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Home My WebLink About18 - Design Report - South University District Ph 1B - Stormwater ApPrs,&L ;'�/t//d Storm Water Design Report South University District — Phase 1 B Subdivision April, 2018 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 ��NEE�jyc STAHLY e 1970 AffDC1 R'f�S�� Engineer of Record: Mark Juras, PE Quality Control Reviewer: y P..►,.( Cordell Pool, PE Storm Water Plan Contents Description of Phase 1 B Project General Information Existing Conditions and General Design Criteria Storm Drainage Plan Description Estimation of Runoff Pre-development Runoff Post-development Runoff Storage/Treatment Facilities OutFall Structure Weir Sizing Curb and Gutter Capacity Inlet Capacity Facilities Maintenance Figures and Tables Figure 1. Inlet Basin Exhibit Table 1. Runoff Calculations Table 2. OutFall Structure Weir Sizing Table 3. Curb and Gutter Capacity Calculations Table 4. Phase 1 Inlet Basin and Pipe Capacity Calculations Table 5. Phase 1 Detention Basin Calculations Storm Water Plan 1 4/23/2018 Description of Phase 1 B Project The South University District (SUD) - Phase 1 B is a subsequent minor subdivision of Lot 2, Phase 1 of the South University District project. The subsequent division will provide right of way dedications for South 14th Avenue and State Street and will result in a 3.66 acre lot (Lot 2A), which will be the location of the Phase 1 B on-site development. In 2014, the storm water infrastructure for SUD Phase 1 was constructed as part of the development of the Stadium View Apartments in Lot 1. This existing network of storm drains and the two detention basins is the backbone for the improvements to Phase 1 B, as originally intended. The previously submitted Storm Water Plan for the South University District Phase 1 subdivision accounted for the storm water that is contributed by the development of Phase 1 B. In the following report, Phase 1 B (referred to as Lot 2A) was designated as an entire drainage basin (Drainage Basin 1A) in the runoff estimates. With the development of a grading plan for Phase 1 B, the drainage basin has been further divided into smaller basins to more accurately represent storm water flow. The amendments updating the Storm Water Plan with improvements from Phase 1 B are as follows: - The Phase 1 B drainage basin has been broken down into sub basins 1 A.1 through 1A.5 as shown in the updated Inlet Basin Exhibit (Figure 1). - Updates have been made to storm water runoff and detention storage calculations to include the sub basins 1A.1 through 1A.5. - Content changes to update the report reflecting the current status of the project within the SUD development. General Information The South University District - Phase 1 subdivision is located within the City of Bozeman adjacent to South 11t" Street and south of Kagy Boulevard. The subdivision is located west of Bobcat Stadium and is south of the existing Stadium Center Subdivision. Phase 1 consists of 2 lots, road right-of-way, and park, with a gross area of 16.23 acres. Lot 1 is a 9.82 acre site that has already been developed into the Stadium View Apartments, which has a total of 499 beds. Lot 2 is a 6.41 Acre lot that is mostly undeveloped. A portion of Campus boulevard was constructed in 2017. The proposed lot 2A, also known as Phase 1 B, is a 3.66 acre lot with an estimated total of 194 beds. The calculated drainage area for the improved areas of Phase 1 is 16.77 acres. The proposed storm