1.5.5: Classification of coastal systems

Due to the intensive interaction between land and water, a large variety of coastal ocean systems has developed. Many classifications of coastal systems have been proposed that try to order the huge variety in classes with similar characteristics. In fact; so many classifications have been proposed that it seems hard to make sense of the large assortment of classifications. We will therefore not attempt to give an overview of all classification schemes, but focus instead on the points of similarity between various classifications.

In order to characterise a given coast we need to describe a minimum of four terms:

1. Geological factors:
   Any coast is the result of slow geological processes (like mountain formation) that require million of years and result in a certain initial state of the solid boundaries (considered as a given by a coastal engineer);
2. Nature and abundance of coastal ‘material’:
   Is the material hard (rocks, coral) and/or soft? Soft material (mud, sand, gravel, cobbles and carbonate sands) is present in depositional coastal features like deltas, beaches and mud flats. They can host vegetation such as mangroves, salt marshes, and dune vegetation;
3. Transgression or regression:
   The gradual relative sea-level changes (rise or fall) that have timescales of thou- sands of years determine – in combination with the amount of sediment supply to the coast – whether a coast during a certain period of time has advanced (for instance a delta that has built out) or retreated (a drowned river valley). In this terminology, transgression and regression are used to describe a horizontal shift of the waterline. Hence, regression (of the sea) is equivalent to advance of the coast. Similarly, transgression (of the sea) implies retreat of the coast. Besides global effects there are regional and local (for instance bottom subsidence) effects;
4. Processes that construct and erode the coast:
   Processes affecting sandy and rocky coasts are for instance wind and hydraulic forcing by waves and tides; for coral coasts and in mangrove environments or salt marshes chemical and biological processes are important. Waves and tides give rise to significantly different shapes of depositional features consisting of sand, mud and gravel. On a global scale these processes are affected by latitude and climate whereas on a local scale they are dependent on local bathymetry.

Coastal classifications are typically based on one or more of the above four identifiers and are dependent on the scale: are we considering an entire continent or a small section of a coastline? Are we looking at developments over thousands of years or at smaller timescales? On regional (tens to hundreds of kilometres) and local scales (only a few kilometres) coastal features are dependent on the forcing by processes such as waves, tides and wind. The present lecture notes take a mainly process-based approach and hence focus – from Ch. 3 onwards – on regional and local scale features. Underlying these smaller-scale features are broader (or first-order) coastal features. The latter cover large geographical distances (thousands of kilometres) and are linked to the long term geological process of plate tectonics and influenced by climate. On long timescales and associated spatial scales, coastal development is also connected with the expansion and retraction of ice-sheets and associated sea-level changes.² History’s legacy to coasts is described in Ch. 2.

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2. During the Quaternary (from 1.8 million years ago till present) the sea level fluctuated over more than 100 m vertically.