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HomeMy WebLinkAbout009 Boulder Creek Lots 3 & 4 Drainage ReportPage 1 of 9 Storm Drainage Calculations Boulder Creek Site Plan – Lots 3 &4 1 Introduction The purpose of this report is to supplement the Stormwater Management Design Report for Boulder Creek Site Plan Lots 1-4, Block 5, Bozeman, MT by Madison Engineering December 2018. The report provides hydraulic calculations for sizing of retention storage facilities and other storm drainage structures for development of Boulder Creek Lots 3 & 4, in accordance with the City of Bozeman Design Standards. The proposed site consists of three residential apartment buildings located east of Abigail Lane and South of Oak Street. The purpose of the report and calculations is to demonstrate the following: - Post- development peak runoff does not exceed pre-development (historic) conditions for the 10-year storm event - The 25-year peak runoff is conveyed by underground storm drain facility with no surface overflow - Storm water treatment is provided to intercept and contain sediment and floating debris on site, for eventual removal and disposal. 2 Drainage Basins Boulder Creek Lots 3 & 4 have a drainage area of approximately 2.47 acres (107,739 ft2). Pre-development topography is gradual, about 1 to 2 percent slope from south to north. Existing groundcover is undeveloped. The proposed property can be divided into flows that will be contained on-site (Basins DA 2 and DA3). Refer to SD1.0 in the original drainage report for Boulder Creek Site Plan Lots 1-4 for the drainage areas for the Boulder Creek Subdivision. See grading and drainage plans for more detail on retention and conveyance design. 3 Approach It will be shown that for the proposed development in Basins DA 3 and Da2, the use of proposed retention facilities will limit the post-development 10-year peak runoff rate to the pre-development rate, and will provide effective treatment of storm runoff. All storm drain pipes have been sized to convey the 25-year peak runoff. Retention ponds shall be sized based on a 10-year, 2-hour storm intensity per the City of Bozeman design standards. See Appendix A for rational method retention pond calculations for Basins DA 3 and DA2. The values used in each column of the table are as follows: C (Rational Method runoff coefficient): Runoff coefficients were calculated using 0.10 for landscaped areas and 0.95 for impervious areas (pavement, concrete and rooftops). Pro-rating by area resulted in the weighted coefficients used in the calculations. Page 2 of 9 Area: Calculated in AutoCAD from Figures 1 and 2 Intensity The rainfall intensity value of I = 0.41in/hr is used per the City of Bozeman design standards for a 10-year, 2-hour storm. Retention Volume Retention volume will be the stormwater runoff from the 10-year, 2-hour storm event or the first 0.5 inch of rainfall over parking impervious surfaces, whichever is greater 3 Retention Ponds As discussed previously, stormwater runoff from major Basins DA2 & DA3 is proposed to be retained by surface storm water retention basins. In accordance with the Design Standards and Specifications Policy, the retention facilities have been designed based on the 10-year, 2-hour storm event. The original drainage report had previously sized Retention Pond #2 the contributing drainage basin DA2 includes drainage from Lots 1 & 2. The same methodology used to design Retention Pond #2 as used to design Retention Pond #3 and is included in Appendix A. A summary of the design parameters and sizing of the reservoirs is highlighted below. Table 1 – Retention