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Home My WebLink About587-01 Stormwater Design Report-Full-1.3.25 (1) 587-01 Babcock 5-plex - Stormwater Design Report Page 1 1/3/2025 TABLE OF CONTENTS Overview ......................................................................................................................................... 2 Existing Conditions ........................................................................................................................... 2 Stormwater Design Criteria ............................................................................................................. 2 APPENDIX Appendix A ........................................................................................................ Drainage Basin Map Appendix B ............................................................................................ Storage Facility Calculations Appendix C ..................................................................................... Stormwater Facilities O&M Plan Appendix D ........................................................................................................ Geotechnical Report 587-01 Babcock 5-plex - Stormwater Design Report Page 2 1/3/2025 OVERVIEW The purposed of this report is to detail the stormwater management design for the multi-family development at 1920 W Babcock Street Bozeman, MT. The property is located on the SE corner of the intersection of W Babcock Street and 29Th Avenue and is legally described as Lot 10 of Kirk Subdivision Block 1, Section 11, Township 2S, Range 5E. The proposed project will include the construction of a 6,860 square foot 5-plex apartment building with associated parking area and sidewalks. EXISTING CONDITIONS Although the site is currently vacant, the property previously contained an office building for Allstate insurance with a paved parking area and perimeter landscaping. The existing building and parking area were demolished within the last two years to prepare the site for the proposed 5-plex. The previous structure had a slightly smaller footprint than the proposed building, but the existing parking area was significantly larger, resulting in a similar lot coverage coefficient to the proposed development. Based on an existing conditions survey and historical street view and aerial imagery, it is estimated that the north / northwestern portion of the property previously drained off site to the curb inlet in W Babcock Street (see pre-development drainage basin 1). The southern and eastern portions of the site appeared to drain to low points on site or on the southern property line (pre-development drainage basin 2). Rawhide Engineering completed a geotechnical investigation for the property in March 2023 which found that the property is generally covered by several feet of either fill or a native sandy lean clay, followed by native gravel starting at depths of 3-6 feet bgs. The test pits were excavated to 7-9 feet bgs and groundwater was not encountered in any of the pits. A groundwater monitoring well will be installed in Spring of 2025 to capture seasonal high groundwater elevations, but based on the geotechnical investigation, groundwater is not expected to impact the proposed stormwater retention system. STORMWATER DESIGN CRITERIA The proposed stormwater water improvements are designed to meet the requirements of the City of Bozeman Standards and Specifications, adopted in March of 2020. The drainage policy requires retention facilities to be designed using the modified rational method to retain the 10-year, two-hour storm intensity. Figure I-3 in the city standards provides a table of rainfall intensities for use in the City of Bozeman. The intensity for a 10-year, two-hour storm is 0.41in/hr. In order to determine the stormwater retention requirements for the proposed improvements, the project site