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Given:
T
d
2
= 2-year critical storm duration, (min).
C
= Developed condition runoff coefficient = 0.59.
A
= Drainage area = 25.0 ac.
t
c
= Post-developed time of concentration = 21 min.
Q
o
2
= Allowable peak outflow = 17 cfs (pre-developed peak rate of discharge).
a
2
= 2-year rainfall constant = 130.3.
b
2
= 2-year rainfall constant = 18.5.
2059
(. )(
25 0 130 3185
.)(
. )(
.
−
21
/
4
)
T
d
2
=
=
2955
.0018−=
.
36.2 min
17
2. Solve for the 2-year critical storm duration intensity (
I
2
) using Equation 25.6 and the 2-year
critical storm duration (
T
d
2
):
a
bT
d
I
=
2
+
2
where
T
d
2
= Critical storm duration = 36.2 min.
a
= 2-year rainfall constant = 130.3.
b
= 2-year rainfall constant = 18.5.
130 3
18 5362
.
I
2
=
=
2.38 in./hr
.
+
.
3. Determine the 2-year peak inflow (
Q
i
2
) using the rational equation and the critical storm
duration intensity (
I
2
):
Q
i
2
=
C
×
I
2
×
A
where
Q
i
2
= 2-year peak inflow (cfs).
C
= Developed condition runoff coefficient = 0.59.
I
2
= Critical storm intensity = 2.38 in./hr.
A
= Drainage area = 25 ac.
Q
i
2
= 0.59 × 2.38 × 25 = 35.1 cfs
4. Determine the 2-year required storage volume for the 2-year critical storm duration (
T
d
2
)
using Equation 25.5:
Qt
QT
3
Qt
ic
2
od
2
2
oc
2
VQT
=
+
−
−
60
2
i
2
d
2
4
2
4
where
V
2
= 2-year required storage (ft
3
).
Q
i
2
= 2-year peak inflow for critical storm = 35.1 cfs.
C
= Developed runoff coefficient = 0.59.
A
= Area = 25.0 ac.
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