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HomeMy WebLinkAbout13 Preliminary Stormwater Engineer's Report 03122021Preliminary Engineering Report Stormwater Design Community First Griffin Place March 2021 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 3/16/2021 Engineering Report Table of Contents General Information and Design Criteria Existing Site Conditions Storm Drainage Plan Description Estimation of Runoff Retention Facilities Storm Event Analysis Pipe Conveyance Capacity Facilities Maintenance Figures and Tables Figure 1. Vicinity Map Figure 2. Stormwater Plan Table 1. Storm Runoff Calculations Table 2. Retention Basin Capacities Table 3. Enhanced Infiltration Capacities Table 4. Stormwater Runoff Rate Calculations Table 5. Pipe Conveyance Capacities Table 6. Curb and Gutter Conveyance Calculations Table 7. Bozeman Rainfall Analysis (1990-2020) Appendices Stormwater Plan 2 3/16/2021 General Information and Design Criteria The proposed Community First Griffin Place is located southeast of the intersection of Manley Road and Griffin Drive. Community First Griffin Place is a planned two building development designed to provide a plethora of community focused programs and services focused on members of the community facing challenges with housing, hunger, and other hardships. The project is located on an undeveloped 5.24-acre lot. The proposed development includes two buildings, referred to as the North Building and the South Building. The North Building will function as a Food Resource Center, providing a new home for both the Gallatin Valley Food Bank as well as the Fork and Spoon, a “pay what you can” restaurant, while the South Building will provide emergency housing. Both buildings will contain additional office space for staff members. Stormwater mitigation is achieved by a using compact, efficient stormwater management design utilizing Low Impact Design (LID) methodology to keep stormwater runoff to pre-development levels. The LID stormwater mitigation will be designed in accordance with City of Bozeman Design Standards and Specifications Policy for retention/infiltration facilities. Plans are provided showing proposed grading, drainage flow paths, and stormwater retention/infiltration details. The developer’s contact information is: HRDC Heather Grenier 32 South Tracy Ave. Bozeman, MT 59715 Stormwater Plan 3 3/16/2021 Figure 1: Vicinity Map, Bozeman, Montana Existing Site Conditions The Community First Griffin Place project is located on the south side of East Griffin Drive at the intersection of Griffin Drive and Manley Road. The site is primarily undeveloped. The site was previously occupied by a single-family residence and associated outbuildings. They have since been demolished. The site is generally flat within the project limits, with some moderately sloped terrain around drainage ditches in the northeast of the site and along a tree stand running north and south across the property. An existing 18” culvert under Griffin Drive is the discharge point for stormwater runoff from the site. This culvert will be replaced as part of the Griffin Drive Improvements project scheduled for Summer 2021. The culvert conveys runoff via approximately 1000 lineal feet of storm main to the Northwestern Energy Detention Pond, ultimately discharging to Glen Lake. A geotechnical evaluation was conducted and is provided in a separate appendix of this submittal. The soils on the site are variable. Geotechnical investigation revealed that the central and western thirds of the site were historically used for ponds that have since been filled. Boring logs reveal encountering this highly variable fill material to a depth of approximately 19’. The eastern third of the site is predominantly native soils, with some limited fill encountered that is typically associated with demolition of historic structures. Full geotechnical evaluation results are available in the attached appendix. Stormwater Plan 4 3/16/2021 Groundwater monitoring was conducted throughout the 2020 groundwater season. During the initial site evaluation and monitoring well installation, groundwater depths ranged from 7.3’ to 13.3’ below grade in the areas of the site proposed for development. Long term monitoring results found groundwater wells dry in all visits, to a depth of approximately 9’ below grade. Based on these results, seasonally high groundwater is assumed to be at least 9’ below grade. Full groundwater monitoring results are available in the attached appendix. The site is not located within a 100-year floodplain. Storm Drainage Plan Description The storm drainage plan for the Community First Griffin Place consists of LID solutions designed to infiltrate stormwater and replicate the natural hydrology of the site. The overall storm drainage plan for Community First Griffin Place is depicted on Figure 2, attached to this