Lessons Learned - Tohoku, Japan Earthquake and Tsunami of 2011 - page 65

sea waves out became barriers for allowing floodwaters to recede. Contingency
planning should include designing barriers that function even if waves overtop or
cause mechanisms to fail; anticipating flow conditions from overtopping or abutment
failure; siting critical facilities; developing evacuation plans; and providing pathways
for return flows. Although designing for extreme events may prove impractical or
even impossible, small design modifications such as stabilizing back slopes or
preventing scour may allow a structure to survive overtopping. Communities may be
better served if most life safety facilities are located outside the extreme inundation
zones, complemented by smaller, ancillary facilities inside the hazard areas if they are
needed there for routine situations . As a life safety measure, evacuation planning is
critical for all low-lying areas, regardless of structural barriers.
The team observed several situations where structural designs, had they considered
extreme conditions, would have prevented the degree of damages sustained during
the tsunami. Scour at the backside of structures was an obvious example of one such
avoidable source of damage. One of the contingency planning efforts that worked
was that many people did evacuate from low-lying areas, despite the apparent
protection from barrier walls. Use of contingency planning will become important to
the decisions on rebuilding and future choices for the damaged communities.
4.2 Scour
All manner of scour was evident in the inundation areas. Scour on the landside of
structures and at ends was very common. Although a nuisance in many places, scour
in some locations was sufficient to damage structures or put them in danger. Options
for scour protection are well-known — deepened piles, scour aprons, and such. Scour
protection is important for many projects, especially when considering contingency
planning or what-if situations. Scour occurred at the inland sides of projects — walls
or breakwaters, for example —that often lacked scour protection. At these locations,
scour appears to have been a major source for structural instability or failure.
Beach scour and loss of sand was a second type of scour. The tsunami deposited
inland sand along coastal roads. Broad-ranging deflation of beach profiles occurred at
several beach areas, such as Noda and Koizumi Beach. Beach scour may have
occurred through the region, but without references to a backshore or fixed elevation,
these changes were often unobservable. Also, the two-month span between the event
and the team's observations allowed opportunity for significant beach recovery. One
observer noted little evidence of area-wide beach deflation in front of remaining
vertical walls. A likely explanation is that the high barriers functioned like dams
downstream of a reservoir and prevented waves from carrying large volumes of sand
inland of the wall.
4.3 Abutments and Tie‐ins
Tie-ins, end attachments, and abutment protection are important design details.
Extreme events highlight the weaknesses of poor connections and tie-ins. The Fudai
River water gate was an example of a well-sited wall tied into stable end points. The
solid end abutment is possibly one feature contributing to the survival of the water
gate.