Coastal flooding occurs when normally dry, low-lying land is flooded by seawater.[1] The extent of coastal flooding is a function of the elevation inland flood waters penetrate which is controlled by the topography of the coastal land exposed to flooding.[1][2] The seawater can flood the land via from several different paths:

  • Direct flooding — where the sea height exceeds the elevation of the land, often where waves have not built up a natural barrier such as a dune
  • Overtopping of a barrier — the barrier may be natural or human engineered and overtopping occurs due to swell conditions during storm or high tides often on open stretches of the coast. The height of the waves exceeds the height of the barrier and water flows over the top of the barrier to flood the land behind it. Overtopping can result in high velocity flows that can erode significant amounts of the land surface which can undermine defense structures.[3]
  • Breaching of a barrier — again the barrier may be natural (sand dune) or human engineered (sea wall), and breaching occurs on open coasts exposed to large waves. Breaching is where the barrier is broken down or destroyed by waves allowing the seawater to extend inland and flood the areas.

Coastal flooding is largely a natural event, however human influence on the coastal environment can exacerbate coastal flooding.[1][4][5][6] Extraction of water from groundwater reservoirs in the coastal zone can enhance subsidence of the land increasing the risk of flooding.[4] Engineered protection structures along the coast such as sea walls alter the natural processes of the beach, often leading to erosion on adjacent stretches of the coast which also increases the risk of flooding.[1][6][7] Moreover, sea level rise and extreme weather caused by anthropocentric climate change will increase the intensity and amount of coastal flooding effecting hundreds of millions of people.[8]