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HomeMy WebLinkAbout20 - Design Report - Bozeman East Mixed - DrainageBozeman East Mixed-Use Storm Sewer Report 1 INTRODUCTION The Bozeman East Mixed-Use development is the final phase of the Bridger Peaks Town Center and consists of 4 apartment buildings, a clubhouse, commercial retail space and an outdoor recreation space. The pre-development topography is gradual, about 1 to 2 percent slope from south east to north west. Existing ground cover is undeveloped, formerly agricultural fields, and is surrounded by previous retail and residential development phases. This report provides hydraulic calculations for sizing of storm water conveyance and detention storage facilities for the development of the Bozeman East Mixed-Use Site in accordance with the City of Bozeman Design Standards. 2 APPROACH The proposed development lies within the drainage basin of the following previously approved drainage submittal, attached at the end of this report: · Revised Drainage Design Report for Bridger Peaks Town Center Bozeman, Montana (October 16, 1998) It will be shown that the proposed detention facilities will limit the post-development 10-year peak runoff rate to the pre-development rate and provide effective treatment of storm runoff. All storm drain pipes were sized to convey the 25-year peak runoff. 3 EXISTING CONDITIONS As described in the Bridger Peaks Town Center Preliminary Plat/PUD application dated June 1998, the project will be constructed in phases. Phases I and II have already been constructed with the remaining final phase being the Bozeman East development. In the original design of Bridger Peaks, the subdivision was divided into 3 historic basins H-1, H-2 and H-3, see Figure 2 in the Revised Drainage Design Report. The post development strategy for the historic basins was planned as follows: H-1 H-1 is the combination of both H-1A and H-1B which had previously drained to the roadside ditch along North 19th Avenue/Simmental Way. H1-A was calculated to have a post- development release rate less than that for the existing conditions, therefore detention is not necessary (p. 30). Detention and treatment for Basin H-1B is provided along with Basin H-2. H-2 Detention for Basins H-1B and H-2 is provided by Pond 2 in the Northeast corner of the project site (see Figure 2). Pond 1 in the Northwest corner of the project site is used primarily to attenuate peak flows in order to reduce pipe sizes in the on-site storm drain system. Detention storage from Pond 1 is neglected in the Pond 2 design; therefore no detention calculations are provided for Pond 1. Pond 3 on the eastern side of the site exists for sediment retention during construction of Phases I and II but is not intended as a permanent detention pond. Pond 2 will limit the combined 10-year release rate of Basins H-1B and H2 to a peak rate less than or equal to the pre-development rate for basin H-2 alone. This requirement has been met for Phase I/II construction and will be the design standard for the fully developed conditions. H-3 Basin H-3 covers a portion of the Oak Street right-of-way and entryway corridor. Pond 4 on the east side of H-3 will be used to fully retain the 10-year, 2-hour storm runoff from this area. Pond 2 As described above and in the Revised Drainage Design Report, most of the Bridger Peaks site (historic basins H-1B and H-2) are graded to drain through storm pipes and surface ditches to Pond 2, a large detention basin at the northeast corner of the site. See the Revised Drainage Design Report for the relevant sizing calculations for the existing on-site surface ditches and storm drain pipe. As a part of Basin H-2, the Bozeman East development will also be designed to drain to Pond 2. The current pond has been sized to detain runoff volume from Phases I & II and will need to be expanded per the Revised Drainage Design Report to store the runoff for the fully developed conditions. Additionally, the existing orifice structure has a release rate equal to the pre-development conditions of Phase I/II. With the construction of the Bozeman East development, the orifice will be modified control runoff under fully developed conditions. The calculations are shown in section 5 of this report. 