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Home My WebLink AboutTHE NORTHERN_FFE Analysis letter_20250411 101 E. Loucks Street, #300 Sheridan, WY 82801 970-227-0714 April 11, 2025 City of Bozeman Engineering Division 121 N Rouse Avenue Bozeman, MT 59715 RE: The Northern - Finish Floor Elevation Analysis To whom it may concern: The Northern Project is located at 621 W. Mendenhall Street, which is located on the northeast corner of the intersection of N. 7th Avenue and W. Mendenhall Street; the Project will consist of a new, mixed-use 7-story multifamily apartment building with structured parking. The Project is bound by N. 7th Avenue to the west, W. Mendenhall Street to the south, a public alley to the north and an existing residence to the east. The Project is located near the upper boundary of a drainage basin that conveys flows east to Bozeman Creek. A FLO-2D model is not available for this area, so the Rational Method was used to calculate the 100-year flow depth in the curb adjacent to the door openings. During the drainage analysis it was determined that the site is located near the upstream limit of a drainage basin that conveys storm drainage to Bozeman Creek. LIDAR contours were obtained from the City of Bozeman website and were used to delineate the approximate tributary basins to the site. Three basins were delineated to determine the depth of flow in the flowline adjacent to proposed building entrances. These basins are depicted on the offsite drainage basin map located in the attachments at the end of this analysis. It should be noted that it has been assumed that all the flows generated in the tributary basins will overtop the street crown in N. 7th Avenue and W. Mendenhall Street in order to conservatively determine the maximum flow that could possibly collect in the curb and gutter adjacent to the building. Basin 1 includes flows generated upstream of the site in N. 7th Avenue to determine the depth in the existing curb and gutter adjacent to the main building entrance at Design Point 1. The depth of flow at Design Point 1 is approximately 0.23’ which correlates with an elevation of 4821.38 which is 0.39’ (4.7”) below the finished floor elevation of 4821.77. Basin 2 includes flows generated upstream of the site in N. 7th Avenue combined with flows generated in W. Mendenhall Street adjacent to the site to determine the depth in the proposed curb and gutter adjacent to the easternmost building entrance at Design Point 2. The depth of flow at Design Point 1 is approximately 0.32’ which correlates with an elevation of 4821.17 which is 0.33’ (4.0”) below the eastern entrance elevation of 4821.50. 101 E. Loucks Street, #300 Sheridan, WY 82801 970-227-0714 Basin 3 includes flows generated upstream of the site in N. 7th Avenue combined with flows generated in N. 7th Avenue adjacent to the site to determine the depth in the proposed curb and gutter adjacent to the northernmost building entrance at Design Point 3. The depth of flow at Design Point 3 is approximately 0.25’ which correlates with an elevation of 4820.04 which is 0.51’ (6.1”) below the northern entrance elevation of 4820.55. It should be noted that in all tributary flow calculation scenarios, at all building entrances, the depth of the conservatively calculated 100-year flow will not exceed the curb height in the street adjacent to the site. Hydraflow Express (Autodesk Civil3D extension software) was