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Home My WebLink About007_StormwaterDesignReport STORM WATER DESIGN REPORT 17032.03 NorthWestern Energy East Lot Improvements Tract 33 Northeast Annexation to Bozeman Bozeman, MT May 26, 2022 May 26, 2022 Project No. 17032.03 STORMWATER DESIGN REPORT FOR NORTHWESTERN ENERGY EAST LOT IMPROVEMNETS TRACT 33 NORTHEAST ANNEXATION TO BOZEMAN PROJECT OVERVIEW & BACKGROUND The purpose of this drainage plan is to present a summary of calculations to quantify the stormwater runoff for the NorthWestern Energy East Lot Improvements project. The proposed project is currently located on Tract 33 of the Northeast Annexation to Bozeman which is owned by the City of Bozeman. The lot is a total of 3.8 acres. The east side of the lot is bounded by North Rouse Avenue. The proposed project will be a gravel storage yard for NorthWestern Energy materials. No work will be done on this lot. The project will include site re-grading and building a gravel access road to existing NorthWestern Energy lots. A stormwater detention facility will be located at the north end of the site. New perimeter fencing will be installed with one entrance gate at the approach entrance. The site is in the East Gallatin River floodplain and 100-year flood area. No portion of the site design is within the floodway. A Floodplain Development Application will be submitted to permit the work and development within the Floodplain. All design criteria and calculations are in accordance with The City of Bozeman Design Standards and Specifications Policy, dated March 2004. The site stormwater improvements have been designed with the intent to meet the current City of Bozeman drainage regulations for the entire site to the extent feasible. HYDROLOGY The Rational Method and rainfall data provided was used to calculate the runoff volumes for the 10- year storm event for the runoff storage facilities and the 25-year storm event for the storm drain conveyance facilities. Watersheds were delineated for both pre- and post-development conditions and are shown in Exhibits A and B in Appendix A. The weighted runoff coefficient “C” was calculated for each contributing area using 0.95 for impervious areas, 0.6 for gravel areas, and 0.2 for undeveloped areas(grassland). Time of concentrations were calculated for both pre-development and V:17032.03_NWE_East_Lot_Stormwater_Design_Report 2 (05/26/22) SNH/bc post-development conditions. The results of the hydrologic analysis for the storm events described above are shown in Appendix B. Hydrologic Equations Cwd = Weighted Runoff Coefficient A = Area (acres) I = Rainfall Intensity (in/hr) Q = Peak Runoff (cfs) Q = Cwd x I x A Water Quantity Volume Q (cfs) = C x 0.41 in/hr x A (acre) V (cf) = 7200 sec x Q (ft3) Water Quality Volume V (cf) = 0.5 in ÷ 12 in x A (sf) EXISTING CONDITIONS The proposed lot is currently vacant with miscellaneous concrete pads and some asphalt surface features. The lot generally slopes to the north with intermittent high and low points throughout the lot. There are no existing structures on the site. Approximately 87% of the lot is generally covered with native grasses with some mature trees and brush lining the edges of the property. The existing lot generally sheet flows to the north of the site and sheet flows into to existing stormwater facilities located under North Rouse Avenue and the trailhead