Fig. 1 shows a cross-section through a river valley upstream of Kathmandu, Nepal. Following intense monsoon rain, a landslide blocked the river and formed a...
Question 1 Report
Fig. 1 shows a cross-section through a river valley upstream of Kathmandu, Nepal. Following intense monsoon rain, a landslide blocked the river and formed a temporary lake behind loose sediment. Table 1 shows measurements made by engineers at 06:00, 12:00 and 18:00. They are concerned that failure of the natural dam could cause a sudden flood downstream.
| Time | Lake water level behind dam (m) | Width of dam crest (m) | Rainfall in previous 6 hours (mm) |
|---|---|---|---|
| 06:00 | 8.4 | 31 | 18 |
| 12:00 | 10.1 | 28 | 34 |
| 18:00 | 12.3 | 24 | 41 |
(a) Which measurement in Table 1 gives the clearest evidence that the natural dam is becoming less stable? [2]
(b) Outline how heavy rainfall can trigger a landslide on a steep valley side. [2]
(c) Explain two actions engineers could take to reduce the flood hazard shown in Fig. 1. [4]
Answer Details
(a) Dam-crest width falls from 31 m to 24 m [1]; this shows erosion / narrowing of the dam [1]. [2] (b) Rainwater saturates soil and sediment [1]; extra weight and reduced friction / cohesion cause material to slide downhill [1]. [2] (c) Any two developed actions: cut a controlled spillway / channel to lower lake level [1] and reduce pressure on the dam [1]; evacuate downstream settlements [1] before a dam-break flood [1]; reinforce the dam with rock / barriers [1] to slow erosion [1]; install monitoring and warnings [1] so people can leave quickly [1]. [4]
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