4 PROPOSED CONDITIONS In accordance with City of Bozeman design standard, the rational method was used to calculate the peak runoff and detention requirements for the entire Bridger Peaks development. The post development detention calculation (Appendix A) is an Excel spreadsheet with formulas that calculate the required detention. The values used in each column of the table are as follows: Area: Calculated in AutoCAD C (Rational Method runoff coefficient): The previously approved Revised Drainage Design Report for Bridger Peaks Town Center calculated the C for the full buildout using the measured C of Phases I/II combined with a conservative assumption of 0.8 for the final phase. The runoff coefficients for the final phase/Bozeman East development were more accurately calculated using 0.2 for pervious area (pre-development and developed landscape areas), 0.65 for gravel area, and 0.9 for impervious areas (pavement, concrete and rooftops). The actual C for the Bozeman East development was determined to be 0.70 which is less than the conservative estimate of 0.8 that was used in the previous calculations. The new runoff coefficient for the proposed development was then combined with the coefficients for Phase I/II (p. 27) to obtain a coefficient for the entire Bridger Peaks development. The following calculations show the original Phase I/II Post Development and Fully Developed Calculations from the Revised Drainage Design Report for Bridger Peaks Town Center. Lastly the C has been recalculated for the Full Development replacing the assumed 0.8 coefficient with the actual coefficient from the site plan of 0.7. From p.27 Detention Calculations: Phase I/II Final Phase Undev. Phase I/II Post Dev. C = 0.9(19.88) + 0.25(2.22) + 0.20(9.80) = 0.64 31.9 acres From p. 28 Fully Developed Calculations: Phase I/II Final Phase Dev. Fully Developed Post Dev. C = 0.9(19.88) + 0.25(2.22) + 0.80 (11.4) = 0.82 33.5 acres Revised Fully Developed Calculations: Phase I/II Final Phase Dev. Fully Developed Post Dev. C = 0.9(19.88) + 0.25(2.22) + 0.70 (11.4) = 0.79 33.5 acres Tc (time of concentration): Determined based on the time required for runoff to travel from the hydraulically most distant point in the watershed to the outlet. The Tc of 9.9 min from the original drainage report was used as this number considers the most hydraulically distant point from Pond 2 (p.27). Storm Intensity: Determines the 10-year storm intensity at the time of concentration Peak Runoff (Qp): Value calculated using the Ration Method formula (Q = C i A) and the intensity formulas in the City’s design standards (Rainfall Intensity – Duration Curve). The Peak Runoff in this case is for the 10-year design storm. Release Rate: From the original drainage design report, the 10-year allowable pre-development release rate for the entire subdivision is 4.08cfs. The resulting detention requirement for the full build out of the Bridger Peaks Town Center is 57,809 ft3, see Appendix A for spreadsheet calculations. Pond 2 will be sized to a volume of 59,432 ft3 based on the following contour information. 