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Here's the Science Behind the Formation of Mystical Beach Cusps

Jhulin Bihari Mar 20, 2019
In many beaches, some periodic patterns having lunette shapes are formed. These shapes consist of cusp-shaped rims, known as beach cusps. It looks spectacular and unique, but some eeriness surrounds it, because the science behind their formations still has the scientists baffled.
Beach cusps are mostly formed before or after the storm, but the surprising fact is that the crescents that are formed have equal gaps and regular periodic shapes.
The two theories that support their formation are "self-organization theory" and "standing edge theory".

Self-Organization Theory

The physiology of a beach and the flow of water, both with a positive interaction, give rise to the projection of figures in a flat surface. The feedback between the fine sand particles and energetic beach waves results in the formation of cusps.
Depressions are created, and these areas get continuously eroded with high energy waves. These patterns of lower relief give rise to the tip of crescent known as "embayment".
After the embayment gets formed, the average beach level decreases, and the areas which were initially at a higher level have decreased their level. This results in the slowing down of water collisions due to which the sediments get deposited, and horns are formed.
Regular spacing occurs because when the waves hit the coast, they strike the horns first. As a result, the water loses its sediment, and it gets deposited on the horns.
During the backwash, the sediment-free water clears the embayments of all sediments. So, during the hit, the slow down of wave speed results in these regular spaces. Gradually, these patterns of beach cusps spread across the length.

Standing Edge Theory

The positive feedback between the shore and waves at shoreline give rise to this theory. Well, the theory states that when the incoming waves constantly reach the shore, it results in the creation of waves perpendicular to the direction of the incoming waves.
The perpendicular waves are called edge waves. The edge waves get trapped near the shoreline and when they meet in opposite direction, then the standing edge is formed.
Well, this concept can be better explained if we consider sinusoidal peaks and troughs, where we have anti-nodes and nodes. It is the anti-node where all actions takes place, like water rises and water falls.
When the incoming waves collide with the peak, then the height of the incoming waves increases, but the reverse happens when the incoming waves collide with the troughs - then the incoming waves' height decreases.
The areas where the wave height increases, the waves grow stronger and more erosion occurs; and in places where wave height decreases, then slow erosion occurs.
Stronger erosion results in embayments, and weaker erosion results in horns. With time, the amplitude of beach cusp may vary.
The theory for regular spacing is supported by the fact that a standing edge has twice the period of the incoming wave; when incoming waves complete one cycle, then the standing edge waves would have completed two cycles.
What once was a peak of the standing edge wave will change to trough in the incoming waves' trough cycle. The initial peaks full of energy now drop down, resulting in equal spaces between the cusps.

Places Where Beach Cusps Are Found

The major locations where cusps are formed are:
• Jurassic Coast, Dorset, England
• Palomarin Beach, Point Reyes, USA
• Ringstead Bay, Dorset, England