8.1: Introduction to Coasts
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The term coastal studies, which is in common use for a great variety of approaches to coastlines, covers a large area of endeavor. What do I mean by the term coastal? There are various definitions or interpretations, depending on how much or how little is included seaward and landward of the shoreline. The shoreline is fairly generally taken to be the line or trace where the sea meets the land, and this is fairly well defined except in areas with estuaries, tidal flats, etc., unless you want to quibble about where the high-water line is.
In this course I’m going to interpret the term coast in a fairly broad sense to mean a marine area extending from the shoreline out onto the continental shelf for some distance, together with some land area immediately landward of the shoreline. This admittedly sloppy definition succeeds in generally including areas both seaward and landward of the shoreline in which a great variety of processes operate that most people would categorize as coastal processes. I intend the definition to include only the innermost part of the continental shelf, the part that’s most strongly affected by the adjacent shoreline. (The continental shelf is the broad belt of shallow water adjacent to the coastline. In many areas of the world it is as much as a few hundred kilometers wide, and water depths at the shelf edge are no more than about two hundred meters.) In many coastal areas there’s a well defined coastal plain that may be well over a hundred kilometers wide; only the part nearest the shoreline, directly affected by modern coastal processes, is included in my definition.
Coastal environments are unusually varied. If you let your mind go blank and I say the word “coast” to you, what image do you first conjure up? Rugged rocky sea cliffs? Wide sandy beaches? Shining coral reefs bathed in transparently blue water? These are only some of the many important coastal environments in the world today. How can I possibly deal with all these environments in one small part of this course, you might ask? Obviously I can’t, so I’ll concentrate on just a few kinds of coastlines and the most important processes that operate around them.
There’s a lot of coastline in the world today: by one estimate, the coastlines of the world are almost 450,000 km long. One difficulty with an estimate like that is that the closer you look at a given stretch of coastline, the longer it comes out to be. On a large scale, that’s because maps always involve a certain degree of generalization, depending upon the scale of the map. But the effect is still there even when you’re looking at a small segment of the coastline right at your feet, because how do you take account of the outlines of the individual little sand grains?
Here’s a list of the important factors that govern the nature of a coast, together with a few initial comments about each. Keep in mind that there’s a rather strong interdependence among these factors.
- sediment supply from land. Clearly this is important in determining whether there is any sediment for the ocean to shape into beaches, deltas, barrier islands, etc. And the size of the sediment is important too: if only mud is supplied to the shoreline (as in certain low-latitude areas with hot and humid climate), you shouldn’t expect to have sandy beaches!
- climate. The climate of the land area inland from the coast has various indirect effects on the nature of the coast: size and supply of sediment (see above), river runoff, and in some cases glaciation.
- sea-level history. Sea level hasn’t stayed the same relative to the land: it rises and falls at rates ranging from something of the order of a millimeter per century, as a representative value throughout much of geologic time, to as much as a meter per century during rapid continental deglaciation, as happened in very recently in geologic time (within the last 20,000 years!). The nature of the coastline is not much affected by very slow changes in sea level, because coastal processes have plenty of time to equilibrate, but rapid changes in sea level are known to have strong effects on the nature of the coastline.
- hydrodynamic setting. By this vague term I mean the picture of waves, tides, and currents that affect the coastline. The relative importance of these three kinds of water motions varies greatly, and the nature of the coastline depends strongly on their relative importance.
- tectonic setting. In many areas the Earth’s crust is unstable, and undergoes both vertical and lateral movements that can be very rapid on geologic time scales and can be substantial even on the time scales of long-term coastal processes. You have to think about the effect of tectonism on the climate and sediment supply landward of the coastline and on the submarine topography offshore, as well as the more direct effects on sea level at the coastline.