Facilities Sizing Summary Drainage Basin(s) Required 10-Year, 2-Hour Retention Volume (ft3) Design System Storage Volume (ft3) Additional Design Notes DA 2 Surface Retention Basin 1,454 1,471 Maximum Water Depth = 1 ft Maximum Depth of Pond = 2.50 ft Side and End Slopes = 4:1 (Horizontal : Vertical) DA 3 Surface Retention Basin 522 634 Maximum Water Depth = 1 ft Maximum Depth of Pond = 2.50 ft Side and End Slopes = 4:1 (Horizontal : Vertical) 4 Groundwater Groundwater data has been collected from the adjacent Northwest Crossing Subdivision to the north of the proposed project and has been included in this report. Groundwater monitoring well #1 is the closest to the Boulder Creek Subdivision and had a high groundwater elevation of 1.92’ below finished grade collected on 8/21/2019. Using this information, the high groundwater at DA 2 Retention pond would be approximately 4738.6 and the pond bottom is 4739.0. Retention pond DA 3 has a high groundwater elevation of approximately 4737.8 with the pond bottom being 39.75. The ponds were placed as high as possible to avoid the high groundwater in the area. 5 Conveyance The storm drain piping system for the proposed development is designed to convey flows from a 25-year storm. Provisions for overland flow of larger, less frequent storms up to the 100-year storm will be incorporated into the site grading. The calculations are Page 3 of 9 provided for representative pipes as required to ensure pipe size is adequate for all pipes; adequacy of other pipe sizes is evident based on the calculated basin areas and flow rates for the representative pipes. A minimum pipe diameter of 12-inches at a minimum slope of 0.5% was chosen for the Basin 3 conveyance design. A 1’ wide rectangular channel sloped at 1% will be used to convey flows from Drainage Basin DA2 to the Retention Pond #2. Appendix B for the conveyance calculations for the storm drainpipe. 6 Maintenance Considerations The owner, Boulder Creek HOA, shall maintain their components of the storm drainage system in accordance with the following maintenance plan and with the applicable City of Bozeman regulations. Maintain a written account of all maintenance and repair activities, such as a logbook, for future reference. Storm Water Conveyance Facilities Storm drain inlets, catch basins, and piping should be inspected at least once per year and following large storm events. Any necessary repair or maintenance should be prioritized and scheduled through the spring, summer, and fall. These items may include inspecting for any damage, removing blockages, cleaning and flushing the length of pipes, establishing vegetation on bare slopes at or near inlets, and sediment removal. Storm Water Retention Facility Maintenance of retention basin is also essential. General objectives of maintenance are to prevent clogging, standing water and the growth of weeds and wetland plants. This requires frequent unclogging of the outlets, inlets, and mowing. Cleaning out sediment with earth-moving equipment may also be necessary in 10 to 20 years. 7 Conclusions The included analyses and calculations show that the storm water management system for the proposed development will adequately handle the design storm events. During the 100-year design storm recurrence interval, ponding may occur at inlets due to their capacities; however, depths are estimated to remain at or below the top-back of curb in the lower areas of the site. If clogging of inlets or larger storm water runoff events occur provisions have been included in the design for overflows without presenting a hazard to the public or adjacent