is divided into 3 individual drainage basins. Dimensionless C-Coefficients used in the Rational Peak Discharge Method were assigned to impervious and pervious areas for each drainage basin. Impervious areas such as concrete, asphalt, and impervious roofing were assigned a C-Coefficient of 0.95. Pervious landscaped and pervious areas were assigned a C-Coefficient of 0.20. A drainage basin map outlining the proposed drainage basin and retention facilities is included in Appendix A of this report. A combination of site grading, curb and gutter, and roof downspouts will be used to convey water to the proposed vegetated swales and on-site drywells. The dry well system will consist of two concrete structures connected via 40’ of 8” HDPE Perforated Pipe surrounded by 24” of washed rock backfill. This system is shown on C5.0 and detailed on C6.0 of the site plan. 587-01 Babcock 5-plex - Stormwater Design Report Page 3 1/3/2025 Post-development Drainage Basins 1 and 2 include the northern and western portions of the site that will sheet flow to the adjacent street right-of-ways. These areas do not generate any additional stormwater runoff when compared to pre-development Drainage Basin 1 and thus meet the allowable runoff from the site. Post-development Drainage basin 3 includes the southern and eastern portions of the site that historically did not drain off-site. Therefore, Drainage Basin 3 is designed to retain all stormwater from the 10-year, 2-hour event on-site in the dry well system. Runoff volumes for each basin are provided in Appendix B and are summarized in Table A below. Drainage Basin Runoff Volume Summary (2-year, 1 hour Storm Event) Drainage Basin Area (Ac.) C- Value I (in/hr) Q (2-yr) (cfs) Vreq (ft3) Vpro (ft3) Facility Pre-1 0.11 0.6 0.41 0.03 189 - Pre-Development Basin 1 Pre-2 0.14 0.67 0.41 0.04 282 - Pre-Development Basin 2 1 0.03 0.58 0.41 0.01 44 - Off - Site (Babcock St Inlet) 2 0.06 0.71 0.41 0.02 131 - Off - Site (Babcock St Inlet) 3 0.16 0.67 0.41 0.04 322 394.5 Drywell System A Table A – Runoff and Retention Volume Summary Appendix A Drainage Basin Map STMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTM STM STM STM STM STM STM STM STM STM STM STM STM STM STMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMN 00°08'37" E74.61'N 89°31'56" E 144.75'S 00°08'35" W75.21'S 89°46'01" W 144.74'OSW C O WEST BABCOCK ST.SOUTH 20TH AVE.4825 4825 4825 4824 482 5 4824 4823 4825 PROJECT NO: DATE: ENGINEER: REVISIONS DATENO.DESC. 587-01 12/23/2024 ELC FIGURE 1 10'0 20' 1" = 10'SCALE N SHEET OF 21BABCOCK 5-PLEXPRE-DEVELOPMENT DRAINAGE BASIN MAPBOZEMAN, MONTANALOT 10, BLOCK 1, KIRK SUBDIVISIONLEGEND DRAINAGE BASIN BOUNDARY DRAINAGE BASIN COVER TYPE - PERVIOUS SYMBOL DESCRIPTION DRAINAGE BASIN COVER TYPE - IMPERVIOUS DRAINAGE BASIN 1 0.11 AC DRAINAGE BASIN 2 0.14 AC EX. CURB INLET (TYP.) DRAINAGE DIRECTION GASGASGASGASGASGASGASGASGASGASGASGASGASSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTM STM STM STM STM STM STM STM STM STM STM STM STM STM STMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMSTMN 00°08'37" E74.61'N 89°31'56" E 144.75'S 00°08'35" W75.21'S 89°46'01" W 144.74'OSW DRYWELL A-1 (2.0') (BEEHIVE GRATE) RIM:4824.10 INV OUT:4820.40 (8")(W) DRYWELL A-2 (2.0') (BEEHIVE GRATE) RIM:4824.10 INV IN:4820.40 (8")(E) 43' of 8" HDPE PIPE @ 0.00% WEST BABCOCK ST.SOUTH 20TH AVE.PROJECT NO: DATE: ENGINEER: REVISIONS DATENO.DESC. 587-01 1/3/2025 ELC FIGURE 2 10'0 20' 1" = 10'SCALE N SHEET OF 22 LEGEND DRAINAGE BASIN BOUNDARY DRAINAGE BASIN COVER TYPE - PERVIOUS STORM MANHOLE STORM INLET STORM CATCH BASIN STORM POND / SWALE SYMBOL DESCRIPTION ST DRAINAGE BASIN COVER TYPE - IMPERVIOUS BABCOCK 5-PLEXPOST-DEVELOPMENT DRAINAGE BASIN MAPBOZEMAN, MONTANALOT 10, BLOCK 1, KIRK SUBDIVISIONCURB CHASE DRAINAGE BASIN 1 0.02 AC DRAINAGE BASIN 2 0.06 AC DRAINAGE BASIN 3 0.16 AC RETENTION SYSTEM REQ. STORAGE VOL: 320 FT3 