13.1 Beaches

For most people, when they think of coastal areas they picture a beach, and the beach that they imagine is probably a typical sandy beach composed of sand grains (section 12.2). But beaches are comprised of whatever types of are dominant in the local area. For example, parts of Hawaii and Iceland are famous for their black sand beaches, made up of eroded and other volcanic materials. The beautiful tropical white sand beaches we see in travel ads are largely composed of the crushed calcium carbonate remains of coral skeletons (much of which has been chewed up and excreted by a fish before we happily run our toes through it!)  Other beaches may lack sand altogether and instead be dominated by small shells, or larger rocks or pebbles (Figure 13.1.1).


Figure 13.1.1 Various beach substrates. Clockwise from top left: Punaluu Black Sand Beach, Hawaii, USA (Diego Delso [CC BY-SA 3.0], via Wikimedia Commons); Shell Beach, Shark Bay, Western Australia (Brian W. Schaller (Own work) [CC BY-NC-SA 3.0], via Wikimedia Commons); white coral sand beach in the Maldives (http://www.elitedivingagency.com/, [CC BY-SA 3.0] via Wikimedia Commons); rocky beach at Killbear Provincial Park in Ontario, Canada (John Vetterli (originally posted to Flickr as Beach) [CC BY-SA 2.0], via Wikimedia Commons).

The shoreline is divided up into multiple zones (Figure 13.1.2). The is the region of the beach above the high tide line, which is only submerged under unusually high wave conditions, such as during storms. The lies between the high tide and low tide lines; it is submerged during high tide and is exposed during low tide. The extends from the low tide line to the depth where wave action is no longer influenced by the bottom, i.e. to where the depth exceeds the (section 10.1).  Finally, the zone represents the depths beyond the nearshore region.

Along the beach itself, the area above the high tide line is called the , which is usually dry and relatively flat. The berm often ends with a berm crest or berm , which is a steeper wall carved out by wave action that leads down to the foreshore. The foreshore has a number of other names, including the , the or , and if the area is fairly flat, the . Just off shore from the beach there are often and longshore troughs running parallel to the beach. The longshore bars are accumulations of sand that are deposited by wave action and (section 13.2). The decrease in depth above longshore bars is what often causes waves to start to break well before reaching the beach (section 10.3).


Figure 13.1.2 The zones of a typical beach (Modified by PW from Steven Earle, “Physical Geology”).

The sand or other particles that make up the beach are distributed by wave action. The water that moves over a beach through incoming waves is called , and it also contains suspended sand grains that can get deposited on the beach. Some of the swash percolates into the sand while the rest of the water washes back out as as the wave recedes. Backwash removes sand from the beach and returns it to the ocean. Sand will therefore be deposited or eroded depending on which process is dominant. If wave action is light, a lot of incoming water gets absorbed by the sand, so swash dominates. Under heavier waves the beach becomes saturated with water, so less can be absorbed, and backwash is dominant. This leads to seasonal cycles in beach structure; waves are heavier during the winter as a result of stormier conditions at sea, so backwash dominates and sand is removed from the beach and deposited offshore in . In the summer the waves are gentler, swash dominates, and the sand is transported from the longshore bar and deposited on the shore to create a wider, sandy beach (Figure 13.1.3).


Figure 13.1.3 The differences between summer and winter on beaches in areas where the winter conditions are rougher and waves have a shorter wavelength but higher energy. In winter, sand from the beach is stored offshore (Steven Earle, “Physical Geology”).



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Introduction to Oceanography by Paul Webb is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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