property. Based on the included analyses and calculations, the proposed storm water management system meets the requirements of the City of Bozeman Design Standards and Specifications Policy. Page 4 of 9 Appendix A Retention Pond #3 Sizing Page 5 of 9 Boulder Creek Site Plan (Lots 1-4, Block 5) Retention Pond #3 Pond ID 3 Accepts flow from Drainage Area 3 (DA3) [see DS1.0] Design Rainfall Freq 10 yr IDF coefficient a 0.64 IDF coefficient b IDF coefficient n 0.65 *Pre-development information (from C&H report) C Areas (ft2): Dense Residential 38501.42 0.50 Retention Pond Calculations Q = CIA C = 0.50 (pre-development) I = 0.41 in/hr (10-yr/ 2-hr storm) A = 0.88 acres Q = 0.18 cfs Required Retention Storage (ft3) = 1,296ft3 *Post-development information C Areas (ft2): Dense Residential 38501.42 0.70 Retention Pond Calculations Q = CIA C = 0.70 (post-development for Residential Apartments) I = 0.41 in/hr (10-yr/ 2-hr storm) A = 0.88 acres Q = 0.25 cfs Required Retention Storage (ft3) = 1,818ft3 Difference between pre and post runoff volumes = 1,818ft3 – 1,296ft3 = 522.4ft3 Page 6 of 9 Note – Detention Pond #5 in the northwest corner of Boulder Creek Subdivision was designed by C&H Engineering to hold the runoff from this lot using a C value of 0.5. Given the nature of the development on this lot, the C value is likely higher than 0.5 due to the actual density proposed. Therefore, a C value of 0.7 was used for the Post- development runoff calculations and the difference between the two runoff volumes will be retained on site. The 0.7 value was selected from the table of Rational Method Runoff C-Coefficients for “Residential Apartments” from Appendix 20.A of the Civil Engineering Reference Manual by Michael R. Lindeburg, PE. Retention Pond Sizing Calculations Design depth of pond 1.5 feet Max Side Slope 4.0 horizontal to 1.0 vertical Volume held between contours: Contour Area (ft2) Delta V(ft) Cumulative Volume Water Depth 4739.75 207.72 4740.75 486 347 347 1 4741.25 663.24 287 634 1.5 Design Storage at 1.5’ depth (ft3) Page 7 of 9 Appendix B Conveyance Page 8 of 9 Conveyance #2 Design Rainfall Freq. 25 year IDF coefficient a 0.78 IDF coefficient b 0.00 IDF coefficient n 0.64 Post-development Calculations (see SD1.0) Total area: 2.51 acres Composite C: 0.70 Conservative Tc: 5 minutes Intensity at tc (Fig. I-2): 3.75 in/hr Post-development peak runoff: 6.59 cfs Conveyance for Rectangular Channel Sidewalk Chaise @ 1%: 9.1 cfs Conveyance #3 Design Rainfall Freq. 25 year IDF coefficient a 0.78 IDF coefficient b 0.00 IDF coefficient n 0.64 Post-development Calculations (see SD1.0) Total area: 0.81 acres Composite C: 0.70 Conservative Tc: 5 minutes Intensity at tc (Fig. I-2): 3.75 in/hr Post-development peak runoff: 2.13 cfs Conveyance for 12” Pipe @ 0.5% slope: 3.17 cfs Conveyance for 15” Overflow @ 2.5% slope: 12.86 cfs Channel Report Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc.Monday, Apr 4 2022 12in Pipe @ 0.5% slope Circular Diameter (ft) = 1.00 Invert Elev (ft) = 100.00 Slope (%) = 0.50 N-Value = 0.011 Calculations Compute by: Q vs Depth No. Increments = 10 Highlighted Depth (ft) = 0.90 Q (cfs) = 3.173 Area (sqft) = 0.74 Velocity (ft/s) = 4.26 Wetted Perim (ft) = 2.50 Crit Depth, Yc (ft) = 0.77 Top Width (ft) = 0.60 EGL (ft) = 1.18 0 1 2 3 Elev (ft)Depth (ft)Section 99.50 -0.50 100.00 0.00 100.50 0.50 101.00 1.00 101.50 1.50 102.00 2.00 Reach (ft) Channel Report Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc.Thursday, Sep 29 2022 15in Pipe @ 2.5% Slope Circular Diameter (ft) = 1.25 Invert Elev (ft) = 100.00 Slope (%) = 2.50 N-Value = 0.011 Calculations Compute by: Q vs Depth No. Increments = 10 Highlighted Depth (ft) = 1.13 Q (cfs) = 12.86 Area (sqft) = 1.16 Velocity (ft/s) = 11.05 Wetted Perim (ft) = 3.13 Crit Depth, Yc (ft) = 1.23 Top Width (ft) = 0.75 EGL (ft) = 3.02 0 1 2 3 Elev (ft)Section 99.50 100.00 100.50 101.00 101.50 102.00 Reach (ft) Page 9 of 9 Appendix C Stormwater Management Design Report for Boulder Creek Site Plan Lots 1-4, Block 5, Bozeman, MT '.STORM STORM STORM STORM STORM STORM N77° 01' 5 9 . 