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HomeMy WebLinkAbout006 STORM DRAINAGE REPORT703 W Mendenhall September 2024 Project: 23-176.10 Bozeman Office . 32 Discovery Drive . Bozeman, Montana 59718 . Ph: (406) 582-0221 . Fax: (406) 582-5770 Page 1 Table of Contents 1 Project Overview and Background ......................................................................................... 2 2 Hydrology ............................................................................................................................... 2 2.1 Pre-Development ............................................................................................................ 3 2.2 Post Development ........................................................................................................... 3 2.3 Summary of Geotechnical/Hydrogeological Evaluation ................................................ 4 3 Post Development Hydraulics ................................................................................................. 5 3.1 Storm Drain Hydraulics .................................................................................................. 5 3.2 Inlet Capacity .................................................................................................................. 6 3.3 Infiltration/Retention Structure Sizing ............................................................................ 6 4 Maintenance Plan .................................................................................................................... 8 Table 1 - Pre-Development Hydrology ........................................................................................... 3 Table 2 – Post Development Hydrology ......................................................................................... 4 Table 3 - Static Groundwater Levels .............................................................................................. 5 Table 4 - Storm Drain Summary (100-year event) ......................................................................... 5 Table 5 – City of Bozeman stormwater volume calculations ......................................................... 6 Table 6 – Retention/Infiltration Structure Stage Storage ................................................................ 7 Table 7 – Stormwater Infiltration versus Time ............................................................................... 7 Attachments: Figure 1.1 – Vicinity Map - Topo Figure 1.2 – Vicinity Map – Aerial Exhibit 2.1 – Pre-Development Drainage Exhibit 3.1 – Post Development Drainage APPENDIX A – Supporting Calculations APPENDIX B – Stormwater Drainage Plans APPENDIX C – Adjacent Well Logs 703 W Mendenhall September 2024 Project: 23-176.10 Bozeman Office . 32 Discovery Drive . Bozeman, Montana 59718 . Ph: (406) 582-0221 . Fax: (406) 582-5770 Page 2 1 Project Overview and Background The proposed site improvements involve the demolition of an existing building and construction of a 5-story mixed-use residential structure. The project site is located on the northwest corner of the intersection of North 7th Avenue and West Mendenhall Street and encompasses approximately 0.14 acres (5,968 square feet). It is bounded on the east and south by these two City streets, respectively; on the west by a mid-block alleyway; and on the north side by an asphalt-covered, commercial property that houses Dairy Queen seen in Figure 1.2. The subject property is platted as Lot 11 and 12, Block 8 of the Springbrook Addition. The legal description for the site is the SW1/4, NE1/4 of Section 12, T2S, R5E, Gallatin County. The latitude/longitudinal coordinates (near the center of the property) are 45.680498°N and -111.046594°W. At present, the project site contains a one-story commercial building that is underlain by a full basement level throughout its foundation footprint area. The building is surrounded by narrow landscape areas on its north, east, and south sides. A small asphalt parking lot occupies the west side of the property and is accessed from the west side alleyway. The existing condition is