HomeMy WebLinkAbout13 - Design Report - Westbrook - Stormwater DESIGN REPORT
WESTBROOK SUBDIVISION
Prepared for:
Four Corners Construction, LLC®
125 Central Avenue, Bozeman, MT 59718
Prepared by:
C&H Engineering and Surveying, Inc.
1091 Stoneridge Drive, Bozeman, MT 5971.8
(406) 587-1.115
Project Number: 12661
NOVEMBER 2013
INTRODUCTION
The Westbrook Subdivision is a proposed 63-lot subdivision located east of Laurel Parkway and
south of Annie Street. The 33.57-acre development is situated in the south half of Section 4,
Township 2 South, Range 5 East of P.M.M., Gallatin County, Montana. This project will require
connection to existing City of Bozeman water and sanitary sewer system.
STORMWATER MANAGEMENT
Storm water runoff from the subdivision will be conveyed to one of four detention facilities. A
plan view of the site highlighting the drainage areas and the storm water features is included in
Appendix A.
The western half of the site will discharge into Detention Pond 1 running south-north near the
western subdivision boundary. This pond has an existing outlet structure that discharges into
Baxter Creek. Detention Ponds 2, 3, and 4 are all proposed to discharge to Baxter Creek. The
soutln/central section of the site will discharge to Detention Pond 2. The eastern section of the site
will discharge to Detention Pond 3. The north/central portion of the subdivision will discharge to
Detention Pond 4 at the north/central section of the site.
DETENTION POND 1
Detention Pond 1 has a volume of 32,939 W and will service Drainage Area #1 with a total area
of 13.66 acres. The existing pond has an outlet structure at the east end which pipes runoff into
Baxter Creek. The pre-development release rate is first calculated to ensure that post-development
discharge rates from the pond do not exceed pre-development rates.
Prc-bevel rnent Release Date
The time of concentration (Tc) is calculated as the flow time from the pre-developed most
hydraulically remote point in the drainage to the proposed discharge point. Pre-developed flow
conditions include overland flow over open land (C=0.20). The.Tc is then used to calculate the
pre-development release rate.
Flow line length= 1,500 ft
Design Report-Page 2 of 13
Slope = 1.25%
Tc= 60.51 mins, 1.008 hrs
Iio=Intensity (10-yr freq.) = 0.64(Tc)(-o.65)=0.636 in/hr
Q i o= CIA= (0.20) (0.636 in/hr) (13.6636 acres) = 1.74 cfs
Drainage Area #I.
The storm water runoff surface areas for Drainage Area 41 are calculated as follows. All runoff
"C" coefficients are from the City of Bozeman's Design Standards and Specifications Policy
(DSSP).
Contributing Areas.-
Right-of-way (C=0.73) = 171,375 f12
Open Land (C=0.20) = 191,121 ft2
Lot Area(C=0.5) = 232,741 ft2
Total = 595,237 ft2
= 13.6648 acres
The composite runoff coefficient for the R/W Area is calculated as follows,this coefficient applies
to all R/W areas in the subdivision:
Crvw= [(0.95 x 42 ft) + (0.20 x 18 ft)] /60 ft = 0.73
The composite runoff coefficient for all of Drainage Area 41 is calculated as:
Comp_ [(0.73 x 171,375 ft2) + (0.20 x 191,121 ft2)+ (0.50 x 232,741 ft2)] /595,237 112=0.47
The maximum required storage is calculated in the table shown in Appendix A by varying the
storm duration with the associated storm intensity, and holding a release rate of L74 cfs with a C
coefficient of 0.47 for the entire drainage area.
Detention Pond 1 was originally sized in the Laurel Glen Phase I Design Report to drain an area
of 15.32 acres in the area that is now being developed as Westbrook. The current area of
Westbrook that is drained to the pond is now only 13.66 acres, and has less impervious area, so
the size of the pond is adequate as previously designed.
Design Report-.Wage 3 of 13
Inlet and Outlet Control Structure Sizing
The existing design for the outlet control structure in Detention Pond 1 was designed and approved
appropriately to handle a 25-year storm and limit discharge rates to below pre-development
conditions. There are no proposed changes to the existing design for the inlet piping, structure or
discharge piping/culvert.
