- 2076 Chaitra · 2+4 marks
DAD curve procedure and isohyetal area data
Short answer
Procedure: prepare isohyetal maps for different durations, compute cumulative areas and average depths, plot DAD curves per duration and envelope curves. For the 12-h data, average depths over cumulative areas 125, 350, 565, 915, 1065 km² are 75.0, 68.6, 63.4, 56.4 and 53.4 cm respectively.
Write down the procedure of developing Depth Area Duration (DAD) curve. From an isohyetal map prepared for 12 hrs precipitation, following data are obtained.
| Isohyetal interval (cm) | Area enclosed (km) |
|---|---|
| >70 | 125 |
| 60-70 | 225 |
| 50-60 | 215 |
| 40-50 | 350 |
| <40 | 150 |
Procedure for developing a DAD curve
- Select a major storm and collect rainfall data of all gauges for the storm period.
- Prepare isohyetal maps of the storm for different durations (for example 6, 12, 24, 48 h) using the maximum depth for each duration at every station.
- For each duration, measure the area enclosed by each isohyet, starting from the storm centre outward, and compute the cumulative area.
- Compute the average depth over each cumulative area = (sum of area × mean depth) / cumulative area.
- Plot average depth (y) against area (x) for each duration, giving one curve per duration.
- Do the same for several storms and draw the envelope curves of the maximum values. These are used for design.
Calculation for the 12-hour isohyetal data
Assumptions: the mean depth of the top class (> 70 cm) is taken as 75 cm and that of the lowest class (< 40 cm) as 35 cm. Other classes use the mid value.
| Class (cm) | Mean depth (cm) | Area (km²) | Cum. area (km²) | Cum. volume (cm·km²) | Avg depth over cum. area (cm) |
|---|---|---|---|---|---|
| > 70 | 75 | 125 | 125 | 9375 | 75.0 |
| 60-70 | 65 | 225 | 350 | 24000 | 68.6 |
| 50-60 | 55 | 215 | 565 | 35825 | 63.4 |
| 40-50 | 45 | 350 | 915 | 51575 | 56.4 |
| < 40 | 35 | 150 | 1065 | 56825 | 53.4 |
Plot the average depth against the cumulative area (125, 350, 565, 915, 1065 km²) for the 12-h duration; the depth falls from 75 cm to 53.4 cm as the area increases.
Answer: Average 12-h depth over 125, 350, 565, 915 and 1065 km² = 75.0, 68.6, 63.4, 56.4 and 53.4 cm respectively.
✓ Answer checked by Claude Sonnet 5.5
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