Tidal Bore

Water with no place to go can rush up into valleys when moving into mountain ranges, and the water level will rise accordingly. Thus when hoping to remain safe within mountains bordering an ocean, the altitude should be increased to take possible tidal bore into consideration. Placing two mountain ranges between the safe location and the ocean allows the water to rise up the side of the first mountain range, down into the valley between the two mountain ranges, but most likely not over the second mountain range. The Britannica describes a tidal bore as:

A body of water that rushes up some rivers that empty into the sea at times of exceptionally high tides. Traveling upstream about two or three times as fast as the normal tidal current, a bore usually is characterized by a well-defined front of one or several waves, often breaking, followed by its main body, which rises higher than the water at its front. The height of the bore is greater near the banks of a river than at midstream. Because of momentum, some bores continue to move upstream for about one-half hour after high water. Not arising in estuaries, tidal bores are formed at a position a short distance upstream, where the river channel has become sufficiently narrow or shallow to concentrate the momentum of the rising tide. Bores occur at spring tides and at several tides preceding and following spring tides but never at neap tides. The formidable tidal bore that occurs on the Lower Seine in France between Rouen and the sea is known as the Mascaret.