What's the name of that place in France...?

Question

which has a shoreline on such a low gradient that when the tide comes in it rushes up across the ground very fast? You can hardly even outrun it.

Answer 1

Aix en Provence?

Answer 2

You are probably referring to the area around Mont. St. Michel. Its possible to walk across the mud flats at low tide but the area rapidly goes underwater when the tide comes in.