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HomeMy WebLinkAbout21 - Appendix P - Stormwater Design ReportEngineering Report Stormwater Design South University District Phase 3 – Block 1 Updated – August 2024 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 Stormwater Plan 1 8/5/2024 Engineering Report Table of Contents General Information and Design Criteria Existing Site Conditions Storm Drainage Plan Description Estimation of Runoff Retention Facilities Groundwater Monitoring Groundwater Impact Analysis Storm Event Analysis Conveyance Capacity Facilities Maintenance Figures and Tables Figure 1. Vicinity Map Figure 2. Stormwater Plan Table 1. Storm Runoff Calculations Table 2 Stormwater System Capacity (Whole Site, Basins 1-4) Table 3 Groundwater Impact on System Capacity Analysis Table 4 Bozeman Rainfall Analysis (1990-2020) Table 5 Curb and Gutter Conveyance Calculations Table 6 Storm Drain Conveyance Calculations Stormwater Plan 2 8/5/2024 General Information and Design Criteria The proposed South University District Phase 3 Block 1 development is located southeast of the intersection of 19th Avenue and Kagy Blvd. This development will consist of 273 residential units on an 8.3-acre lot. This project consists of 3 different building types including three flat-style apartments, one clubhouse and one commercial building. The intent of this project is to bring 273 high-quality residential units and a commercial building to a growing part of town adjacent to the university. This report seeks feedback on the conceptual design and associated site improvements. Stormwater mitigation is achieved by using a compact, efficient stormwater management design utilizing Low Impact Design (LID) methodology to mitigate the impacts of stormwater runoff. The LID stormwater mitigation has been designed in accordance with City of Bozeman Design Standards and Specifications Policy for retention/infiltration facilities. Survivability of LID systems through conventional development is difficult. Erosion control Best Management Practices (BMP’s) will be utilized during construction to reduce the impacts of construction site runoff, until the LID systems are constructed. Plans are provided showing proposed grading, drainage flow paths, and stormwater retention/infiltration details and locations. The Owner /Developer’s contact information is: Alpenglow of Bozeman LLC Jim Rooney 13167 County Hwy 00 Chippewa Falls, WI 54729 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 The location of the South University District Phase 3, Block 1 Development is shown on the Vicinity Map in Figure 1. Stormwater Plan 3 8/5/2024 Figure 1. Vicinity Map, Bozeman, Montana Existing Site Conditions SUD Phase 3 Block 1 is a block parcel created by the South University District Phase 3 Major Subdivision. The SUD Block 1 development site was previously used agriculturally prior to its subdivision development. The existing ground slopes north at grades between 1-3%. Existing offsite drainage is minimal, if present it would flow west to an existing irrigation ditch on the east side of 19th Avenue right-of-way, east to Mandeville Creek or north to an irrigation ditch on the south side of Kagy Boulevard, which then either infiltrates or makes its way to Mandeville Creek. Block 1 cannot utilize Mandeville Creek as an outfall due to the topography, creek location and adjacent properties. The Block 1 Development will have its own on-site retention/infiltration facilities. Stormwater Plan 4 8/5/2024 A geotechnical evaluation was conducted and is provided in a separate appendix of this submittal. This site is typical of Bozeman with fine-grained soils overlying gravel deposits. Geotechnical borings have determined the depth to gravel ranges from 3.5 to 5 feet below existing grade. The underlying gravelly sandy material will provide long- term infiltration of stormwater. Groundwater monitoring