drainage improvements include modifications to those already installed in Phase 1 including; new curb inlets on existing main, Storm Water Plan 2 4/23/2018 extending a stub with three new inlets, adjusting finish grade on existing manholes, and replacing existing manhole inlets as needed to meet the new design. The developer's contact information is: CCC — Bozeman LLC 431 Office Park Drive Birmingham, AL 35223 Existing Conditions and General Design Criteria The existing condition at the Phase 1 B site is undeveloped agricultural land. Phase 1 has been developed into the Stadium View Apartments. Down-gradient development has restricted storm drainage paths leaving the property to the north. An existing creek (Mandeville Creek) runs along the west edge of Phase 1 and is the only acceptable location for storm water to leave the site. Mandeville Creek continues to flow north through the City to the East Gallatin River. In accordance with the City of Bozeman Design Standards and Specifications Policy (COB Standards) an engineering analysis has been conducted for the South University District (SUD) property to limit post-development runoff to pre- development rates. Storm runoff originating on the site will be detained on-site and released at rates less than the pre-development rates. Furthermore, detention basins will reduce storm water velocity and allow solids, silt, oils, grease, and other pollutants to settle from the runoff before the flows leave the site. Inlets and storm sewers are sized to pass the 25-year event without surcharging or overtopping roadways. Should a larger event occur, the roadways were designed to divert storm water away from structures. Storm Drainage Plan The overall storm drainage plan for Phase 1 and Phase 1 B is depicted on Figure 1 at the end of this report. Overall drainage direction on the proposed development is indicated by arrows showing the proposed drainage patterns and inlet locations. Estimation of Runoff The rational method was used to estimate peak flow rates and volumes from storm runoff. 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 Storm Water Plan 3 4/23/2018 land use. Time of concentration was determined from the slope of the basin, rational runoff coefficient and flow length (COB Standards — Equation II.E.6). Pre-development Runoff A runoff coefficient of 0.20 for open land was selected for pre-development conditions. An average overland slope of 2.1% over a basin length of 1,650 feet provides a time of concentration of 53.4 minutes. The Bozeman IDF curves show a rainfall intensity for the 10-year design storm event of 0.69 in/hr. Applying this over the 16.77 acre site produces a runoff flow rate of 2.32 cfs at the downstream end of the basin. Post-development Runoff Post-construction runoff was calculated using a runoff coefficient of 0.60 (commercial neighborhood) to account for future site development in accordance with the master plan. The rational method peak runoff rates are displayed in the following table. Table 1: Runoff Calculations Pre-development Post-Development Post-Development Open Land Commercial Commercial Land Use Neighborhood Neighborhood Drainage Area (acres) 16.77 16.77 16.77 Slope % 2.1 2.1 2.1 Overland Runoff Coefficients 0.2 0.6 0.6 Frequency Adjustment Factor, Cf 1 1 1.1 Design Storm Frequency 10- ear 10- ear 25- ear Basin Length ft 1,650 425 425 Time of Concentration, Tc min 53.4 15.1 13.2 Intensity at Tc (Figure 1-2 pg. 29) (in/hr 0.69 1.57 2.05 Runoff Rate(cfs) 2.32 