PRO. STORAGE VOL: 394.5 FT3 BOT. OF STONE ELEV: 4817.40' SHGW ELEV: 4817.10' EX. CURB INLET TO RECEIVE RUNOFF FROM DB 1 AND 2 DRAINAGE DIRECTION Appendix B Drainage Basin & Storage Facility Calculations Design Rainfall Frequency (year):10 Storage Method:N/A Rational Method Peak Runoff Equation:Q = CIA Discharge Method:N/A Q = Peak Runoff Rate (cfs)Facility Type:N/A C = Runoff Coefficient Facility Make/Model:N/A I = Rainfall Intensity (in/hr):0.41 A = Drainage Basin (acres) Land Use Drainage Basin Number Contributing Area (sf)Contributing Area (Ac) Runoff Coe. (C)A (Ac) x C DB - Impervious 1 2484 0.057 0.95 0.0542 DB - Pervious 1 2117 0.049 0.2 0.0097 Totals 4,601 0.11 0.0639 Weighted C:0.60 Total Area (Acres) = 0.11 Weighted C =0.60 Intensity (in/hr) = 0.41 Duration (hr)2.00 Q (cfs) =0.03 Pre-Development Runoff Volume (ft3)=189 Generated Runoff Volume - 10 year Storage Facility Calculations Pre-Development Basin 1 Basis For Calculations Storage Facility Information Contributing Area & Runoff Coefficient Tabulation Design Rainfall Frequency (year):10 Storage Method:N/A Rational Method Peak Runoff Equation:Q = CIA Discharge Method:N/A Q = Peak Runoff Rate (cfs)Facility Type:N/A C = Runoff Coefficient Facility Make/Model:N/A I = Rainfall Intensity (in/hr):0.41 A = Drainage Basin (acres) Land Use Drainage Basin Number Contributing Area (sf)Contributing Area (Ac) Runoff Coe. (C)A (Ac) x C DB - Impervious 2 3889 0.089 0.95 0.0848 DB - Pervious 2 2352 0.054 0.2 0.0108 Totals 6,242 0.14 0.0956 Weighted C:0.67 Total Area (Acres) = 0.14 Weighted C =0.67 Intensity (in/hr) = 0.41 Duration (hr)2.00 Q (cfs) =0.04 Pre-Development Runoff Volume (ft3)=282 Generated Runoff Volume - 10 year Storage Facility Calculations Pre-Development Basin 2 Basis For Calculations Storage Facility Information Contributing Area & Runoff Coefficient Tabulation Design Rainfall Frequency (year): 10 Storage Method: N/A Rational Method Peak Runoff Equation: Q = CIA Discharge Method: N/A Q = Peak Runoff Rate (cfs) Facility Type: N/A C = Runoff Coefficient Facility Make/Model: N/A I = Rainfall Intensity (in/hr): 0.41 A = Drainage Basin (acres) Land Use Drainage Basin Number Contributing Area (sf) Contributing Area (Ac) Runoff Coe. (C)A (Ac) x C DB - Impervious 1 577 0.013 0.95 0.0126 DB - Pervious 1 517 0.012 0.2 0.0024 Totals 1,094 0.03 0.0149 Weighted C:0.60 Total Area (Acres) = 0.03 Weighted C = 0.60 Intensity (in/hr) = 0.41 Duration (hr) 2.00 Q (cfs) = 0.01 Req. Runoff Volume (ft3)=44 Storage Facility Calculations Drainage Basin 1 Basis For Calculations Contributing Area & Runoff Coefficient Tabulation Generated Runoff Volume - 10 year Storage Facility Information Design Rainfall Frequency (year):10 Storage Method:N/A Rational Method Peak Runoff Equation:Q = CIA Discharge Method:N/A Q = Peak Runoff Rate (cfs)Facility Type:N/A C = Runoff Coefficient Facility Make/Model:N/A I = Rainfall Intensity (in/hr):0.41 A = Drainage Basin (acres) Land Use Drainage Basin Number Contributing Area (sf)Contributing Area (Ac) Runoff Coe. (C)A (Ac) x C DB - Impervious 2 1856 0.043 0.95 0.0405 DB - Pervious 2 857 0.020 0.2 0.0039 Totals 2,714 0.06 0.0444 Weighted C:0.71 Total Area (Acres) = 0.06 Weighted C =0.71 Intensity (in/hr) = 0.41 Duration (hr)2.00 Q (cfs) =0.02 Req. Runoff Volume (ft3)=131 Generated Runoff Volume - 10 year Storage Facility Calculations Drainage Basin 2 Basis For Calculations Storage Facility Information Contributing Area & Runoff Coefficient Tabulation Design Rainfall Frequency (year): 10 Storage Method: Retention Rational Method Peak Runoff Equation: Q = CIA Discharge Method: N/A Q = Peak Runoff Rate (cfs) Facility Type: Dry Well C = Runoff Coefficient Facility Make/Model: N/A I = Rainfall Intensity (in/hr): 0.41 A = Drainage Basin (acres) Land Use Drainage Basin Number Contributing Area (sf) Contributing Area (Ac) Runoff Coe. (C)A (Ac) x C DB - Impervious 3 