was conducted in 2023 during the high groundwater season for the SUD Phase 3 subdivision development. Groundwater monitoring found groundwater depths ranged from 6.5’ to 10.0’ below existing grade in the areas of the site proposed for development. The site is not located within a 100-year floodplain. Storm Drainage Plan Description The storm drainage plan for the SUD Phase 3 Block 1 development consists of four (4) subsurface infiltration systems and fourteen (14) at-grade bio-swale infiltration systems distributed evenly throughout the project site and are designed to capture and infiltrate stormwater in a manner that more closely replicates the natural hydrology of the site. The overall storm drainage plan for Block 1 is depicted in attached Figure 2. This plan shows proposed drainage basin areas, flow paths, and retention/infiltration system locations. In general, grades will be sloped away from buildings towards the center of the site. The sub drainage areas typically drain towards the center of the site via curb and gutter, and sloping of the parking and drive aisles. The runoff is conveyed to inlets, bioswales and storm drains connected to subsurface retention/infiltration systems located within the drive aisles or landscape areas. Prior to infiltration into the native gravels below the systems, stormwater runoff will be treated through sediment removal in the pretreatment chambers or in the bioswales. Where feasible, each subsurface infiltration system was sized to capture and infiltrate the design storm event to reduce the chances of overloading down gradient stormwater infrastructure. Stormwater runoff will bypass retention systems once they are full and be conveyed to the next down- gradient storm system that has capacity remaining. The site will be divided into 6 sub- drainage areas as shown in attached Figure 2. Stormwater Plan 5 8/5/2024 Estimation of Runoff Runoff estimates were obtained for each sub-drainage area using City of Bozeman standards. The site’s stormwater facilities have been sized to fully capture the 10-year, 2-hour storm event. The Bozeman IDF (Intensity, Duration, and Frequency) curves show a rainfall intensity of 0.41 in/hour for the 10-year, 2-hour storm, totaling a 0.82- inch storm event. Table 1 shows the storm runoff calculations for the 6 sub-drainage basins. Specifically, the design uses the volume of the 10-year, 2-hour storm for retention calculations. Table 1 - Storm Runoff Calculations Drainage Area Characteristics Whole Site 1 2A 2B 3A 3B 4 Area (ft²) 357122 61016 29767 8597 74533 11696 171513 Area (acre) 8.20 1.40 0.68 0.20 1.71 0.27 3.94 Pervious Area (ft²) 71424 12203 5953 1719 14907 2339 34303 Impervious Area (ft²) 285698 48813 23814 6878 59626 9357 137210 Pervious ( C ) 0.2 0.2 0.2 0.2 0.2 0.2 0.2 Impervious ( C ) 0.9 0.9 0.9 0.9 0.9 0.9 0.9 Weighted ( C ) 0.76 0.76 0.76 0.76 0.76 0.76 0.76 Time of Concentration (min) 5 5 5 5 5 5 5 Runoff Volume (cf) 10-year 2-hr storm 18547 3169 1546 446 3871 607 8907 Flowrate Q (cfs) 25yr 5min storm 23.839 4.073 1.987 0.574 4.975 0.781 11.449 Retention Facilities The site is divided into 6 sub-drainage areas, each with its own sub-surface infiltration system or bioswale system designed to capture and infiltrate the design storm event. Sub-Drainage Areas Each sub-drainage area has a subsurface or bioswale infiltration system which is designed to capture and infiltrate stormwater runoff from hardscapes, buildings, and landscaped areas. The underground infiltration systems are approximately 2.5 feet in depth starting approximately 2 feet below finished grade and extend down to the native gravel soil horizon. Storage media is comprised of a minimum of 3.5 feet of crushed gravel with a minimum of 6 inches at both the top and bottom of the system. Each retention system is hydraulicly connected to the down gradient retention system via storm piping or overland grading