Coastal Erosion and Shoreline Change

Primary reference(s)

Mentaschi, L., M.I. Vousdoukas, J. Pekel, E. Voukouvalas and L. Feyen, 2018. Global long-term observations of coastal erosion and accretion. Scientific Reports, 8:12876.

Additional scientific description

The coast is a dynamic environment that is subject to constantly changing energy inputs. This leads to variable process rates as reflected in the changing ratios of weathering to erosion. The land-based processes deliver sediment to the shoreline environment that marine processes mobilise and transport. The sediment may lead to local accretion. Rates of erosion reflect the consequences of environmental change (human modifications and climate change) superimposed on the natural variability in the underlying process-driven rates of erosion (Mentaschi et al., 2018). While zones of high sediment accretion mitigate against erosion in some areas, the landward movement of the coastline can be as high as several metres per year. Therefore, understanding patterns of sediment migration is fundamental to modelling coastal erosion. Processes at the coast are wide-ranging and both the marine and land processes (landsliding and fluvial processes) present hazards to coastal environments including infrastructure, business, people, and ecosystem services in the coastal zone (Wong et al., 2014; Mentaschi et al., 2018).

Marine processes include tides and tidal range, tidal surges, coastal flooding, waves, tsunamis, long-shore drift and a range of types of current. Landsliding processes are comparable with on-shore processes, including falls, topples, slides and flows reflecting the local geological and groundwater conditions. While estuarine environments naturally dominate the context for the fluvial processes, anthropogenic impacts are commonly further inland. For example, Mentaschi et al. (2018) found that dams are among the most prominent contributors to erosion because they retain sediment that would otherwise naturally supply the coastal zone with beach sediment.

Metrics and numeric limits

Mentaschi et al. (2018) used satellite data to evaluate global coastal morphodynamics over the period 1984 to 2015. They established a land loss of 28,000 km2; gained land of 14,000 km2; an increase in the active zone (essentially the intertidal zone together with areas frequently inundated by other causes, such as rivers or waves) of 25,000 km2, and a loss in the active zone of 11,500 km2. The region with the highest change per unit coast was the Caspian Sea with an average cross-shore erosion of 600 m, followed by southern Asia, Pacific Asia, South America, eastern Africa and western Australia with an average crossshore erosion of 50 m.

Examples of drivers, outcomes and risk management

Coastal environments host a range of infrastructure, including the landing points for telecommunications, ecosystem services and in the order of 40% of the world’s population (Mentaschi et al., 2018).

Extreme weather events associated with higher intensity storm surges and flooding as well as tsunamis and waves and currents extend the reach and rate of erosion of landforms. Extreme rainfall events contribute to soil saturation and the associated shear strength reduction associated with landsliding. Anthropogenic factors impact most significantly on sediment supply primarily through sediment retention.

Coastal erosion becomes a hazard when society does not adapt to its effects on people, the built environment and infrastructure (UNDRR, 2017). Therefore, adaptation and adaptive pathways are a key policy in managing coastal hazards. In some parts of the world a range of engineered interventions have been successful in protecting specific infrastructure and populations, but the consequences of engineered intervention need to be fully appraised to avoid unplanned consequences. Engineered solutions vary widely, for example: rock armour, breakwaters, groynes and seawalls, wharfs and harbours, offshore barriers, a range of styles of revetments, beach nourishment or replenishment, mangrove protection and dredging (Climate-ADAPT, 2015).

Examples of regional and national programmes for coastal erosion and shoreline change include the first pan-European shoreline-migration map, prepared by the European Marine Observation and Data Network (EMODnet, 2020) and the UK national coastal erosion risk map, prepared by the UK Environment Agency (no date).