roughly completely impermeable on site with minimal boulevard trees and bushes present. An aerial overview can be seen in Figure 1.2. The site currently does not have an underground storm sewer network or dedicated detention/retention facilities. Stormwater drains off site to North 7th Avenue and West Mendenhall Street. Storm water is conveyed along curbs and gutters where it is later intercepted by curb inlets connected to City of Bozeman Storm Drains. See Exhibit 2.1 for present stormwater drainage conditions. A modular underground retention/infiltration structure will be installed to meet peak runoff rate requirements. A sand oil separator will be installed upstream of the underground system to provide pre-treatment. Stormwater from the roof of the building will be collected at downspouts by storm grate inlets, then piped to an oil-water separator. A trench drain will collect stormwater runoff from the western portion of the property. Runoff from the courtyard of the building and surrounding sidewalks will direct discharge to the existing curb and gutter along West Mendenhall Street and North 7th Avenue. See Appendix B for proposed stormwater drainage infrastructure plans and details. Subsurface conditions were investigated along the north side of the project site on December 20, 2023, and January 10, 2024. Three boreholes were drilled to depths of 36.5 feet yielding groundwater depths ranging from 22 to 25 feet. Additional information is provided later in this report and in the Geotechnical Investigation and Report by Allied Engineering dated April 11th, 2024. 2 Hydrology The rational method was utilized to estimate peak flow rates for collection and conveyance sizing as outlined by the City of Bozeman Design Standards and Specifications. A weighted runoff coefficient “C” was determined based on a combination of open space (C=0.2) and pavement 703 W Mendenhall September 2024 Project: 23-176.10 Bozeman Office . 32 Discovery Drive . Bozeman, Montana 59718 . Ph: (406) 582-0221 . Fax: (406) 582-5770 Page 3 (C=0.95). This runoff coefficient was adjusted by a correction factor, Cf, based on the return interval of the design storm as specified by the HEC-22 manual. Time to concentration was estimated using the overland flow method from the City of Bozeman Design Standards for pre- development conditions, and sheet flow for post development conditions (applicable for flow lengths less than 100 feet). The following design storms follow design criteria provided in the City of Bozeman design criteria and standards. A. Minor Drainage System (10-Year Recurrence interval): Used to design general stormwater interception and conveyance facilities. B. Major Drainage System (100-Year Recurrence interval): Used to determine the flood risks generated by the site for public infrastructure via drainage through receiving systems (i.e. surrounding streets and alleyways) 2.1 Pre-Development One basin was assumed for the site. A summary is provided in Table 1 - Pre-Development Hydrology. Time to concentration was measured from one property corner to another assuming open space for the runoff coefficient. Time to concentration for pre-development conditions was calculated using overland flow and yielded a value of 4 minutes. As this value was smaller than the City of Bozeman’s tabulated hyetograph, a time to concentration of 5 minutes was used. Basin A.1 was conservatively assumed to have no impervious area prior to development. This area encompasses the entire footprint of the property. Calculations for the pre-development drainage included assumptions for conditions prior to any site development. The following assumptions were made for pre-development calculations. i. The current slope is the same as the historic slope. ii. The pre-development landcover was all open space and assumed to be a grassy field. This is a conservative assumption as grassy fields promote infiltration and have high roughness coefficients. Table 1 - Pre-Development Hydrology Return Interval Total Area Impervious Open Space C*Cf Tc Intensity Peak Runoff acres ft2 ft2 min in/hour ft3/s 10-Year 0.14 0 5968 0.2 5 3.87 0.11 100-Year 0.14 0 5968 0.25 5 6.09 0.21 2.2 Post Development The post development conveyance infrastructure is required to consider the 10-year design event as outlined in the City of Bozeman Design Standards for interception and conveyance of the Minor Drainage System. However, the 100-year event was considered for conservative sizing of the underground retention system. A summary of post development sub-basins is shown in Table 2 – 703 W Mendenhall September 2024 Project: 23-176.10 Bozeman Office . 