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HomeMy WebLinkAbout09 - Design Report - Gallatin County Detention Center - Stormwater EEr STAH LY Engineers and Surveyors �' t7585 Shedhorn Drive, Bozeman, MT 59718 •phone:406-522-9526 fax:406-522-9528•seaengnr@MT.net January 8, 2009 Bob Murray, PE City of Bozeman Engineering Department 20 East Olive Bozeman, MT 59715 RE: Gallatin County Detention Center Storm Water Plan Submittal Bob: u ' Please find the enclosed documents for your review of the proposed Storm "_`4`-"-`"'-��-__ Water Plan for the Gallatin County Detention Center. Included with this submittal are the Storm Water Plan (report) and plan sheets representing the portion of the overall set showing grading and drainage improvements. Project specifications are the City of Bozeman Modifications to Montana Public Works Standard Specifications and no additional specification packets are proposed. Sincerely, Greg Benjamin, PE Storm Water Plan Gallatin County Detention Center January 2009 Submitted to: City of Bozeman Engineering Department 20 East Olive Bozeman, MT 59715 Prepared by: Stahly Engineering and Associates 7585 Shedhorn Drive Bozeman, MT 59718 (406) 522-9526 �y��1NEE�f�c C�0iJ STAH LY B�G �3 � IOPE ��' Gallatin County Detention Center Storm Water Plan Contents Existing Conditions and General Design Criteria Storm Drainage Plan Description Estimation of Runoff Pre-development Runoff Post-development Runoff Storage/Treatment Facilities Discharge Structure and Storm Sewer Curb Capacity Facilities Maintenance Figures and Tables Figure 1. Hydraflow Hydrographs Stage-Discharge Graph (separate figure at end of report) Table 1. Runoff Calculations Table 2. Post-Development Runoff Routing (separate table at end of report) Plan Sheets Plan Sheet C 1.04 Grading and Drainage Plan (included separately) Plan Sheet C 4.01 Grading and Drainage Details (included separately) Storm Water Plan 1 1/7/2009 Existing Conditions and General Design Criteria The existing Gallatin County Law and Justice Center and Detention Center occupy 18.95 acres of property within the City of Bozeman. Past development of the Gallatin County Law and Justice property has not considered storm drainage. Additionally, the property has been surrounded by residential and office development disrupting historical flow patterns. Down-gradient development has restricted storm drainage paths leaving the property to the north, which has contributed to runoff ponding on the western portion of the property. An existing open drainage runs along the east edge of the property and is the only acceptable location for storm water to leave the site. This drainage flows into a 24" storm sewer running north through neighboring developments. The storm drain crosses Main Street to the west of the Hastings Shopping Center flowing to the northeast behind the shopping center. At North 15th Avenue and Beall Street the drainage splits, flowing to the north or to the east into Mandeville Creek. Although development of the Gallatin County Detention Center will only affect approximately % of the site, storm water facilities have been proposed which will provide for the ultimate development of the property in accordance with master planned development presented to the public in 2008. Site grading will direct storm runoff to the northeast corner of property into the existing 24" storm sewer leaving the property. Proposed grading reduces the potential for future storm water to flow onto property to the north. In accordance with the City of Bozeman Design Standards and Specifications Po/icy(COB Standards) an engineering analysis has been conducted for the Gallatin County 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. Culverts