15.82 20.64 The rational method provides a 10-year peak runoff rate of 15.82 cfs. The same analysis for a 25-year event provides a peak runoff rate of 20.64 cfs. In addition to the overall storm runoff calculations, the 25-year storm runoff flow rate for each storm drainage inlet basin has been determined based on the drainage plan shown on Figure 1. These calculations were used for sizing of conveyance pipes and are provided in Table 4 at the end of this report. Storage/Treatment Facilities Pond routing using the modified rational method was utilized to determine the hydraulics of the detention basin. Calculations are provided for Phase 1 in Table 5 at the end of the report. The inflow hydrograph and limited pond outflow resulted in a required maximum storage of 14,851 cubic feet of runoff for the 10- Storm Water Plan 4 4/23/2018 year event for Phase 1. The Phase 1 pond has a bottom area of 9075 square feet and 4:1 side slopes. The bottom elevation is 4928.00 and surface overflow to the west is provided at an elevation of 4930.50. The maximum depth is 1.5 feet, with a total volume of approximately 16,344 cubic feet. The Phase 1 detention basin is located at the northwest (down-gradient) end of the site and is vegetated with grass. The Phase 1 detention basin is labeled "Detention Basin A" on Figure 1. Detention Basin B on Figure 1 is a separate system provided for Phases 2 and 3 of the South University District (SUD) development. Outfall Structure Weir Sizing The detention basin outflow for the Phase 1 development is restricted by a typical control structure as depicted in COB Standards Figure A-2. Using the weir calculation equation given in Section II.D.2 of the COB Standards, the slot width of the weir structure was determined. The slot width for the Phase 1 pond discharge structure is 5 inches. This slot width limits the discharge from the structure to pre-developed rates at the maximum depth of 1.5 feet. Table 2: Outfall Structure Weir Sizin Phase 1 Pre-developed Runoff(cfs 2.32 Weir Coefficient"C" 3.33 Head ft 1.5 Slot Width in 5 In the occurrence of an event greater than the 10-year event, the pond will overflow to the west to Mandeville Creek. Curb and Gutter Capacity Storm runoff will be collected in street and parking lot curb and gutters. The hydraulic capacity of the curb and gutter was analyzed to determine if the curb would be overtopped in a 25-year event. The maximum 25-year event for any of the curb inlet basins produces approximately 2.70 cfs of runoff. The capacity of the curb at the lowest designed slope of 0.5% is 5.03 cfs at a depth of .15 feet below the top of the curb. Therefore, curb capacities in the development are adequate to carry the 25-year event. Storm Water Plan 5 4/23/2018 Table 3: Curb and Gutter Capacity Calculations Right-side slope X:1 0.05 Left-side slope X:1 33.00 Channel bottom width ft 0 Flow Depth ft 0.350 Flow Area ft^2 2.024 Wetted Perimeter ft 11.906 Width 0.035 Hydraulic Radius ft 0.170 Mannin 's Roughness 0.013 Slope ft/ft 0.005 Average Velocity ft/sec) 2.49 Flow ft^3/sec 5.03 Inlet Capacity The curb inlets on the roadways in the development were analyzed to ensure they have capacity to accept flows conveyed to them without reaching a point 0.15 feet from the top of the curb. The COB approved Neenah Foundry R-3067- L inlet structure has a capacity of 5.98 cfs at 0.35 feet of head (0.15 below top of curb). This capacity will convey the necessary flows. Facilities Maintenance The proposed storm drainage facilities will be operated and maintained by the property owner. The owner is responsible for quarterly maintenance of all storm water facilities. The maintenance includes, but is not limited to, visual inspection and removal of sediment from curbs, inlets, detention basins, and outlet structures as well as any necessary repairs to the facilities. Storm Water Plan 6 4/23/2018 STAH LY CO) PIPE 6A PIPE 12A PIPE 15A w DETENTION BASIN A PIPE 1A PIPE 4A PIPE 5A om� C I PIPE 3 1BASM TA.2 ol1r . 