Beaches - Products of
Waves and Sand
are collections of loose sediment deposits, usually consisting of
well-sorted sand, in a coastal zone. They exist between the height
of storm surge waves and the level of the low tide. A beach consists
of two regions: foreshore and backshore. Nearshore regions are always
underwater, but are still affected by coastal processes such as
storm wave action and currents. The foreshore is comprised of the
normal intertidal region and generally has a 2-10 degree slope.
The angle is controlled by a combination of grain size and typical
wave energy for that shore. The backshore area slopes landward,
is generally wider than the foreshore and may contain one or more
berms, or terrace-like area.
The beach shown to the left is an example of a backshore at Canaveral
National Seashore. If you look closely you may be able to tell
that the beach's angle steepens as it meets the waves - this, depending
on the height of the tide, could be the plunge step, or a slight
Wave energy is determined by a number of factors: prevailing winds,
current, and slope of the shore. Wave
energy increases with increased winds and current. A shorter,
steeper slope will result in more erosive intensity than a long
slope. If the slope is too shallow the waves will break before reaching
the beach, diminishing the erosive power. Longshore
drift, or current that runs parallel to the coast, moves sand
lengthwise down the beach. This drag influences the migration of
sand and, over time, barrier islands.
Assateague Island National Seashore (in the lower half of the image
at right) has moved coastward, or west, in relation to Fenwick Island
since the construction of a sea wall, en placed to protect the channel,
an important shipping conduit. This shift is caused by a combination
of factors, including waves, current, and the disruption of sediment
supply by the sea wall.
To read more about coastal issues, including those at Assateague
Island National Seashore, click here,
or on the image of the island.
Sand comes from many locations and environments, and may be derived
from either rock or biotic
sources. It can have a source as close as rockfalls from seacliffs,
or be transported hundreds to thousands of miles by streams and
rivers. A portion is even carried as dust and sand in the air. Once
the sand is has been washed into the ocean longshore drift moves
and collects it in locations - prompting dune formation. The majority
of the sand you'll see in temperate regions is composed of silica-based
rocks. Silica, what both quartz and glass are made of, is very resistant
to erosion. It lasts after other minerals have been broken down
and altered. In tropical regions another mineral can also be found
in sand - calcium carbonate, or CaCO3.
Dig deeper - silica sand
Dig deeper - calcareous
Dig deeper - green sand
Dig deeper - black sand
Circumstances such as storms or rising sea level can have significant
effects on the width and slope of the beach. Barrier islands, due
to their lack of hard substrate (base layer) such as bedrock, are
particularly vulnerable to these extreme conditions. Much of the
recent research on beaches concerns the change in amount of beach
sand, and effects of storms on deposition and erosion of those beaches.
New methods of quantifying coastal change have recently been developed.
SWASH, a ground-based
GPS system, and LIDAR,
employing lasers for aerial mapping of topography, are now being
used to monitor the status of beaches of the coastal US. Aerial
photography is also used to track changes, and has been used
extensively in post-hurricane measurements. Pictured below is the
SWASH dune buggy. Click on it to learn more about SWASH
and the instruments used.