5 DETENTION BASIN OUTLET STRUCTURE The detention basin outlet structure for Pond 2 is located under the berm which impounds Pond 2, and consists of a 15-inch inlet and outlet pipes connected to a 24-inch riser within a concrete manhole. The pipe and riser is made of corrugated outer wall, smooth interior HDPE pipe. The control structures inlet and outlet pipe is sized to convey the maximum allowable site release rate of 6.41 cfs as calculated in the Preliminary Plat/PUD application. The orifices on the riser will be places such that the maximum releaser rate will not be exceeded during periods of low tailwater. With the final phase of construction, the Pond 2 orifice will be modified to control runoff under fully developed conditions. Orifice Calculations for Fully Developed Conditions: Dewatering Orifice (2” dia.) = 2ℎ Where: C= 0.6 a = orifice area, sq. ft G = 32.2 ft/sec2 h = head to center of orifice 15” Outlet Pipe I.E. = 16.21 = 0.6 112 2(32.2)(20.8 − 16.21)= 0.23 Primary Orifice (square): Side dimension = . Where: C= 0.6 a = orifice area, sq. ft G = 32.2 ft/sec2 Inlet control on 15” outlet pipe = 17.49 Side dimension = .!".# .$(#.)(.%"&%. ’) .= 0.67)= 8.02" +,-. Therefore, the control structure will be as follows under fully developed conditions: Top of Riser Elevation = 20.7 Dewatering Orifice = 2” diameter, center elevation 15.00 Primary Orifice = 8.0” square, center elevation 17.10 10-yr. peak release rate (post-development) = 4.08cfs 6 CONVEYANCE The storm drain piping being proposed for the site will convey both existing and proposed runoff to Pond 2 at the northeast corner of the site. The proposed site has been divided into 3 basins, see Appendix B. Inlets throughout the site will collect the storm water. The piping, in accordance with City Standards shall be designed to accommodate the 25-year storm event. Existing Conveyance At the northwest corner of the site, an existing 30” RCP pipe daylights to the north of Building 4/D, these existing flows then swale to the east and into Pond 2. Based on Figure 1 Phase I/II Developed Drainage Map in the approved Revised Drainage Design Report this pipe, P1, has a contributing area of 14.5 acres. Existing flows at the south of the site will be conveyed by proposed storm drains connecting to two existing southern storm drain stubs. Based on Figure 1 Phase I/II Developed Drainage Map in the approved Revised Drainage Design Report that the stubs are P16 and P18 with contributing areas of 1.6 and 5.8 acres respectively The original detention calculations for Phase I/II Post development used a C value of 0.64 and a Tc of 9.9 for the existing flows. These will be used when calculating the contribution of the existing flows in the development. Proposed Conveyance With the construction of Bozeman East, this existing pipe P1 will be extended to Pond 2 while also collecting contributing flows from proposed Basin 1 & Basin 4. The flows from P16 and P18 will be conveyed to Pond 2 while also collecting flows from Basin 5. The pipe sizing is shown in Appendix C while the following table summarizes the existing and proposed conveyance calculations. Existing Conveyance Area C Tc Q25 Pipe Dia. Slope Capacity Full P16 - Ex 18" Stub 1.60 0.64 9.9 2.53 18" 3.13 23.41cfs P18 - Ex 24" Stub* 5.80 0.64 9.9 9.17 24" P1 - Ex 30" Stub 14.50 0.64 9.9 22.93 30" 0.16 20.67cfs *The slope for the 24" Stub is unknown Proposed Conveyance Area C Tc Q25 Pipe Dia. Slope Capacity Full Basin 1 4.80 0.79 5.0 14.51 24" 0.30 15.61 cfs Basin 2 1.10 0.79 5.0 3.32 12" 0.75 3.89 cfs Basin 3 0.52 0.79 5.0 1.57 8" 1.20 1.69 cfs Basin 4 w/ P1** 15.27 0.65 9.7 24.83 30" 0.45 34.66cfs Basin 5 w/ P16 & P18** 9.58 0.67 8.8 17.23 21" 1.10 21.86 cfs ** The combined flows were calculated using the total areas weighting the C and Tc values Appendix A Bozeman East