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HomeMy WebLinkAbout04_Bzm_Sports_Park_Ph2_Storm_Drainage_Engineer's_ReportEngineering Report Stormwater Design Bozeman Sports Park - Phase 2 Updated – April 2025 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 4/21/2025 Engineering Report Table of Contents General Information and Design Criteria Existing Site Conditions Storm Drainage Plan Description Estimation of Runoff Retention Facilities and Sub-Drainage Areas 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 Table 3 Groundwater Monitoring Depths Table 4 Groundwater Impact on System Capacity Analysis Table 5 Bozeman Rainfall Analysis (1990-2020) Stormwater Plan 2 4/21/2025 General Information and Design Criteria The Bozeman Sports Park is located northeast of the intersection of Cottonwood Road and West Oak Street. The Bozeman sports park is approximately 80 Acres. Phase 2 of the development will be constructed on the south quarter of the site (see vicinity map in figure 1). This project intent is to provide additional access to sports in the community and for the adjacent high school, by providing additional sports fields and associated parking along with desired amenities. Stormwater mitigation is achieved by using a single infiltration retention basin to minimize the impacts of stormwater runoff. The stormwater mitigation has been designed in accordance with City of Bozeman Design Standards and Specifications Policy adopted in March 2004 for retention/infiltration facilities. Erosion control Best Management Practices (BMP’s) will be utilized during construction to reduce the impacts of construction site runoff, until the storm 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: City of Bozeman Parks and Recreation Department P.O. Box 1230 Bozeman, MT 59771 Stormwater Plan 3 4/21/2025 The location of the Bozeman Sports Park is shown on the Vicinity Map in Figure 1. Figure 1. Vicinity Map, Bozeman, Montana Existing Site Conditions Bozeman Sports Park was previously used as irrigated agriculture prior to development. The site slopes generally to the north-west at a 1% to 2% slope and contains a ditch that flows to the north running along the eastern site boundary. Existing offsite drainage is minimal if it exists at all. Phase 1 of the Bozeman Sports Park has been previously developed and is located on the north third of the park. Phase 1 resulted in the development of 2 artificial turf athletic fields, 4 natural turf athletic fields and hard surfaced parking areas with supported sidewalks. The Phase 1 development drains into stormwater ponds located on the north-east and north-west corners of the site. A geotechnical evaluation was conducted and is provided in a separate appendix of this submittal. This site has a lean clay layer resting on a gravely sand layer. The lean clay ranges from 1.2 to 10 feet below existing grade as determined through test pits. Estimated groundwater elevation was determined to be 4 feet below existing ground. Stormwater Plan 4 4/21/2025 Storm Drainage Plan Description The storm drainage plan for the Bozeman Sports Park development consists of one infiltration retention pond sized to capture the runoff from the developed impervious surfaces. The storm system is designed to capture and infiltrate stormwater in a manner that closely replicates the natural hydrology of the site. The overall storm drainage plan for the Sports Park is depicted in attached Figure 2. This plan shows proposed drainage basin areas, flow paths, and retention/infiltration system locations. The general grading for the site will be to the north-west. The parking lot is graded to the north-east causing the stormwater to flow into the infiltration retention pond located on the north-east end of the parking lot. The sub-drainage areas typically drain towards the retention pond via curb and gutter and sloping of the parking and drive aisles. The runoff is conveyed overland into the retention pond. 