12879_0005.tif r
Pipe Detention Calculat��ns
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Vst= 60 * (( 1 +m) / * (qpa) * (tca) * ( 1 - a) Z
V Storage Volume Required (ft')
M = ration of hydrograph recession to time of peak j
q after development peak discharge (cfs)
q = outflow peak discharge (cfs)
tca after development time of consentration (min)
a = gpb/gpa
M = 1
q = 11 cfs E
q = 6 cfs
tca 7 min
a = 0.545455
Vst = 954.5455 ft'
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Capacity Calculations (30" Pipe)
Item Capacity (ft) Units Total Capacity (W)
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Double Manifold 61.4 2 122.8
10' pipe length 31.6 26 821.6
End Cap 6.74 4 26.96
Reducer 6.74 1 6.74
Total 978.1 fe
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Capacity Calculations (36" Pipe)
Item Capacity (ft) Units Total Capacity (fe)
Double Manifold 93.6 2 187.2
10' pipe length 70.7 11 777.7
End Cap 9.09 4 36.36
Reducer 9.09 1 9.09
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Total 1010.35 fe
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Rational Method Computation
Proposed Conditions
Q =CIA
Site: Blair, Nebraska j
Project No. 2006 -0134
Kirpich Equation for Computing Tc
Tc= 0.0078 ((L/(s^0.5))^0.77
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L=J 600 ft
s- 0.0200 ft/ft
Tc= 2 min
V= 5.26
Steel Formula for Computing Intensity
i= k/(Tc +b)
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Region = 2
k2= 140
b2= 21
i 6.1 in /hr
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k1o= 230
blo= 29
i 7.4 in /hr
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k, 00 = 375
bloo= 36
i1oo= 9.9 in /hr
Calculation of C
S= 2.0000
CN= 92
Imperviousness= 85 %
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C2= 0.62
C1o= 0.75
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C1oo= 0.82
Drainage Area=1 1.89 Acres
0 7 ft /s
Q10= 11 ft /s
Q100= 15 ft 3 /s
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Rational Method Computation
Existing Conditions
Q =CIA
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Site: Blair, Nebraska
Project No. 2006 -0134
Kirpich Equation for Computing Tc
Tc= 0.0078 ( (U(s^0.5))^0.77
L= 800 ft
s= 0.0130 ft/ft
Tc= 7 min
V = 1.87
Steel Formula for Computing Intensity
i= k/(Tc +b)
Region = 2
k2= 140
b2= 21
i 5.0 in /hr
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k1o= 230
bio= 29
i 6.4 in /hr
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kioo= 375
bioo= 36
iioo= 8.7 in /hr
Calculation of C
s= 1.3000 %
CN= 69
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Imperviousness= 0 %
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C2= 0.20
Cio= 0.26
Cioo= 0.31
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Drainage Area=1 3.93 Acres
0 4 ft /s
Quo= 6
ft /s
Q100= 11 ft /s
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k C C . —_
ASSOCIATES 0
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December 26, 2007 .
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Mr. Al Shoemaker
City of Blair
218 South 16 Street
Blair, NE 68008
RE: Walgreens - Blair, NE
13 Street and Washington Street
Dear Al:
Enclosed p lease find the following for the above- mentioned project:
1. Calculations for Existing Conditions using the Rational Method
2. Calculations for Proposed Conditions using the Rational Method
3. Calculations for the volume to be detained
4. Exhibit of the plan that will be implemented
Based on the Rational Method Computation, we found existing conditions to have an
overall runoff of 6 ft /s at the 10 -year storm. The proposed conditions left us with 11 ft /s
of runoff at the 10 -year storm. We will detain 5 ft /s by means of pipe detention. Refer to
the attached exhibit for the configuration that we will use for pipe detention. Based on l
the calculations it will take us 110 LF of 36" HDPE pipe to detain the additional 5 ft /s.
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Don't hesitate to contact me if you have any questions or require further information.
Sincerely,
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Nathaniel Buss, Olsson Associates
Enclosures
cc: File;
1111 Lincoln Mall, Suite 111
P.O. Box 84608 TEL 402.474.6311
Lincoln, NE 68501 -4608 FAX 402.474.5160 www.oaconsulting.com
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PROJECT NO: 006 -0134 1111 Lincoln ,, , , ,Suite 111 EXHIBIT
WALGREENS- BLAIR, NE P.O. Box 84608
DRAWN BY: NKB PIPE DETENTION EXHIBIT
(AO LSSON TEL 402.474 6
DATE: 12.26.07 ASSOCIATES FAX 402.474.5160
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