flow paths. Each system is designed to allow for surfacing bypassing via valley gutters or curb and gutter drainage to be captured by a down gradient retention system. Stormwater Plan 6 8/5/2024 Table 2 below shows that sub-drainage areas meet the retention of the 10-year, 2-hour storm event. Table 2 - Stormwater System Capacity (Basins 1-4) Drainage Area Characteristics Whole Site 1 2A 2B 3A 3B 4 Runoff Volume (cf) 10-year 2-hr storm 18547 3169 1546 446 3871 607 8907 Total Storage (cf) 27,518 5,355 1,656 464 4,878 2,211 12,954 Storm Event Captured (in) 1.22 1.39 0.88 0.85 1.03 2.99 1.19 Groundwater Impact Analysis Groundwater monitoring data was collected weekly in 2023 between March 31st and July 5th. Table 3 below shows the chamber bottom elevations for each system in relation to the extrapolated seasonally high groundwater elevation at that retention system’s location. As shown in Table 3, the separation between chamber bottom and seasonally high groundwater ranges from 3.7 feet to 10.3 feet with a site system average of 7.4 feet of separation. Ideally, a separation of 3 feet from seasonally high groundwater is preferred to ensure long-term survivability and performance of the retention system. The site stormwater facilities all provide excess of 3 feet if separation from the extrapolated seasonally-high groundwater observed. Table 3 – Groundwater Impact on System Capacity Analysis Storm Facility ID Chamber Bottom Elevation Seasonal High GW at Chamber GW Separation FT Reduced Storage Capacity (3ft above High GW) 1 4916.78 4906.50 10.3 NA 2A 4918.83 4911.78 7.1 NA 3 4918.51 4908.27 10.2 NA 4A 4923.74 4920.06 3.7 NA 4B 4921.95 4917.48 4.5 NA TOTAL AVG 7.1’ Storm Event Analysis A storm event analysis was previously conducted to determine the percentile storm events in the City of Bozeman. The exercise analyzed 30 years of recorded 24-hour rainfall events using precipitation data collected at MSU rain gauge Station USC00241044. The analysis excluded snow fall events and 24-hour rainfall events that Stormwater Plan 7 8/5/2024 accumulated less than 0.1-inches. Table 4 below summarizes the percentile storm events produced by the analysis. As indicated by the table, the site stormwater facilities as designed would retain a storm volume in excess of the 95th percentile storm event. Table 4- Bozeman Rainfall Analysis (1990-2020) Percentile Storm Event Volume (in) 80th 0.41 85th 0.49 90th 0.57 95th 0.74 99th 1.2 100th 2.29 Conveyance Capacity As designed, each retention system has the capacity to completely capture the 100- year 5-minute (0.44 inch) storm event. The distributed retention systems throughout the site reduce the likelihood of overloading any single conveyance system. Each retention system has a primary overflow and a secondary overflow. Each overflow has adequate capacity to convey the designed high intensity storm event without overloading the system. The stormwater retention systems work in tandem to reduce the compounding effects that come with a single large drainage basin. The site conveyance systems were each individually analyzed to ensure adequate capacity exists in the event there is a failure within the system (i.e. clogging). Curb and Gutter Stormwater runoff in most sub-drainage basins will be collected in curb and gutter on the site to direct runoff into the underground storm pipe network. To ensure the standard curb design will provide adequate conveyance capacity for peak flows, curb and gutter with the greatest flows and least amount of redundant or secondary storm conveyance options was identified and analyzed for conveyance capacity. The critical curb required to convey the maximum flow was identified as the approximately 184’ length of curb running from east to west along the north side of Aisle “1” on the north end of the site, serving a catchment area of approximately 27076 sf (ie Most of Basin 2A Drainage Area). This