used to determine the 100-year storm depth for the three basins described above. This analysis results in the determination that the conservatively calculated 100-year flow rate generated within the entire street section adjacent to the site will not pond to a depth that will reach the proposed door elevations of the building. See attached documents for the basin delineation, hydrology analysis and channel reports. Please do not hesitate to contact us with questions or comments regarding this analysis. Respectfully Submitted, Ryan Meisel, P.E. Owner/Principal Attachments: - Rational Method Hydrologic Calculations - Rainfall Data - Channel Reports - Offsite Drainage Basin Map Final Tc Design Pt.Basin IDAreaCfCCfCACfCCfCALength (300' max)Slopeti3 LengthSlopeVelocity4tt5 tc=ti+tt2-year10-year100-year2-year10-year100-yearacres ft % min. ft % fps min min in/hr in/hr in/hr cfs cfs cfs 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 Tributary Basin 1 0.33 1.00 0.95 0.31 1.25 0.95 0.39 37 5.0%0.7 158 0.6%1.6 1.7 5.0 2.68 4.21 0.8 1.7 Tributary Basin 2 0.52 1.00 0.95 0.49 1.25 0.95 0.61 37 5.0%0.7 260 0.3%1.1 3.9 5.0 2.68 4.21 1.3 2.6 Tributary Basin 3 0.54 1.00 0.95 0.51 1.25 0.95 0.64 37 5.0%0.7 292 1.1%2.1 2.3 5.0 2.68 4.21 1.4 2.7 Notes: 2. I = 28.5P1/(10+Tc)0.786 3. ti = (0.42/(P20.5))*((0.013*L)/S0.5)0.8 4. Velocity = KukSp0.5 5. tt = L/60*V 6. Minimun tc is 5.0 minutes for urban areas. Table A-2 Rational Method Hydrologic Calculations - Offsite Tributary Basins The Northern Bozeman, Montana Intensity2, I Peak Discharge1, QSubbasin Data Time of Concentration, Tc 1. Flows calculated using the rational method, based on the methods provided in Section 4.2.2 of the HEC-22 Manual, 4th Edition. Overland Time (ti) Travel Time (tt)100-year10-year C Values 1 of 1Hydrology Calcs.xls Q Peak Recurrence Interval (yrs)1-hr Rainfall Depth (in) 2 0.42 5 0.64 10 0.79 25 0.97 50 1.10 100 1.24 Values obtained from City of Bozeman Design and Construction Standards Table 6.5.2 Table A-3 Rainfall Data - City of Bozeman The Northern Note: Hydrology Calcs_THE NORTHERN_offsite basins.xls Rainfall Channel Report Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc. Tuesday, Mar 18 2025 <Name> Gutter Cross Sl, Sx (ft/ft) = 0.040 Cross Sl, Sw (ft/ft) = 0.040 Gutter Width (ft) = 1.21 Invert Elev (ft) = 4821.00 Slope (%) = 0.60 N-Value = 0.013 Calculations Compute by: Known Q Known Q (cfs) = 1.70 Highlighted Depth (ft) = 0.23 Q (cfs) = 1.700 Area (sqft) = 0.68 Velocity (ft/s) = 2.51 Wetted Perim (ft) = 6.06 Crit Depth, Yc (ft) = 0.26 Spread Width (ft) = 5.83 EGL (ft) = 0.33 0 2 4 6 8 10 12 14 16 18 Elev (ft) Depth (ft)Section 4820.75 -0.25 4821.00 0.00 4821.25 0.25 4821.50 0.50 4821.75 0.75 4822.00 1.00 Reach (ft) Channel Report Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc. Tuesday, Mar 18 2025 <Name> Gutter Cross Sl, Sx (ft/ft) = 0.040 Cross Sl, Sw (ft/ft) = 0.040 Gutter Width (ft) = 1.21 Invert Elev (ft) = 4820.88 Slope (%) = 0.26 N-Value = 0.013 Calculations Compute by: Known Q Known Q (cfs) = 2.60 Highlighted Depth (ft) = 0.32 Q (cfs) = 2.600 Area (sqft) = 1.28 Velocity (ft/s) = 2.03 Wetted Perim (ft) = 8.33 Crit Depth, Yc (ft) = 0.31 Spread Width (ft) = 8.00 EGL (ft) = 0.38 0 2 4 6 8 10 12 14 16 18 Elev (ft) Depth (ft)Section 4820.00 -0.88 4820.50 -0.38 4821.00 0.12 4821.50 0.62 4822.00 1.12 Reach (ft) Channel Report Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc. Tuesday, Mar 