Precipitation
Computing IDF curves
1. Computation from rainfall records
2. Wenzel method
3. Chen method
Computation from rainfall records
1. Choose duration Dt and determine maximum
rainfall depths for this duration in each year.
2. Rank the precipitation amounts from high to low
3. Compute the return period.
p( x xm ) 
m
n 1
 T 
n 1
m
4. Repeat for the next duration.
Similar to Lec. 4
5. Interpolate to the desired return period
6. Compute intensity = depth/duration.
Example
Given: 39 year record, top two depths (mm) are listed.
Find:
Intensity vs. duration for 30-year storm
Rank
1
10
20
Dt (min)
20
30
30
34
2
18
25
29
40
36
32
Example
Given: 39 year record, top two depths (mm) are listed.
Find:
Intensity vs. duration for 30-year storm
Rank
1
T (yrs)
40
10
20
2
30
20
19
18
Dt (min)
20
30
30
34
27.5
25
31.5
29
40
36
34
32
Example
Wenzel method
10-year storm computed for some major U.S. cities
a
i
c1
(t  b1 )
t in minutes, i = in/hour
Example:
Compute 10-year, 1-hour
storm for Chicago.
City
a
b1
c1
Atlanta
64.1
8.16
0.76
Chicago
60.9
9.56
0.81
Cleveland
47.6
8.86
0.79
Denver
50.8
10.50
0.84
Helena
30.8
9.56
0.81
Houston
98.3
9.30
0.80
Los Angeles
10.9
1.15
0.51
Miami
79.9
7.24
0.73
New York
51.4
7.85
0.75
Olympia
6.3
0.60
0.40
Santa Fe
32.2
8.54
0.76
St. Louis
61.0
8.96
0.78
Chen method
10
100
10
1. Determine R1 , R1 , and R24 from the contour maps,
T
where Rt is the rainfall with return period T (years)
and duration t (min).
2. Find a1, b1, and c1 from the nomograph.
3. Compute
a
i
(t  b1 ) c1
t in minutes, i = in/hour
where
a  a1 R110 ( x  1) log 10 T p / 10   1
x  R1100 / R110
T p  1 / ln 1  1 / T 
Chen method: example
Determine the intensity of the 1-hour storm with 10year return period for Hammonton, NJ
Chen method: example
10
100
10
1. Determine R1 , R1 , and R24 from the contour maps,
T
where Rt is the rainfall with return period T (years)
and duration t (min).
2. Find a1, b1, and c1 from the nomograph.
3. Compute
a
i
(t  b1 ) c1
t in minutes, i = in/hour
where
a  a1 R110 ( x  1) log 10 T p / 10   1
x  R1100 / R110
T p  1 / ln 1  1 / T 
Chen method: example
R110  2.35 inches
Chen method: example
10
R24
 5.3 inches
Chen method: example
R1100  3.4 inches
Chen method: example
10
100
10
1. Determine R1 , R1 , and R24 from the contour maps,
T
where Rt is the rainfall with return period T (years)
and duration t (min).
2. Find a1, b1, and c1 from the nomograph.
3. Compute
a
i
(t  b1 ) c1
t in minutes, i = in/hour
where
a  a1 R110 ( x  1) log 10 T p / 10   1
x  R1100 / R110
T p  1 / ln 1  1 / T 
Chen method: example
10
R110 / R24
 44%
a1 = 25.5
b1 = 8.4
c1 = 0.77
Chen method: example
10
100
10
1. Determine R1 , R1 , and R24 from the contour maps,
T
where Rt is the rainfall with return period T (years)
and duration t (min).
2. Find a1, b1, and c1 from the nomograph.
3. Compute
a
i
(t  b1 ) c1
t in minutes, i = in/hour
where
a  a1 R110 ( x  1) log 10 T p / 10   1
x  R1100 / R110
T p  1 / ln 1  1 / T 
Chen method: example
in/h