and storm sewers are sized to pass the 25-year event without overtopping roadways. Should a larger event overtop the roadways grading will divert overflows to South 15th Ave instead of property to the north. Storm Drainage Plan A Drainage and Grading Plan has been prepared for the Detention Center site and is included separately as Plan Sheet C1.04. The plan identifies new and existing buildings, culverts, storm sewers, detention basins, and roadways. Existing grade contours are shown at one foot intervals, and finish grades are depicted by spot elevations. Details necessary for construction are provided on Plan Sheet 4.01 Grading and Drainage Details, also included separately. Storm Water Plan 2 1/7/2009 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 land use. Time of concentration was determined from the slope of the basin, rational runoff coefficient and flow length. Pre-development Runoff A runoff coefficient of 0.20 for open land was selected for pre-development conditions. An average overland slope of 2% over a basin length of 1110 feet provides a time of concentration of 44.5 minutes. The Bozeman OF curves show a rainfall intensity for the 10-year design storm event of 0.78 in/hr. Applying this over the 18.95 acre site produces a runoff flow rate of 2.95 cfs at the downstream end of the basin. Post-development Runoff Post-construction runoff was calculated using a runoff coefficient of 0.80 (industrial or commercial downtown) to account for future site development in accordance with the master plan. The increased impervious area reduces the time of concentration to 15.4 minutes. The Bozeman IDP curves show a 10-year storm intensity of 1.55 in/hr. The rational method provides a 10-year peak runoff rate of 23.46 cfs. The same analysis for a 25-year event provides a peak runoff rate of 28.21 cfs. Table 1 Runoff Calculations Pre-development Post-Development Post-Development Land Use Open Land Industrial Industrial Drainage Area acres 18.95 18.95 18.95 Sloe % 2 2 2 Overland Runoff Coefficients 0.2 0.8 0.8 Fre uenc Adjustment Factor, Cf 1 1 1 Design Storm Frequency 10- ear 10- ear 25- ear Basin Length ft 1110 1200 1200 Time of Concentration, Tc min 44.5 15.4 15.4 Intensity at Tc (Figure 1-2 pg. 29) (in/hr 0.78 1.55 1.86 Runoff Rate (cfs) 2.95 23.46 28.21 Based on area approximately 57% of the runoff flows west of the Law and Justice Center, 11% flows between the Law and Justice Center and the new Detention Center, and 20% flows to the east of the new Detention Center. The remaining 12% originates north of the Law and Justice Center and the Detention Center. Storm Water Plan 3 1/7/2009 Storage/Treatment Facilities Simple pond routing using the modified rational method is provided in Table 2 at the end of the report. This table shows a required storage of 24,511 cubic feet of runoff for the 10-year event. The required storage is provided by 2 interconnected basins with a bottom area of 21281 square feet and 5:1 side slopes. The bottom elevation is 4821.65 and an surface overflow to the east is provided at elevation 4823.0. The 10-year water depth is 1.08 feet with a detention basin volume of 24,750 cubic feet at this depth. The maximum depth is 1.35 feet, with a total volume of approximately 32,000 cubic feet. The detention basins are located at the northern (down-gradient) end of the site and will be vegetated with grass. Discharge Structure and Storm Sewer The detention basin outflow is restricted by a typical control structure and hydraulic resistance in the outlet pipe. The outlet structure was designed using a stage-discharge relationship obtained using Hydraflow Hydrographs. This program considers tail water depth in the storm sewer and