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Coastal Erosion and Deposition

Coastal erosion is the term used for the removal of land or sediment from a coastline, whereas coastal deposition is the addition of sediment to a coastline. Factors influencing erosion and deposition include the rate of sediment supply to the coast, the shape of the underlying geology, the rate at which coastal processes transport sediment, and the rate and direction of sea-level change. These can ultimately be attributed to tectonics, climate, geology, and eustasy, all of which vary spatially and temporally. It is important to understand these processes, given the significance of the coast for habitat, recreation, and resources.

Coastal morphodynamics, a systematic approach to integrating the processes and boundary conditions that influence coastal erosion and deposition, was introduced by Don Wright and Bruce Thom in 1977. This integrates the effects of instantaneous processes (e.g., wave and swash processes including incident waves, rip currents, bed return flows, and infragravity waves), long-term processes such as sea-level change, and geological processes (e.g., tectonic changes). Using this approach, the coasts can be divided into three categories: prograding, eroding, and stationary (where net sediment movement is zero) coasts.

The Twelve Apostles, on the southern coast of Australia, formed by bedrock eroding at a variable rate

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Source: Natalie Townsend.

Coastal Erosion

Eroding coasts, often termed retrograding, encroaching, or receding coasts, result from a net negative sediment budget. This can occur due to factors including wave energy increase, wave direction change, currents, relative sea-level rise, and losses to the coastline by tidal and aeolian transport. The less aggregated and finer grained the beach, the easier it is to transport and remove the sediment from the coast.

On rocky coasts, constant wave pounding, chemical and mechanical weathering, abrasion, and hydraulic action play a major role in degrading the parent rock into finer particles for deposition elsewhere. The ease with which the bedrock is eroded depends on the characteristics of the parent material. This may result in the formation of rock stacks such as the Twelve Apostles on the southern coast of Australia (see photo).

Other coastal features formed by coastal erosion include beach and dune scarps on sandy coasts. This often leads to infrastructure being compromised or stranded on the beach or, ultimately, the loss of human life. An example of eroded dunes due to a net loss of sediment is on the northern French coast (see photo), where stationary World War II concrete bunkers are gradually being exposed.

Coastal Deposition

Coastal deposition, often referred to as progradation or transgression, occurs when there is a net sediment input. Deposition frequently results in the development of large alluvial deltas (e.g., Mississippi and Rhone deltas), beach ridge plains (e.g., Normandy coast, France), Chenier plains (e.g., Princess Charlotte Bay, Northern Australia; see photo), or mud flats. The nature of each shoreline depends on the origin of the sediment supply (i.e., whether it is dominated by river sediments or reworked beach sediment) and on wave conditions.

The erosion along the northern French coast has left relict World War II bunkers exposed along the coast.

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Source: Denis Marin.

Factors Affecting Coastal Erosion and Deposition

In the following sections the most common factors influencing coastal erosion and deposition are reviewed.

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