18 2025 <Name> Gutter Cross Sl, Sx (ft/ft) = 0.040 Cross Sl, Sw (ft/ft) = 0.040 Gutter Width (ft) = 1.21 Invert Elev (ft) = 4820.10 Slope (%) = 1.10 N-Value = 0.013 Calculations Compute by: Known Q Known Q (cfs) = 2.70 Highlighted Depth (ft) = 0.25 Q (cfs) = 2.700 Area (sqft) = 0.76 Velocity (ft/s) = 3.54 Wetted Perim (ft) = 6.43 Crit Depth, Yc (ft) = 0.32 Spread Width (ft) = 6.18 EGL (ft) = 0.44 0 2 4 6 8 10 12 14 16 18 Elev (ft) Depth (ft)Section 4819.75 -0.35 4820.00 -0.10 4820.25 0.15 4820.50 0.40 4820.75 0.65 4821.00 0.90 Reach (ft) T 4" 4" PROPOSED BUILDING 621 W. MENDENHALL STREET BOZEMAN, MT 59715 FFE = 4821.77N00°04'35"E 120.04'W. MENDENHALL STREET (50' ROW) ALLEY (20' ROW)S00°04'52"E 140.13'N89°38'30"E 144.94' S89°39'46"W 126.32' PROPERTY LINE PROPERTY LINE PROPERTY LINE PROPERTY LINE L = 30.34' R = 20.00' Δ = 86°55'29" PROPERTY LINE EXISTING PUBLIC SIDEWALK CURB RAMP CONSTRUCTED PER MDT FEDERAL AID PROJECT CMDP 118-1(5)0 PROPOSED STANDARD CATCH CURB & GUTTER RE: COB DWG. NO. 02528-1 PROPOSED ALLEY APPROACH RE: COB DWG. NO. 02529-7A 4820 4820 4820 4820 482 0 482 04819 4820482148224822 4822 4821 48 2 148204821 48214821 PARKING GARAGE INGRESS/EGRESS FFE = 4821.77 PARKING GARAGE INGRESS/EGRESS FFE = 4820.81 N. 7TH AVENUEPEDESTRIAN INGRESS/EGRESS FFE = 4821.50 PEDESTRIAN INGRESS/EGRESS FFE = 4821.77 PEDESTRIAN INGRESS/EGRESS FFE = 4821.77 PEDESTRIAN INGRESS/EGRESS FFE = 4820.67 PEDESTRIAN INGRESS/EGRESS FFE = 4820.55 1 2 3 0.33 AC 0.95 1 0.52 AC 0.95 2 0.54 AC 0.95 3 * BASIN 3 INCLUDES A COMBINATION OF BASIN 1 AND BASIN 3 AREAS SHOWN ON THIS EXHIBIT * BASIN 2 INCLUDES A COMBINATION OF BASIN 1 AND BASIN 2 AREAS SHOWN ON THIS EXHIBIT PROPOSED DRIVE APPROACH RE: COB DWG. NO. 02529-5A HIGH POINT IN N. 7TH AVENUE DR-2 2 OFFSITE DRAINAGE BASIN MAP0'20'40' SCALE: 1" = 20' N FOR THE MARKING OF UNDERGROUND BEFORE YOU DIG, GRADE, OR EXCAVATE CALL 2 BUSINESS DAYS IN ADVANCE CENTER OF MONTANA CALL UTILITY NOTIFICATION 1-800-424-5555 MEMBER UTILITIES. EXISTING FIRE HYDRANT EXISTING WATER VALVE EXISTING POWER POLE EXISTING GUY WIRE EXISTING WATER METER EXISTING LIGHT POLE EXISTING FIBER OPTIC LINE EXISTING GAS LINE EXISTING WATER LINE EXISTING OVERHEAD POWER LINE EXISTING SANITARY SEWER LINE EXISTING UNDERGROUND POWER LINE EXISTING UNDERGROUND TELEPHONE LINE PROPOSED MINOR CONTOURS5024 5025 PROPOSED MAJOR CONTOURS EX RIGHT-OF-WAY/ PROPERTY BOUNDARY LEGEND: EXISTING MINOR CONTOURS5024 5025 EXISTING MAJOR CONTOURS CONTROL POINT SHEET NUMBER:THE NORTHERN621 WEST MENDENHALL STREETBOZEMAN, MT 597152 CHECKED BY: DESIGNED BY: DRAWN BY:DESCRIPTION:DATE:REVISED BY:REVIEWED BY:REVISION #1234567PROJECT #: 2024017 DATE: OF DRAWING NAME:PROJECT NAME & LOCATION:RECORD DRAWING BY: 101 E. Loucks Street, #300 Sheridan, WY 82801 PHONE: 970-227-0714 RCM RCM RCMOWNER:4/11/25DRAKE BUILDING & DEVELOPMENTBENCHMARK: ELEVATION DATUM IS NAVD 88 BASIS OF BEARING: BOBCAT LDP COORDINATE SYSTEM POINT NORTHING EASTING ELEVATION DESCRIPTION 30 120391.19 380214.59 4821.11 MDT Cu Plug LS59853 96 120340.70 380215.51 4822.33 CH MAG DISK 97 120549.24 380217.82 4819.94 MAG NAIL 98 120536.01 380339.37 4820.54 MAG NAIL 99 120440.54 380329.09 4821.26 MAG NAIL CONTROL POINT TABLE: DRAINAGE LEGEND PROPOSED DRAINAGE BASIN 2 BASIN DESIGNATION BASIN AREA IN ACRES RUNOFF COEFFICIENT 0.33 AC 0.95 1 PROPOSED DRAINAGE BASIN 1 DRAINAGE DESIGN POINTDRAINAGE DESIGN POINT1 PROPOSED DRAINAGE BASIN 3 PROPOSED DRAINAGE FLOW ARROW