HomeMy WebLinkAbout15 - Design Report - Valley Meadows - Stormwater DESIGN REPORT
STORMWATER MANAGEMENT
VALLEY MEADOWS SUBDIVISION
Prepared for:
Fortin Construction, LLC
P.O. Box 11811, Bozeman, MT 59719
Prepared by:
C&H Engineering and Surveying, Inc.
1091 Stoneridge Drive, Bozeman, MT 59718
(406) 587-1115
1 b
4 P
Project Number: 14328
May 2015
INTRODUCTION
The proposed Valley Meadows Subdivision will extend Meriwether Avenue north from Westglen
Subdivision to Durston Road. Villard Street will also be extended west approximately 330 feet
from where it currently dead ends in North Meadows Subdivision. The majority of the stormwater
runoff from the Valley Meadows Subdivision will be conveyed via surface flow in catch curb and
gutter to two storm sewer curb inlets.The two proposed inlets flow to a proposed detention pond to
the east of Meriwether Avenue on the north side of the property. The portion of Villard Street
which is west of Meriwether Avenue will flow offsite to the west. The portion of Meriwether
Avenue which is north of the two proposed inlets will flow offsite onto Durston. A drainage area
snap is included in Appendix A. Calculations for each individual drainage area(contributing area,
weighted C factor, and time to concentration) are included in Appendix B. For the purposes of this
report,each individual drainage area has been named to match the name of the inlet that it flows to.
STORM SEWER FACILITIES DESIGN
Storm sewer facilities were sized for the 25-yr storm using Hydraflow Storm Sewers software. For
each inlet,the contributing area,weighted C factor, and time to concentration were calculated(see
Appendix B). These values were input into the Hyrdraflow model to check capacity and flow
characteristics for inlets and storm drain pipes. For the purposes of this report, each pipe section
was named to match the associated upstrearn structure. Per the City of Bozeman's Design
Standards and Specifications Policy (DSSP), March 2004, pipes were sized to keep the hydraulic
grade line below the crown of the pipe. The Hydraflow Storm Sewer Summary Report is included
in Appendix C.
Gutter spread widths were also checked using Hydraflow. Maximum gutter spread width was
found to be 2.38' at inlet 1 B. This is well below the maximum spread width of 9.5' specified in the
DSSP. Gutter spread widths are included in the Inlet Report in Appendix C.
RETENTION/DETENTION POND DESIGN
Ponds have been sized according to City of Bozeman Design Standards. Retention ponds are sized
to capture the entire volume of the 10-year 2-hour storm event. Detention ponds are sized to limit
discharge to pre-development rates for the 10-year storm event.
Design Report-Page 1 of 2
Retention Pond #1
Retention Pond #1 is located to the east of Meriwether on the north side of the site. It receives
runoff from Drainage Areas IA and 113, totaling 7.76 acres. The required pond volume was
calculated to be 9,873 cubic feet. The preliminary pond design has a volume of 12,640 cubic feet.
The pond has been designed to handle a 10-yr storm. In the case of a storm exceeding the design
storm, runoff will overflow the pond banks and flow north onto Durston Road. Pond sizing
calculations are included in Appendix D.
Offsite Flows
The predevelopment runoff rate for the site was calculated to be 1.29 cfs (see supporting
calculations in Appendix B). This predevelopment runoff rate was then checked against the runoff
rates for portions of the site where flows will not be able to be captured due to topography. The
portion of Villard Street which is west of Meriwether Avenue will flow offsite to the west. This
area has been designated as Drainage Area#2 and has a post-development 10-yr runoff rate of 0.36
cfs. The portion of Meriwether Avenue which is north of the two proposed inlets will flow offsite
onto Durston. This area has been designated as Drainage Area 43 and has a post-development
10-yr runoff rate of 0.65 cfs. The combined post-development 10-yr runoff rate for these two areas
is 1.01 cfs which is below the pre-development runoff rate of 1.29 cfs. Please see Appendix B for
supporting calculations for these drainage areas.
