Storm Surge, Shelter Places and a Bridge That Will Go Under
A cyclone warning for the Puri coast forecasts landfall in 36 hours with a storm surge of 2.2 m above astronomical tide. The district control room's table shows, for each block, the people living within 5 km of the shoreline and the pucca cyclone-shelter capacity inside the block: Block A: 6,800 people, 5,200 places. Block B: 3,100 people, 3,900 places. Block C: 4,400 people, 2,800 places; C sits at a river mouth where the surge is modelled at 2.6 m, and its only approach road is a 3 km earthen embankment that becomes impassable in heavy rain. Block D: 2,200 people, 2,400 places; D's shelters are 12 km inland. Two panchayats of Block A, with 900 people in all, are reached only by a bridge that the irrigation department says will submerge once the surge crosses 1.8 m, roughly 12 hours before landfall.
1. Compute the shelter shortfall or surplus for each block and rank the blocks for priority, justifying the ranking.
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Arithmetic first:
- A: 6,800 - 5,200 = shortfall of 1,600 places.
- B: 3,100 - 3,900 = surplus of 800 places.
- C: 4,400 - 2,800 = shortfall of 1,600 places.
- D: 2,200 - 2,400 = surplus of 200 places.
Ranking by numbers alone would tie A and C. The hazard data breaks the tie.
- C first. Its surge is modelled at 2.6 m, the highest on the table, and its single approach is an earthen embankment that fails in rain. A shortfall inside a zone that becomes unreachable is worse than the same shortfall inside a zone that stays reachable: the evacuation must be completed before the road goes, which means earliest.
- A second, but with its bridge panchayats pulled out of the general plan and handled separately (see next task). Its 1,600 shortfall is the largest single gap, and B can absorb only 800 of it.
- B is a receiving block, not a priority for evacuation. Its 800 spare places should be formally allotted to A's overflow before families start moving on their own.
- D last. It has a surplus and shelters 12 km inland; it needs monitoring, not evacuation.
Ranking must therefore be by exposure and access as well as by headcount.
2. What is the correct decision on the two panchayats behind the bridge, and why should they not wait for the general evacuation?
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Move those 900 people first, now, and do not fold them into the general evacuation of Block A.
The constraint is arithmetic. The bridge submerges when the surge crosses 1.8 m, about 12 hours before landfall, so the usable window is roughly 24 hours from the warning, not 36. After that point the 900 are both exposed and unreachable, and no amount of later effort reaches them.
Decision:
- Start their movement immediately, ahead of the main A evacuation, with the first bus wave.
- Shelter them inland in A or B, and count those 900 against A's 5,200 places before allotting the rest.
- Keep a hard cut-off: no vehicle crosses the bridge after T-26 hours, with a margin over the forecast, because surge forecasts move.
- If buses cannot make the round trips in time, pre-position people in the nearest pucca school or high-ground building on the far side before the bridge goes, rather than gamble on the return trip.
Waiting for the general evacuation is the standard failure in this situation: the general plan is sized to the biggest number, and the smallest group with the hardest deadline gets absorbed into it and lost.
3. Fourteen buses are available before landfall, each able to make three round trips of 40 minutes carrying 45 people. Check whether this is enough and state how you would allocate the seats.
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Capacity: 14 x 3 x 45 = 1,890 person-trips.
Demand, if the buses were used for everything: 900 behind the bridge plus A's remaining 1,600 shortfall is 2,500 people. Capacity falls 610 short, before allowing for the fact that a round trip of 40 minutes on an embankment road will not hold in rain.
Allocation, in order:
- The 900 behind the bridge first - about 20 bus-loads of 45, comfortably inside the 1,890 if the first wave starts immediately. This is non-negotiable because the deadline is hard.
- The remaining 990 seats go to the most exposed part of A's 1,600 - those nearest the shoreline.
- The rest do not wait for a bus. They walk to shelters within 5 km with ward volunteers, which is the normal and faster method for the last stretch, and B's 800 spare places are allotted in advance.
Conclusion for the control room: buses are adequate for the bridge group and a large part of A, but not for the whole demand, so the plan must combine buses with walking relocation and a request for additional vehicles from neighbouring blocks. Allocating buses to the general evacuation first and the bridge group later inverts the only deadline that cannot be missed.