Tsunami evacuation: Lessons from the Great East Japan earthquake ...

2012
negative impacts of significant hard defences, which can induce false impressions of safety
and an inability to see the natural warning of a disturbed ocean. Even in small events,
unforeseen issues (e.g. break-down and non-closure of gate systems/earthquake damage)
may result in unexpected inundation.
The tsunami impacts in Tarō have been widely reported due to the damage the town
sustained, despite having a concrete sea wall of 10 m height, in three sections with a
combined length of 2,433 m (Tarō Town Municipal Government, n d). This sea wall
prevented damage in the town due to the 1960 distant Chile tsunami, and would likely
prevent damage to the main town in the vast majority of tsunami in its design lifetime.
However, on March 11 th , the perceived safety in which residents lived may have contributed
to a number of deaths – as already discussed in Section 2.5, the presence of the wall and
the estimated 3 m tsunami height combined such that some people believed they would be
unaffected behind the wall. Sixteen people died while driving through Tarō, possibly as a
result of not feeling the earthquake or at least not being able to sufficiently recognise the
arriving tsunami due to their views of the sea being obstructed. The disaster prevention
official also reported that some people believed that the tsunami spray hitting the wall was
fire rather than waves arriving.
This issue was also apparent in Ryoishi, where several vehicles were observed driving along
the coastal road immediately adjacent to a sea wall (Bombaadi, 2011). At the time these
vehicles were on the road, the tsunami was rising rapidly on the other side of the wall
unseen by the drivers before it poured over the top of the wall, which was several metres tall.
The reduction in visibility out to sea also endangered the lives of fire-fighters in Setai, Miyako
City, who were manually closing tsunami gates when they saw the tsunami at the last minute
through a small window in the gate and were able to drive away.
New Zealand and Washington presently have sea walls (mostly around harbours) designed
only to mitigate normal storm waves and prevent long-term erosion. On coastlines at risk of
a local subduction zone earthquake, it is unlikely to be feasible to construct a defence of
sufficient size to protect against the maximum potential events – exactly as shown in Tarō.
Sea walls may reduce tsunami velocity and wave heights even if breached or damaged,
however, the population of the area ‘protected’ by the defence must be suitably educated to
know that the defence is not a 100% guarantee of safety.
2.7 Vertical evacuation buildings
Vertical evacuation buildings are designed to provide refuge in the inundated area by
providing sufficient elevation above the maximum water level. This type of refuge proved to
be extremely valuable in reducing life loss on March 11 th 2011. These buildings are different
to regular evacuation centres, which are generally designated in Japan for use in multiple
hazards, and are located outside of the previously estimated tsunami hazard zone.
The consistent education message delivered to the public in Tōhoku was to evacuate inland
or to high ground as priority, but the presence of vertical evacuation buildings was valuable
in situations where people were unable to get inland or to high ground prior to tsunami
arrival. The successes are numerous; however, some important lessons can also be learnt
from the experiences of evacuees who used these buildings. Appendix 4 provides a list of
vertical evacuation buildings in the locations visited, with details on construction, signage,
access, inundation level on 11 th March and number of people saved.
GNS Science Report 2012/17 37