Design Report-Page 2 of 2
APPENDIX A
DRAINAGE AREA MAP
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APPENDIX B
DRAINAGE AREA CALULATIONS
STORM INLET# 1 A
1. Calculate ROW Hardscape and Landscape Areas
Width Are a(ft2)
ROW Hardscape 45 26977
ROW Landscape 15 :'a992'
ROW 60 35969'
2. Calculate Area and Weighted C Factor
Contributing Area C Area (ft 2) C *Area
ROW Hardscape ��`26977, 25628
ROW Landscape 0.28992 1798
Open Land 02. 03167 4633�
Low-Med Residential '108923`,-- : 38:123
Dense Residential 5: 0' 0
Commercial Neighborhood 0 6 0 ' Q
Commercial Downtown 0.8 0. 0
Industrial
e
Total 168059, 70,1 83'
A =Area(acres)
C= Weighted C Factor 0.42
3. Calculate Tc(Time to Concentration)
Tc Overland Flow
Tc= 1.87 (1.1-CCf)D'/2/Sfi3
-----------------------------
�Storm
S = Slope of Basin (%} 2 'Return (yrs) Cf
C = Rational Method Runoff Coefficient 0.35 12 to 10 1
Cf= Frequency Adjustment Factor 1.1 ;11 to 25 1.1
D = Length of Basin (ft) 172 126 to 50 1.2
51 to 100 1.25
Tc Overland Flow(minutes) 1392
Tc Gutter Flow
Tc= L/W60
V= (1.486/n)R213 S1i2
n =Mannings Coefficient
R = Hydraulic Radius A/P (ft) 0.13; (0.15'below top of curb)
S =slope (%) 0.95%
L= length of gutter(ft) 1050
V = mean velocity(ft/s) � 2:92
Tc Gutter Flow(minutes)
Tc Total= 9:91
STORM INLET# 1 B
1. Calculate ROW Hardscape and Landscape Areas
Width Area(ft 2)
ROW Hardscape 45 31288
ROW Landscape 15 10429
ROW 60 41717
2. Calculate Area and Weighted C Factor
Contributing Area C Area (ft2) C *Area
ROW Hardscape - 0:95: 31288 29.723
ROW Landscape ;0,2 1�0429 `2f)86
Open Land ! 2; 78921578
Low-Med Residential 0:35 : 120343 421 20
Dense Residential 0,5:: 0 - :0
Commercial Neighborhood0,60 ,}
Commercial Downtown ` t1.8 ; 0 0
Industrial (3:8 00
Total 1'69951 ,75507
A =Area(acres) 3.9015
C= Weighted C Factor 0.44
3. Calculate Tc(Time to Concentration)
Tc Overland Flow
Tc= 1.87 (1.1-CC')D1/2/S1/3
;Storm
S= Slope of Basin (%) 2 ;Return (yrs) Cf
C = Rational Method Runoff Coefficient 0.35 12 to 10 1
Cf= Frequency Adjustment Factor 1.1 ;11 to 25 1.1
D = Length of Basin (ft) 175 126 to 50 1.2
51 to 100 1.25
Tc Overland Flow(minutes) 14.04
To Gutter Flow
Tc= L/V/60
V = (1.486/n)R2/3 S1/2
n = Mannings Coefficient : OY013"
R = Hydraulic Radius A/P (ft) -0103, (0.15'below top of curb)
S =slope (%) 0.95%
L= length of gutter(ft) 1028
V= mean velocity(fVs) 2:92
Tc Gutter Flow(minutes)
Tc Total
DRAINAGE AREA# 2 (OFFSITE)
1. Calculate ROW Hardscape and Landscape Areas
Width Area ft2
ROW Hardscape 45 4801
ROW Landscape 15 :,100,0
ROW 60 6401
2. Calculate Area and Weighted C Factor
Contributing Area C Area ft2 C `Area
ROW Hardscape '0.95. 4801, : 4561
ROW Landscape 30
Open Land 0.2 .0. 0
Low Med Residential �.0 `0
Dense Residential 0.5;� 0 0
Commercial Neighborhood
Commercial Downtown0:8`� 0 0
Industrial 0
Total 6401', 4881'<
A=Area(acres) 0.1469
C= Weighted C Factor 0.76'
3. Calculate Tc(Time to Concentration)
Tc Overland Flow
Tc= 1.87(1.1-CCf)Dl"/S"
'-----------------------------
tStorm
S=Slope of Basin (%) 1 ;Return (yrs) Cf
C=Rational Method Runoff Coefficient 0.35 12 to 10 1
CE= Frequency Adjustment Factor 1.1 il1 to 25 1.1
D=Length of Basin (ft) 0; 126 to 50 1.2
1.25
Tc Overland Flow(minutes) ,'Uo0
Tc Gutter Flow
Tc=UV/60
V=(1.486/n)R21I S112
n=Mannings Coefficient 0.013
R=Hydraulic Radius A/P (ft) 0 13 (0.15'below top of curb)
