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Intensity-duration curve from storm record

Short answer

Maximum consecutive depths are divided by duration: 13 mm in 0.5 h gives 26.0 mm/h; 22 mm in 1 h gives 22.0 mm/h; 35 mm in 2 h gives 17.5 mm/h; 65 mm in 5 h gives 13.0 mm/h.

The rainfall depth with time during a storm at a station is given below. Compute maximum average intensities of the rainfall for durations 30 minutes, 1 hr, 2 hr, and 5 hr and plot the resulting intensity duration curve.

Time (hr)06:0006:3007:0007:3008:0008:3009:0009:3010:0010:3011:0011:3012:00
Rainfall (mm)06658591364320

The maximum average intensity for a duration DD is the greatest rainfall depth in any DD consecutive hours divided by DD. Rainfall in each 30-min interval (mm): 6, 6, 5, 8, 5, 9, 13, 6, 4, 3, 2, 0 (total 67 mm).

DurationCritical periodMax depth (mm)Max intensity (mm/h)
30 min09:00-09:301326.0
1 h08:30-09:3022 (9+13)22.0
2 h07:30-09:3035 (8+5+9+13)17.5
5 h06:00-11:006513.0

Checks: 2 h = 8 + 5 + 9 + 13 = 35 mm; 5 h = 6+6+5+8+5+9+13+6+4+3 = 65 mm (the five-hour window starting 06:00; the window 06:30-11:30 gives only 61 mm).

Intensity-duration curve: plot duration (x-axis) against maximum intensity (y-axis):

 mm/h
 26 |*
 22 |   *
 17.5|        *
 13 |                       *
    +------------------------
     0.5   1    2         5   h

The curve falls with duration because the intense part of the storm is spread over a longer time.

Answer: 26.0, 22.0, 17.5 and 13.0 mm/h for 30 min, 1 h, 2 h and 5 h.

✓ Answer checked by Claude Sonnet 5.5

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