Characteristics and formation of beaches, sand dunes, spits and bars.
• Bar - Where a spit grows across a bay, a bay bar can eventually enclose the bay to create a lagoon. Bars can also form offshore due to the action of breaking waves.
• Beach - The zone of deposited material that extends from the low water line to the limit of storm waves. The beach or shore can be divided in the foreshore and the backshore.
• Deposition - Occurs when material being transported by the sea is dropped due to the sea losing energy.
• Longshore drift - The zigzag movement of sediment along a shore caused by waves going up the beach at an oblique angle (wash) and returning at right angles (backwash). This results in the gradual movement of beach materials along the coast
• Sand dune - Coastal sand hill above the high tide mark, shaped by wind action, covered with grasses and shrubs.
• Spit - A depositional landform formed when a finger of sediment extends from the shore out to sea, often at a river mouth. It usually has a curved end because of opposing winds and currents.
If rocks and cliffs are being continually weathered, eroded and moved then it stands to reason that this will generate a lot of material that will need to be deposited (or laid down) somewhere else along the coastline. The major deposition landforms are beaches, sand dunes, spits and bars. Deposition occurs when wave velocities slow, or when ocean currents slow due to encountering frictional forces such as the sea bed, other counter currents and vegetation.
Beaches are areas of sand, pebbles and shingle that are formed by deposition produced by wave processes. Beaches are by no means uniform and contain a huge variety of sediment types and sizes, and have many different shapes.
The formation of a beach:
Gently sloping beaches are formed by strong destructive waves that backwash more material away from the beach that they swash up the beach.
Steeply sloping beaches occur by constructive waves that swash more material up the beach than they backwash away, building up a steep beach gradient.
Sand dunes are accumulations of sand and other sized sediments that gather on a beach. Sand dunes are created around obstacles on the beach, these could be natural such as a rock or human things such as some waste drift wood or a fence.
The sea brings sediment to the beach and then the wind redistributes that sediment. When the wind encounters the beach obstacles velocity falls and sediment is DEPOSITED. This makes amount of sand or sediment at the front of the sand dune system, known as an EMBRYO dune.
Over time, tough plants known as PIONEERS such as Marram grass take root on the dune, their root systems helping to stabilise the sand and fix it in place.
As these plants die off they add nutrients and humus to the sand dune improving the soil, so more complex plants can move in, such as brambles.
Eventually, the climatic climax vegetation is reached, which in the UK would be forest.
SPITS and BARS
Spits are created by the process of Longshore drift. Some eroded material ends up caught up within the waves and is carried by the sea along the coastline in cells known as littoral cells. Material is carried along the shore in a zigzag fashion by waves as they swash material up the beach at an angle and backwash material down the beach at a right angle. The angle of swash is determined by the prevailing wind (the dominant or main direction in which the wind blows).
Wherever there is a break in the coastline (e.g. across a river or a change in coastline direction) then material is deposited closest to the shore. This is because there are often counter currents and a loss in velocity, so material is dropped or deposited.
Eventually this material builds up out into sea to form a spit. As seen in the picture opposite of Spurn Head. The spit often curves inwards towards the land as a result of the prevailing wind directing the waves which push the sediment in towards the shore.
Spits often have salt marshes build up behind them because the spit offers protection from the stronger waves and the wind, allowing salt tolerant plants to grow.
How spits are created
If a spit extends from headland to headland then a bar will be created.