(A little long--sorry.) A beach is the sediment which occupies the transition zone between land and open water. They come in many forms, from mud to boulders, with low wave activity or high, tiny tidal influence or great. In all cases, the sediment of the beach is acted on by water and wind. Today we are focusing on sand beaches, being the most typical.
Barrier beach with marsh or lagoon behind.
Sand is sediment—fragments of rock and shells—which ranges in diameter from 0.0625 mm (4 on the phi scale) to 2 mm (-1 phi). Sand grains are large enough that they are not picked up and carried great distances by wind (dust storms are silt- and clay-sized grains, not sand), but are readily transported by rapidly moving water and crashing waves. For this reason, sandy beaches are dynamic environments, constantly responding to changing conditions.
The most typical sandy beach form is a barrier beach (not to be confused with a barrier island). Sand covers the entire transition from offshore depth to terrestrial land, with the beach itself as the barrier to the ocean water. The form a barrier beach takes is dictated by several factors, the principal ones being wind, waves, tides, and sediment supply. If sediment supply is consistent, and given that a beach’s tidal regime is as well (even if that regime is of irregular tides), the beach’s form changes mostly with alternation between fair (summer) and stormy (winter or hurricane) weather.
A barrier beach is largely a series of berms backed by a large grassy dune. A beach berm is essentially wave-built pile of sand with a sloping front and more or less flat top (slopes can vary based on wave height and sand grain size). The height of a berm demonstrates the wave energy which built it: the berm’s top is the limit to which typical wave energy can carry grains of sand.
North Topsail Beach, North Carolina, before and after Hurricane Florence, 2018.
Generally berms are built at low-tide and high-tide elevations, with other berms being built on large beaches at higher storm wave heights. Larger waves with more energy erode lower berms, and either carry that sand farther up to build higher berms (or create an overwash deposit on the landward side of the dune), or they transport the sand offshore. Smaller waves with less energy tend to slowly carry offshore sand back to the shore and rebuild the lower-elevation berms.
A very large hurricane can transport sand far enough away
from shore that it is lost to the beach system, because smaller fair-weather
waves don’t reach deep enough to move it again. Depending on the storm’s
severity, it diminishes or destroys the fair weather dune system. But the
alternation between the larger, lower berms of fair weather and the smaller
storm beach demonstrates the dynamic
nature of the coastal and beach environment.
A pocket beach between rocky headlands.
Tides, waves and sediment supply can be used to very simply categorize beaches. The tidal range is defined as the difference between average high and average low tides. Microtidal zones have tidal ranges less than 2 m; mesotidal coasts have tidal ranges between 2 and 4 m; macrotidal zones greater than 4 m. Wave energy is harder to categorize but is done so in comparison to tidal action, based on the form of the coast. An area is described as being dominated by tidal or by wave energy. A sheltered mud flat with little wave activity might be described as macrotidal, tide-dominated coast. A barrier beach with live surf might be a mesotidal, wave-dominated environment.
A barrier island.
Sediment supply matters greatly. New England coasts, particularly to the north, tend to be sediment starved: there is a large amount of exposed bedrock, with sandy beaches only in sheltered areas (“pocket beaches”). Meanwhile, from the mid-Atlantic south, the coasts are sediment rich, and not only are sandy beaches ample, but barrier islands are common, built further offshore almost entirely out of sand.
Tomorrow: dynamic equilibrium.
Be brave, and be well.
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