Normal ocean waves result from the frictional drag of wind on the surface
of the water. The waves you observe at a given beach may originate thousands of
kilometers away, whereas tsunami waves are caused by
displacement of the crust caused by an earthquake. Waves have a crest and a
trough, the distance between crests being known as the wavelength. Waves travel in
groups called a wave train. Waves are not all moving at the same speed. When one
wave catches up to another you can end up with an even higher wave. This is
called constructive interference. Ocean waves can travel hundreds of kilometers
before they reach the land. The distance of water over which the wind has blown
is called the fetch. A long fetch will result in big waves and big waves carry
with them a tremendous amount of energy. Storms create destructive waves thus
a lot of erosion takes place during storms, Even on calm days it is possible to get
big waves as these ones have been generated in a storm somewhere else some
time ago. Waves that have been generated somewhere else are called swell. Swell
waves have a long wavelength and are considered desirable by surfers compared
to the choppy locally produced waves which have short wavelength. Waves on the
east coast of Australia are often big as may have traveled a long distance across
the Pacific Ocean before reaching Australia. Waves that build up coastlines
by transporting and depositing sediments on the shore are called constructive
waves.
So what happens when waves reach the coast. There is a maximum depth to which
waves will cause any significant water movement. We call this the wave base.
Thus below the wave base waves have no impact on the sediments below.
However when waves enter shallow water the usual orbital motion of water
becomes elliptical, the waves slow down and becomes steeper and higher. This is
called shoaling. When the waves become too high relative to the waters depth
the waves break. Since the water is shallow the breaking waves are able to
push the sediment up towards the beach. This flow of water moving up the beach
is called the swash. The returning flow of water down the beach is called backwash,
and also drags sediment back with it. The offshore topography of the seafloor will
impact on the way waves break. Spilling breakers have wave crests spill
forward creating foam and turbulent water. They occur where waves travel over
a wide gentle sloping seafloor. Plunging breakers have the classic
barrel shape where the waves curl and their crests fall forward with a lot of force.
They occur on steep beaches, on sandbars and the outside edges of reefs. In these
places the waves are moving from deep to shallow very quickly and they have to
slow down fast.
Surging breakers are waves that rapidly surge up a beach without appearing to
break. They occur on beaches where the beach profile is too steep for waves to break
properly. These types of waves have a very strong backwash. The shape of the
coastline also affects the way waves travel towards it. When waves approach
an uneven coastline of bays and headlands they are refracted and become
increasingly parallel to the coastline. Refraction occurs because the waves
slow down first in the shallow water, forcing the refracting part of the wave to
break first at the headland, while the other part of the wave that is traveling
faster towards the beach is in deeper water. Refraction increases erosion at
headlands and deposition tends to take place in the lower energy bays and
beaches. Diffraction occurs when waves meet a gap such as a gap between a break
wall or between two islands. Once past the barrier they spread out in all
directions rather than carrying on straight.