report. The plan shows proposed drainage path flows, mitigation areas, and drainage basins. With development, the site will be divided into 4 primary basins, with one of these further divided into 2 sub-basins. Basin 1 contains the northwestern portion of the site, with a small sliver of catchment area extending along the drive access nearly the length of the site. Basin 2 contains the southern portion of the site, extending to the north edge of the South Building. This basin is split into two sub basins, as Basin 2A is captured in a small retention facility (referred to as the “south basin”) designed to capture stormwater runoff generated by the southern parking lot. Basin 3 is a relatively small basin in the northeast corner of the site, capturing runoff from the east drive access. Basin 4 includes the remaining area of the site, generally centered around the northern retention facility (referred to as the “north basin”) will runoff directly into the retention facility via overland flow. Storm runoff from all basins is directed to the north basin located in the northeast corner of the site. Runoff is routed through site grading, curb and gutter, a series of stormwater inlets connected through a network of storm sewer piping, and connection of building roof drains directly into the storm sewer network. The inclusion of the south basin serving Sub-basin 2 provides numerous hydraulic benefits, including reduction of peak flows in the conveyance system as well as overall runoff volume reduction. Both retention facilities will utilize enhanced infiltration to improve performance. 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. Stormwater Plan 5 3/16/2021 Estimation of Runoff Runoff estimates were obtained for each sub-basin using City of Bozeman standards. For the stormwater retention calculations, the design storm is a 10- year, 2-hour storm. 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 0.81-inch storm event. For the purposes of sizing conveyance facilities, the design storm is a 25-year event at a time of concentration of 5 minutes. This storm has an intensity of 3.826 in/hour, but due to its short duration, only represents a 0.32-inch event. Table 1 shows the storm runoff calculations for the 5 drainage sub-basins. Specifically, the design uses the volume of the 10-year, 2-hour storm (for retention calculations), and the flow rate from the 25-year event (for conveyance calculations). Table 1 – Storm Runoff Calculations Basin Characteristics Drainage Basin 1 Drainage Basin 2 Drainage Basin 3 Drainage Basin 4 Drainage Basin 5 Area (ft²) 90042 57087 29279 7570 44263 Area (acre) 2.07 1.31 0.67 0.17 1.02 Pervious Area (ft²) 31515 19980 10248 2650 15492 Impervious Area (ft²) 58527 37107 19031 4921 28771 Pervious C 0.2 0.2 0.2 0.2 0.2 Impervious C 0.9 0.9 0.9 0.9 0.9 Weighted C 0.66 0.66 0.66 0.90 0.66 Time of Concentration (min) 5 5 5 5 5 Runoff Volume (cf) 10-yr 2-hr storm 0.559 0.355 0.182 0.064 0.276 Flow Rate Q (cfs) 25-yr 5-min Storm 5.220 3.309 1.697 0.598 2.576 Retention Facilities The site contains 2 retention facilities for stormwater management. Site constraints dictate the use of the north basin as the primary retention facility, supported by the south basin facility that captures runoff from the largely impervious southern portion of the site. Overflow from the south retention basin facility discharges into the storm sewer system, where any overflow is conveyed to the north retention basin facility. Table 2 describes the characteristics of each retention basin and total site storage. Stormwater Plan 6 3/16/2021 Table 2 – Retention Basin Capacities Basin ID South Basin North Basin Contributing Drainage Basins 2 1, 2, 3, 4, 5 Top Area (sf) 1270 9155 Bottom Area (sf) 416 6208 Average Area (sf) 843 7681.5 Ponding depth (ft) 1 1.5 TOTAL Ponding Storage (cf) 843 11522.25 12365 Gravel bed surface area (sf) 193 1415 Gravel depth (ft) 2 2 Total Cube Volume (cf) 386 2830 3216 Gravel Porosity 0.3 0.3 Gravel Storage 115.8 849 965 Total Basin Storage Volume (cf) 959 12371 13330 Captured Basin Area Rain Event Captured (in) 0.31 1.09 1.06 Captured Rain Event Required (in) 1.00 Design Storm In addition to providing adequate ponding storage, both retention facilities will have enhanced infiltration. Approximately 25% of the surface area of the bottom of each retention basin will have a strip of gravel that extends to native gravel to facilitate infiltration. The south basin will feature a 2 ft wide strip of gravel running the length of inflow to outflow. The north basin will feature an 8 ft wide strip of gravel. This design provides superior infiltration as compared to a typical retention basin. The effects of enhanced infiltration have not been included in design values, so it is possible for both basins to provide greater mitigation performance than indicated in this design report. Table 3 – Enhanced