Coastal deposition is the laying down of material on the coast by the sea. It occurs when waves lose energy or when large inputs of sediment are made into the coastal system - perhaps due to the arrival of fluvial sediment at a river estuary. Wave refraction in bays also encourages deposition due to the dispersal of wave energy. Lower-frequency constructive waves often contribute to deposition due to their strong swash, moving beach material inland.
In addition to beaches, a range of unique depositional landforms exist, including the bar, spit, tombolo and cuspate foreland. The formation of these landforms additionally depends upon the process of longshore drift. This occurs when waves approach a coast-line at an angle, due to the dominant wind. There is thus a sideways component to the swash which helps move beach material diagonally up the beach (it travels laterally as well as inshore). Backwash under gravity returns water and beach material directly to the sea (perpendicularly to the beach profile). The net result is a zig-zag or saw-tooth motion that can carry material past the end of a headland.
Depositional landforms can be highly vulnerable to erosion during extreme storm events unless vegetation colonisation has taken place. Plant roots can help anchor sediments, making them more resistant to the action of destructive waves.
Climate change threatens depositional landforms in two main ways:
1. Rising waters mean that landforms such as beaches and bars will be increasingly at risk of day-to-day erosion and submergence. Beaches in Miami already need to be artificially replenished at regular intervals using deposits dredged from further offshore. Similar actions could soon be needed in many other places, especially where expensive developments have taken place. On the Studland Peninsula in Dorset, the six kilometres of sandy beach (which attracts over a million visitors a year) is being eroded by two to three metres a year. The cafes, toilets, a shop, car parking and beach huts on the eroding southern section are under threat. The Trust has moved the beach huts twice and is now seeking a way to relocate many of the other buildings and infrastructure.
2. Extreme weather events may also become more common as a result of climate change and these can wreak sudden havoc on depositional landforms such as spits and bars. Rising sea temperatures provide greater energy for hurricanes and depressions (as moist water becomes even more unstable when it is warmed). The combination of wind and low atmospheric pressure forces sea levels upwards by many centimetres or even metres in some cases. This can lead to the breaching of bars and spits. For instance, in 2004, Hurricane Charley was one of a series of hurricanes that devastated the coast of Florida. The North Captiva spit was breached by the high tides and strong winds, leaving a 450m gap between a newly created island and the mainland.
Fluvial depositional landforms are also vulnerable to these same effects where they are found close to the coastline. The devastation caused by Hurricane Katrina in New Orleans in 2005 was partly due to the breaching of river levees. Elsewhere, many of the world's major deltas are at risk of submersion and erosion. Already, several small islands in the Indian ocean have been partially submerged and 30,000 km2 of land in Bangladesh and India is expected to disappear in the event of a 1-metre rise in sea-levels overwhelming parts the Ganges delta. This could lead to the displacement of tens of millions of people.
Day-to-day factors include wave energy, supply of sediment and the direction of dominant and prevailing winds. However, the time factor is also important. Extreme weather events and tsunamis can entirely re-shape depositional environments overnight, but their influence can only be discerned when studying an environment over longer time-scales - as they occur infrequently. Students can ask whether climate change may increase the regularity of high-impact storm surges or hurricanes, causing a step-change in the development of depositional landforms (spits may breach, for instance).