DETENTION POND 2
Detention Pond 2,located in the north/central area of the subdivision,is proposed to have a storage
volume of 6,627 ft3, and will service Drainage Area #2 with a total area of 434,311 ft2. The
detention pond will discharge to the Baxter Creek.
Pre-(Development Release Date
The time of concentration (Tc) is calculated as the flow time from the pre-developed most
hydraulically remote point in the drainage to the proposed discharge point. Pre-developed flow
conditions include overland flow over open land (C=0.20). The Tc is then used to calculate the
pre-development release rate.
Flow line length=990 ft
Slope = 1.75%
Tc=43.94 mins, 0.732 hrs
I I o =Intensity(10-yr freq.) =0.64(Tc)(465)= 0.784 in/hr
Qlo= CIA =(0.20) (0.784 in/hr) (9.97 acres) = 1.56 cfs
Drainage Area#2
The storm water runoff surface areas for Drainage Area#2 are calculated as follows:
Contributing Areas:
Right-of way (C=0.73) = 89,921 ft2
Open Land (C=0.20) = 80,451 ft2
Lot Area(C=0.50) =263,939 ft2
Total =434,311 ft2
Design Report-Page 4 of 13
=9.9704 acres
The composite runoff coefficient for all R/W areas is 0.73, as calculated above. The composite
runoff coefficient for all of Drainage Area#2 is calculated as:
Comp_ [(0.73 x 89,921 ft2)+(0.20 x 80,451 ft2)+ (0.50 x 263,939 ftz)] /434,3 l l ft2=0.49
The maximum required storage is calculated in the table shown in Appendix A by varying the
storm duration with the associated storm intensity, and holding a release rate of 1.56 cfs with a C
coefficient of 0.49 for the entire drainage area.
Detention Pond 2 will have a volume of 6,627 ft3 which is adequate to handle the runoff from the
drainage area.
Inlet Pipe Sizing
The storm sewer pipes are designed to handle a 25-year storm event. The post-development time
of concentration is first calculated to determine the peak discharge:
Overland flow(990 ft @ 1.75% slope, C=0.49)
Total Time of Concentration=27.39 min (0.457 hrs)
For a 25-year storm event, 125 = 0.78X-o-"= 0.78 (0 457)'0-64= 1.288 in/hr
Q25 = CIA= 0.49 (1.288 in/hr) (9.97 acres) = 6.29 cfs
A 14-inch PVC Schedule 40 pipe is proposed for the storm sewer pipe for Drainage Area#2. The
capacity of a 14-inch PVC at the proposed slope is checked using Manning's equation:
Qfau = (1.486/n)AR2/3Sl/2
Manning's n=0.013 for PVC
Slope = 0.0165 ft/ft
A=area= (3.14/4)d 2= (3.14/4)(14/12)2= 1.0684 ft2
P =perimeter= 2(3.14)r= 2(3.14)(7/12) =3.6633 ft
R=hydraulic radius=A/P = 1.0684/3.6633 = 0.2916 ft
R2/3 = 0.4380 ft
Design Report-Page 5 of 13
S12= 0.1285 ft/ft
Qfull = (1.486/0.013)(1.0684)(0.4380)(0.1285) =6.87 cfs
The 14-inch PVC will be adequate to handle the stormwater discharge rates to the detention
pond.
Outlet Control Structqre Sizing
The outlet control structure in Detention Pond 2 needs to limit the release rate to the
predevelopment runoff rate of 1.56 cfs. The slot width is sized using the following equation:
Q= CLI-13/2
The maximum pond depth is 1.5 ft which represents the maximum head. Therefore:
L = (1.56 cfs)/(3.33 x 1.5312) = 0.255 ft
The weir needs to be 3.06" wide in order to limit the release rate to 1.56 cfs.
Outlet Structure Pine Sizing
The outlet structure discharge pipe must be sized to handle the maximum flow of 1.56 cfs into the
structure. A 10-inch PVC pipe is proposed for the discharge pipe. The capacity of a 10-inch PVC
line is calculated as follows:
Qfuil = (1.486/n)AR2/3S1r2
Manning's n = 0.013 for PVC Pipe
Minimum Slope= 0.0165 ft/ft
A =area= (3.14/4)d 2= (3.14/4)(10/12)2= 0.5451 ft2
P =perimeter=2(3.14)r= 2(3.14)(5/12) = 2.6180 ft
R=hydraulic radius =A/P =0.5451 /2.6180=0.2082 ft
R2/3 =0.3513 ft
S =0.0165 ft/ft
S v2= 0.1285 ft/ft
Qf,,n= (1.486/0.013)(0.5451)(0.3496)(0.1285) =2.80 cfs
The 10-inch PVC pipe will be adequate to handle the discharge rates from the outlet structure.