7a. . . . . . . ad ST TE STREET Corn w e§ BASIN 4A 1.03 ac 0 pe)2 Lo 4) Imp W)m 0.79 ac @a E U) acC 0 PIPE l9j �16A W -".ASIN z J.. . . . . . . w'e BASIN LU 2 u) 1 18 !0.32 ac U) 7A— U)ca t PIPE 13A z tPIPE LU L6 L I 17TMM PIPE PIPE '9� PIPE 14A A BASIN 6A Efl 2.11 ac 0 Li Co p ;- co.;41 j j3ASIN B FOR Jl PH,ASES 2&3 0 THE SUD D 1 -� l�- 14aL �- ^ .� L ; 1 W K a N BA J1111 m ASTER PLAN IN 1A BASIN 1A.5 Cn M —M 71-10 5 cl ac 3.66 ac G7777797777M 17 1777-711' 177 PIPE 1 1A flTN BASIN A BASIN 9A 1.83 2.09 a 17 1 . 4a 6 > s II II ; — p — l l — — — — l _ _� x 26 LU 4MMM- Z< ZO Z W > in�C-AM Rus; BEND 00 Z 2� uj U) lu z 3 - - - \ - - - - r - - - > m 0N Z 0 [,Fl-tC.NEB CDP DRAWN CDP CHECKED Cl)p DATE 2/20/15 iv 0 5.0 100 200 SHEET IN FEET /\, _FIGURE I,) Table 4 Phase 1 Inlet Basin and Pipe Capacity Calculations Inlet Drainage Basin Inlet Basin 1A.1 1A.2 1A.3 1A.4 1A.5 2A 3A 4A 5A 6A 7A 8A 9A 10A 11A Land Use REMU REMU REMU REMU REMU REMU REMU REMU REMU REMU REMU REMU REMU REMU REMU Drainage Area(acres) 0.55 0.17 0.53 0.15 3.66 0.32 0.79 1.03 1.84 2.11 0.52 0.69 1.83 2.09 0.49 Slope(% 2.1 2.1 2.1 2.1 2.1 2.1 2.1 2.1 2.1 2.1 2.1 2.1 2.1 2.1 2.1 Runoff Coefficients 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 Frequency Adjustment Factor,Cf Basin Length(ft) 660 190 630 180 425 180 1 307 410 357 377 263 581 387 439 340 Time of Concentration(min) 16.5 8.9 16.1 8.6 13.2 8.6 11.3 13.0 12.1 12.5 10.4 15.5 12.6 13.5 11.8 Peak Flow Calculations Design Storm 25yr 25 r 25yr 25yr 25yr 25yr 25yr 25yr 25yr 25yr 25yr 25yr 25yr 25yr 25yr Intensit of Tr_. Fi ure I-2 p .29 (inlhr) 1.78 2.65 1.81 2.70 2.05 2.70 2.28 2.07 2.17 2.13 2.39 1.86 2.11 2.03 2.20 Peak Runoff Rate at Tc(Q =CIA)(cfs) 0.59 0.27 0.58 0.24 4.50 0.52 1.07 1.28 2.39 2.70 0.75 0.7f 2.32 255 0.64 Detention Basin A- Pipe Network A Capacity Pipe Number 1A 2A 17 3A 4A 5A 19 6A 20 7A 8A 9A 10A 11A 12A 13A 1 .0 150 0 15.000 18.00 16 18 Pipe Size in 30.00 18.00 18.00 30.00 15.00 30.00 12.00 30.00 12.00 18.00 15.00 15.00 18.00 15.00 15.00 15.00 15.00 15.0 Pi Mannin 's"n"(PVC) 0.013 0.013 0.013 0.013 0.013 0.0,13 0.013 0.013 0.013 0.013 0.013 0.013 0.013 0.0'13 0.013 0.013 0.013 0.013 0.013 0.013 Area (ft`) 4.91 1.77 1.77 4.91 1.23 4.91 0.79 4.91 0.79 1.77 1.23 1.23 1.77 1.23 1.23 1.23 1.23 1.23 1.23 1.77 Wetted Perimeter(ft) 7.85 4.71 4.71 7.85 3.93 7.85 3.14 7.85 3.14 4.71 3.93 3.93 4.71 3.93 3.93 3.93 3.93 3.93 3.93 4.71 Hydraulic Radius ft) 0.63 0.38 0.38 0.63 0.31 0.63 0.25 0.63 0.25 0.38 0.31 0.31 0.38 0.31 0 0 .01 0.31 .00 0.31 0.3II Slope(fUft 0.0027 0.0050 0.0050 O.U027 0.0100 0.0027 0.0050 0.0027 0.0050 0.0100 0.0050 0.0050 0.0100 0.0050 U.0050 0.0100 0.0100 0.0050 0.0050 0.0050 Full Flow Capacity(cfs) 21.37 7.45 7.45 21.37 6.48 21.37 2.53 21.37 2.53 10.53 4.58 4.58 10.53 4.58 4.58 6.48 6.48 4.58 4.58 7.45 Velocity(ft1sec) 4.35 4.21 4.21 1 4.35 5.28 4.35 3 22 4.35 3.22 5.96 3.73 3.73 5.96 3.73 3.73 5.28 5.28 3.73 3.73 4.21 ContributingWet Basin NONE 1A.1. 