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30
2-yr peak inflow
Q
2post
= 25.9 cfs (TR-20)
Peak 25.9 cfs, 12.13 hr
20
Calculations:
S
2
= (0.398 in.
2
)(10 cfs/in.)(2.5 hr/in.)(3600 sec/hr)
= 35,820 ft
3
S
2
= 0.82 ac-ft
2-yr storage required
S
2
= 35,820 ft
3
(shaded area)
10
Q
2pre
2-yr allowable release = 8.0 cfs (TR-20)
Straight-line approximation of outlet rating curve
0
10
15
20
25
t
2
Time (hr)
FIGURE 25.9
SCS runoff hydrograph, 2-yr post-developed.
60
10-yr peak inflow
Q
10post
= 61.1 cfs (TR-20)
Peak = 61.1 cfs, 12.1 hr
50
Calculations:
S
10
= (0.89 in.
2
)(10 cfs/in.)(2.5 hr/in.)(3600 sec/hr)
= 80,100 ft
3
S
10
= 1.84 ac-ft
40
S
10
= 80,100 ft
3
(total shaded area)
S
2
= 35,820 ft
2
(shaded area
preceding
t
2
)
30
Q
10pre
10-yr allowable release = 25.5 cfs (TR-20)
20
Straight-line approximation
of outlet rating curve
10
2-yr release = 8.0 cfs (TR-20)
0
8
10
t
2
t
10
15
Time (hr)
20
25
FIGURE 25.10
SCS runoff hydrograph, 10-yr post-developed.
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