Mixed Use MODIFIED RATIONAL METHOD Qp = C i A POST-DEVELOPMENT 33.50 AC 0.79 9.9 MIN Release rate = 4.08 cfs 2.06 IN/HR TIME STEP 54.50 CFS DURATION = 5.0 min. Max. Volume, Max. Volume,Required Detention Triangle Rel. (cu. ft.) Constant Rel. (cu. ft.)Volume (cu. ft.) 68907.02 46710.03 57,809 Triangle Release Constant Release DURATION INTENSITY Qp POND VOLUME POND VOLUME (MIN) (IN/HR) (CFS) (CF) (CF) 9.41 2.13 56.35 29,433 27,246 19.31 1.34 35.31 37,322 34,027 29.21 1.02 26.98 42,486 38,066 39.11 0.85 22.32 46,360 40,805 ORIFICE CALCULATIONS 49.01 0.73 19.27 49,451 42,754 MATCHING Qp = 4.08 cfs 58.91 0.65 17.10 52,008 44,165 Max. Depth = 3.21 ft 68.81 0.59 15.46 54,173 45,180 78.71 0.54 14.16 56,036 45,888 88.61 0.50 13.11 57,656 46,353 SQUARE ORIFICE (as-built): 98.51 0.46 12.24 59,077 46,616 DIAMETER = 8.000 inches 108.41 0.44 11.50 60,331 46,710 Area = 0.44 ft 2 118.31 0.41 10.87 61,443 46,660 ORIFICE FLOW = 3.83 cfs 128.21 0.39 10.31 62,431 46,486 138.11 0.37 9.83 63,312 46,203 148.01 0.36 9.39 64,098 45,823 157.91 0.34 9.01 64,798 45,356 167.81 0.33 8.66 65,422 44,813 177.71 0.32 8.34 65,977 44,199 187.61 0.31 8.05 66,469 43,522 197.51 0.30 7.79 66,903 42,787 207.41 0.29 7.54 67,285 41,997 217.31 0.28 7.32 67,618 41,159 227.21 0.27 7.11 67,906 40,275 237.11 0.26 6.92 68,152 39,348 247.01 0.26 6.73 68,359 38,382 256.91 0.25 6.56 68,529 37,379 266.81 0.24 6.41 68,665 36,341 276.71 0.24 6.26 68,770 35,270 286.61 0.23 6.11 68,843 34,169 296.51 0.23 5.98 68,889 33,038 POST-DEV Qp = POND VOLUME CALCULATIONS: Total Detention Requirement (10 year storm) BASIN AREA POST = POST-DEV C = POST-DEV Tc = STORM INTENSITY = DURATION INTENSITY Qp POND VOLUME POND VOLUME (MIN) (IN/HR) (CFS) (CF) (CF) 306.41 0.22 5.85 68,907 31,880 316.31 0.22 5.73 68,900 30,697 326.21 0.21 5.62 68,868 29,488 336.11 0.21 5.51 68,814 28,256 346.01 0.20 5.41 68,737 27,002 355.91 0.20 5.31 68,640 25,727 365.81 0.20 5.22 68,522 24,431 375.71 0.19 5.13 68,386 23,116 385.61 0.19 5.04 68,231 21,783 395.51 0.19 4.96 68,059 20,431 405.41 0.18 4.88 67,870 19,063 415.31 0.18 4.80 67,665 17,678 425.21 0.18 4.73 67,444 16,277 435.11 0.18 4.66 67,208 14,861 445.01 0.17 4.59 66,959 13,431 454.91 0.17 4.53 66,695 11,987 464.81 0.17 4.47 66,418 10,529 474.71 0.17 4.40 66,128 9,057 484.61 0.16 4.35 65,826 7,574 494.51 0.16 4.29 65,512 6,078 504.41 0.16 4.23 65,186 4,570 514.30 0.16 4.18 64,849 3,051 524.20 0.16 4.13 64,501 1,521 534.10 0.15 4.08 64,142 -20 544.00 0.15 4.03 63,773 -1,572 553.90 0.15 3.98 63,395 -3,133 563.80 0.15 3.94 63,007 -4,704 573.70 0.15 3.89 62,609 -6,285 583.60 0.15 3.85 62,202 -7,875 593.50 0.14 3.81 61,787 -9,474 603.40 0.14 3.77 61,363 -11,082 613.30 0.14 3.73 60,930 -12,698 623.20 0.14 3.69 60,490 -14,323 633.10 0.14 3.65 60,041 -15,956 643.00 0.14 3.62 59,585 -17,596 652.90 0.14 3.58 59,121 -19,244 662.80 0.13 3.55 58,650 -20,900 672.70 0.13 3.51 58,172 -22,563 682.60 0.13 3.48 57,687 -24,233 692.50 0.13 3.45 57,194 -25,910 702.40 0.13 3.41 56,696 -27,594 712.30 0.13 3.38 56,191 -29,285 722.20 0.13 3.35 55,679 -30,982 732.10 0.13 3.32 55,161 -32,685 742.00 0.12 3.29 54,637 -34,394 751.90 0.12 3.27 54,108 -36,110 761.80 0.12 3.24 53,572 -37,831 771.70 0.12 3.21 53,031 -39,559 781.60 0.12 3.18 52,484 -41,292 791.50 0.12 3.16 51,932 -43,030 801.40 0.12 3.13 51,374 -44,774 811.30 0.12 3.11 50,811 -46,524 821.20 0.12 3.08 50,243 -48,278 831.10 0.12 3.06 49,670 -50,038 841.00 0.12 3.04 49,092 -51,803 850.90 0.11 3.01 48,509 -53,572 860.80 0.11 2.99 47,922 -55,347 870.70 0.11 2.97 47,330 -57,126 880.60 0.11 2.95 46,733 -58,910 890.50 0.11 2.93 46,132 -60,699 900.40 0.11 2.91 45,526 -62,492 910.30 0.11 2.88 44,916 -64,289 920.20 0.11 2.86 44,302 -66,091 Appendix B HYD HYDHYD HY DHYDHYDEEEEEESDSD SD SD SD