Design Report-Page 6 o f 13
DETENTION POND 3
Detention Pond 3 is proposed to have a storage volume of 4,900 ft3,and will service Drainage Area
#3 with a total area of 294,628 112. The detention pond will discharge to Baxter Creek. There is a
proposed storm drain discharging to Detention Pond 3 on Annie Street.
Pre-Development Release hate
The time of concentration (Tc) is calculated as the flow time from the pre-developed most
hydraulically remote point in the drainage to the proposed discharge point. Pre-developed flow
conditions include overland flow over open land (C=0.20). The Tc is then used to calculate the
pre-development release rate.
Flow line length= 1,130 ft
Slope= 1.20%
Tc = 53.24 mins, 0.887 hrs
I i o = Intensity (10-yr freq.)=0.64(Tc)t"o-61>=0.692 in/hr
Quo= CIA =(0.20) (0.692 in/hr) (6.76 acres) = 0.94 efs
Drainne Area 93,
The stormwater runoff surface areas for Drainage Area#3 are calculated as follows:
Contributing Areas:
Right-of-way (C=0.73) =25,820 ft2
Open Land (C=0.20) =0 ft2
Lot Area (C=0.50) =209,887 ft2
Total =294,628 ft2
= 6.7637 acres
The composite runoff coefficient for all R/W areas is 0.73, as calculated above. The composite
runoff coefficient for all of Drainage Area#3 is calculated as:
Ccomp= [(0.73 x 25,820 ft2)+(0.50 x 209,887 ft2)] /294,628 ft2=0.53
Design Report-Page 7 of 13
The maximum required storage is calculated in the table shown in Appendix A by varying the
storm duration with the associated storm intensity, and holding a release rate of 0.94 efs with a C
coefficient of 0.53 for the entire drainage area.
Detention Pond 3 will have a volume of 4,900 ft3 which is adequate to handle the runoff from the
drainage area.
Inlet Pipe Sizing
The storm sewer pipes are designed to handle a 25-year storm event. The post-development time
of concentration is first calculated to determine the peak discharge:
Overland flow(1130 ft cr 1.20% slope, C=0.37)
Total Time of Concentration=40.99 min(0.683 hrs)
For a 25-year storm event, 125 = 0.78X-0.64=0.78 (0.683)-0.64= 0.995 in/hr
Q25 = CIA = 0.53 (0.995 in/hr) (6.76 acres) = 3.57 efs
A 12-inch PVC Schedule 40 pipe is proposed for the storm sewer pipe for Drainage Area#3. The
capacity of a 1.2-inch PVC is checked using Manning's equation:
Qnuil= (1.486/n)AR2i3Sv2
Maiming's n= 0.013 for PVC
Slope = 0.0165 ft/ft
A=area= (3.14/4)d 2=(3.14/4)(12/12)2=0.7850 ft2
P=perimeter= 2(3.14)r= 2(3.14)(6/12) =3.14 ft
R=hydraulic radius=A/P =0.7850/3.14 =0.2499 ft
R2'3 = 0.3967 ft
51/2= 0.1285 ft/ft
Qf,,Il = (1.486/0.013)(0.7850)(0.3467)(0.1285) =4.5725 cfs
The 12-inch PVC will be adequate to handle the storm water discharge rates to the detention pond.
Design Report-Page 8 of 13
Outlet Control Structure Sizing
The outlet control structure in Detention Pond 3 needs to limit the release rate to the
predevelopment runoff mate of 0.94 cfs. The slot width is sized using the following equation:
Q=CLH3/2
The maximum pond depth is 1.5 ft which represents the maximum head. Therefore:
L= (0.94 cfs)/(3.33 x 1.5"2) = 0.153711
The weir needs to be 1.84" wide in order to limit the release rate to 0.94 cfs.
