Beach cusp

Beach cusps are shoreline formations made up of various grades of sediment in an arc pattern. The horns are made up of coarser material and the embayment contains finer sediment.

They can be found all over the world and are most noticeable on shorelines with coarser sediment such as pebble beaches. However, they can occur with sediment of any size. They nearly always occur in a regular pattern with cusps of equal size and spacing appearing along stretches of the shoreline. These cusps are most often a few metres long. However, they may reach 60 m (200 ft) across. Although the origin of beach cusps has yet to be proven, once cusps have been created they are a self-sustaining formation. This is because when an oncoming wave hits the horn of a beach cusp, it is split and forced into two directions. The crashing of the wave into the cusps slows its velocity, causing coarser sediment to fall out of suspension and be deposited on the horns. The waves then flow along the embayments (picking up finer sediment) and run into one another in the middle. After this collision these waves attempt to flow back out to sea where they are met by incoming waves. Therefore, once the cusp is established, coarser sediment is constantly being deposited on the horn and finer sediment is being eroded away from the embayments. This process causes the horners and embayments to at least maintain their size, if not grow larger.

There are a number of theories as to why beach cusps are formed but currently, there are only two explanations with any real credibility.

The standing edge wave theory is based on an interaction, near the shoreline, between the waves that are approaching the shore and waves that have been set up perpendicular to the shoreline called 'edge waves'. The regular arrival of incoming waves in the near shore waters causes the development of waves perpendicular to the direction of the incoming waves; these are termed 'edge waves'. These edge waves become trapped near the shoreline and when two of them come together from opposite directions, a standing edge wave is formed. The movement patterns of these waves are fixed and so can be defined as two regions of interest, the nodal and antinodal points.

The antinodal point is where all the movement takes place as the water rises and falls, creating a series of peaks and troughs. Between these antinodal points are the nodal points where no vertical movement takes place. An incoming wave has an almost uniform height but when it collides with a standing edge wave, this is changed. If it collides with a peak, then the wave height is increased and if it collides with a trough, then its height is decreased. In the diagram on the right, the two waves cancel each other out, creating a flat surface. However, this is a highly simplified version of events. The incoming wave has the same wave period as the edge wave, so the incoming wave changes from a peak to a trough over the same period as it takes the standing wave to change so they keep the same pattern. These are known as synchronous waves and are very uncommon.

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