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 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 2 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 Basin 1 Basin 2 Area (ft²) 108496 69491 39005 Area (acre) 2.49 1.60 0.90 Runoff Coefficient (C) 0.8 0.8 0.8 Time of Concentration (min) 5 5 5 Runoff Volume 10yr-2hr Storm (ft³) 5859 3753 2106 Retention Facilities and Sub-Drainage Areas The site is divided into 2 sub-drainage areas, that will both be drained into the same retention basin designed to capture and infiltrate the design storm event. Figure 2 displays 2 areas that are depicted as “not-runoff generating area”. These areas are designed to have no impact from the existing runoff conditions. These areas will meet the pre-construction runoff by either; being undisturbed, or by the stormwater from the artificial sports fields being drained into the site soils located under the fields. Sub- drainage area 1 is to be constructed in phase 2 and will drain into the retention basin upon completion of phase 2 construction. Sub-drainage basin 2 is a future phase basin Stormwater Plan 5 4/21/2025 and is calculated within the current phase to ensure the retention basin is sized adequately to accommodate future developments. The retention basin is to be 1.5 feet deep and retain storm water to a depth of 1.5 feet. Table 2 below shows that sub-drainage areas for the site meet the retention of the 10- year, 2-hour storm event. Table 2 : Stormwater System Capacity Drainage Area Characteristics Whole Site Required Storage Volume (ft³) 5859 Design Storage Volume (ft³) 11303 Storm Event Captured (in) 1.56 Groundwater Impact Analysis Groundwater was studied using open water sources that surround the site. Maynard Ditch that flows north along the east side of the site has a 4-foot drop from existing ground to the surface of the running water. The water surface in the ditch is a representative marker of groundwater elevation. Groundwater monitoring is being conducted in an existing on site well and will be continuously monitored to confirm the groundwater levels are matching the current design elevations. Current groundwater observations can be seen in the table 3 below. The current observed depth to groundwater is expected to rise as much as 1.5’ as groundwater elevations peak. Due to the water elevation in the ditch with relation to the existing ground, site groundwater is predicted to be located 4 feet below existing grade across the site. The impact of the anticipated seasonal high groundwater on the storm water retention pond can be seen in table 4. Table 3: Groundwater Monitoring Depths Ground Elevation Groundwater Elevation Depth to Groundwater (FT) DATE 4743.18 4737.50 5.68 3/24/2025 4743.18 4737.41 5.77 3/28/2025 4743.18 4737.33 5.85 4/1/2025 Table 4 : Groundwater Impact on System Analysis Storm Facility ID Bottom/Invert Elevation Seasonal High GW at Chamber GW Separation FT Retention Pond 4740.30 4737.30 3.0 Stormwater Plan 6 4/21/2025 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 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 99th percentile storm event. Table 5- 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, the retention pond has the capacity to completely capture and infiltrate in excess of the 10-year 2-hour (.81 inch) storm event. Overflow of the retention pond will result in the stormwater being conveyed into the sports fields north of the retention pond. Secondary overflow will be the storm system that is piped running north down the center of the site that collects stormwater from under the artificial turf fields. The piped storm drains to the retention ponds on the northeast and northwest corners of the site. The retention ponds on the north side of the site are sized to retain the storm flows of the entire site. The site conveyance systems was analyzed to ensure adequate capacity in the event there is a failure within the system (i.e. clogging). Facilities Maintenance The periodic maintenance of the stormwater systems is necessary to keep the system operating within the intended design limitations. The stormwater on the site is considered a private storm system operated by the City of Bozeman. At a minimum the storm facilities are to be visually inspected once a month. 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. Maintenance to ensure adequate operation of the stormwater system will involve the removal of any trash and debris to prevent clogging within the system. Sediment Stormwater Plan 7 4/21/2025 removal along flow paths will be required to keep the system operating at its intended flow capacities. The retention pond will require inspections and sediment removal will be required once sediment has accumulated and is covering the grass at the bottom of the pond. Weed control will be required within the retention pond. 12W OAK STREETCOTTONWOOD ROAD0NOT RUNOFFGENERATING AREA0NOT RUNOFFGENERATING AREAFIGURE 2STORMWATERFIGURE1 4/22/2025 SITE PLAN SUBMITTAL ZWL JWF PROFESSIONALENGINEERS &SURVEYORSSTAHLYENGINEERING& ASSOCIATESBOZEMAN SPORTS PARK PHASE 2 CITY OF BOZEMAN GREAT FALLS Phone:(406)604-4644BOZEMAN Phone:(406)522-9526CODY Phone:(307)509-5541BILLINGS Phone:(406)601-4055HELENA Phone:(406)442-8594