Tsunami evacuation and shelter investment in a coastal ward
A coastal ward of 6,200 people has one elementary school at 4 m elevation and 900 m from the sea, a community hall at 12 m and 1.4 km inland, and a gymnasium at 7 m and 2.0 km inland. A 1946 tsunami reached the ward in about 20 minutes and flooded to 5 m; a new forecast says the next event could arrive in 18 minutes. The ward has 1,100 residents aged 75+, 480 elementary pupils, and 70 people who use wheelchairs. Walking speed is estimated at 0.8 m/s for pupils, 0.6 m/s for frail older people, and 1.0 m/s for adults without mobility aids. The road to the hall crosses a bridge that may be closed after an earthquake; the road to the gymnasium is wider but 400 m longer. The town can fund either a new elevated shelter at the school (capacity 800, cost ¥900 million) or retrofitting the hall (capacity 500, cost ¥450 million) plus a school evacuation bridge.
1. Using the school as the starting point, calculate whether pupils can reach the hall or the gymnasium before the 18-minute forecast arrival. What does this imply about the school itself?
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For pupils walking at 0.8 m/s to the hall, time = 900 m ÷ 0.8 m/s = 1,125 seconds = 18.75 minutes. That is already longer than the 18-minute forecast, before allowing for warning time, crowding, or the bridge. To the gymnasium, time = 2,000 m ÷ 0.8 m/s = 2,500 seconds ≈ 41.7 minutes, far beyond the forecast.
The implication is that neither the hall nor the gymnasium is a reliable destination for pupils starting at the school. The school itself is only at 4 m elevation, while the 1946 tsunami flooded to 5 m, so staying in the current building is also unsafe. The school needs vertical evacuation - an elevated shelter - or pupils need transport that is available immediately. The 18-minute forecast leaves almost no margin: even the hall is missed by 45 seconds under ideal walking conditions, so a plan that depends on pupils walking there is not defensible.
2. Which investment should the town prioritize: the elevated school shelter or the hall retrofit plus bridge? Consider capacity, cost, access, and the 18-minute forecast.
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The elevated school shelter is the stronger priority for the school population. It costs ¥900 million, twice the hall option, but it places 800 spaces at the site where 480 pupils already are and removes the 18.75-minute walk to the hall. Because the school is at 4 m and the flood reached 5 m, elevation is essential, not optional. The shelter also avoids the bridge, which may be closed after an earthquake - the very condition that triggers a tsunami.
The hall retrofit plus bridge has real advantages: the hall is at 12 m, safer from flooding, and the option costs less. But the hall is 1.4 km away, and a frail older person walking at 0.6 m/s would need 1,400 ÷ 0.6 ≈ 2,333 seconds, or about 38.9 minutes - more than twice the forecast. Capacity is also only 500. The bridge helps school access but adds a closure risk. The town should therefore fund the school shelter and use transport, signage, and volunteers to move older and disabled residents to high ground. The school shelter alone does not cover 6,200 people, so it must be part of a layered plan.
3. What additional map data would you request to test whether the community hall is a safe shelter?
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I would request a detailed inundation map showing predicted depth and arrival time by street, not just a single ward-wide figure. The hall is at 12 m, but the road to it may cross lower ground, and the bridge could be impassable. I would also request the bridge’s seismic rating and closure history, road widths and slopes along the route, and the distribution of older residents and wheelchair users relative to the hall. A shelter is safe only if people can reach it: average ward distances can hide a hamlet that is much farther away. Finally, I would ask for the hall’s structural assessment and actual capacity, including accessibility, because a capacity of 500 is useless if the entrance or toilets cannot serve wheelchair users. These data would show whether the hall is a true high-ground refuge or only appears safe on a coarse map.