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USGS Geology in the Parks

Beaches - Products of Waves and Sand

Figure showing labeled idealized beach cross section Beaches 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.

Cape Canaveral National Seashore Beach 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 berm.

Waves

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 drift

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

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 sand
Dig deeper - green sand
Dig deeper - black sand

Fragile Beaches

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.

SWASH buggy

 

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