32 Discovery Drive . Bozeman, Montana 59718 . Ph: (406) 582-0221 . Fax: (406) 582-5770 Page 4 Post Development Hydrology. Basin A.1 was conservatively assumed to have no permeable area post development. See Exhibit 3.1 for post development basin delineation. Table 2 – Post Development Hydrology Basin Area Area C Tc 10-yr Peak Runoff 100-yr Peak Runoff ft2 acres min ft3/s ft3/s Roof Area A 1706 0.04 1 5 0.15 0.24 Roof Area B 1233 0.03 1 5 0.11 0.17 Deck Area 1 939 0.02 1 5 0.08 0.13 Deck Area 2 418 0.01 1 5 0.04 0.06 Valley Gutter 632 0.01 1 5 0.06 0.09 Direct Discharge 1039 0.02 1 5 0.09 0.15 Total 5968 0.14 1 5 0.50 0.83 2.3 Summary of Geotechnical/Hydrogeological Evaluation Geotechnical/ hydrogeological conditions were investigated on December 20, 2023, and January 10, 2024. Three boreholes were dug at 36.5-foot depths on the Northern portion of the project site (The existing building inhibited southern exploration). Allied Engineering conducted geotechnical explorations on the site and produced a report dated April 11th, 2024, including greater detail with relevant information listed below. Subsurface Soil Layers: i. 0 to 20 feet: silt/clay ii. >20 feet: Sandy gravel with cobbles extending to the total depth of the bore holes. Groundwater: The groundwater in the bore holes ranged from 22 to 25 feet below ground surface. Bore holes were dug during annual periods of reduced groundwater depth. Based on historic patterns, it is likely groundwater fluctuates approximately 3-4 feet annually. Well logs from adjacent properties contained static water levels that fluctuating between 19.5 to 26 feet seen Table 3 - Static Groundwater Levels. Well logs from Table 3 can be seen in Appendix C. 703 W Mendenhall September 2024 Project: 23-176.10 Bozeman Office . 32 Discovery Drive . Bozeman, Montana 59718 . Ph: (406) 582-0221 . Fax: (406) 582-5770 Page 5 Table 3 - Static Groundwater Levels Location GWIC ID Static Water Level Date of Record feet BGS North 8th Ave 274720 21.3 6/18/1996 North 8th Ave 274722 19.5 6/25/1996 North 7th Ave 277104 26 3/13/2014 North 7th Ave 277108 26 3/13/2024 West Mendenhall Street 277110 26 3/14/2024 West Mendenhall Street 277115 26 3/24/2024 3 Post Development Hydraulics On-site stormwater drainage is to be captured, retained, and infiltrated using a modular detention facility buried on the western side of the property. Drains and gutters will convey water from rooftops and balconies to downspouts. Downspouts will be collected by grates at ground level and feed the storm sewer network shown on C1.5 in Appendix B. Runoff from the western portion of the site not covered by the existing structure will be collected by a trench drain and routed to the sand/oil separator. A portion of runoff from the courtyard and perimeter sidewalks will direct discharge to adjacent streets (see Table 2). This area is relatively small and is outlined in more detail below. All runoff collected will be routed to a pre-treatment sand-oil separator before passing to the underground retention system. The volume of the underground system was conservatively sized based on the 100-year, 24-hour storm. The design storm event was chosen based on the proximity to the structure and foundation and risk associated with saturating the native silt/clay. 3.1 Storm Drain Hydraulics The storm drain hydraulics were conducted using Autodesk Hydraflow Express Extension for Autodesk Civil 3D (see Appendix A). Headwater depths at the pipe entrances are all below top of pipe, no pipes are shown to be surcharged. Storm drains were designed under the criteria outlined in section 6.7.3 in