catchment area produces a peak flow of 1.81 cfs for the 25-year-5-minute design storm. As shown in Table 5 below, at the minimum slope of 0.6%, the curb on site has a Gutter Spread of 0.22ft during a flow of 1.81cfs. Stormwater Plan 8 8/5/2024 Table 5 - Curb and Gutter Conveyance Calculations Curb and Gutter Capacity Calcs Max Curb Conveyance Req'd (ft^3/sec) 1.81 Manning's Roughness 0.013 Cross Slope (ft/ft) 0.06 Longitudinal Slop (ft/ft) 0.006 Gutter Spread (ft) 0.22 Storm Drains The hydraulic capacity of the storm drains were analyzed to determine the required pipe size to convey the 25-year, 5-minute storm event. The maximum 25-year event for any of the basins draining to a 12” storm pipe produces approximately 1.90 cfs of runoff. The capacity of a 12” storm drain at 0.5% slope is 2.58 cfs at 90% full pipe flow. Therefore, 12” storm drains have the capacity to convey the 25-year peak runoff of any drainage basin from the site to the retention systems. The maximum 25-year event for any of the basins draining to a 15” storm pipe produces approximately 1.30 cfs of runoff. The capacity of a 15” storm drain at 0.5% slope is 4.68 cfs at 90% full pipe flow. Therefore, 15” storm drains have the capacity to convey the 25-year peak runoff of any onsite defined drainage areas. The maximum 25-year event for any of the basins draining to a 24” storm pipe produces approximately 3.80 cfs of runoff. The capacity of a 24” storm drain at 0.5% slope is 16.39 cfs at 90% full pipe flow. Therefore, 24” storm drains have the capacity to convey the 25-year peak runoff of any onsite defined drainage areas. Table 6 - Storm Drain Conveyance Calculations Manual Input Parameters Pipe Diameter (in) 12 15 24 Flow Depth in Pipe (in) 10.12 12.65 20.25 Coefficient of Roughness (n) 0.013 0.013 0.013 Slope (ft/ft) 0.005 0.005 0.005 Calculated Parameters Radius (in) 6 7.50 12 Radius (ft) 0.50 0.63 1.00 Flow Depth in Pipe (ft) 0.84 1.05 1.69 Wetted Perimeter (ft) 2.33 2.91 4.66 Area of Flow (sq ft) 0.71 1.10 2.83 Hydraulic Radius (ft) 0.30 0.38 0.61 Average Velocity (ft/sec) 3.7 4.2 5.8 Flowrate (cfs) 2.58 4.68 16.39 Stormwater Plan 9 8/5/2024 Facilities Maintenance The proposed stormwater mitigation systems are located within private rights-of-ways and will be operated and maintained by the owners’ association. A detailed operations and maintenance plan has been developed and included in the final covenants. Applicable portions of the Block 1 POA CCRs Stormwater Maintenance plan is included as an attachment to this report. At a minimum, the owners’ association is responsible for quarterly inspection and annual maintenance of all stormwater facilities. The inspection and maintenance responsibilities include, but are not limited to, general housekeeping responsibilities, visual inspection for performance, removal of sediment from mitigation facilities, vegetative maintenance, as well as any necessary repairs to the facilities themselves. FOPTFOPTFOPTFOPTFOPTSDSDSDSDSDSDSDSDSDSD SD SDFOPTFOPTFOPTFOPTFOPTFOPTFOPTFOPTFOPTFOPTSDSDSDSDSDSDSDSDSDSDSDSD SD SD SD SD SD SD SD SD SD SD SDSDSDSDSD SDSDSDSDSDSD SD SD SD SD SD SD SD SD SD SD SD SD SD SD O.S 1O.S 2AISLE "C"AISLE "A"AISLE "B"AISLE "C"AISLE "6"AISLE "D"MAIL LOADINGZONE12A3A4AISLE "1"AISLE "2"AISLE "3"AISLE "4"AISLE "5"12B3A44NO DIVING NO DIVING NO DIVING NO DIVINGNO DIVINGNO DIVING NO DIVINGNO DIVINGNO DIVING NO DIVINGPOOL >>>>>>>>>>>>>>>>>>3BEV STATIONEV STATIONEV STATIONEV STATION O.S 3COMMERCIALBUILDINGBUILDING 3BUILDING 1BUILDING 2BUILDING 3CLUBHOUSEFIGURE 2STORMWATERFIGURE1 3/31/2023 CONCEPT SITE PLAN ZWL CDP 2 9/29/2023 SITE PLAN SUBMITTAL ZWL CDP 3 1/12/2024 RC SITE SUBMITTAL ZWL CDP 4 5/31/2024 RC 2 SITE SUBMITTAL ZWL CDP 5 8/1/2024 BUILDING PERMIT SUBMITTAL ZWL CDP 6 8/9/2024 RC 3 SITE PLAN SUBMITTAL ZWL CDP PROFESSIONALENGINEERS &SURVEYORSSTAHLYENGINEERING& ASSOCIATESALPENGLOW APARTMENTS SUD PH3 BLOCK 1 ALPENGLOW OF BOZEMAN, LLC