Precipitation

1. Precipitation

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

3. Similar to Lec. 4 Computation from rainfall records • 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

5. Example Given: 39 year record, top two depths (mm) are listed. Find: Intensity vs. duration for 30-year storm

6. Example

7. Wenzel method 10-year storm computed for some major U.S. cities 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. inches Chen method: example

12. inches Chen method: example

13. inches Chen method: example

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

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