Infiltration Calculations Detention Basin ID South Basin North Basin Total Storage Capacity (cf) 959 12371 Gravel Strip Surface Area (sf) 193.00 1415 Gravel Infiltration Rate (in/hr) 1.67 1.67 Time for Complete Infiltration (hr) 35.7 62.8 As shown in Table 3, the south basin will completely infiltrate in under 36 hours through the gravel alone. The north basin would completely drain in approximately 63 hours with no other infiltration taking place. Considering the remaining 75% of basin bottom surface area in each basin also facilitates infiltration, both basins will drain completely in under 48 hours following the design storm event. Stormwater Plan 7 3/16/2021 Conveyance Capacity Stormwater runoff in each sub-basin will be collected into a storm sewer system for conveyance to retention areas. The pipe network was designed to provide adequate conveyance capacity for the 25-year, 5-minute design storm event. Table 4 shows the storm runoff calculations for the 5 drainage sub-basins, specifically, the design uses the volume of the 10-year, 2-hour storm (for retention calculations), and the flow rate from the 25-year event (for conveyance calculations). Table 4 – Storm Runoff Rate Calculations Basin Characteristics 1 2 3 4 5 Area (ft²) 90042 57087 29279 7570 44263 Area (acre) 2.07 1.31 0.67 0.17 1.02 Weighted (C) 0.66 0.66 0.66 0.90 0.66 10yr-2hr Q (cfs) 0.559 0.355 0.182 0.064 0.276 25yr-5min Q (cfs) 5.220 3.309 1.697 0.598 2.576 Table 5 – Pipe Conveyance Calculations Manual Input Parameters pipe diameter (in) 15 15 depth in culvert (in) 14.00 14.00 coefficient of roughness (n) 0.013 0.013 slope (ft/ft) 0.0050 0.0070 Calculated Input Parameters radius (in) 7.50 7.50 radius (ft) 0.63 0.63 depth of flow (ft) 1.17 1.17 wetted perimeter 3.27 3.27 area of flow (ft2) 1.19 1.19 hydraulic radius 0.36 0.36 Avg. Velocity (ft/sec) 4.1 4.9 Max Velocity (ft/sec) 6.2 7.3 Calculated Operating Parameters Flowrate culvert (cfs) 4.91 5.81 The hydraulic capacity of the site storm drains was 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 produces approximately 5.75 cfs of runoff. Analysis was performed at a minimum slope of 0.5%. The capacity of a 15” storm drain at 0.5% slope at 90% flow is 4.91 cfs. For any pipes requiring greater Stormwater Plan 8 3/16/2021 conveyance capacity, a 15” pipe with a slightly steeper slope will be used. A 15” pipe at a 0.7% slope will convey 5.81 cfs at 90% capacity. Table 6 – Curb and Gutter Conveyance Calculations Curb and Gutter Capacity Calcs Max Curb Conveyance Req'd (ft^3/sec) 0.391 Right-side Slope X:1 0.06 Left-side Slope X:1 33.00 Channel Bottom Width (ft) 0 Flow Depth (ft) 0.350 Flow Area (ft^2) 2.025 Wetted Perimeter (ft) 11.906 Width 0.044 Hydraulic Radius (ft) 0.170 Manning's Roughness 0.013 Slope (ft/ft) 0.005 Average Velocity (ft/sec) 2.49 Flow (ft^3/sec) 5.04 Curb and gutter are used on the site to direct runoff into the underground storm sewer network. To ensure the standard curb design will provide adequate conveyance capacity for peak flows, the length of curb with the greatest flow was identified and analyzed for conveyance capacity. The critical curb required to convey the maximum flow was identified as the approximately 435’ length of curb channeling runoff to STMH-7, serving a catchment area of approximately 4,950 sf. This catchment area produces a peak flow of 0.391 cfs for the 25-year design storm. As shown in Table 6 above, At the minimum slope of 0.5%, the curb on site has a conveyance capacity of 5.04 cfs. Stormwater Plan 9 3/16/2021 Storm Event Analysis To get a sense of how the site stormwater facilities would have performed in past storm events, a storm event analysis was 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 accumulated less than 0.1-inches. The table below summarizes the percentile storm events produced by the analysis. As indicated by the table below, the site’s stormwater system will retain the 97th percentile of storms on site. Table 7 – 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 Facilities Maintenance The proposed stormwater mitigation systems are located within public rights-of- ways and common open spaces and will be operated and maintained by the owners’ association. A detailed operations and maintenance plan will be developed and will be included in the final covenants. 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. GRIFFIN DRIVEPROJECT SITE5.24 ACRESNORTH BUILDING27,086 SF FOOTPRINT202 E. GRIFFIN DR.SOUTH BUILDING14,500 SF FOOTPRINT206 E. GRIFFIN DR.MONTANA RAIL LINKPHASE 1PHASE 2DRAINAGE BASIN 1DRAINAGEBASIN 2DRAINAGE BASIN 3DRAINAGEBASIN 5DRAINAGEBASIN 4DRAINAGE BASIN AREASDRAINAGE BASIN IDAREA (SF)12345900425708729279757044263INDICATES STORMRUNOFF FLOW PATHFIGURE 2STORMWATERFIGUREPROFESSIONALENGINEERS &SURVEYORSCOMMUNITY FIRST GRIFFIN PLACE STAHLYENGINEERING& ASSOCIATESHRDC 202 & 206 EAST GRIFFIN DR. BOZEMAN, MT