A 3 m tsunami isn't like a 3 m wave from wind action. It is like a 3 m tide or storm surge added on top of whatever normal tide level you are at. So if it hits

A 3 m tsunami isn't like a 3 m wave from wind action. It is like a 3 m tide or storm surge added on top of whatever normal tide level you are at. So if it hits at low tide it might be somewhat negated, but if it hits at high tide it may overwhelm sea walls and other coastal infrastructure. When people talked above about the long wavelength of a tsunami, they mean it doesn't crest and possibly break in seconds like a surface wave. It can keep building for many minutes and continue to flow inland after the crest. Look for some tourist video of the 2004 tsunami that flowed over islands in Thailand. It looks more like a river or levee breach flooding over the whole field of view. In steeper coastal terrain, the wave can sometimes be funneled or focused into a narrower port or valley and then reach much higher elevations than you would expect.