the COB design standards criteria. Table 4 summarizes the carrying capacity for the storm drain conduits. All pipes were designed to convey the 100-year peak flow rate. See Exhibit 3.1 for contributing areas and storm drain locations. Estimated pipe flow rates, velocities, and depths for the Major Storm event can be seen in Table 4. Table 4 - Storm Drain Summary (100-year event) Storm Drain Contributing Area Size/Material Grade Peak Flow Velocity Flow Depth Pipe Full ft2 % ft3/s ft/s ft % SD-1 1706.32 6" SDR-35 1 0.24 3.1 0.21 42.8 SD-2 2172.36 8" SDR-35 1 0.30 3.19 0.21 31.9 SD-3 5968 12" SDR-35 1 0.83 4.05 0.31 31.7 SD-4 2984 12" CMP 1.5 0.39 2.14 0.28 28.0 703 W Mendenhall September 2024 Project: 23-176.10 Bozeman Office . 32 Discovery Drive . Bozeman, Montana 59718 . Ph: (406) 582-0221 . Fax: (406) 582-5770 Page 6 Diameters shown in Table 4 are nominal pipe diameters. Calculations used inside diameters see Appendix A. Fittings and connections will include tee-connections, and 45-degree fittings. Casing pipe is anticipated for all storm drains running under the footprint of the structure. The casing will host spacers to prevent pipe and outer wall contact. 3.2 Inlet Capacity Inlets were designed to capture the peak runoff from gutter downspouts. Grate inlets are to be sized with diameters larger than feeding downspouts (8-10” in diameter). Regular maintenance and inspection of inlet grates is an important factor for their performance (see 4.0 Maintenance Plan). Complete failure of grate inlets would release a maximum of 0.3 cfs to North 7th Avenue and or 0.24 cfs to West Mendenhall Street during the Major Storm event. See Table 4 and C1.5 in Appendix B. 3.3 Infiltration/Retention Structure Sizing Hydrologic calculations for the retention system used the 24 hour 100-year storm event for rainfall intensity and duration. The total water volume for this event was used to size the volume of the retention system. Table 5 provides estimates of rainfall volume calculations for the Major Storm, Minor Storm, Water Quality, and Runoff Treatment volumes. Runoff treatment volume is defined as the difference between design storm volume and Water Quality volume. Table 5 – City of Bozeman stormwater volume calculations Storm Event Area Depth Rainfall Volume Water Quality Volume Runoff Treatment Volume Return Interval ft2 in ft3 ft3 ft3 Q10 5968 1.70 845 249 597 Q100 5968 2.34 1164 249 915 The retention volume of the facility included the void space of the modular retention/infiltration structure and pore volume of crushed gravel around and below the structure. The structure will be encased in a Mirafi Filter liner on the top and sides to prevent introduction of fine sediments. The void volume was specified as 95% by the manufacturer and the pore space of crushed gravel was assumed to be 30% as specified by DEQ Circular 8. The depth of the crushed gravel was assumed to extend 1-2 feet below the retention structure with an areal footprint extending approximately 1- 2 feet around the outer perimeter of the retention/infiltration structure (see C2.1 and C2.2 in Appendix B). Crushed gravel will provide pore space and connection for infiltration as well as a platform for installation. Aprons of impermeable PVC liner will be installed around the areal footprint extending to native gravels to prevent lateral water percolation. Due to construction constraints, the specific extent will be dictated by the contractor. A stage storage relationship for volumetric sizing can be seen below Table 6. As can be seen in Table 5 above and Table 6, the water quality and treatment volumes were encompassed into storm drainage system design. 703 W Mendenhall September 2024 Project: 23-176.10 Bozeman Office . 