The URL can be used to link to this page

Home My WebLink About20 - Design Report - Mountain Vista - Stormwater DESIGN REPORT STORMWATER MANAGEMENT MOUNTAIN VISTA 2829 W. BABCOCK ST. Prepared for: GLEN HAVEN PROPERTIES, INC 1516 W. Babcock St., STE 1 Bozeman, MT 59715 Prepared by: C&H Engineering and Surveying, Inc. 1091 Stoneridge Drive, Bozeman, MT 59718 (406) 587-1115 Project Number: 180334 October 2019 Updated: December 2019 INTRODUCTION The proposed site plan for 2829 W. Babcock Street includes 11 duplex residential buildings and 3 single residential buildings, for a total of 25 units. The property is legally described as the tract of land described in Document No. 2046120, EXCEPT that part thereof conveyed to the City of Bozeman in Document No. 2190840, all on file and of record in the office of the Clerk and Recorder, Gallatin County, Montana, and located in the NE 1/4 of the SW 1/4 of Section 11, T. 2 S.,R. 5 E.of P.M.M.,City of Bozeman,Gallatin County,Montana. The site plan shows a number of asphalt drive aisles, concrete sidewalks, concrete curb and gutter, and driveways that are proposed with the development. A combination of site grading of pavement, curb and gutter, a retention pond and an underground infiltration chambers will be used to manage stormwater runoff on the site. A Drainage Area Map is included in Appendix A. Calculations for each individual drainage area(total area and weighted C factor) are included in Appendix B. STORM WATER FACILITIES DESIGN The existing site currently drains stormwater to the north, towards West Babcock Street. The eastern portion of the site also drains into Upper Cattail Creek. The proposed development would direct runoff from the additional impermeable surfaces to a retention pond and underground infiltration chamber located near the northwest corner of the site. The proposed stormwater facilities are designed to retain the difference in the pre-development conditions and the post-development conditions. With a 10-year, 2-hour design storm per the City of Bozeman Design Standards for retention facilities, the pre-development runoff from the site is 1,582 cubic feet. Runoff from the proposed site plan is 4,800 cubic feet. The total amount of runoff required to be retained onsite is 3,218 cubic feet. The runoff will be collected and transmitted via a combination of surface and pavement flow and curb and gutter to 2' wide curb cut. From there,the water will directly in to Retention Pond#1. The proposed volume of the pond is 1,669 cubic feet. An underground infiltration system will be installed directly below the retention pond to provide additional retention and infiltration capacity.As stormwater fills Retention Pond#1,the water will either infiltrate in to the void space of the infiltration chamber, or it will reach the inlet of the catch basin set 0.5' below the top of Retention Pond#1. The required volume of the infiltration chamber is 1,549 cubic feet. The proposed volume of the chamber is 1,551 cubic feet. The total amount of retention provided with these systems is 3,220 cubic feet. A Drainage Area Map can be found in Appendix A. Calculations for the drainage areas can be found in Appendix B. APPENDIX A DRAINAGE AREA MAP EXISTING CONDITIONS LOCATED IN THE SW 114, SECTION 11, T. 2 S., R. 5 E. OF P.M.M., CITY OF BOZEMAN, GALLATIN COUNTY, MONTANA 8 W.s�acocu 8T1EET 55 SS r - T - LEGEND '°"W EXISTING HARDSCAPED AREA 3 0 HARDSCAPED 3,000 SF °D [S89'09'57"E] 0 LANDSCAPED I — W 104.60 �OOT07'42" : :2'07'42"E] I N� Ut 110.00 090050'03" g [S89'09'57"E] . .' . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . as� c O .'.' N EXISTING LANDSCAPED AREA o M ' Lo 103,052 SF �.. fN ' u C) ' . G C) . 0)N p 0 CN Z ' Sri co N SS B. ,sSS.s 4 / 301.65 270'48'04" \ IN89-11'56"W] Scale In Feet 60 0 60 18 0 18 Engineering and Surveying Inc. Scale In Meters 1091 Stoneridge Drive•Bozeman, MT 59718 Contour Intervals: I Foot Phone(406)587-1115•Fax(406)587-9768 www.chengineers.com•info@chengineers.com #180334 PROPOSED CONDITIONS LOCATED IN THE SW 114, SECTION 11, T. 2 S., R. 5 E. OF P.M.M., CITY OF BOZEMAN, GALLATIN COUNTY, MONTANA g W.enaooat arnar — ss ss LEGEND PROPOSED 10"W RETENTION/INFILTRATION CHAMBER REQ. VOL.