Outlet Structure Pipe Sizing
The outlet structure discharge pipe must be sized to handle the maximum flow of 0.94 cfs into the
structure. An 8-inch PVC line is proposed for the discharge pipe. The capacity of an 8-inch PVC
line is calculated as follows:
Qfuii = (1.486/n)AR213S U2
Manning's n= 0.013 for PVC Pipe
Minimum Slope=0.0165 ft/ft
A=area= (3.14/4)d 2=(3.14/4)(8/12)2= 0.3489 ft2
P =perimeter=2(3.14)r= 2(3.14)(4/12) =2.0933 ft
R=hydraulic radius =A/P =0.3489 /2.0933 = 0.3011 ft
R2/3 = 0.3011 ft
Sv2=0.1285 ft/ft
Qf,,u = (1.486/0.013)(0.3489)(0.3011)(0.1285) = 1.5431 cfs
As calculated above, the capacity of an 8-inch PVC line flowing full is 1.5431 cfs, which is
adequate to handle the anticipated discharge rate.
DETENTION POND 4
Detention Pond 4 is proposed to have a storage volume of 5,342 ft3,and will service Drainage Area
##4 with a total area of 166,241 ft2. The detention pond will discharge to Baxter Creek directly east
of the detention pond.
Design Report-Page 9 of 13
Pre-Development Release Date
The time of concentration (Tc) is calculated as the flow time from the pre-developed most
hydraulically remote point in the drainage to the proposed discharge point. Pre-developed flow
conditions include overland flow over open land (C=0.20). The Tc is then used to calculate the
pre-development release rate.
Flow line length=450 ft
Slope =3.00%
Tc=24.75 mins, 0.413 hrs
Iio=Intensity (10-yr freq.) = 0.64(Tc)(-0.61) = 1.138 in/hr
Qto=CIA = (0.20) (1.138 in/hr) (3.8164 acres) = 0.87 cfs
brain Area #4
The stormwater runoff surface areas for Drainage Area#4 are calculated as follows:
Contributing Areas:
Right-of-way(C=0.73) = 41,866 ft2
Open Land (C=0.20) = 115,980 ft2
Lot Area(C=0.50) = 8,395 112
Total = 166,241 ft2
= 3.8164 acres
The composite runoff coefficient for all R/W areas is 0.73, as calculated above. The composite
runoff coefficient for all of Drainage Area 44 is calculated as:
Ceo,,,p_ [(0.73 x 41,866 ft2) + (0.20 x 115,980 ft)+ (0.50 x 8,395 ft2) / 166,241 ft2= 0.35
The maximum required storage is calculated in the table shown in Appendix A by varying the
storm duration with the associated storm intensity, and holding a release rate of 0.87 cfs with a C
coefficient of 0.35 for the entire drainage area.
Detention Pond 4 will have a volume of 5,342 ft3 which is adequate to handle the runoff from the
Design Report-Page 10 of 13
drainage area.
Inlet Pike Sizing
The storm sewer pipes are designed to handle a 25-year storm event. The post-development time
of concentration is first calculated to determine the peak discharge:
Overland flow (450 ft @ 3.00% slope, C=0.35)
Total Time of Concentration = 19.67 min (0.328 hrs)
For a 25-year storm event, Its = 0.78X`0.64=0.78 (0.328)-0.64= 1.59 in/hi
Q25 = CIA=0.35 (1.59 in/hr) (3.8164 acres) =2.13 cfs
A 10-inch PVC Schedule 40 pipe is proposed for the storm sewer pipe for Drainage Area#4. The
capacity of a 10-inch PVC is checked using Manning's equation:
Qfull = (1.486/n)AR2/3S1/2
Manning's n= 0.013 for PVC
Slope =0.0165 ft/ft
A=area= (3.14/4)d 2= (3.14/4)(10/12)2=0.5451 f12
P =perimeter=2(3.14)r=2(3.14)(5/12) =2.6167 ft
R=hydraulic radius =A/P = 0.5451 /2.6167 = 0.2083 ft
R213 = 0.3496 ft
S1/2= 0.1285 ft/ft
Qfu11 = (1.486/0.013)(0.5451)(0.3496)(0.1285) =2.80 cfs
The I0-inch PVC will be adequate to handle the storm water discharge rates to the detention pond.
