Geography Reference
In-Depth Information
between twice the wave length and a twentieth of the wavelength,
then the wave motion will be affected by friction on the seabed.
This means that the water motion beneath the waves becomes
more elliptical with the ellipses being smaller and flatter nearer the
bed so that at the bed water just moves back and forth. In shallow
water very close to the shore where the water depth is less than a
twentieth of the wavelength then the water movement is just hori-
zontal in a back and forth direction. In reality what this all means is
that as waves move from deep to shallow water they get closer
together and slow down. Waves also get higher as they travel into
shallower water, a process known as
shoaling
. They also change
their shape from a more symmetrical wave shape to a shape with
more peaked crests and flatter troughs. In deep areas, water move-
ment produced by waves is back and forth at the same velocity (i.e.
it is the shape of waves and their energy that is moving across the
ocean but not the water itself within the waves). However, close to
the shore the onshore side of the wave is stronger and this pro-
motes sediment transport more favourably in the onshore direction.
Wave
refraction
occurs as the wave moves close to the shore.
This is when the section of the wave in shallower water travels
slower due to friction on the bed than the section of the wave in
deeper water. The outcome is that the wave crest rotates to
become parallel with the contours of the seabed so that wave direc-
tion 'bends' as it approaches the shore.
When the water depth becomes too shallow for the wave then
the wave breaks up. When the water depth is just slightly greater
than the wave height then the wave breaks up creating an area
known as the
surf zone
. The energy released by breaking waves
can be significant and help generate nearshore currents and sedi-
ment transport. When waves break they cause a rise in the water
level on the beach with the returning water running back down
the slope into the sea. This motion is called swash and the up-
beach movement is better at transporting sediment than the return-
ing flow and so this helps maintain the gradient of the beach.
Nearshore currents
are important for landform development in
coastal areas. These currents gain their energy from wave breaking
so if the waves are stronger (e.g. during storms) then the currents
will be stronger. Longshore currents flow parallel to the shore
within the surf zone driven by waves entering the surf zone with
