Tsunami Evacuation Decision for a Coastal School
A coastal school in Fukushima Prefecture has 240 students. The building floor is 6 m above sea level and 900 m from the shore. At 11:58 a strong earthquake shakes for 55 s. Students assemble outdoors at 11:59:30. The local tsunami model gives a first wave 18 min after the earthquake, with 5 m amplitude at the coast and a possible later wave. Route A to a hill is 1.2 km, reaches 35 m elevation, and crosses a bridge and a narrow street. Normal walking speed is 4 km/h, but with 240 students and congestion the estimate is 20 min. Route B is vertical evacuation to a reinforced 4-story building 250 m away with a rooftop at 16 m and capacity for 300 people; it is not officially designated as a tsunami evacuation building. The principal must choose before the first wave.
1. Calculate the earliest arrival time by Route A and by Route B if departure is at 11:59:30. What do the times imply?
Read the worked answer
The first wave is due at 11:58 + 18 min = 12:16. Route A at 4 km/h takes 1.2/4 = 0.30 h = 18 min; with congestion the planning time is 20 min. Departing at 11:59:30, arrival is about 12:19:30, which is after 12:16. The hill route therefore cannot be relied on for the first wave, and the bridge could be damaged or blocked. Route B is 0.25 km; at 4 km/h that is 0.0625 h = 3.75 min, arriving about 12:03:15, before the wave. Even allowing several minutes to climb to the roof, there is a large time margin. The comparison implies that the hill is not a viable first-choice route for a 240-student group starting after shaking, while the nearby reinforced building is the only option that can be reached before the modeled arrival time.
2. Recommend a decision and state the conditions under which you would change it. Address the building's lack of official designation.
Read the worked answer
Recommend immediate vertical evacuation to the reinforced 4-story building, because 55 s of strong shaking is a natural tsunami warning and the hill route arrives after the first wave. On the way, staff should check the building's exterior for obvious structural damage, tilting, falling glass, or blocked entrances; if it looks sound, move students to the upper floors or roof and keep them there. Avoid elevators and stay away from windows. Lack of official designation is a planning gap, not proof that a sound building is unsafe; a nearby reinforced building is preferable to being caught on a bridge or low road. Change the decision if the building is visibly damaged, access is blocked, or official information directs people elsewhere. In that case, move inland or to higher ground by a route that avoids the bridge, and do not return until officials say the tsunami sequence is over.