Geomorphology is the analysis of geological forms. It focuses on the surface expression of components, such as bedrock, soil, gravel and sand, and the natural forces which act on them, such as rain, wind, waves, and gravity. The term comes from Greek “ge”, earth, and “morphe”, shape. Coastal geomorphology focuses on the zone where land and ocean interact. A “beach” is defined as the sediment which covers any stretch of coast. That sediment need not be sand!
Wave rose for one location, showing direction, probability density, and typical heights of incoming waves.
Our minds tend to run, out of habit, to sandy beaches when we think of the coast. And with reason: it’s estimated that slightly less than a third of the world’s total coastline is covered with sand. Sand is defined as pieces of rock ranging in diameter from 0.0625 to 2 mm. The range might seem arbitrary but it is based, at least loosely, on physical behavior. Smaller sediment such as silt and clay tend to be cohesive, and behave at times like a fluid. Larger sediment, due to its mass, is far more resistant to the push of water and air. But beaches can be composed of smaller sediment grains, like silt and clay, and larger, like gravel, cobbles or even boulders.
Mudflat beach.
Your typical sandy barrier beach.
A number of influences act on sand and other sediment along the coastline. First among these are waves. Based on the direction and shape of the coast, waves could arrive from a range of different directions, and be of varying height. A rose diagram (similar diagrams exist for wind) can show several different types of information at once: the direction from which waves tend to arrive at a given location, and the relative percentage for each. With color coding, the diagram can also show percentages of typical wave heights for any one direction. This is useful information in analyzing the form of a beach.
A rather steep gravel beach.
Cobble beach.
Tides can be another major influence, ranging from the large tides of Cook Inlet, Alaska to the relatively tiny tides of the Mediterranean. Wind is a factor. The overall climate, including difference between summer and winter conditions, is very important. Along much of the coast of New England, there is a clearly visible difference between beaches in summer or fair weather, which are larger, and winter or after a large storm, where much of the sand has been carried offshore by larger waves. This is an aspect of “dynamic equilibrium”, a concept we will return to in a future post.
Now that's a beautiful boulder beach, isn't it?
Tomorrow: intro to coastal geomorphology 2: sediment.
Be brave, and be well.
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