1. Precipitation

2. Computing IDF curves Computation from rainfall records Wenzel method
Chen method

3. Computation from rainfall records
Similar to Lec. 4
Choose duration Dt and determine maximum rainfall depths for this duration in each year.
Rank the precipitation amounts from high to low
Compute the return period.
Repeat for the next duration.
Interpolate to the desired return period
Compute intensity = depth/duration.

4. Example Given: 39 year record, top two depths (mm) are listed.
Find: Intensity vs. duration for 30-year storm
Dt (min)
Rank 10 20 30 40 1 34 36 2 18 25 29 32

5. Example Given: 39 year record, top two depths (mm) are listed.
Find: Intensity vs. duration for 30-year storm
Dt (min)
Rank
T (yrs) 10 20 30 40 1 34 36 19
27.5
31.5 2 18 25 29 32

6. Example

7. Wenzel method 10-year storm computed for some major U.S. cities
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
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
St. Louis
61.0
8.96
0.78
t in minutes, i = in/hour
Example:
Compute 10-year, 1-hour storm for Chicago.

8. Chen method
Determine from the contour maps, where is the rainfall with return period T (years) and duration t (min).
Find a1, b1, and c1 from the nomograph.
Compute
t in minutes, i = in/hour
where

9. Chen method: example
Determine the intensity of the 1-hour storm with 10-year return period for Hammonton, NJ

10. Chen method: example
Determine from the contour maps, where is the rainfall with return period T (years) and duration t (min).
Find a1, b1, and c1 from the nomograph.
Compute
t in minutes, i = in/hour
where

11. Chen method: example
inches

12. Chen method: example
inches

13. Chen method: example
inches

14. Chen method: example
Determine from the contour maps, where is the rainfall with return period T (years) and duration t (min).
Find a1, b1, and c1 from the nomograph.
Compute
t in minutes, i = in/hour
where

15. Chen method: example
a1 = 25.5
c1 = 0.77
b1 = 8.4

16. Chen method: example
Determine from the contour maps, where is the rainfall with return period T (years) and duration t (min).
Find a1, b1, and c1 from the nomograph.
Compute
t in minutes, i = in/hour
where

17. Chen method: example
in/h