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Home My WebLink About17 - Design Report - Story Mill Community Park - Stormwater Storm Water Plan Story Mill Community Park Infrastructure May, 2017 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 ��1NEE�f�c STAH LY -Ne • CORDELL D. OL • No.P0fi45P_E Storm Water Plan Contents General Information Existing Conditions and General Design Criteria Storm Drainage Plan Description Estimation of Runoff Post-development Runoff Storage/Treatment Facilities Facilities Maintenance Figures and Tables Table 1 Runoff Calculations Table 2 Retention Calculations Figure 1 Basin Exhibit Storm Water Plan 1 5/15/2017 A_ General Information The Story Mill Community Park is a redevelopment of areas located south of and adjacent to Bridger Drive in Bozeman, Montana. The park is approximately 60.24 acres and will include a Community Center, amphitheater, events lawn, playground, pavilions, trail system, dog park, river access, and new parking areas. Existing Conditions and General Design Criteria The park site is currently the home of the Boys & Girls Club, an abandoned mobile trailer park, and open space. The Boys & Girls Club includes an activities building, large lawn area, parking lot, and driveway. The abandoned trailer park consists of roads, driveways, and open areas. In accordance with the City of Bozeman Design Standards and Specifications Policy (COB Standards) an engineering analysis has been conducted for Story Mill Park property to limit post-development runoff to pre-development rates. Most of the high-intensity park uses are located in the northern portion of the park. Even though the East Gallatin River flows through the park, the existing topography flows away from the river to the north. Since this would direct stormwater off-site onto adjacent properties, storm water runoff is mitigated through the use of on-site retention. In addition to the COB Standards all storm water within Story Mill Community Park will be mitigated through green storm water management techniques. Most park improvements such as sidewalks, playground, and pavilions will be located adjacent to lawn areas without storm water collection. Storm runoff from these low-density improvements will be mitigated by infiltration into adjacent park land. For this reason storm water mitigation is only proposed for parking lots and hardscapes where runoff will be collected by curb and gutter. Storm Drainage Plan Description The overall storm drainage plan for Story Mill Park is depicted on Figure 1 at the end of this report. Figure 1 shows the individual drainage basins and their adjacent retention areas. Storm water collection is typically by surface flow in curb and gutters, or in the case of Griffin Drive in roadside ditches. Storm water treatment and retention will be primarily mitigated by bio-swales (bioretention) located downgradient of impervious areas. Bio-swales will typically treat the smaller more common storm events (<0.5"). Where site grades and space allows larger storm events will be directed to graded lawn areas in the park. These areas were selected where occasional shallow ponded water will not interfere with park uses or overflow off-site. This accomplishes a beneficial use of storm water to provide supplemental irrigation to park natural areas. In locations where Storm Water Plan 2 5/15/2017 this is not feasible bio-swales are sized to provide retention of larger storm events. Estimation of Runoff Post development runoff estimates are provided for the 0.5" storm, the 10-year 2- hr storm (City of Bozeman required mitigation), and the 100-year 24-hr storm. The storm water volume for the 0.5" storm was estimated using a weighted runoff C factor and areas for each individual basin. The rational method (COB Standards — Equation I.C.5) was used to estimate the runoff volume for the 10-year 2-hr storm. Standard assumptions in using the rational method include that rainfall is uniformly distributed over a watershed for the duration of a storm event, the peak runoff rate occurs when the storm duration equals the time of concentration, and the runoff coefficient for a watershed is constant for similar land use. A rainfall intensity of 0.41 in/hr (COB Standards — Figure 1-2, 1-3 for 10-year 2-hr) was used for runoff estimates. The 100-year 24-hr storm water volume for each basin was estimated by using SCS Hydrographs. A total rainfall of 3" was used for the total precipitation for the storm. Post-development Runoff The individual basin areas and characteristics were estimated and used for runoff calculations. Open land and lawn areas were given a runoff coefficient (C) of 0.2 and a SCS curve number (CN) of 70. Impervious areas were given a C of 0.9 and a CN of 98. Storm Water Plan 3 5/15/2017 Table 1: Runoff Calculations Basin Characteristics 1 2 3 4 5 Area ft2 40936 6929 45553 12350 9089 Area acre 0.94 0.16 1.05 0.28 0.21 Pervious Area ft2 10518 0 2672 5043 0 Impervious Area ft2 30418 6929 42881 7307 9089 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 0.72 0.90 0.86 0.61 0.90 Impervious CN 98 98 98 98 98 Pervious CN 70 70 70 70 70 Weighted CN 91 98 96 87 98 Runoff Volume 10 r-2hr Q cfs 0.277 0.059 0.368 0.071 0.077 0.5" Rainfall volume ft3 1228 260 1630 316 341 10 r-2hr ft3 1998 423 2652 514 554 100 r 24hr [3.0 in] ft3 6626 1507 9119 1657 1978 Storage/Treatment Facilities Storm water is first routed to bio-swales that have a 30% porosity in their treatment media. The water captured by the bio-swales is treated and infiltrated in place. The design of the bio-swales also allows for ponding, above the bio- swale media, which acts as a retention area for runoff for larger storms. Runoff that is in excess of the bio-swale capacity is then routed to adjacent graded lawn areas that serve as further retention areas for the largest storms. Table 2: Retention Calculations Basin Characteristics 1 2 3 4 5 Bio-Swale Area ftz 2925 616 1724 933 620 Bio-Swale Porosity % 30% 30% 30% 30% 30% Bio-Swale Volume ft3 1755 V370 1034 V560 372 Bio-swale pondin Vol ft3 2616 496 1367 794 509 Lawn retention Area ft2 2992 1069 20488 0 1441 Lawn retention Vol ft3 1995 713 13659 0 961 Total Retention ft3 6366 1578 16060 1354 1842 All retention volumes exceed the City of Bozeman requirements. Storm Water Plan 4 5/15/2017 Facilities Maintenance The proposed storm water retention facilities will be operated and maintained by the City of Bozeman Parks and Recreation Department. Due to this constraint the storm water mitigation plan focused on low maintenance facilities. Since the storm water collection is only surface flow and the mitigation is by bio-swale and graded lawns, the maintenance requirements are in-line with typical park landscape maintenance. Typical maintenance requirements are keeping the parking lots and curb and gutters free of debris. The bio-swale maintenance is similar to any planting bed and excess vegetation should be removed at the end of the growing season. The graded lawns are typically located in the natural vegetation zones and should not require any special maintenance. Storm Water Plan 5 5/15/2017