parking area. The site is 87% vegetated with native grasses. The pre-development runoff coefficient is 0.36 and the time of concentration was estimated to be 10 minutes resulting in a 2-year storm event peak flow of 1.46 cfs. The predevelopment calculations can be found in Appendix B, Table 1. POST-DEVELOPMENT The runoff will sheet flow to the northeast of the site to a retention pond that slopes to the northwest along the perimeter of the site. Site slopes range from 1.65% to 3%, designed to prevent ponding and not convey sediment throughout the site. The site will have majority cut volume and no additional fill brought to the site. Since the project is located in a floodplain, the site was graded to match existing conditions. The site is in a floodplain, so the base flood elevation is not raised in this location. The Floodplain storage area is not used for stormwater storage. A runoff coefficient for gravels was chosen to be 0.6 for all stormwater calculations. The post- development peak flow was calculated to be 8.72 CFS and the volume required to store for the 10- year, 2-hour storm is 6,695.74 CF. The volume required for the first one half inch of rain, the water V:17032.03_NWE_East_Lot_Stormwater_Design_Report 3 (05/26/22) SNH/bc quality volume, is 6,897.33. The design volume for the proposed detention pond will be the water quality volume of 6,897.33. The post-development basin calculations can be found in Appendix C, Table 2. And a reference exhibit for post-development conditions in Appendix A. Pipes One 15-inch RCP culvert is proposed under the new approach to the site to connect the roadside ditches on either side of the approach. Only runoff from the Manley Access Road enters the culvert. The proposed pipe slope is 1.6% and maintains a minimum velocity of 3 fps. The runoff is conveyed to existing stormwater facilities under North Rouse Avenue and within the trailhead parking area which flow to the East Gallatin River. Calculations are in Appendix C. Detention Facility The total required volume for the detention facility is 6,897.33 CF. The post-development basin calculations in Appendix C, Table 2. This calculates the storage required for a 10-year, 2-hour storm event with a time of concentration of 5 min. The maximum basin depth is 1.5 feet. Runoff enters the detention facility via sheet flow across the site. Runoff will sheet flow through proposed landscaping to help provide a buffer to filter out sediment entering the detention facility. The proposed pond has a bottom elevation of 4700.5’ and a maximum top elevation of 4702.00’. The designed volume to be detained is 7,659 CF. The depth to ground water is assumed to be 9.5 ft blow existing surface based on adjacent site geotechnical report from 2017. An outlet structure with a 6-inch orifice will provide a design discharge rate of 1.07 cfs which is less than the allowable discharge rate is the 2-year pre-development flow rate of 1.46 cfs. The rim of the outlet structure will be set to the top elevation of the pond and will be used as an emergency overflow. An 18-inch outlet pipe will be designed to discharge to the existing storm drain system under North Rouse Avenue. The 18-inch outlet pipe will need to be an 18-inch arch equivalent pipe in order to fit in the outlet structure. The basin area is designed to allow for settling velocity of 40-micron particles which is equal to 0.0069 