Dunes

Dunes

Coastal dunes, often referred to as sand dunes, form where there is a readily available supply of sand-sized sediment and are located landward of the beach in the supratidal zone. The size of the sediment, duration, velocity, and direction of winds in the coastal zone, as well as the size and extent of vegetation, are fundamental properties that govern the size and shapes of dunes in coastal settings. The development and growth of dunes derive from the beach when the wind is blowing in an onshore direction. When the wind is blowing offshore, sediment in the dunes is transported back onto the beach or offshore into open water where it may later be carried back to the beach by waves.

Sand accumulates to create a dune system when the wind carrying the sand encounters an obstacle. Pieces of driftwood, trash, or piles of seaweed can all provide such an obstacle, causing the velocity of the wind to locally decrease, at which point the transport of the sand ceases and it is deposited. Most often, the obstacle that creates large continuous sand dunes is salt-water tolerant vegetation, either beach grasses or shrubs and trees depending upon the climate of the region. Vegetation, therefore, promotes the deposition of sand and acts to stabilize the dune system because of rooting.

During fair weather conditions, the base of the dunes is not affected by wave energy when a beach is in equilibrium with the prevailing conditions, because the waves dissipate on the beachface. During a storm, when water levels are elevated because of storm surge and large waves are being produced, a dune system may, however, be subjected to breaking waves that can cause erosion and the removal of significant volumes of sediment from the dunes. In extreme situations, dunes can be completely washed over by storm waves, completely flattened, and the sediment that was removed can be carried offshore, alongshore, or farther inland to create a wash over platform.

Overall, the presence of well-established dune systems act as barriers against storm waves and, thus, help to protect infrastructure that is located landward of the dune systems. For this reason, as well as the unique coastal ecosystems that they provide, dunes are very often protected environments. Fences around dunes and walkways above dune vegetation are common features in high traffic areas, and walking through dunes or riding motorized vehicles across them can be met with hefty fines and punishment because of the damage these activities can cause to the dune vegetation.

Beach grass growing Dune system at Ocean City New Jersey, United States.

Credit: Beach Grass, Ammophila Arenaria, by Ellywa, "Helmgras kijkduin februari 2005" is licensed under CC BY-SA 3.0

Picture showing the erosional effects of Hurricane Isabel (2003) to a dune system in Nags Head, North Carolina. In this case, the erosive power of the hurricane storm surge and waves removed the entire front half of the dune system.

Credit: USGS: St. Petersburg Coastal and Marine Science Center

Vegetated and protected dune system at Ocean City, New Jersey, United States. The purpose of the fencing along the backside of the dune is to prevent people from walking through the sensitive vegetation of the dunes and to help trap sand that is being moved or carried by the wind so that it is not lost to areas outside of the dunes.

Credit: M. Kulp