Two camps, a saturated slope and a rainfall trigger that fires too late
Terra is a Himalayan block of 340 households in three settlements. Dhara (1,210 m) sits on the crest of a convex ridge. Terra proper (1,180 m) lies on a concave slope below a spring line and has 60 households, including 22 people over 70 and 14 children under five. Bazar (1,150 m) is on the floodplain where a seasonal nala meets the river, and holds 190 households, the primary health centre and both schools. The only motorable road runs along the valley floor and crosses the nala at a culvert 1.2 km from Bazar. In the past ten days 310 mm of rain has fallen; the forecast is 190 mm in the next 30 hours, with 60 per cent of it in one six-hour spell. The district disaster plan activates evacuation at 100 mm in 24 hours. A landslide scar from 2019 is visible on the slope 300 m above the road. The two proposed relief camps are the school at Dhara and the panchayat ground at Bazar.
1. Choose the camp site and justify it using slope form, drainage and access. State what is wrong with the rejected site and what must be fixed at the chosen one.
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Camp at Dhara, with fixed deficiencies.
- Slope form: Dhara is on a convex ridge crest, where water sheds away from the settlement. Terra proper's concave slope concentrates water from a wide catchment, and Bazar is on a floodplain at a confluence - the exact place a nala expands into during a burst.
- The forecast supplies the mechanism: 60 per cent of 190 mm in six hours is about 114 mm in a single spell. On ground already holding 310 mm from the past ten days, almost all of that becomes runoff within hours, and it arrives at Bazar first, through and over the culvert.
- Bazar's panchayat ground is close to the health centre and the road, which is precisely why it is the wrong camp: it places the healthy population in the hazard zone and competes for the same rescue capacity as the people who will need evacuating.
What Dhara lacks must be planned, not assumed: no motorable access (mark a helipad or dropping zone and a flagged walking route), no stored water (tank and purification), and no shelter for 340 households (school plus tents, with a priority order). Bazar's health centre should stay functioning as a forward medical post until the water forces it out, and the school buildings there used only as a staging point, never as sleeping accommodation.
2. Which settlement do you evacuate first, and in what order? Justify with terrain, timing and dependency.
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Move Bazar first; pre-position Terra's vulnerable people early; Dhara is the destination, not the problem.
- Bazar is first on hazard, not on numbers. It is on the floodplain at the confluence, downstream of the culvert, with a six-hour burst forecast; the safe window is roughly the four to five hours before the nala rises. It also holds 190 households, the schools and the health centre, so a single decision protects most people and the medical capacity they will need.
- Terra proper is at risk from a different mechanism - saturation-induced shallow failure on a concave slope after 310 mm of antecedent rain - and that risk builds over 24 to 48 hours, not six. There is more time, but less speed: 22 people over 70 and 14 children under five cannot self-evacuate, so their movement to Dhara should start at the same time as Bazar's, even though the general evacuation of Terra comes later.
- Route matters more than order. Bazar to Dhara must go up the ridge path, not along the valley road past the culvert, which will be cut early. Vehicles, if used at all, only in the first two hours, and never as the plan of record.
Sequence: alert and move the dependent elderly and children from Terra; evacuate Bazar by the ridge path; then move Terra proper if the rain total crosses the revised trigger.
3. Critique the 100 mm in 24 hours evacuation trigger using the data given, and propose a better one.
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The trigger uses one variable, one threshold and no hazard logic.
- It ignores antecedent rainfall. After 310 mm in ten days the soil is close to saturation, so the same 100 mm generates far more runoff than it would in a dry month. A threshold that does not know what fell last week will fire too late in exactly the conditions that cause disasters.
- It ignores intensity. 114 mm in six hours is a burst; spread over 24 hours it is a wet day. The nala responds to intensity, while the slope responds to the cumulative total, so the two settlements need different triggers.
- It ignores the specific hazards on the map: the culvert, which can block and back water up, and the 2019 scar 300 m above the road, whose failure would cut the only access route.
A better trigger would combine 48-hour cumulative rainfall, 6-hour intensity and a simple soil-moisture proxy, with separate thresholds for Bazar (rainfall in the nala catchment) and Terra (cumulative total and spring discharge). It must also name the decision-maker, the action and the alarm - a siren audible at 2 a.m., since a burst can arrive at night - and it must be tested in a monsoon drill before it is relied upon in a real one.