Outlet Control Structure Sizing
The outlet control structure in Detention Pond 4 needs to limit the release rate to the
predevelopment runoff rate of 0.87 cfs. The slot width is sized using the following equation:
Q=CLI43/2
The maximum pond depth is 1.5 ft which represents the maximum head. Therefore:
L= (0.87 cfs)/(3.33 x 1.5"2) = 0.1422 ft
Design Report- Page 11 of 13
The weir needs to be 1.71" wide in order to limit the release rate to 0.87 cfs.
Outlet Structure Pipe Siziai
The outlet structure discharge pipe must be sized to handle the maximum flow of 0.87 cfs into the
structure. An 8-inch PVC line is proposed for the discharge pipe for Detention Pond 4. The
capacity of an 8-inch PVC line is calculated as follows:
Qrull = (1.486/n)AR2/3Sv2
Manning's n=0.013 for PVC Pipe
Minimum Slope = 0.0165 ft/ft
A = area= (3.14/4)d 2= (3.14/4)(8/12)2= 0.3489 {12
P =perimeter=2(3.14)r=2(3.14)(4/12) =2.0933 ft
R=hydraulic radius=A/P = 0.3489/2.0933 = 0.3011 ft
R2/3 =0.3011 It
Si/2 = 0.1285 ft/ft
Qfuu = (1.486/0.013)(0.3489)(0.3011)(0.1285) = 1.5431 efs
The 8-inch PVC pipe will be adequate to handle the discharge rates from the outlet structure.
Design Report-Page 12 of 13
Design Report-Page 13 of 13
AREA 1 -DETENTION POND
Pre-Developed Conditions
Overland Flow Time
Flow Length= 1500.00
Slope(%)= 1.25
C coeficient= 0.20
Cf = 1.00
Tc=(1.87`(1.1-C`Cf)(L)("))/(s)(v3)
Tc(min)= 60.5099
Total Basin Tc(hr)= 1.008
Rational Method For.Calculating Flow Rate
Storm Return Interval= 10
I(in/hr)for 10 year storm 1=0.64t(-o.65) 0.636
Rational Method: Q=C`I*A
C coefficient= 0.20
1(in/hr)= 0.64
Area(ft2)= 595187.00
Area(ac)= 13.6636
Total Runoff For 10yr Storm(cfs)= 1.74 cfs
Post Developed Weighted C Calculations
Landscaped C= 0.2 ROW C= 0.73 Lot C= 0.5
Area(ft)= 191121 Area(fe)= 171375 Area(ft)= 232741
Area(ac)= 4.39 Area(ac)= 3.93 Area(ac)= 5.34
Weighted C= 0.47
Weighted C= 0.47
Total Area= 13.66 acres
Release Rate = 1.74 cfs
Storm Storm Runoff Release Required
Length(min) Length(hrs) Intensity Q future Volume Volume Storage
24 0.40 1.16 7.45 10734.14 2504.65 8229.49
25 0.42 1.13 7.26 10888.61 2609.01 8279.60
26 0.43 1.10 7.08 11039.11 2713.37 8325.74
27 0.45 1.08 6.90 11185.90 2817.73 8368.16
28 0.47 1.05 6.74 11329,19 2922.09 8407.09
29 0.48 1.03 6.59 11469.19 3026,45 8442.74
30 0.50 1.00 6.45 11606.09 3130,82 8475.27
31 0.52 0.98 6.31 11740.05 3235,18 8504.88
32 0.53 0.96 6.18 11871.24 3339.54 8531.70
33 0.55 0.94 6.06 11999.78 3443.90 8555.89
34 0.57 0.93 5.94 12125.82 3548,26 8577.56
35 0.58 0.91 5.83 12249A7 3652.62 8596.85
36 0.60 0.89 5.73 12370.85 3756.98 8613.87
37 0.62 0.88 5.63 12490.05 3861.34 8628.71
38 0.63 0.86 5.53 12607.17 3965.70 8641.48
39 0.65 0.85 5.44 12722.31 4070.06 8652.25
40 0.67 0.83 5.35 12835.55 4174.42 8661.13
41 0.68 0.82 5.26 12946.96 4278.78 8668.18
42 0.70 0.81 5.18 13056.62 4383.14 8673.48
43 0.72 0.79 5.10 13164.59 4487.50 8677.09