32 Discovery Drive . Bozeman, Montana 59718 . Ph: (406) 582-0221 . Fax: (406) 582-5770 Page 7 Table 6 – Retention/Infiltration Structure Stage Storage Stage Elevation Rock Void Volume StormTank Void Volume Provided Volume ft ft ft3 ft2 ft3 0 4807 0 0 0 1 4808 46 0 46 2 4809 92 0 92 3 4810 92 436 528 4 4811 92 581 673 5 4812 92 727 819 6 4813 92 872 964 7 4814 92 1018 1109 8 4815 92 1163 1255 The City of Bozeman requires a maximum drain down time of 72 hours post-storm cessation for the design stormwater runoff volume. The design stormwater runoff volume for this calculation was the 100-year 24-hour storm comparable to design of all other stormwater infrastructure. The minimum area determined from these calculations (75 ft2), references surface area below the retention/infiltration structure to be fully excavated down to native gravels and backfilled with clean crushed drain rock. Native gravels were assumed to have an infiltration rate of 2.6 in/hour as recommended by DEQ-8. Drain-Down Time and corresponding stormwater volumes can be seen in below Table 7. Table 7 – Stormwater Infiltration versus Time Drain-Down Time Retained Volume Infiltration rate Area hours ft3 in/hour ft2 0 1170 2.6 75 6 1072.5 2.6 75 12 975 2.6 75 18 877.5 2.6 75 24 780 2.6 75 30 682.5 2.6 75 36 585 2.6 75 42 487.5 2.6 75 48 390 2.6 75 54 292.5 2.6 75 60 195 2.6 75 66 97.5 2.6 75 72 0.0 2.6 75 703 W Mendenhall September 2024 Project: 23-176.10 Bozeman Office . 32 Discovery Drive . Bozeman, Montana 59718 . Ph: (406) 582-0221 . Fax: (406) 582-5770 Page 8 4 Maintenance Plan All proposed storm drainage features will be privately owned and maintained. Storm infrastructure consists of grate inlets, downspouts, conveyance piping, sand oil separator, and an underground retention system. • The sand oil separator will be maintained after 90 days or if the combined thickness of floating oil and settled solids is equal to or greater than 25% of the total liquid depth. Details regarding specific timing and details of maintenance will be established by installers and refined by the contracted plumber. Logs of maintenance will be kept for the structure by the plumbing company. Inspection and maintenance will be more frequent directly after installation. After performance is established, they may become less frequent. Maintenance considerations include removal of built-up sludge and sediments. Maintenance of the sand oil separator will be decreased if sand and sediments are removed preemptively from contributing areas. The separator will be maintained using a vac truck if accessible, or by manual labor if vehicle access is not feasible via access risers shown in Exhibit C1.6 in Appendix B. Failure to maintain the sand oil separator can result in critical failure of the subsurface retention/infiltration system. Failure of the underground retention facility may entail excavation and will incur significant costs. • Inspection of the modular retention system will be conducted frequently after instillation, and an inspection schedule will be established by maintenance staff after performance has been validated. An inspection log will be kept by staff documenting the time of inspection/maintenance and condition of the structure. Maintenance will be conducted after debris buildup. Excessive debris buildup in the retention facility is indicative of poor performance of the sand oil separator. Two access risers will enable jet/vac equipment to remove debris and provide ports for inspection. • Roof downspouts, sewer grates, and trench drains will collect debris and should be inspected in the spring and fall and cleaned if debris is present. Sumps will be included with trench drain inlets and grate inlets. Sumps should be inspected and cleaned out by hand or using a vac truck once a year or as debris is collected. • Storm drainage pipes passing under the building footprint will consist of a casing pipe. Casing material will facilitate removal and replacement of pipes in the event of cracking and or failure. Cleanouts will be included at locations where pipes bend, or connections are necessary. Cleanout inspection will be conducted on an annual basis and when debris buildup inhibits performance. 703 W Mendenhall September 2024 Project: 23-176.10 Bozeman Office . 