SD SD SD S D S D S D SDS D SDSDS DSD SDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSD SD SLDSLDSLDSLDSLDSLDSLDSLDSLDSLDSLDSLD SLD SLD SLD SL D SLD SDSDSDSD SD SD S D SDSDSDSDSD SD SDPSD SD SD SD SD SD SD SD SDSDSDSDSDSDSDSDWWWWWWSDSDW SDP PSDSDSD SD SD SD SD SDPWVWVDYHWVWVPPDYHDYHPPDYHWVWVWVWVWVWVWVWVWVWVWVPPWWWWWWWWWWWWWWWSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSNGNGNGNGNGNGBPBPBPBPBPBPBPBFBFBFBFBTBTBTBCTVBCTVBCTVBCTVBCTVBCTVBCTVBCTVBCTVBCTVBCTVBCTVBCTVBCTVBCTVBCTVBCTVBCTVBCTVBCTVBCTVBCTVBCTVBCTVBCTVBCTVBCTVSDSDHYDHYDHYDHYDHYDHYDEEEEEEN1°29'24"E 551.27'S87°55'12"E 799.23'S2°21'23"W 493.32'N87°39'57"W 152.02'S 4 6 ° 2 9 ' 2 4 "W 2 8 0 . 5 4 ' S1°26'03"W 105.31'N88°30'36"W 189.83'N43°30'20"W 355.88'HYD HYDHYD HY DHYDHYDEEEEEEPROPOSED STORMDETENTION PONDBUILDING 4/DFFE: 4724.75BUILDING 1/A4723.50BUILDING 2/B4727.00BUILDING 3/CFFE: 4727.50BASIN 21.1 ACRES1REVIEWSUBMITTAL DEVELOPMENT APARTMENT UNITS IN BOZEMAN, MONTANA FARRAN REALTY PARTNERS BOZEMAN EAST MIXED-USE 4202 BELTWAY DRIVE ADDISON, TX 75001hedk.com214.520.8878 BASIN 14.8 ACRESBASIN 52.2 ACRESBASIN 3 0.52 ACRESBASIN 40.77 ACRES Appendix C Channel Report Hydraflow Express Extension for Autodesk® AutoCAD® Civil 3D® by Autodesk, Inc.Tuesday, Feb 18 2020 24in PVC @ 0.3% Circular Diameter (ft) = 2.00 Invert Elev (ft) = 100.00 Slope (%) = 0.30 N-Value = 0.011 Calculations Compute by: Q vs Depth No. Increments = 10 Highlighted Depth (ft) = 1.80 Q (cfs) = 15.61 Area (sqft) = 2.98 Velocity (ft/s) = 5.24 Wetted Perim (ft) = 5.00 Crit Depth, Yc (ft) = 1.43 Top Width (ft) = 1.20 EGL (ft) = 2.23 0 1 2 3 4 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 2.00 102.50 2.50 103.00 3.00 Reach (ft) Channel Report Hydraflow Express Extension for Autodesk® AutoCAD® Civil 3D® by Autodesk, Inc.Tuesday, Feb 18 2020 12in PVC @ 0.75% Circular Diameter (ft) = 1.00 Invert Elev (ft) = 100.00 Slope (%) = 0.75 N-Value = 0.011 Calculations Compute by: Q vs Depth No. Increments = 10 Highlighted Depth (ft) = 0.90 Q (cfs) = 3.886 Area (sqft) = 0.74 Velocity (ft/s) = 5.22 Wetted Perim (ft) = 2.50 Crit Depth, Yc (ft) = 0.84 Top Width (ft) = 0.60 EGL (ft) = 1.32 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® AutoCAD® Civil 3D® by Autodesk, Inc.Tuesday, Feb 18 2020 8in PVC @ 1.2% Circular Diameter (ft) = 0.67 Invert Elev (ft) = 100.00 Slope (%) = 1.20 N-Value = 0.011 Calculations Compute by: Q vs Depth No. Increments = 10 Highlighted Depth (ft) = 0.60 Q (cfs) = 1.689 Area (sqft) = 0.33 Velocity (ft/s) = 5.05 Wetted Perim (ft) = 1.68 Crit Depth, Yc (ft) = 0.60 Top Width (ft) = 0.40 EGL (ft) = 1.00 0 1 Elev (ft)Section 99.75 100.00 100.25 100.50 100.75 101.00 Reach (ft) Channel Report Hydraflow Express Extension for Autodesk® AutoCAD® Civil 3D® by Autodesk, Inc.Tuesday, Feb 18 2020 30in RCP @ 0.45% Circular Diameter (ft) = 2.50 Invert Elev (ft) = 100.00 Slope (%) = 0.45 N-Value = 0.011 Calculations Compute by: Q vs Depth No. Increments = 10 Highlighted Depth (ft) = 2.25 Q (cfs) = 34.66 Area (sqft) = 4.66 Velocity (ft/s) = 7.45 Wetted Perim (ft) = 6.25 Crit Depth, Yc (ft) = 2.00 Top Width (ft) = 1.50 EGL (ft) = 3.11 0 1 2 3 4 Elev (ft)Section 99.50 100.00 100.50 101.00 101.50 102.00 102.50 103.00 Reach (ft) Channel Report Hydraflow Express Extension for Autodesk® AutoCAD® Civil 3D® by Autodesk, Inc.Tuesday, Feb 18 2020 21in @ 1.1% Circular Diameter (ft) = 1.75 Invert Elev (ft) = 100.00 Slope (%) = 1.10 N-Value = 0.011 Calculations Compute by: Q vs Depth No. Increments = 10 Highlighted Depth (ft) = 1.58 Q (cfs) = 20.93 Area (sqft) = 2.28 Velocity (ft/s) = 9.18 Wetted Perim (ft) = 4.38 Crit Depth, Yc (ft) = 1.62 Top Width (ft) = 1.05 EGL (ft) = 2.88 0 1 2 3 Elev (ft)Section 99.50 100.00 100.50 101.00 101.50 102.00 Reach (ft)