Geoscience Reference
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T
d
2
= Critical storm duration = 36.2 min.
t
c
= Developed condition time of concentration = 21 min.
Q
o
2
= 2-year allowable peak outflow = 17 cfs.
(.)(
35 121
4
)
(
17
)(
36 2
. )
31721
4
(
)(
)
+
−
−
3
=
V
2
=
(.)(
35 1362
.)
60
=
52 764
,
ft
1.21 ac-ft
2
Repeat Steps 2 through 4 for the 10-year storm, as follows:
5. Determine the 10-year critical storm duration
T
d
10
, using Equation 25.9.
Given:
T
d
10
= 10-year critical storm duration (min).
C
= Developed condition runoff coefficient = 0.59.
A
= Drainage area = 25 ac.
t
c
= Post-developed time of concentration = 21 min.
Q
o
10
= 24 cfs.
a
10
= 189.2.
b
10
= 22.1.
2059
(. )(
25 0 189 2221 21
.)(
. )(
.
−
/
4
)
T
d
10
=
−
22 1
. =
3918 6 21
.
−
.
=
40 5
.min.
24
6. Solve for the 10-year critical storm duration intensity (
I
10
) using Equation 25.6 and the
10-year critical storm duration (
T
d
10
):
a
bT
d
189 2
22 1405
.
I
=
=
=
3.02 in./hr
10
+
.
+
.
10
7. Determine the 10-year peak inflow (
Q
i
10
) using the rational equation and the critical storm
duration intensity (
I
10
).
Given:
Q
i
10
= 10-year peak inflow.
C
= Developed condition runoff coefficient = 0.59.
I
10
= Critical storm intensity = 3.02 in./hr.
A
= Drainage area = 25.0 ac.
Q
i
10
=
C
×
I
10
×
A
= 44.5 cfs
8. Determine the required 10-year storage volume for the 10-year critical storm duration
(
T
d
10
) using Equation 25.5.
Given:
V
10
= Required storage (ft
3
).
Q
i
10
= 44.5 cfs.
C
= 0.59.
A
= 25 ac.
T
d
10
= 40.5 min.
t
c
= 21 min.
Q
o
10
= 24 cfs.
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