pipe flow depth resulting in submerged weirs, in addition to multiple stage weirs. The storm sewer outlet elevation is set by the grade of the existing 24" storm sewer. A tail water depth of 1.0' or the 24" pipe flowing Y2 full was used. The length of the new storm sewer is 115' feet from the control structure to the 24" storm sewer. Weir size and pipe size was then iterated to provide a discharge of 2.95 cfs at a stage of 1.08', equivalent to the 10-year pre-development release rate. An overflow weir beginning at a depth of 1.08' provides free flowing capacity for flows greater than the 10-year design event. An RCAP outlet pipe of 18" equivalent diameter with an 11" wide weir 13" (1.08') high provides a release rate of 2.95 cfs at 1.08 feet of head. At depths above this the overflow weir provides additional flow for a release rate up to 4.8 cfs at 1.35 feet of head. The resulting stage-discharge relationship is provided in Figure 1 at the end of this report. A 11" wide free-flowing weir would normally flow greater than 2.95 cfs at 1.08 feet of head, however, the weir capacity is reduced due to submergence caused by headlosses at the entrance of the 18" outlet pipe. This dual outlet control (weir and pipe) is well balanced and still allows the outlet piping to pass the 25-year event at a maximum depth of less than 1.35'. Routing a 25-year event through the detention basin results in an outflow of approximately 4.0 cfs at a depth of 1.2 feet. Construction details shown on Sheet C4.01 Grading and Drainage Details show a typical detention basin discharge structure as provided in Fig A-2 in the COB Standards with the required weir dimension and elevations. Storm Water Plan 4 1/7/2009 Curb Capacity Where runoff flows around the east side of the Detention Center it will be carried in the curb and gutter of a 12' wide access road. Capacity of this curb and gutter was analyzed to determine if the curb would be overtopped in a 25-year event. Since this is a security access only, drivability was not a consideration, merely capacity. A 25-year event produces approximately 5.6 cfs of runoff flowing east of the Detention Center. The access road curb and gutter carries 5.85 cfs at a maximum depth (curb height) of 0.45'. Facilities Maintenance The proposed storm drainage facilities will be owned by Gallatin County up to the 24" storm sewer leaving the property. The county is responsible for quarterly maintenance of all storm water facilities up to this point. The maintenance includes, but is not limited to, visual inspection and removal of sediment from curbs, culverts, detention basins, and outlet structures as well as any necessary repairs to the facilities. Storm Water Plan 5 1/7/2009 Table 2. Post Development Runoff Routing Time of Concentration Calculations Preconstruction Postconstruction Land Use Open Land Industrial Drainage Area(acres) 18.95 18.95 Slope(%) 2 2 Runoff Coefficients 0.2 0.8 Frequency Adjustment Factor,Cf 1 1 Basin Length(ft) 1110 1200 Time of Concentration(min) 44.5 15.4 Peak Flow Calculations Design Storm 10-Year 10-Year Intensity at Tc (Figure 1-2 pg.29)(in/hr) 0.78 1.55 Peak Runoff Rate at Tc(Q=CIA)(cfs) 2.95 23.46 (max release rate) Required Storage Volume MAX VOL. 24510.99 Storm Duration Intensity Future Runoff Rate Runoff Volume Release Volume Required Storage Notes (minutes) (in/hr) (Q=CIA)(cfs) (cf) (cf) (cf) 2 5.84 88.51 10621,58 353.46 10268.13 4 3.72 56.41 13537.85 706.92 12830.94 6 2.86 43.34 15602.05 1060.37 14541.68 8 2.37 35.95 17254.81 1413.83 15840.98 10 2.05 31.09 18656.44 1767.29 16889.15 12 1.82 27.62 19885.76 2120.75 17765.01 14 1.65 24.99 20988.12 2474.20 18513.92 16 1.51 22.91 21992.30 2627.66 19164.64 18 1.40 21.22 22917.86 3181.12 