Return Interval
(year)
|
Twenty-four-hour Rainfall Total
(inches)
|
---|---|
1
|
2.13
|
2
|
2.56
|
10
|
3.71
|
25
|
4.46
|
50
|
5.09
|
100
|
5.76
|
Time of concentration = Tc = [(Tlag/.6)*60] (minutes)
|
Where:
| ||
Tlag
|
=
|
Lag time (hours)
|
L
|
=
|
Hydraulic length of watershed (feet)
|
Y
|
=
|
Average overland slope of watershed (percent)
|
S
|
=
|
Maximum retention in watershed as defined by S = [(1000/CN)
- 10]
|
CN
|
=
|
NRCS curve number for watershed
|