Beach cusps: shoreline symmetry; By Gary Griggs

Posted In Beach of the Month, Features
Dec
1

1cusps

By Gary Griggs, Distinguished Professor of Earth and Planetary Sciences, Director Institute of Marine Sciences, University of California, Santa Cruz, California

There are many strikingly regular patterns in nature that have long intrigued scientists and non-scientists alike. Beach cusps are one of these. Coastal geologists and careful beach observers or frequent visitors may have noticed these evenly spaced, semicircular, scalloped-shaped patterns along the shoreline from time to time, or perhaps not at all. These very uniform patterns seem to be far more frequent on certain beaches than others, and are much more visible from an elevated vantage point, like a cliff top or in an aerial photograph, than when standing on the beach. They can form on sand or gravel beaches and can range in width or diameter from 25 to over 200 feet, but are very uniform or strikingly regular on any particular shoreline at any point in time.

The cusp spacing is shorter in gravel beaches and longer on finer-grained sandy beaches. These very symmetrical features are far easier to recognize and appreciate, however, than to figure out or understand, and we could just leave it at that and appreciate them for their symmetry. But being curious scientists, we usually look for answers to the mysteries we find in nature.

Published writing on these features goes back nearly a century to 1919 when one of the first coastal geomorphologists, Douglas Johnson, described these unique landforms.

“There are many strikingly regular patterns in nature that have long intrigued scientists and non-scientists alike. Beach cusps are one of these. ”
— Gary Griggs

Beach cusps seem to form most often when waves approach normal or at a right angle to the shoreline. The portion of the broken wave that washes up the beach face is called the swash, and the maximum difference in the run-up of the swash seems to be the dominant influence on the spacing of the cusps.

There have been two prevailing ideas on the formation of these symmetrical shoreline features, although neither is easy to explain and both probably will leave some lingering doubts in the minds of most readers. The earliest idea on the formation of cusps was that they were due to distinct properties of the waves breaking on the shoreline and were, therefore, essentially imposed on the beach by nearshore wave interactions.

In recent years, however, this theory has been essentially displaced by another idea, which involves the somewhat complicated concept known as self-organization. This principle, which has now been reproduced in models, shows that interactions between wave runup on a beach and sediment transport can combine to give rise to instability that soon produces cusps. On a flat beach, areas will develop that have slightly lower relief or elevation than adjacent areas. As waves wash up the beach, they will accelerate or speed up over these lower areas, and cause erosion. These lower areas will deepen gradually to form an embayment. On the other hand, those areas on the beach that are slightly higher will slow down the uprush of the waves, causing sediment to be deposited on top of them. These will evolve to form the horns between the embayments, and together they known as beach cusps.

Somehow, as these cusps begin to form, and they can develop quite quickly as wave conditions along the shoreline change, these features interact or communicate. As a result the patterns of erosion and deposition along the beach face (the cusps) begin to develop a very uniform size and spacing as the beach tries to rearrange itself through erosion and deposition to reduce variations along its surface.

This self-organization process, at least for now, is the favored mechanism used to explain the formation of these widespread and interesting shoreline features. They occur in both small pocket beaches but also may extend for miles down the beach, and can then disappear within a few hours as wave conditions change. Beach cusps are one of those somewhat mysterious natural phenomena that for most of us may simply be best appreciated without trying to completely understand just how they form.

Tags:

More / Beach Of The Month

Santa Veronica Beach, Atlantico, Caribbean coast, Colombia: A model of small community, beach loss, wrong responses; By Nelson Rangel-Buitrago, Adriana Gracia & William J. Neal

July 1st, 2019

Santa Veronica is one of numerous recreational beach developments along Colombia’s Caribbean Coast most sharing a similar history of shoreline retreat, perceived as shoreline erosion, and the attempt to hold the shoreline in place through the use of shore-hardening structures.

Read More

Big Talbot Island’s Blackrock Trail; By Cecelia Dailey

June 1st, 2019

The locals call it “lava beach”—a misnomer which leads some to believe the unique formation found here are igneous in origin. But these mystifying “black rocks” crumble to the touch, staining the hands, feeling gritty with sand. Although many are black, these “rocks” are sometimes light colored, deep red or burnt brown.

Read More

Terraces and Towns; By Gary Griggs

April 1st, 2019

The geologic history of California’s north coast is evident in the typically steep relief and coastal landforms. This is an area where a drive along much of the narrow lanes of State Highway One along the often steep coast is always an adventure and where it’s never wise to take your eyes off the road for very long. Most of the beaches occur at the mouths of the coastal streams.

Read More

A Special Beach: Nauthólsvík Geothermal Beach, Iceland; By Norma J. Longo & Orrin H. Pilkey

February 1st, 2019

Iceland is a land of black beaches, usually with a large gravel component. But one Icelandic beach near Reykjavík is different.

Read More

“Beach Robbers”; By Charles O. Pilkey

December 1st, 2018

“Beach Robbers”, is a book chapter written and illustrated by Charles O. Pilkey, excerpted from “The Magic Dolphin: A Young Human’s Guide to Beaches, Sea Level Rise and Living with the Sea” by Charles O. Pilkey with Orrin H. Pilkey.

Read More

California’s Coastal Harbors, Beach Compartments and Sand Dredging; By Gary Griggs

October 1st, 2018

Every year the dredge at the Santa Cruz Small Craft Harbor along central California’s northern Monterey Bay sucks up about 250,000 cubic yards of sand, on average, from the entrance channel and pumps it out onto Twin Lakes Beach where it continues its journey down coast. If it were put in dump trucks, it would fill about 25,000 of them, but the waves can move all that sand without any human labor, and without any noise or carbon emissions.

Read More

Beyond Preservation: The Coral Restoration Foundation Bonaire; By Andrew Jalbert

August 1st, 2018

When avid scuba diver and famed Jurassic Park author Michael Crichton first visited Bonaire decades ago, he eloquently described the underwater environment as, “a world of riotous, outrageous color.” Years later, Bonaire has seen some changes but his assessment still largely rings true.

Read More

Management Strategies for Coastal Erosion Processes; By Nelson Rangel-Buitrago

June 1st, 2018

The Special Issue Management Strategies for Coastal Erosion Processes (MSforCEP) presents an international collection of papers related to the implementation of various management strategies for coastal erosion under specific objectives.

Read More

Archive / Beach Of The Month