- 2070 Chaitra · 2+6 marks
Hyetograph and intensity-duration curve, 6:00-12:00 storm
Short answer
30-min hyetograph uses given depths; 2-h hyetograph groups four intervals. Maximum intensities: 30-min = 13 cm (26 cm/h, 8:00-8:30); 1-h = 13+10=23 cm (23 cm/h, 8:00-9:00); 2-h = 8+9+13+10=40 cm (20 cm/h, 7:00-9:00). Plot intensity vs duration.
The rainfall depth with time during a storm at a station is as given:
| Time | 6:00 | 6:30 | 7:00 | 7:30 | 8:00 | 8:30 | 9:00 | 9:30 | 10:00 | 10:30 | 11:00 | 11:30 | 12:00 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Rainfall (cm) | 0 | 7 | 5 | 8 | 9 | 13 | 10 | 8 | 6 | 5 | 3 | 1 | 0 |
i) Construct the hyetograph of this storm for 30 min and 2 hours interval. ii) Compute maximum average intensity of rainfall for 30 min, 1 hour, 2 hour in this storm and plot the resulting intensity duration curve.
The values are the rainfall (cm) recorded in each 30-minute interval ending at the stated time. Total rainfall cm.
(i) Hyetographs
30-min interval: the bar chart of the 30-min depths (intensity = depth × 2 cm/h).
| Interval | Depth (cm) | Intensity (cm/h) |
|---|---|---|
| 6:00-6:30 | 7 | 14 |
| 6:30-7:00 | 5 | 10 |
| 7:00-7:30 | 8 | 16 |
| 7:30-8:00 | 9 | 18 |
| 8:00-8:30 | 13 | 26 |
| 8:30-9:00 | 10 | 20 |
| 9:00-9:30 | 8 | 16 |
| 9:30-10:00 | 6 | 12 |
| 10:00-10:30 | 5 | 10 |
| 10:30-11:00 | 3 | 6 |
| 11:00-11:30 | 1 | 2 |
2-hour interval (sum of four 30-min depths):
| Interval | Depth (cm) | Intensity (cm/h) |
|---|---|---|
| 6:00-8:00 | 29 | 14.5 |
| 8:00-10:00 | 37 | 18.5 |
| 10:00-12:00 | 9 | 4.5 |
cm/h (30-min hyetograph; one bar per 30-min interval from 6:00 to 11:30)
26 | ##
22 | ##
18 | ## ## ##
14 |## ## ## ## ## ##
10 |## ## ## ## ## ## ## ## ##
6 |## ## ## ## ## ## ## ## ## ##
2 |## ## ## ## ## ## ## ## ## ## ##
+---------------------------------
6:00 11:00
(ii) Maximum average intensities
| Duration | Max depth (cm) | Critical period | Max intensity (cm/h) |
|---|---|---|---|
| 30 min | 13 | 8:00-8:30 | 26.0 |
| 1 h | 23 | 8:00-9:00 (13+10) | 23.0 |
| 2 h | 40 | 7:00-9:00 (8+9+13+10) | 20.0 |
(For 2 h the window 7:00-9:00 holds 8 + 9 + 13 + 10 = 40 cm.)
Intensity-duration curve: plot duration (x) against maximum intensity (y): points (0.5 h, 26), (1 h, 23), (2 h, 20) joined by a smooth falling curve.
Answer: Maximum intensities = 26, 23 and 20 cm/h for 30 min, 1 h and 2 h.
✓ Answer checked by Claude Sonnet 5.5
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