Beach Dune Erosion
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Vegetated sand ridges called dunes, built up by dry beach sand blown inland and trapped by plants and other obstructions, back most beaches. As sand accumulates, the dunes become higher and wider.
Plants play a vital role in this process, acting as a windbreak and trapping the deposited sand particles. A characteristic of these plants is their ability to grow up through the sand and continually produce new stems and roots as more sand is trapped and the dune grows.
Stable sand dunes play an important part in protecting the coastline. They act as a buffer against wave damage during storms, protecting the land behind from salt-water intrusion. This sand barrier allows the development of more complex plant communities in areas protected from salt-water inundation, sea spray and strong winds. The dunes also act as a reservoir of sand, to replenish and maintain the beach at times of erosion.
Frontal sand dunes are vulnerable. The vegetation can be destroyed by natural causes such as storms, cyclones, droughts or fire, or by human interference such as clearing, grazing, vehicles or excessive foot traffic. If the vegetation cover is damaged strong winds may cause blowouts or gaps in the dune ridge. Unless repaired, these increase in size, the whole dune system sometimes-migrating inland covering everything in its path. Meanwhile, with a diminished reservoir of sand, erosion of the beach may lead to coastal recession.
To avoid this, protecting the vegetation is vital. The beach, between high and low tides, is hard-wearing but the sensitive dunes, which we cross to reach it, must be protected also. For this reason damaged and sensitive dunes might need to be fenced and access tracks for vehicles and people provided.
Processes such as waves, near shore currents and tides continually modify shorelines. The ability of beaches to maintain themselves is achieved through these natural forces. The natural process of beach renourishment, sometimes called “dynamic equilibrium”, is how the beach responds to weather. When waves are high during storms or when hurricanes hit the shore, sand is carried from the beach and deposited on the ocean floor. This makes the ocean bottom flatter and makes waves break further from shore and smaller. During subtle weather or erosion, smaller waves slowly shift the sand back to the shore and replenish the beach.
When people build homes or resorts on beaches, the buildings interrupt this natural process because the sand that is usually taken by storms is removed so that humans may build. When erosion catches up with them they scream for help and defense against the angry sea.
Hard structures, like bulkheads, seawalls and groins, are built on beaches to prevent erosion, but they usually backfire. In the long run after a beach has been “defended” by a hard structure, the beach will have been carried out to sea while the homes still stand.
Bulkheads and seawalls protect banks and bluffs by completely separating land from water. Bulkheads act as retaining walls, keeping the earth or sand behind them from crumbling or slumping. Seawalls are primarily used to resist wave action. Design considerations for these types of structures are very similar.
However, these structures do not protect the shore in front of them. In fact, when bulkheads and seawalls are used in areas where there is significant wave action, they may actually accelerate beach erosion. This happens because much of the energy of the waves breaking on the structure is redirected downward, to the toe where the wall meets the soft sand or earth. The shore on this side of the bulkhead or seawall is thus subjected too much more of the force of the waves than if there were no wall, and it erodes quickly. Man made structures interrupt wave-driven drift, stealing sand from down shore beaches.
In the US, many coastal states, including South Carolina, Georgia and California, have passed laws preventing the destruction of beach dunes. These laws state that people cannot build houses on the dunes. Boardwalks and other structures that allow people to reach the beach must be approved and be constructed in a way so as to not interfere with the preservations of surrounding dunes. Furthermore, it is illegal to “pick” or otherwise remove vegetation such as sea oats, which help maintain the dunes.
Most people who visit the beaches annually are quite impressed by the beauty of the sea oats growing on the primary dunes along the oceanfront. But this hearty grass performs a far more important function than adding its good looks to the scenery. It helps to hold