2006-06-23 / Columnists

Drawing On Science

by Stephen Yaeger



Sand Dunes, Again

Coastal sand dunes serve to protect the upper beach and inland regions by acting as a barrier to excessive high tides or storm surges. Their development is a way in which beaches "protect themselves" in a sense like our own immune system protecting us from disease. Because of this protection they are deemed an integral part of the beach ecosystem. To better understand this natural phenomenon let's see how they form.

Sand dunes rise mainly on wide beaches when ocean and on-shore winds lift and carry sand particles up onto the beach. If there is an obstacle, such as American Beach grass, 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. 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. The continuous depositing of sand particles in the shadows results in a gentle slope forming 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 34 a 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 34 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. Without human interference all coastal (and lake) beaches will develop dunes on one type or another. The presence of coastal dunes is 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 also protect inland areas and property. They are an additional benefit: they add natural beauty to a beach. They provide habitat to a number of flowering plants such as beach pea, dusty miller, sea rocket and cocklebur, which are among the most common. The root system of American beach grass, especially, helps keep the dunes viable. Dunes play host to a number of insects such as some species of ants, but do not, themselves, attract larger animals. But sand dunes must be protected and appreciated. Littering, walking on or otherwise destroying dunes serve no purpose other than allowing possible storm surges a greater chance to come up on and over the beach to affect inland development. Natural vegetation should be allowed to take root for the build up of the dunes.

Once a dune has matured designated paths should be laid out to prevent destruction of the structure.

Questions/comments? E-mail

Steve: Drawingonscien ce@aol. com

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