I've been thinking about this ever since I/we saw the pathetic job that the so-called 'break-waters' did in not doing their jobs for the nuke plants. Now, it may be that the walls were simply not tall enough to do the job. A 10m wave is pretty hard to plan for, let alone protect people/infrastructure against, after all.
Take the jump for my layman's solution. Fluid dynamic engineers/experts are invited to shoot it down...
Below is a schematic I threw together, representing a protective sea wall. As you can no doubt see from the arrows depicting water flow, the tsunami is ultimately canceled out by destructive interference as indicated by the 'Xs'. The height and length of the wall would of course be determined by local conditions.
This is obviously a simple diagram, and the best configuration of the wall would itself be a semi-circle to accommodate a wave from any direction.
I invite criticism from experts in fluid dynamics/tide dynamics.
If you want to add to the tags, please do if appropriate.
Updated by Clive all hat no horse Rodeo at Sat Mar 19, 2011 at 05:47 PM PDT
Here's a mock-up of the underwater situation...