[WJW]

I wonder about this too. We have several dikes already that are much higher than seven metres. From everything I hear, we're not anywhere near the limits of feasible dike height, not in terms of engineerable strength nor in economic feasability (they're mostly just earth walls, after all).

[jacquesm]

That, and dikes need not be made of earth, they can have concrete as well.

The biggest risks with dikes are seepage and shifting, especially a problem with old river dikes before we properly understood how to make them.