32 Discovery Drive . Bozeman, Montana 59718 . Ph: (406) 582-0221 . Fax: (406) 582-5770 APPENDIX A – Supporting Calculations Channel Report Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc. Tuesday, Sep 10 2024 SD-1 Circular Diameter (ft) = 0.49 Invert Elev (ft) = 4817.37 Slope (%) = 1.00 N-Value = 0.011 Calculations Compute by: Known Q Known Q (cfs) = 0.24 Highlighted Depth (ft) = 0.21 Q (cfs) = 0.240 Area (sqft) = 0.08 Velocity (ft/s) = 3.10 Wetted Perim (ft) = 0.70 Crit Depth, Yc (ft) = 0.25 Top Width (ft) = 0.49 EGL (ft) = 0.36 0 1 2 3 Elev (ft)Depth (ft)Section 4817.00 -0.37 4817.25 -0.12 4817.50 0.13 4817.75 0.38 4818.00 0.63 Reach (ft) Channel Report Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc. Tuesday, Sep 10 2024 SD-2 Circular Diameter (ft) = 0.66 Invert Elev (ft) = 4817.77 Slope (%) = 1.00 N-Value = 0.011 Calculations Compute by: Known Q Known Q (cfs) = 0.30 Highlighted Depth (ft) = 0.21 Q (cfs) = 0.300 Area (sqft) = 0.09 Velocity (ft/s) = 3.19 Wetted Perim (ft) = 0.79 Crit Depth, Yc (ft) = 0.26 Top Width (ft) = 0.62 EGL (ft) = 0.37 0 1 2 3 Elev (ft)Depth (ft)Section 4817.00 -0.77 4817.50 -0.27 4818.00 0.23 4818.50 0.73 4819.00 1.23 Reach (ft) Channel Report Tuesday, Sep 10 2024Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc. SD-3 Circular Diameter (ft)= 0.98 Invert Elev (ft) = 4816.44 Slope (%)= 1.00 N-Value = 0.011 Calculations Compute by:Known Q Known Q (cfs) = 0.83 Highlighted Depth (ft)= 0.31 Q (cfs)= 0.830 Area (sqft)= 0.20 Velocity (ft/s)= 4.05 Wetted Perim (ft) = 1.17 Crit Depth, Yc (ft) = 0.39 Top Width (ft)= 0.91 EGL (ft)= 0.56 0 1 2 3 Elev (ft)Depth (ft)Section 4815.50 -0.94 4816.00 -0.44 4816.50 0.06 4817.00 0.56 4817.50 1.06 4818.00 1.56 Reach (ft) Channel Report Thursday, Sep 12 2024Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc. SD-4 Circular Diameter (ft)= 1.00 Invert Elev (ft) = 4816.44 Slope (%)= 1.50 N-Value = 0.025 Calculations Compute by:Known Q Known Q (cfs) = 0.39 Highlighted Depth (ft)= 0.28 Q (cfs)= 0.390 Area (sqft)= 0.18 Velocity (ft/s)= 2.14 Wetted Perim (ft) = 1.12 Crit Depth, Yc (ft) = 0.26 Top Width (ft)= 0.90 EGL (ft)= 0.35 0 1 2 3 Elev (ft)Depth (ft)Section 4815.50 -0.94 4816.00 -0.44 4816.50 0.06 4817.00 0.56 4817.50 1.06 4818.00 1.56 Reach (ft) 703 W Mendenhall September 2024 Project: 23-176.10 Bozeman Office . 32 Discovery Drive . Bozeman, Montana 59718 . Ph: (406) 582-0221 . Fax: (406) 582-5770 APPENDIX B – Stormwater Drainage Plans Civil EngineeringGeotechnical EngineeringLand Surveying32 DISCOVERY DRIVEBOZEMAN, MT 59718PHONE (406) 582-0221FAX (406) 582-5770www.alliedengineering.comDiverse Proj e c ts Solutions that Las t SHEET703 W MENDENHALLSTORM DRAIN DETAILBOZEMAN, MTC2.2703 W MENDENHALLSTORM DRAIN SECTION Civil EngineeringGeotechnical EngineeringLand Surveying32 DISCOVERY DRIVEBOZEMAN, MT 59718PHONE (406) 582-0221FAX (406) 582-5770www.alliedengineering.comDiverse Proj e c ts Solutions that Las t SHEET703 W MENDENHALLSTORM DRAIN DETAILBOZEMAN, MTC2.3703 W MENDENHALLSTORM DRAIN SECTION Civil EngineeringGeotechnical EngineeringLand Surveying32 DISCOVERY DRIVEBOZEMAN, MT 59718PHONE (406) 582-0221FAX (406) 582-5770www.alliedengineering.comDiverse Proj e c ts Solutions that Las t SHEET703 W MENDENHALLSTORM DRAIN DETAILBOZEMAN, MTC2.4703 W MENDENHALLSTORM DRAIN SECTION Civil EngineeringGeotechnical EngineeringLand Surveying32 DISCOVERY DRIVEBOZEMAN, MT 59718PHONE (406) 582-0221FAX (406) 582-5770www.alliedengineering.comDiverse Proj e c ts Solutions that Las t SHEET703 W MENDENHALLSTORM DRAIN DETAILBOZEMAN, MTC2.5703 W MENDENHALLSTORM DRAIN SECTION Civil EngineeringGeotechnical EngineeringLand Surveying32 DISCOVERY DRIVEBOZEMAN, MT 59718PHONE (406) 582-0221FAX (406) 582-5770www.alliedengineering.comSHEET703 W MENDENHALLSTORM DRAIN DETAILBOZEMAN, MTC2.6703 W MENDENHALLSTORM DRAIN DETAILDETAIL1C2.6SAND OIL SEPARATOR Civil EngineeringGeotechnical EngineeringLand Surveying32 DISCOVERY DRIVEBOZEMAN, MT 59718PHONE (406) 582-0221FAX (406) 582-5770www.alliedengineering.comSHEET703 W MENDENHALLSTORM DRAIN DETAILBOZEMAN, MTC2.7703 W MENDENHALLSTORM DRAIN DETAILDETAIL1C2.7TRENCH DRAINDETAIL2C2.7DOWNSPOUT AND GRATE INLETDETAIL3C2.7TRENCH DRAIN AND VALLEY GUTTER 703 W Mendenhall September 2024 Project: 23-176.10 Bozeman Office . 32 Discovery Drive . Bozeman, Montana 59718 . Ph: (406) 582-0221 . Fax: (406) 582-5770 APPENDIX C – Adjacent Well Logs