The Evacuation Tower and the Narrow Bridge
A coastal town in Iwate has three districts along a narrow bay. District A has 520 residents, many of them elderly, and lies 300 m from the sea. District B has 410 residents and a designated evacuation hill 25 m high, but the path to it crosses a bridge only 4 m wide. District C has 270 residents and a new evacuation tower 12 m high, built after the last tsunami. A town survey estimates that a tsunami could arrive 20 minutes after a major earthquake. In a drill, District A reached high ground in 18 minutes, District B in 14 minutes, and District C in 11 minutes. However, the drill assumed that everyone could walk without assistance. The town has one public address system, and 38% of residents in District A live alone. The town must decide whether to widen the bridge, raise the tower, or buy small motorized carts for District A.
1. Identify the most serious bottleneck in the evacuation plan and explain why the drill may understate the risk.
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The most serious bottleneck is District B's bridge. Four metres is narrow enough that two-way movement, a stalled vehicle, or fallen debris could block the route, and 410 residents must cross it to reach the hill. The drill time of 14 minutes looks safe against a 20-minute arrival estimate, but the margin is only six minutes. The drill also assumed that everyone could walk without assistance, which is unrealistic in a district with elderly residents. District A is also a concern: 18 minutes leaves only two minutes, and 38% of its residents live alone, so there may be no one to help a person who needs assistance. The drill therefore measures fit walkers in good conditions, not the actual population. The bottleneck is not just distance; it is the combination of a narrow crossing and a thin time margin.
2. A resident says, "The tower is useless because the last tsunami was higher than 12 metres." Evaluate this statement.
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The statement is too strong. Whether the tower is useful depends on the height and arrival time of the specific tsunami, not on the height of the last one. A 12 m tower gives District C residents a vertical refuge that is closer than the hill, and the drill shows they can reach it in 11 minutes, leaving nine minutes of margin. If the next tsunami is lower than 12 m, the tower saves lives; if it is higher, the tower may fail. The resident is right to demand that the town not treat the tower as a complete solution. The proper judgement is conditional: the tower is useful for some scenarios and inadequate for others. The town should combine it with faster warnings, a route to higher ground, and a rule for when to use the tower and when to keep moving to the hill.
3. Describe one piece of data you would collect first and explain how it would affect the choice among the three options.
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I would first collect data on how many residents in each district cannot walk 300 m without help, and where those residents live. This matters because the drill's times assume unassisted walking, and District A has many elderly residents and 38% living alone. If, say, 90 residents in District A need assistance, motorized carts would directly address the largest gap and could be cheaper than widening the bridge. If the number is small but the bridge is used by 410 people, widening the bridge might save more total minutes. If the tower is the only close refuge for District C, raising it would protect the group with the least time. The data would not decide by itself, but it would show which option protects the most people within the 20-minute limit.