= 1,549 CF 1 1.00 090'50 03 HARDSCAPED PRO. VOL.= 1,551 CF 00 [$$9'Q9'S7"E] 0 LANDSCAPED PROPOSED RETENTION POND ... TIME OF PRO. VOL.=1,669 CF W CONCENTRATION 104.60 �OOT07'42" 2'07'42"E] ;: ''`' `:: ' OD '• 110.00 090'50'03" M. [S89'09'57"E] 4821.90 �. ,:. ..: PROPOSED ,• a"ss HARDSCAPED AREA ,. .. .:. g5,805 SF to M O Qo N Z PROPOSEDUi } 007= - LANDSCAPED C4 ss„e AREA 39,402 SF 4826.55 301.65 270'48'04" \ [N89'11'56"W] Scale In Feet 60 0 60 18 0 18 Engineering and Surveying Inc. Scale In Meters 1091 Stoneridge Drive•Bozeman, MT 59718 Contour Intervals: I Foot Phone(406)587-1115•Fax(406)587-9768 uwwv.chengineers.com•info@chengineers.com #180334 APPENDIX B DRAINAGE AREA CALCULATIONS DRAINAGE AREA-EXISTING 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft`) C *Area Hardscape 0.95 3000 2850 Landscape 0.20 103052 20610 Total 106052 23460 A =Area(acres) 2.4346 C= Weighted C Factor 0.22 2. Calculate Discharge Volume Q = CIA V=72000 C = Weighted C Factor 0.22 1 = intensity (in/hr) 0.41 (10 yr, 2hr storm) A=Area (acres) 2.43 Q = runoff (cfs) 0.22 V = REQUIRED VOL (ft) 1582 DRAINAGE AREA 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft`) C *Area Hardscape 0.95 66650 63318 Landscape 0.20 39402 7880 Total 106052 71198 A =Area(acres) 2.4346 C= Weighted C Factor 0.67 2. Calculate Tc(Time to Concentration) Tc Overland Flow Tc = 1.87 (1.1-CC')D'/2/S'/3 ..----------------------------- Storm S = Slope of Basin (%) 1.50% Return (yrs) Cf C = Rational Method Runoff Coefficient 0.95 12 to 10 1 Cf= Frequency Adjustment Factor 1.1 11 to 25 1.1 D = Length of Basin (ft) 103 ;26 to 50 1.2 51 to 100 1.25 ,.----------------------------- Tc Overland Flow(minutes) 1.7 Tc Gutter Flow Tc = L/V/60 V= (1.486/n)R213 S112 n = Mannings Coefficient 0.013 R = Hydraulic Radius A/P (ft) 0.13 (0.15' below top of curb) S = slope (%) 1.50% L= length of gutter (ft) 223 V= mean velocity (ft/s) 3.67 Tc Gutter Flow(minutes) = 1.0 Tc Total= 5.0 (5 minute minimum) 2. Calculate Discharge Volume Q = CIA V=7200Q C = Weighted C Factor 0.67 1 = intensity (in/hr) 0.41 (10 yr, 2hr storm) A=Area (acres) 2.43 Q = runoff (cfs) 0.67 V= REQUIRED VOL (ft) 4800 Requirea Retention Volume (pre-develompent runoff-post-development) (ft) 3218 CONVEYANCE STRUCTURES 25-yr Flow Rate REQUIRED CAPACITY-DA1 1. Calculate Area and Weighted C Factor(Post-Development) Contributing Area C Area (fV ) C *Area Hardscape 0.95 66650 63318 Landscape 0.20 39402 7880 Total 106052 71198 A =Area(acres) 2.4346 C= Weighted C Factor 0.67 2. Calculate Rainfall Intensity(Duration=Max Tc from Contributing Drainage Areas) i = 0.78x-0.64 (10-yr Storm, Fig. 1-3, COB Design Standards) x = storm duration (hrs) 0.08 (DA#1) i=rainfall intensity(in./hr.) 3.83 3. Calculate 25-yr Pond Outflow Rate Q = CiA C = Rational Method Runoff Coefficient 0.67 (calculated above) i = rainfall intensity (in./hr.) 3.83 (calculated above) A=Area (acres) 2.43 (calculated above) Q=25-yr Flow Rate(cfs) 6.25 MANNING'S EQUATION for OPEN CHANNEL FLOW-CURB CHASE Project: Mountain Vista Location: Bozeman, MT By: AMK Date: 10/10/2019 Chk By: Date: INPUT z (sideslope)= 0 Mannings Formula - - -.... -.... - -.-.-...- -.-.-.-.-.... - - - z (sideslope)= 0 1 d 1 w (btm width, ft)= 1.5 Q = (1.486/n)ARh2i3S1i2 Z d (depth, ft)= 0.5 R = A/P z S (slope, ft/ft) 0.111 A= cross sectional area d w I n low= 0.013 P=wetted perimeter n high = 0.013 S= slope of channel V= (1.49/n)Rh2i3S1i2 Clear Data n = Manning's roughness coeffi Q =V x A Entry Cells Low N High N Wetted Perimete Hydraulic Velocity, Depth, ft Area, sf r, ft Radius, ft Velocity, fps Flow, cfs fps Flow, cfs 0.5 0.75 2.50 0.30 17.07 12.80 17.07 12.80 T= 1.50 Dm = 0.500 Sc low = 0.0061 Sc high = 0.0061 so = critical slope ft/ft T = top width of the stream .7 Sc 1.3 Sc .7 Sc 1.3 Sc dm = a/T = mean depth of flow 0.0043 0.0080 0.0043 0.0080 Conc 0.013 Nyloplast 30"Dome Grate Inlet Capacity Chart 16.00 14.00 12.00 10.00 s 8.00 U 6.00 4.00 2.00 0.00 0.00 0.05 0.10 0.15 020 0.25 0.30 0.35 0,0 0.45 0 0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.00 1.05 1.10 Head(ft)