19736.74 20 1.31 19.82 23778.76 3534.58 20244.19 24 1.16 17.60 25345.61 4241.49 21104.12 26 1.10 16.71 26065.70 4594.95 21470.75 28 1.05 15.92 26750.63 4948.41 21802.23 30 1.00 15.22 27404.46 5301.86 22102.59 32 0.96 14.60 28030.52 5655.32 22375.20 34 0.93 14.04 28631.65 6008.78 22622.87 36 0.89 13.52 29210.21 6362.24 22847.97 38 0.86 13.06 29768.23 6715.69 23052.54 40 0.83 12.63 30307.47 7069.15 23238.32 42 0.81 12.23 30829.47 7422.61 23406.86 44 0.78 11.87 31335.54 7776.07 23559.47 46 0.76 11.53 31826.87 8129.52 23697.35 48 0.74 11.22 32304.51 8482.98 23821.53 50 0.72 10.92 32769.38 8836.44 23932.94 52 0.70 10.65 33222.32 9189.90 24032.42 54 0.69 10.39 33664.06 9543.35 24120.71 56 0.67 10.15 34095.30 9896.81 24198.49 58 0.65 9.92 34516.64 10250.27 24266.37 60 0.64 9.70 34928.64 10603.73 24324.91 62 0.63 9.50 35331.81 10957.18 24374.62 64 0.61 9.30 35726.60 11310.64 24415.96 66 0.60 9.12 36113.46 11664.10 24449.36 68 0.59 8.94 36492.77 12017.56 24475.22 70 0.58 8.78 36864.90 12371.01 24493.89 72 0.57 8.62 37230.18 12724.47 24505.71 74 0.56 8.47 37588.92 13077.93 24510.99 Required Storage 76 0.55 8.32 37941.41 13431.39 24510.03 78 0.54 8.18 38287.93 13784.84 24503.08 80 0.53 8.05 38628.71 14138.30 24490.41 82 0.52 7.92 38964.00 14491.76 24472.25 84 0.51 7.80 39294.02 14845.22 24448.81 86 0.51 7.68 39618.97 15198.67 24420.30 88 0.50 7.56 39939.04 15552.13 24386.91 90 0.49 7.45 40254.42 15905.59 24348.83 92 0.48 7.35 40565.28 16259.05 24306.23 94 0.48 7.25 40871.77 16612.50 24259.27 96 0.47 7.15 41174.06 16965.96 24208.10 98 0.47 7.05 41472.28 17319.42 24152.86 100 0.46 6.96 41766.56 17672.88 24093.69 102 0.45 6.87 42057.05 18026.34 24030.71 104 0.45 6.79 42343.86 18379.79 23964.06 106 0.44 6.70 42627.10 18733.25 23893.85 108 0.44 6.62 42906.89 19086.71 23820.18 110 0.43 6.54 43183.33 19440.17 23743.17 112 0.43 6.47 43456.53 19793.62 23662.91 114 0.42 6.39 43726.57 20147.08 23579.49 116 0.42 6.32 43993.55 20500.54 23493.01 118 0.41 6.25 44257.56 20854.00 23403.56 120 0.41 6.18 44518.67 21207.45 23311.22 Pond Report Hydraflow Hydrographs Extension for AutoCAD®Civil 3D®2008 by Autodesk, Inc.v6.052 Tuesday,Jan 6,2009 Pond No. 1 - DC Pond Pond Data Trapezoid-Bottom L x W=212.0 x 100.0 ft, Side slope=5.00:1, Bottom elev.=4821.65 ft, Depth=1.35 ft Stage/Storage Table Stage(ft) Elevation(ft) Contour area(sgft) Incr.Storage(cuft) Total storage(cuft) 0.00 4821.65 21,200 0 0 0.14 4821.79 21,623 2,891 2,891 0.27 4821.92 22,050 2,948 5,838 0.41 4822.06 22,480 3,006 8,844 0.54 4822.19 22,914 3,064 11,908 0.68 4822.33 23,352 3,123 15,031 0.81 4822.46 23,793 3,182 18,213 0.95 4822.60 24,238 3,242 21,455 1.08 4822.73 24,686 3,302 24,758 1.22 4822.87 25,138 3,363 28,121 1.35 4823.00 25,594 3,424 31,545 Culvert/Orifice Structures Weir Structures [A] [B] [C] [PrfRsr] [A] IBl [C] [D] Rise(in) = 18.00 0.00 0.00 0.00 Crest Len(ft) = 0.92 3.08 0.00 0.00 Span(in) = 18.00 0.00 0.00 0.00 Crest El.(ft) = 4821.65 4822.73 0.00 0.00 No.Barrels = 1 0 0 0 Weir Coeff. = 3.33 3.33 3.33 3.33 Invert El.(ft) = 4821.50 0.00 0.00 0.00 Weir Type = Rect Rect --- --- Length(ft) = 115.00 0.00 0.00 0.00 Multi-Stage = Yes Yes No No Slope(%) = 1.00 0.00 0.00 n/a N-Value = .013 .013 .013 n/a Orifice Coeff. = 0.60 0.60 0.60 0.60 Exfil.(in/hr) = 0.000(by Contour) Multi-Stage = n/a No No No TW Elev.(ft) = 0.00 Note:Culvert/Orifice outflows are analyzed under inlet(ic)and outlet(oc)control. Weir risers checked for orifice conditions(ic)and submergence(s). Stage(ft) Stage/Discharge Elev(ft) 2.00 4823.65 1.80 4823.45 1.60 4823.25 1.40 4823.05 1.20 4822.85 1.00 4822.65 0.80 4822.45 0.60 4822.25 0.40 000, 4822.05 0.20 4821.85 0.00 4821.65 0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50 5.00 Total Q Discharge(cfs)