cfs) = 1.50 Q Capt (cfs) = 1.50 Q Bypass (cfs) = -0-Depth at Inlet (in) = 2.29Efficiency (%) = 100Gutter Spread (ft) = 14.21Gutter Vel (ft/s) = -0- Bypass Spread (ft) = -0- Bypass Depth (in) = -0- INLET - CENTRAL 3B Inlet ReportHydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc. Wednesday, Jan 14 2026<Name>Combination InletLocation = SagCurb Length (ft) = 3.00Throat Height (in) = 5.00Grate Area (sqft) = 2.10Grate Width (ft) = 1.50Grate Length (ft) = 3.00GutterSlope, Sw (ft/ft) = 0.040Slope, Sx (ft/ft) = 0.030Local Depr (in) = -0-Gutter Width (ft) = 1.50Gutter Slope (%) = -0-Gutter n-value = -0-CalculationsCompute by: Known QQ (cfs) = 2.08Highlighted Q Total (cfs) = 2.08 Q Capt (cfs) = 2.08 Q Bypass (cfs) = -0-Depth at Inlet (in) = 3.21Efficiency (%) = 100Gutter Spread (ft) = 8.41Gutter Vel (ft/s) = 0.74 Bypass Spread (ft) = -0- Bypass Depth (in) = -0- INLET - CENTRAL 4 Inlet ReportHydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc. Wednesday, Jan 14 2026<Name>Combination InletLocation = SagCurb Length (ft) = 3.00Throat Height (in) = 5.00Grate Area (sqft) = 2.10Grate Width (ft) = 1.50Grate Length (ft) = 3.00GutterSlope, Sw (ft/ft) = 0.040Slope, Sx (ft/ft) = 0.030Local Depr (in) = -0-Gutter Width (ft) = 1.50Gutter Slope (%) = -0-Gutter n-value = -0-CalculationsCompute by: Known QQ (cfs) = 2.36Highlighted Q Total (cfs) = 2.36 Q Capt (cfs) = 2.36 Q Bypass (cfs) = -0-Depth at Inlet (in) = 3.46Efficiency (%) = 100Gutter Spread (ft) = 9.10Gutter Vel (ft/s) = 0.74 Bypass Spread (ft) = -0- Bypass Depth (in) = -0- INLET - SOUTH 5A & 5B Inlet ReportHydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc. Wednesday, Jan 14 2026<Name>Combination InletLocation = SagCurb Length (ft) = 3.00Throat Height (in) = 5.00Grate Area (sqft) = 2.10Grate Width (ft) = 1.50Grate Length (ft) = 3.00GutterSlope, Sw (ft/ft) = 0.040Slope, Sx (ft/ft) = 0.030Local Depr (in) = -0-Gutter Width (ft) = 1.50Gutter Slope (%) = -0-Gutter n-value = -0-CalculationsCompute by: Known QQ (cfs) = 1.81Highlighted Q Total (cfs) = 1.81 Q Capt (cfs) = 1.81 Q Bypass (cfs) = -0-Depth at Inlet (in) = 2.96Efficiency (%) = 100Gutter Spread (ft) = 7.71Gutter Vel (ft/s) = 0.74 Bypass Spread (ft) = -0- Bypass Depth (in) = -0- INLET - SOUTH 6