S=slope (%) 0.95%'
L=length of gutter(ft) 152'
V=mean velocity(ft/s) .2 92
Tc Gutter Flow(minutes)
Tc Total= 5 00': (5 minutes minimum)
4. Calculate Post-Development Flow(Rational Formula)
O=CIA
C=Weighted C Factor 0 76> (calculated above)
I=0.64 Tc 0-"(in/hr) 3 2Z (10-yr storm)
A=area(acres) (calculated above)
Q=Post-Development Flow(cfs)
DRAINAGE AREA# 3 (OFFSITE)
1. Calculate ROW Hardscape and Landscape Areas
Width Area ft 2
ROW Hardscape 45 $516
ROW Landscape 152839
ROW 60 11355:
2. Calculate Area and Weighted C Factor
Contributing Area C Area ft2 C "Area
ROW Hardscape 0.95 8516 $090
ROW Landscape 0>2 .283968
Open Land
Low-Med Residential19211 6724'
Dense Residential 0.5 0 `
Commercial Neighborhood 0;6; 0r: 0
Commercial Downtown `0
Industrial
Total 34273," 116123i
A=Area(acres) O.M68
C= Weighted C Factor U.47
3. Calculate Tc(Time to Concentration)
Tc Overland Flow
Tc= 1.87(1.1-CC')D112/Sv3
'-----------------------------
IStorm
S=Slope of Basin(%} 2 ;Return (yrs) Cf
C=Rational Method Runoff Coefficient 0.35 ;2 to 10 1
C,= Frequency Adjustment Factor 1.1 111 to 25 1.1
D=Length of Basin (ft) 128 126 to 50 1.2
1.25
Tc Overland Flow(minutes) 12.01
Tc Gutter Flow
Tc=UV/60
V=(1.486/n)R13 S112
n=Mannings Coefficient0, 13
R=Hydraulic Radius A/P (ft) p; 3, (0.15'below top of curb)
S=slope (%) 0.95%
L=length of gutter(ft) 141
V=mean velocity(ft/s)
Tc Gutter Flow(minutes)
Tc Total_ 2.87. (5 minutes minimum)
4. Calculate Post-Development Flow(Rational Formula)
Q=CIA
C=Weighted C Factor 0 4'7` (calculated above)
I=0.64 Tc ose(in/hr) (10-yr storm)
A=area(acres) 0.79' (calculated above)
Q=Post-Development Flow(cts) .0: 6
PRE-DEVELOPMENT RUNOFF
1. Calculate Tc(Pre-Development)
Tc Overland Flow
Tc = 1.87 (1.1-CCs)D112/Sv3
r-----------------------------
'Storm
S = Slope of Basin (%) 141 'l Return (yrs C,
C = Rational Method Runoff Coefficient 0.2 ,2 to 10 1
Ct= Frequency Adjustment Factor 1.1 ;11 to 25 1.1
D = Length of Basin (ft) 1072 !26 to 50 1.2
51 to 100 1.25
Tc(Pre-Development)(minutes) 48
2. Calculate Rainfall Intensity(Duration=Pre-Development Tc)
i = 0.64x-0.65 (10-yr Storm, Fig. 1-3, COB Design Standards)
x =storm duration (hrs) 0.80 (Tc Pre-Development)
i=rainfall intensity(in./hr.) 0.74
3. Calculate Runoff Rate(Pre-Development)
Q= CiA
C = Rational Method Runoff Coefficient 0.2'(open land)
i = rainfall intensity(in./hr.) 0.74 (calculated above)
A=Area (acres) 8.69;
Q=Runoff Rate (Pre-Development)(cfs) 1.29
APPENDIX C
STORM SEWER FACILITIES
CALCULATIONS
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APPENDIX D
POND SIZING CALCULATIONS
RETENTION POND # 1
REQUIRED VOLUME
1. Calculate ROW Hardscape and Landscape Areas
Width Area(ft2)
ROW Hardscape 45 . 58265,
ROW Landscape 15 1,9422;
ROW 60 77686
2. Calculate Area and Weighted C Factor
Contributing Area C Area (ft2) C *Area
ROW Hardscape 0"95 �58265 55352
ROW Landscape ..0:2j9422: 3884
Open Land 0.2. 31.058� <` 6212
Low-Med Residential 8024 0:35 .22g266 � 3
Dense Residential 0;5 q ' 0
Commercial Neighborhood 0 6 0 0
Commercial Downtown 0:8 Q 0
Industrial 0.8 0` 0
Total 33801 1': 1456911'
C=Weighted C Factor 0.43
3. Calculate Required Volume
Q = CIA
V=7200Q
C =Weighted C Factor0,;43
I = intensity (inlhr) 0.41;(10 yr, 2hr storm)
A=Area (acres) 776
Q = runoff (cfs) 1.37'
V= REQUIRED VOL (ft) 9$73