1A.2 1A,3, 1A.4 NQNE 1`12:af3A 1/20 of4A 1l2tif , N aA = -'Q;A, NNE NONE 8h1 1A,5 Contributing Pipes 2A 17A 20A 4A NONE 6A NONE 7A NONE 8A NONE NONE 11A NONE 13A 14A I NONE I6A NONE NONE 3A 5A 18A 12A 9A 15A 19A 10A Actual Pipe Flow(cfs) 21.18 6.18 5.32 15.00 0.52 14.48 0.54 13.41 0.64 9.97 2.39 2.70 4.87 2,55 2 16 1.40 0.64 0.76 0.76 4.50 % Capacity 99% 83% 71% 70% 8% 68% 21% 63% 25% 95% 52% 59% 46% 56% 47% 22% 10% 17% 17% Fi0% Table 5 Phase 1 Detention Basin Calculations Time of Concentration Calculations Preconstruction Postconstruction Storm Sewer Flow Land Use Open Land REMU REMU Drainage Area(acres) 1677 16.77 16.77 Slope(%) 2-1 2.1 2-1 Runoff Coefficients 0.2 0.6 0.6 Frequency Adjustment Factor,Cf 1 1 1.1 Basin Length(it) 1650 425 425 Basin length based on typical inlet area Time of Concentration(min) 53.4 15.1 13.2 Peak Flow Calculations Design Storm 10-Year 10-Year 25-Year Intensity at Tc (Figure 1-2 pg.29)(in/hr) 0.69 1.57 2.05 Peak Runoff Rate at Tc(Q=CIA)(cfs) 2.32 15.82 20.64 (max release rate) Required Storage Volume MAX VOL 14850.09 Storm Duration Intensity Future Runoff Rate Runoff Volume Release Volume Required Storage Notes (minutes) (hdltr) (Q=CIA)(cfs) (co (cf) (cf) 2 5.84 58.75 7049.76 277.91 6771.85 4 3.72 37.44 8985.35 555.81 8429.53 6 2.86 28.76 10355.40 833.72 9521.68 8 2.37 23.86 11452.37 1111.63 10340.74 10 2.05 20.64 12382.66 1389,54 10993.12 12 1.82 18.33 13198.58 1667.44 11531.14 14 1.65 16.58 13930.24 1945.35 11984.89 16 1.51 15.20 14596.74 2223.26 12373.48 18 1.40 14.08 15211,05 2501.16 12709.89 20 1.31 13.15 15782.45 2779.07 13003.38 24 1.16 11.68 16822.39 3334.89 13487.51 26 1.10 11.09 17300.34 3612,79 13687.54 28 1.05 10.57 17754.94 3890.70 13864.24 30 1.00 10.10 18188.89 4168.61 14020.29 32 0.96 9.69 18604.43 4446.52 14157.91 34 0.93 9.32 19003.41 4724.42 14278.98 36 0.89 8.98 19387.41 5002.33 14385.08 38 0.86 8.67 19757.78 5280.24 14477.54 40 0.83 8.38 20115.69 5558.14 14557,54 42 0.81 8.12 20462.14 5836.05 14626.09 44 0.78 7.88 20798.03 6113.96 14684.08 46 0.76 7.65 21124.14 6391.87 14732.28 48 0.74 7.44 21441.16 6669.77 14771.39 50 0.72 7.25 21749.70 6947.68 14802.02 52 0.70 7.07 22050.33 7225.59 14824.74 54 069 690 22343.52 7503.49 14840.03 56 0.67 6.74 22629.74 7781.40 14848.34 58 0.65 6.58 z22909.40 8059.31 14850.09 Required Storage 60 0.64 644 23182.85 8337.22 14845.63 62 0.63 6.30 23450.44 8615.12 14835,31 64 0.61 6.18 23712.47 8893.03 14819.44 66 0.60 605 23969.24 9170.94 14798.30 68 0.59 5.94 24220.99 9448.85 14772.15 70 0.58 5.83 24467.98 9726.75 14741.23 72 0.57 5.72 24710.43 10004.66 14705.77 74 0.56 5.62 24948.53 10282.57 14665.96 76 0.55 5.52 25182.49 10560-47 14622.01 78 0.54 5.43 25412.47 10838.38 14574.09 80 0.53 5.34 25638.66 11116.29 14522.37 82 0.52 5.26 25861.20 11394.20 14467.01 84 0.51 5.17 26080.24 11672.10 14408.14 86 0.51 5.10 26295.92 11950.01 14345.91 88 0.50 5.02 26508.36 12227.92 14280.44 90 0.49 4.95 26717.68 12505.82 14211.85 92 0.48 4.88 26924.00 12783.73 14140.27 94 0.48 4.81 27127.43 13061.64 14065.79 96 0.47 4.74 27328.06 13339.55 13988.51 98 0.47 4.68 27525.99 13617,45 13908.54 100 0.46 4.62 27721.32 13895.36 13825.96 102 0.45 4.56 27914.12 14173.27 13740.85 104 0.45 4.50 28104.48 14451.17 13653.30 106 0.44 4.45 28292.47 14729.08 13563.39 108 0.44 4.39 28478.18 15006.99 13471.19 110 0.43 4.34 28661.66 15284.90 13376.76 112 0.43 4.29 28842.98 15562.80 13280.18 114 0.42 4.24 29022.21 15840.71 13181.50 116 0.42 4.20 29199.41 16118.62 13080.80 118 0.41 4.15 29374.64 16396.53 12978.11 120 0.41 4.10 29547.95 16674.43 12873.51