44 0.73 0.78 5.03 13270.95 4591.86 8679.09
45 0.76 0.77 4.95 13375.74 4696.22 8679.52
46 0.77 0.76 4.88 13479.03 4800.58 8678.45
47 0.78 0.75 4.82 13580.88 4904.94 8675.93
48 0.80 0.74 4.75 13681.32 5009.30 8672.01
49 0.82 033 4.69 13780.41 5113.67 8666.74
AREA 2-DETENTION POND
Pre-Developed Conditions
Overland Flow Time
Flow Length= 990.00
Slope(%)= 1.75
C coeficient= 0.20
Cf = 1.00
Tc=0.87*(1.1-C*Cf)(L)t1r2>)l(s)(113)
Tc(min)= 43.94293
Total Basin Tc(hr)= 0.732
Rational Method For Calculating Flow Rate
Storm Return Interval= 10
1(inlhr)for 10 year storm 1=0.64t("0 65) 0.784
Rational Method: Q=C*I*A
C coefficient= 0.20
1(in(hr)= 0.78
Area(ft)= 434311.00
Area(ac)= 9.9704
Total Runoff For 10yr Storm(cfs)= 1.56 cfs
Post Developed Weighted C Calculations
Landscaped C= 0.2 ROW C= 0.73 Lot C= 0.5
Area(ft)= 80451 Area(ft)= 89921 Area(ft)= 263939
Area(ac)= 1.85 Area(ac)= 2.06 Area(ac)= 6.06
Weighted C= 0.49
Weighted C= 0.49
Total Area= 9.97 acres
Release Rate = 1.56 cfs
Storm Storm Runoff Release Required
Len thq (min) Lenath(hrs; Intensity Q future Volume Volume Storage
18 0.30 1.40 6.87 7416.41 1687.58 5728.83
19 0.32 1.35 6.63 7558.09 1781.34 5776.76
20 0.33 1.31 6.41 7695.01 1875.09 5819.91
21 0.35 1.27 6.21 7827.54 1968.85 5858.69
22 0.37 1.23 6.03 7956.03 2062.60 5893.43
23 0.38 1.19 5.86 8080.78 2156.36 5924.42
24 0.40 1.16 5.70 8202.05 2250.11 5951.94
25 0.42 1.13 5.55 8320.08 2343.87 5976.21
26 0.43 1.10 5.41 8435.08 2437.62 5997.46
27 0.45 1.08 5.28 8547.24 2531.38 6015.86
28 0.47 1.05 5.15 8656,73 2625.13 6031.60
29 0.48 1.03 5.04 8763.71 2718.89 6044.82
30 0.50 1.00 4.93 8868.31 2812.64 6055.67
31 0.52 0.98 4.82 8970.68 2906.39 6064.28
32 0.53 0.96 4.72 9070.91 3000.15 6070,77
33 0.55 0.94 4.63 9169.14 3093.90 6075.23
34 0.57 0.93 4.54 9265.44 3187,66 6077.78
35 0.58 0.91 ;' 4.46 9359,93 3281 Al` 6078 51
36 0.60 0.89 4.38 9452.67 3375.17 6077.50
37 0.62 0.88 4.30 9543.75 3468.92 6074.83
38 0.63 0.86 4.23 9633.25 3562.68 6070.57
39 0.65 0.85 4.15 9721,23 3656.43 6064.80
\
/
' AREA 3 - DETENTION POND
Pre-Developed Conditions
Overland Flow Time
Flow Length 1130�00
Slope (96) = 1.20
C coeficiend= 0.20
{X = 1.00
To= (1.87^(1.1-C°Cf)(L)("2)VYe>(//3)
To (m/n)= 63.23896
Total Basin Tc(hr) = 0,887
Rational Method For Calculating Flow Rate
Storm Return Interval = 10
| (in/hh for 1O year storm |=0.04t(-0a5) 0.682
Rational Method: O = C^/*4
C coefficient= 0.20
| /in/hh = 8.69
Area (ft) = 294628.00
Area (uo) = 6,7637
Total Runoff For 1OyrStorm (cfs) = 0.94 cfo
Post Developed We�ghted C Calculations
Landscaped = 0.2 ROW C = 0.73 Lot = 0.5
Area (*) = O Area (ft) = 2S82O Area mz\ = 208887
Area /an\ = 0.00 Area (eo) = 0.59 Area (ac) = 4.82
Weighted C = 0.53
Weighted C = 0-53
Total Area = 6.78 acres
Release Rate = 0'94 cfn
Storm 8hornn Runoff Release Required
Length(min) jq9_qjh(hrs Intensity Q(future) Volume Volume Storage