ft/sec. Settling velocity of 40-micron particles = 0.0069 ft/sec Design Release Rate = 1.07 cfs Minimum Area Required: 1.07 cfs ÷ 0.0069 ft/sec = 155 SF < proposed 3,265 SF APPENDICES Appendix A – Existing Conditions and Proposed Grading Plan Sheets Appendix B – Pre-development Calculations Appendix C – Post-development Calculations APPENDIX A Existing Conditions and Proposed Grading Plan Sheets NorthWestern Energy East Lot Improvements 17032.03 NORTH NOTE: -EXISTING UNDERGROUND INSTALLATIONS & PRIVATE UTILITIES SHOWN ARE INDICATED ACCORDING TO THE BEST INFORMATION AVAILABLE TO THE ENGINEER. THE ENGINEER DOES NOT GUARANTEE THE ACCURACY OF SUCH INFORMATION. SERVICE LINES (WATER, POWER, GAS, STORM, SEWER, TELEPHONE & TELEVISION) MAY NOT BE STRAIGHT LINES OR AS INDICATED ON THE PLANS. STATE LAW REQUIRES CONTRACTOR TO CALL ALL UTILITY COMPANIES BEFORE EXCAVATION FOR EXACT LOCATIONS. -ALL IMPROVEMENTS SHALL BE PERFORMED IN ACCORDANCE WITH MONTANA PUBLIC WORKS STANDARD SPECIFICATIONS 6TH EDITION, APRIL, 2010, AND THE CITY OF BOZEMAN STANDARD MODIFICATIONS, DATED MARCH 31, 2011, WITH ADDENDUM. -UNLESS OTHERWISE SPECIFIED, ALL CONSTRUCTION LAYOUT AND STAKING SHALL BE PERFORMED UNDER THE RESPONSIBLE CHARGE OF A LAND SURVEYOR LICENSED IN THE STATE WHERE THE PROJECT IS LOCATED AND BY A PARTY CHIEF OR ENGINEERING TECHNICIAN EXPERIENCED IN CONSTRUCTION LAYOUT AND STAKING TECHNIQUES AS ARE REQUIRED BY THE SPECIFIC TYPE OF WORK BEING PERFORMED.FILE:PROJECT NO:CAD:QUALITY ASSURANCE:DRAWING HISTORYDATE DESCRIPTIONSTORAGE YARD LAYOUTEXISTING TOPOGRAPHY AND DEMOLITION PLANNORTHWESTERN ENERGY EAST LOTPROPOSED TRACT 2BOZEMAN, MTC3.0 -17032_03_EXISTING_PROD_SP.DWGSNH/RRB/KF05/18/221ST SP SUBMITTAL--------------17032.030 30 SCALE: 1" = 30' 601530 REGULATORY FLOODWAY FLOODPLAIN HAZARD AREA 0.2% ANNUAL FLOOD HAZARD AREA V:\17032_03_NorthWestern_Energy_East Lot&North_Access_Road\CADD_C3D\PRODUCTION_DWG\17032_03_EXISTING_PROD_sp.dwg, existing, 5/31/2022 1:34:56 PM, Shudock, 1:1 FILE:PROJECT NO:CAD:QUALITY ASSURANCE:DRAWING HISTORYDATE DESCRIPTIONSTORAGE YARD LAYOUTGRADING AND DRAINAGE PLANNORTHWESTERN ENERGY EAST LOTPROPOSED TRACT 2BOZEMAN, MTC5.0 -17032_03_GRADING & DRAINAGE_PROD_SP.DWGSNH/RRB/KF05/18/221ST SP SUBMITTAL--------------17032.030 30 SCALE: 1" = 30' 601530 NORTH NOTE: -EXISTING UNDERGROUND INSTALLATIONS & PRIVATE UTILITIES SHOWN ARE INDICATED ACCORDING TO THE BEST INFORMATION AVAILABLE TO THE ENGINEER. THE ENGINEER DOES NOT GUARANTEE THE ACCURACY OF SUCH INFORMATION. SERVICE LINES (WATER, POWER, GAS, STORM, SEWER, TELEPHONE & TELEVISION) MAY NOT BE STRAIGHT LINES OR AS INDICATED ON THE PLANS. STATE LAW REQUIRES CONTRACTOR TO CALL ALL UTILITY COMPANIES BEFORE EXCAVATION FOR EXACT LOCATIONS. -ALL IMPROVEMENTS SHALL BE PERFORMED IN ACCORDANCE WITH MONTANA PUBLIC WORKS STANDARD SPECIFICATIONS 6TH EDITION, APRIL, 2010, AND THE CITY OF BOZEMAN STANDARD MODIFICATIONS, DATED MARCH 31, 2011, WITH ADDENDUM. -UNLESS OTHERWISE SPECIFIED, ALL CONSTRUCTION LAYOUT AND STAKING SHALL BE PERFORMED UNDER THE RESPONSIBLE CHARGE OF A LAND SURVEYOR LICENSED IN THE STATE WHERE THE PROJECT IS LOCATED AND BY A PARTY CHIEF OR ENGINEERING TECHNICIAN EXPERIENCED IN CONSTRUCTION LAYOUT AND STAKING TECHNIQUES AS ARE REQUIRED BY THE SPECIFIC TYPE OF WORK BEING PERFORMED. REGULATORY FLOODWAY FLOODPLAIN HAZARD AREA 0.2% ANNUAL FLOOD HAZARD AREA V:\17032_03_NorthWestern_Energy_East