3 3 " W 1 0 6 . 8 9 ' N89° 18' 14.00"E 213.54'N89° 18' 14.00"E 293.22' N 3 5 ° 5 5 ' 2 4 . 9 0 "W 1 0 7 . 7 3 '43.84'N2° 42' 42.11"W 53.31'9.47'S23° 00 ' 31 . 4 3 "E 1 41 .93 ' N90° 00' 00.00"W 391.12' N90° 00' 00.00"W 391.12' S23° 00' 31.43"E 32.59' 100.00'100.00'107.45' N90° 00' 00.00"E 368.21' N23° 00' 31.43"W 12.77'N11° 12' 45.64"W 130.85'N11° 12' 45.63"W 90.42'S0° 43' 38.03"W 66.41'N0° 43' 38.02"E 557.51'N0° 00' 00.00"E 63.01'R=330.00'L=93.33'Δ=16°12'13"R=330.00'L=39.20'Δ=6°48'19"S23° 00 ' 31 .43 "E 1 1 2 . 12 ' S23° 00' 31.43"E 32.59'S23 ° 0 0 ' 3 1 .43 "E 67 .27 'R=270.00'L=81.02'Δ=17°11'38"R=270.00'L=27.40'Δ=5°48'54"S0° 00' 00.00"E 125.06'S0° 00' 00.00"E 120.01'R=330.00'L=65.93'Δ=11°26'51"R=330.00' L=10.70' Δ=1°51'25"S23° 00 ' 31 . 4 3 "E 6 4 .09 ' S23° 00' 31.43"E 30.00'S2 3 ° 00 ' 31 .43 "E 69 .5 5 'S2 3 ° 00 ' 31 .43 "E48. 3 4 'R=27 0 .00'L=26.46'Δ=5° 3 6 '54" R=270.00' L=7.16' Δ=1°31'13"N0° 00' 00.00"E 63.01'N90° 00' 00.00"E 90.19' N0° 00' 00.00"E 3.39' N90° 00' 00.00"E 64.50' S0° 00' 00.00"E 3.30' S89° 58' 04.16"E 85.54' N86° 58' 55.72"E 30.35' N78° 49' 46.19"E 20.98'S11° 13' 48.14"E49.73'N78° 49' 28.94"E 72.00' '.STORM STORM STORM STORM STORM STORM N77° 01' 5 9 . 3 3 " W 1 0 6 . 8 9 ' N89° 18' 14.00"E 213.54'N89° 18' 14.00"E 293.22' N 3 5 ° 5 5 ' 2 4 . 9 0 "W 1 0 7 . 7 3 '43.84'N2° 42' 42.11"W 53.31'9.47'S23° 00 ' 31 . 4 3 "E 1 41 .93 ' N90° 00' 00.00"W 391.12' N90° 00' 00.00"W 391.12' S23° 00' 31.43"E 32.59' 100.00'100.00'107.45' N90° 00' 00.00"E 368.21' N23° 00' 31.43"W 12.77'N11° 12' 45.64"W 130.85'N11° 12' 45.63"W 90.42'S0° 43' 38.03"W 66.41'N0° 43' 38.02"E 557.51'N0° 00' 00.00"E 63.01'R=330.00'L=93.33'Δ=16°12'13"R=330.00'L=39.20'Δ=6°48'19"S23° 00 ' 31 .43 "E 1 1 2 . 12 ' S23° 00' 31.43"E 32.59'S23 ° 0 0 ' 3 1 .43 "E 67 .27 'R=270.00'L=81.02'Δ=17°11'38"R=270.00'L=27.40'Δ=5°48'54"S0° 00' 00.00"E 125.06'S0° 00' 00.00"E 120.01'R=330.00'L=65.93'Δ=11°26'51"R=330.00' L=10.70' Δ=1°51'25"S23° 00 ' 31 . 4 3 "E 6 4 .09 ' S23° 00' 31.43"E 30.00'S2 3 ° 00 ' 31 .43 "E 69 .5 5 'S2 3 ° 00 ' 31 .43 "E48. 3 4 'R=27 0 .00'L=26.46'Δ=5° 3 6 '54" R=270.00' L=7.16' Δ=1°31'13"N0° 00' 00.00"E 63.01'N90° 00' 00.00"E 90.19' N0° 00' 00.00"E 3.39' N90° 00' 00.00"E 64.50' S0° 00' 00.00"E 3.30' S89° 58' 04.16"E 85.54' N86° 58' 55.72"E 30.35' N78° 49' 46.19"E 20.98'S11° 13' 48.14"E49.73'N78° 49' 28.94"E 72.00'ABIGAIL LANE PHASE 1 PHASE 2 SHERWOOD WAY BASIN DA3 BASIN DA2 BASIN DA1 VERIFY SCALE! THESE PRINTS MAY BE REDUCED. LINE BELOW MEASURES ONE INCH ON ORIGINAL DRAWING. MODIFY SCALE ACCORDINGLY! 2022COPYRIGHT © MORRISON-MAIERLE, INC., SHEET NUMBER PROJECT NUMBER DRAWING NUMBER DATEDESCRIPTIONNO.BY N:\4192\025 - THE BOULDERS SITE DEVELOPMENT\ACAD\SHEETS\EX 1 - BOULDER CREEK LOTS 1 - 4.DWG PLOTTED BY:CELINE SAUCIER ON Sep/29/2022 REVISIONS DRAWN BY: DSGN. BY: APPR. BY: DATE: Q.C. REVIEW DATE: BY: 2880 Technology Blvd West Bozeman, MT 59718 406.587.0721 www.m-m.net engineers surveyors planners scientists MorrisonMaierle BOULDER CREEK APARTMENTS 4192.025 MONTANA STORMWATER DRAINAGE EXHIBIT POST DEVELOPMENT DELINEATION SD1.0 CMS MEE BOZEMAN 40 8020400 SCALE IN FEET RETENTION POND #3 RETENTION POND #2 RETENTION POND #1