Lessons Learned - Tohoku, Japan Earthquake and Tsunami of 2011 - page 68

barriers to protect from future sea level conditions, storms, and typhoons. Large-scale
subsidence poses a significant regional concern: a property-by-property approach,
effective at addressing differential settlement and lateral spreading, is ineffective on a
grand scale.
4.11 Vegetated Barriers
Several coastal communities had developed wide barriers of beach and wooded belts
of vegetation seaward of the main development areas. These barriers undoubtedly
provided many benefits to the area, but they proved ineffective as protection from
tsunami inundation and fast moving currents. The Great East Japan event exceeded
the protective function of these green belts. In locations where the tsunami ripped out
the vegetation, the trees became woody debris that added to the impact loads, became
floating projectiles, or — when trapped between pile supports or bridge piers —
created localized dams. The ecological, aesthetic, and social benefits of these coastal
greenbelts may provide justification for reestablishing them as part of a community
redevelopment plan. However, quantifying the expected storm protection these
features can provide requires additional work.
On the Sendai plain, a thicket or copse of trees surrounded a number of buildings that
survived the tsunami. The structures experienced flooding, but the vegetation seemed
to dampen the currents; waves did not knock buildings off their foundations or sweep
them away. The surviving communities and buildings were located inland from the
beach and possibly positioned on a slight rise. No evidence suggests that this
development style would have been effective immediately adjacent to the shoreline;
however, these features may provide some protection for more inland development.
Identifying that level of protection and, in general, determining the most effective
alternative measures available to the surviving communities require additional work.
4.12 Sharp Wall Angles
Failures often occurred at sharp 90-degree angles in walls. While the angular
connection can provide a strong point for a vertical wall, increasing the resistance to
overturning, these features can become points of weakness if overtopped. When
waves overtop structures, angled segments experience concentrated flows from both
the fronting wall and the angled wall. This focusing of overflow caused extremely
deep scour pockets that undermined the foundation and reduced the passive resistance
provided by inland soils.