2004-08-20 / Columnists

Drawing On Science

by Stephen Yaeger

Coastal sand dunes are an integral part of the beach ecosystem. They serve to protect the upper beach and inland regions by acting as a barrier to excessive high tides or storm surges.

Sand dunes are formed when ocean and on-shore winds lift and carry sand particles up onto the beach. If there is an obstacle in the path of the wind it is diverted and creates what is called a wind shadow. Such a shadow is formed leeward or beyond the obstacle.

Another, smaller wind shadow is formed windward, which is in front of the obstacle. Particles are deposited in the windward shadow as the wind strikes the obstacle. This occurs because the wind is losing energy as it is diverted and it releases the particles.

Also deposited are those sand particles that strike the obstacle. Particles carried past the obstacle are deposited in the leeward shadow. As a result of this process a small sand mound is formed, which will develop into a sand dune. As the wind continues to deposit sand particles in the shadows a gentle slope forms on the windward and leeward sides of the developing dune.

As long as wave action is relatively gentle and as much sand is deposited on average as is removed by the tides, wind action will continue to build the dune. In time vegetation takes root stabilizing the dune.

Sand dunes will migrate inland at a slow pace. This movement of landward travel is due to sand slides on the leeward side of the dune. When the leeward slope reaches an angle of about 34U0 /Ua small sand slide occurs, forming what is known as a slip face. The continuous deposition of sand tends to form a new slope and, when that slope reaches the 34U0/U mark another sand slide occurs. This loss and buildup of sand causes the dune to move landward.

In some areas, just above the highest tide mark, pioneer dunes form. As organic matter is deposited by the wind, plants such as sea rockets and other low growing herbs begin to take root. As the vegetation traps more and more sand the dunes increase in height. The pioneer dunes are now fore dunes. Fore dunes are common along the shores of the Atlantic coast from Massachusetts southward as well as the coasts of California and points along Oregon and Washington. Such dunes are also found on the coastline of Lake Michigan. The extensive root systems of the vegetation help maintain the stability of the dunes.

Further back on the beach larger barrier dunes may form. Generally speaking these large barrier dunes are not very stabile. They would tend to move inland, as explained above, except for the fact that vegetation such as beach grasses and other plants that can grow under the terribly harsh conditions of a sand dune serve to slow such a migration and help keep the dunes in place—or at least slow down their movement. Keep in mind that the plants growing on sand dunes must be able to withstand constant salt spray from the ocean, off shore winds, and a very small amount of fresh water and nutrients.

Dunes are a natural process following specific physical laws. Their presence serves to protect the inland areas and property. They are the best natural defense against erosion due to storm surges and flooding. As such they provide the most economical way of protecting and maintaining coastal areas and recreational beaches. They are an additional benefit: they add natural beauty to a beach. But sand dunes must be protected and appreciated. Natural vegetation should be allowed to take root and designated paths must be laid out to prevent destruction of the dunes.


E-mail Steve: Drawingonscience


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