When the moon and the sun act on the oceans, it creates a kind of immense bulge that creates the phenomenon of the tides. In the Atlantic Ocean, which is very deep, there is a huge volume of water. Whereas in the Channel Sea, its smaller neighbor, there is much less depth and therefore less water.
Imagine then that the large bulge of water in the Atlantic Ocean enters the Channel Sea in a matter of hours. The latter then fills up much more significantly, and its water level is then much higher.