The tidal rise in the Bay of St.-Malo is on average the second greatest in the world (14m, compared with those in Eastern Canada-Bay of Fundy, etc-17m). On average ocean tides are 0.8m. The phenomenon is due to the funnel shape of the sides and bottom of the bay funneling into the estuary of the Rance River. To our knowlege nowhere else has this been utilized as a source of energy (See Tips on the Usine mare-motrice in various pages for St. Malo or St.-Servan). More important to the unsuspecting tourist, is the effect upon the beach size and facilities. It also presents a hazard to access to the National Fort and the Ile de Grand Be where Chateaubriand's tomb is located. Warnings are posted but visits must be timed for low time and length of stay. I am sure the tourist office can give the precise daily schedule. At times this same bay-tide effect involves Mont St.-Michel changing it from an island to a rocky outcropping in a sea of sand. (Look at the appended pictures. Note also the deserted beach off-season when the wind is strong and cold).
We decided to take a walk out to the National Fort from the beach and were surprised when a lifeguard in a wetsuit turned us away, as it didn't look like the tide was coming in quickly from the small island....but within 5 minutes, amazingly it had not only turned, but completely covered the spot where we were standing, with water rushing in from both sides. I've never seen a tide rush so quickly - it even separated a couple of families from their children, who were only about 7 feet away - the lifeguard rescued them from knee deep water that was really rushing in.
The sea was very calm at St Malo when we were there, but we saw postcards and photos of huge waves which actually can come up over the ramparts (see sign in Photo 2) - winter time perhaps. Something to be aware of anyway, not to go up on the ramparts in really bad weather.
When the tide comes in - get out of the water, it covers the sandbars and all the beach area.