44 0.73 0.78 2.78 7342.48 2470.29 4872.19
45 0.75 0,77 2.74 7400.40 2526.43 4874.03
46 0.77 0.76 3.70 7457.61 2582.67 4875.84
E:==��0.8P�O74�2 W�7569 W 7694.8P48046P
48
40 0.82 0.73 2.59 7624.35 2751.00 4873.35
50 0.83 0.72 2.58 7678,46 2807.14 4871.31
} 51 0.85 8.71 2.53 7731.88 2863.20 4868.57
-/ 52 0.87 8.70 2.50 7784.59 2919.43 4865.16
AREA 4-DETENTION POND
Pre-Developed Conditions
Overland Flow Time
Flow Length = 450.00
Slope (%) = 3.00
C coeficient= 0.20
Cf = 1.00
Tc= (1.87*(1.1-C*Cf)(L)(v2))/(s)(1/3)
Tc(min)= 24.75426
Total Basin Tc (hr) = 0.413
Rational Method For Calculatinq Flow Rate
Storm Return Interval = 10
(in/hr)for 10 year storm 1=0.64t(-0.65) 1.138
Rational Method: Q = C*I*A
C coefficient= 0.20
1 (in/hr) = 1.14
Area (ft) = 166241.00
Area (ac) = 3.8164
Total Runoff For 10yr Storm (cfs) = 0.87 cfs
Post Developed Weighted C Calculations
Landscaped C = 0.2 ROW C = 0.73 Lot C = 0.5
Area (ft) = 115980 Area (ft) = 41866 Area (ft) = 8395
Area (ac) = 2.66 Area (ac) = 0.96 Area (ac) = 0.19
Weighted C= 0.35
Weighted C= 0.35
Total Area = 3.82 acres
Release Rate = 0.87 cfs
Storm Storm Runoff Release Required
Len.qth(min) Length(hrs) Intensity Q future Volume Volume Storage
4 0.07 3.72 4.95 1188.11 208.45 979.66
5 0.08 3.22 4.28 1284.62 260.56 1024.06
6 0.10 2.86 3.80 1369.27 312.67 1056.60
7 0.12 2.59 3.44 1445.17 364.78 1080.39
8 0.13 2.37 3.15 1514.32 416.89 1097.43
9 0.15 2.20 2.92 1578.05 469.01 1109.04
10 0.17 2.05 2.73 1637.33 521.12 1116.21
11 0.18 1.93 2.56 1692.87 573.23 1119.64
12 0.2t�. 1.82 2. 2 1745.22 625.34 111988.
13 0.22 1.73 2.30 1794.80 677.45 1117.35
14 0.23 1.65 2.19 1841.96 729.56 1112.40
DRAINAGE PLAN
WESTBROOK SUBDIVISION
LOCATED IN THE S 112 OF SECTION 4, T. 2 S., R. 5 E.
OF P.M.M., CITY OF BOZEMAN, GALLATIN COUNTY, MONTANA
LEGEND
L01 10 LOT 12 1 1-01' LOT 7
nnJJ
M M DRAINAGE AREA BOUNDARY • opo t 41-ft R
LOT 1
P R
K LOT 8 LOT 8 DRAINAGE ARROW PUBLIC PA K -0.f �N
LOT I
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--LOT 88
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I LOT A,
PUB RK
LOT 12 DhAINA- LOT ifi,
LOT 1 -.00
g�r1011 LOT 5B
#4
ci
LOT 13 .......... LOT
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—LOT LOT F,
LOT 14 ■ LOT 4
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0 ARK I
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`7
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it Ai LOT 3
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LOT 17
LOT 2
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4 Aq
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e p e LOT 11 %
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04. ".9 i
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Scale In Feet
80 0 80 m beed
24 0 24 v
Scale In Meters
9L
DRAWING DATE- 1/28/14
3211,
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.......... ---------
r"g." u-qJ Su Ning Inc.