Lot&North_Access_Road\CADD_C3D\PRODUCTION_DWG\17032_03_GRADING & DRAINAGE_PROD_SP.dwg, GRADING AND DRAINAGE, 5/31/2022 1:38:10 PM, Shudock, 1:1 APPENDIX B Pre-development Calculations NorthWestern Energy East Lot Improvements 17032.03 Project:NorthWestern Energy East Lot Project No.: 17032.03 Date: 5/26/2022 Time of Concentration=10 mins Intensity10-Year, 2-Hr=0.41 in/hr Water Quality=0.5 inches WS Area (sf) Area (ac.) Area Impervious (sf) Area Gravel (sf) Area Pervious (sf) RC % IC I2 (in/hr)I10 (in/hr)I25 (in/hr) Q2 Peak Flow (cfs) Q10 Peak Flow (cfs) Q25 Peak Flow (cfs) 10-Yr, 2- Hr (in/hr) V10 year, 2 hour (cf)V0.5 (cf) EXISTING 165,536 3.80 14,056 6,827 144,653 0.36 8% 1.055 2.051 2.455 1.456 2.83 3.39 0.408 4,054.49 6,897.33 HYDROLOGY WORKSHEET - TABLE 1 PRE-DEVELOPMENT WATERSHED APPENDIX C Post-development Calculations NorthWestern Energy East Lot Improvements 17032.03 Project:NorthWestern Energy East Lot Project No.:17032.03 Date:5/26/2022 Time of Concentration=7 mins Intensity10-Year, 2-Hr=0.41 in/hr POST DEVELOPMENT WATERSHED Water Quality=0.5 inches WS Area (sf) Area (ac.) Area Impervious (sf) Area Gravel (sf) Area Pervious (sf) RC % IC I10 (in/hr)I25 (in/hr)Q25 Peak Flow (cfs) 10-Year, 2- Hour (in/hr) V10 year, 2 hour (cf) V0.5 (cf) POST 165,536 3.80 0 165,536 0 0.60 0% 2.371 3.085 7.03 0.41 6,695.74 6,897.33 HYDROLOGY WORKSHEET - TABLE 2 Project: NWE East Lay Down Yard Project #: 17032.03 Date: 5/26/2022 Design Storm Frequency =10 years Length (ft)Width (ft)Infiltration Rate (in/hr) Discharge Rate, d =1.07 cfs 3846.3836 Input values for runoff coefficients from appropriate tables. Area Area Runoff Coefficient Frequency Factor Calculation Value A A/(43560 ft2/acre)C Cf C x Cf C' C' x A (ft2)(Acres)=(C x Cf) < or = 1 (Acres) 0.00 0.000 0.9 1 0.90 0.90 0.000 0.00 0.000 0.3 1 0.30 0.30 0.000 165536.00 3.800 0.6 1 0.60 0.60 2.280 0 1 0.00 0.00 0.000 0 1 0.00 0.00 0.000 165536 3.800 2.280 Weighted Runoff Coefficient, Cwd SCjAj SAj Cwd x Cf x SAj =2.28 Where Cj is the adjusted runoff coefficient for surface type j and Aj is the area of surface type j Rainfall Rainfall Peak Flow Duration, t Intensity, i = Cwd x SAj x i (min) (in/hr)(ft3/s) 1 9.16 20.89 5 3.22 7.34 10 2.05 4.68 15 1.58 3.59 18 1.40 3.19 19 1.35 3.08 20 1.31 2.98 21 1.27 2.89 25 1.13 2.58 30 1.00 2.29 35 0.91 2.07 40 0.83 1.90 45 0.77 1.76 50 0.72 1.64 55 0.68 1.54 60 0.64 1.46 75 0.55 1.26 90 0.49 1.12 105 0.44 1.01 120 0.41 0.93 150 0.35 0.80 180 0.31 0.71 360 0.20 0.46 720 0.13 0.29 1440 0.08 2,292.97 ft3 20.89 (ft3/s) 5095.80 3531.00 1564.80 12535.79 46224.00 ----- 15977.63 92448.00 ----- 7716.68 11556.00 ----- 9835.38 23112.00 ----- 6695.74 7704.00 ----- 7239.65 9630.00 ----- 6054.39 5778.00 276.39 6390.01 6741.00 ----- 5253.37 3852.00 1401.37 5680.11 4815.00 865.11 4750.18 2889.00 1861.18 4928.61 3210.00 1718.61 4350.20 2247.00 2103.20 4558.34 2568.00 1990.34 3866.91 1605.00 2261.91 4121.71 1926.00 2195.71 3446.92 1155.60 2291.32 3638.00 1348.20 2289.80 3512.77 1219.80 2292.97 3576.40 1284.00 2292.40 2805.99 642.00 2163.99 3233.83 963.00 2270.83 1253.39 64.20 1189.19 2201.53 321.00 1880.53 = Cwd x SAj x i x t = d x t = Runoff Volume - Discharge Volume (ft3) (ft 3) (ft 3) = 0.6000 Cwd x Cf =0.60 Runoff Volume Discharge Volume Site Detention = Landscape Gravel Totals Impervious RATIONAL METHOD FOR RUNOFF CALCULATIONS EXISTING Surface Type = Detention Pond Outlet Orifice OUTLET PIPE CAPACITY