HomeMy WebLinkAboutStormwater Design Report 01-13-2020 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)