Precipitation
Computing IDF curves Computation from rainfall records Wenzel method Chen method
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.
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
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
Example
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.
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
Chen method: example Determine the intensity of the 1-hour storm with 10-year return period for Hammonton, NJ
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
Chen method: example inches
Chen method: example inches
Chen method: example inches
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
Chen method: example a1 = 25.5 c1 = 0.77 b1 = 8.4
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
Chen method: example in/h