The Coastal System

Constructive waves

Associated with swell, these have short amplitude, long wavelength, frequency around 6-8/min, strong swash, weak backwash, deposits material and produce berms as more material is deposited and pushed up the beach.

Destructive waves

Associated with storm conditions, these have high amplitude, short wavelength, frequency around 10-14/min, weak swash, strong backwash, material is removed to form a steep beach. When powerful destructive waves fire material to the back of the beach, a ridge known as a storm beach forms.

Tides

Tides = regular rising and falling movements of the surface of the sea caused by the effects of the gravitational pull of the moon and the sun on the oceans.

The tide cycle (the gap between two periods of high tide) is 12 hours and 25 minutes, meaning we experience about two high tides and two low tides each day.

Spring tides occur when the moon is between the Earth and the Sun, as their gravitational pulls combine to produce the greatest tidal range.

Neap tides occur when the sun and the moon form a right angle, as their gravitational pulls interfere with one another (tidal range is lowest at this time).

This tide pattern is modified by the nature of the ocean bed, land mass proximity and the effect of Coriolis force.

Tidal range determines vertical range of erosion, deposition and the length of time the littoral zone is exposed to subaerial weathering.

Tidal ranges are classified as:

• Macrotidal – more than 4m
• Mesotidal – 2 to 4m
• Microtidal – less than 2m

In low-pressure systems, high tides and storm surges may combine to result in enhanced erosion.

Sediment Sources

Sediment sources include cliff erosion, biogenic input (e.g. shells), marine deposits, material carried by the wind and rivers (90%).

Sediment Cells

A sediment cell is a length of coastline that is relatively self-contained as far as the movement of sand or shingle is concerned, and where interruption to such movement should not have a significant effect on adjacent sediment cells.

There are 11 sediment cells on the England and Wales coastline.

Each major littoral cell is divided into a number of sub-cells.