Recommendations for Additional Investigation - Tohoku, Japan Earthquake and Tsunami of 2011 - page 69

Chapter5

Recommendations for Additional Investigation

Field investigations not only identify lessons learned and provide observations of the
strengths and weaknesses of various structures and systems but also remind us of the
limitations of our current understanding of how these structures and systems respond
to extreme events. Therefore, it is important to examine the strengths and weaknesses
in our current project planning and engineering design. While not intended to cover
the full range of knowledge gaps identified during the field investigation, the list
below covers some of the questions that the COPRI coastal structures team
determined to be of greatest importance to this event and to the focus of the survey.
5.1 Structural Response to Dynamic Tsunami Forces
While tsunami inundation height is an important criterion for designing a coastal
structure, the performance of a structure during a tsunami must also take into account
hydrodynamic loads such as vertical uplift, fluid and debris impacts, hydrostatic
loads, and drag. The design of i nternalandfoundationconnections,tie‐ins,end
attachments,andabutmentsmustaccountfortheseloadsaswell.Currently,
recommendationsexistforloadingofverticalevacuationstructures(FEMAP‐
646,2008),buttheserecommendationsdonotextendtogeneralbuildingcodes.
UsinglessonsfromtheGreatEastJapantsunamitoevaluateand,insomecases,
toestablishtsunamidesignloadsforallcoastalstructureswouldreducethe
damagesfromfutureextremeevents.

5.2 Beach Responses
The beaches in Japan responded to the tsunami event in a variety of ways. Some
beach areas exhibited a noticeable beach deflation and massive loss of sediment.
Others showed little indication of a significant drop in beach elevation. A number of
causes could account for these divergent responses — shoreline structures that
prevented significant inland transport of sand, beach orientation relative to the
incoming wave, or natural beach recovery during the two months between the
tsunami event and the field investigation. The team's observations of various beaches
throughout the inundation area suggest the need for addition attention paid to beach
response to tsunami wave conditions both with and without coastal structures.

5.3 Scour
Pervasive scour caused any number of structural failures. Observed scour patterns
were quite erratic; scour behind a continuous structure varied greatly from one
location to another. Scour patterns around structures were irregular. For piles and
other cylindrical structures, this report documents attempts to relate scour depths to
pile diameters. These “rules of thumb” do not help with much of the scour patterns
observed inland of coastal dikes or around building pads. Because scour can

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