As the waves roll in and strike the beach, they slow down when they feel the bottom and
grow in height and steepness as they become compressed and closer together. Surf height on
a gently sloping beach may grow in size to twice the height of the swell in deep water
before they break. Very long period swell of 25 to 30 second periods may grow to close to
triple their deep water swell height. As the waves become too steep, and the top portion
of the wave moves much faster than the lower portion, they will break. The threshold for
steepness is somewhere close to the point where the wavelength becomes shorter than 7
times the wave height.