4,461 0.1024 0.95 0.097283 DB - Pervious 3 2,573 0.0591 0.2 0.011815 Totals 7,034 0.16 0.1091 Weighted C:0.68 Total Area (Acres) = 0.16 Drywell A-1 - Inner Diameter (ft) 2 Weighted C = 0.68 Depth (ft) 6 Intensity (in/hr) = 0.41 Arywell A-2 - Inner Diameter (ft) 2 Duration (hr)2.00 Depth (ft) 6 Q (cfs) = 0.04 Lateral Diamter (inches) 8 Req. Runoff Volume (ft3)=322 Lateral Length (ft) 40 Volume of Lateral (ft3)14.0 Washed Rock Porosity 40% Lateral Washed Rock Offset (inches) 24 Volume of Washed Rock Around Lateral (ft3)342.9 Volume of Drywell 5A (ft3)19 Volume of Drywell 5B (ft3)19 Total Ret Volume (ft3)394.5 Generated Runoff Volume - 10 year, 2 hour Storm Provided Retention (Drywell) Storage Facility Information Storage Facility Calculations Drainage Basin 3 Basis For Calculations Contributing Area & Runoff Coefficient Tabulation Appendix C Stormwater Facilities Operation and Maintenance Plan Babcock 5-Plex – Stormwater Facilities Operation & Maintenance Plan Page 1 January 2025 STORMWATER FACILITIES OPERATION & MAINTENANCE PLAN FOR: Babcock 5-plex – Lot 10, Block 1, Kirk Subdivision PROPERTY DESCRIPTION Lot 10, Block 1, Kirk Subdivision, City of Bozeman, Gallatin County, Montana RESPONSIBLE PARTY Maloney Holdings LLC C/O Gallardo Trust Company 136 S Dakota Ave Sioux Falls, SD 57104 (406) 586-5599 STORMWATER FACILITIES DESCRIPTION Vegetated Swales: Swales should be maintained as detailed on the approved “Babcock 5-Plex Civil Site Plan Set” by Alpine Surveying and Engineering. These swales should be fully vegetated with grass that is trimmed regularly. Swales should maintain a minimum depth of 6” and should have a 1% longitudinal slope to convey water to storm chases or inlets. Storm Chases: Storm chases should adhere to City of Bozeman Standard Drawing No. 02529-14 and should have 6” minimum depth and 1’ minimum width. Storm Sewer Network: The site storm sewer network is comprised of HDPE Perf Pipe and Drywell Structures to capture runoff from the rear of the site and alley parking area. This system is installed as depicted on the approved “Babcock 5-Plex Civil Site Plan Set” by Alpine Surveying and Engineering. INSPECTION & MAINTENANCE SCHEDULE The stormwater facilities on site will be inspected and maintained based on the below schedule. Inspection and maintenance will be performed by the responsible party, the facility employees, or a contracted third party. Inspection: Routine inspection of stormwater facilities will include visual inspection to ensure the facilities are functioning properly and there is no debris accumulated in or clogging the stormwater facilities. Inspections should also be performed after major storm events producing approximately 0.5” of rainfall in a 24-hour period. These inspections shall include observations of the stormwater facilities for areas of erosion or areas of ponding, and removal of accumulated trash or debris in facilities. Maintenance: Maintenance shall be performed as necessary based on inspections, and at regular intervals throughout the year based on the schedule below. Routine Maintenance will generally include removal of trash, overgrown vegetation, grass clippings, leaves, accumulated sediment, and obstructions in stormwater facilities. Major maintenance will be scheduled as necessary to Babcock 5-Plex – Stormwater Facilities Operation & Maintenance Plan Page 2 January 2025 repair or replace damaged storm structures and to revegetate areas of erosion or dead vegetation. Inspection & Maintenance Schedule BMP Inspection Frequency A=Annual, M=Monthly, S=After Major Storm, Q=Quarterly, SA=Semi Annually Maintenance Frequency Catch Basin / Storm Inlet Q 1 / year Swale M 2-3 / year Storm Chase Q 2-3 / year Dry Well A 1 / year Underground Storage System SA 1 / year Infiltration Trenches A, S 2-3 / year Table Source: Stormwater Equipment Manufacturers Association Appendix D Geotechnical Report