Coastal flooding occurs when normally dry, low-lying land is flooded by seawater. 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. The seawater can flood the land via from several different paths:
Coastal flooding is largely a natural event, however human influence on the coastal environment can exacerbate coastal flooding. Extraction of water from groundwater reservoirs in the coastal zone can enhance subsidence of the land increasing the risk of flooding. 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. 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.
Coastal flooding can result from a variety of different causes including storm surges created by storms like hurricanes and tropical cyclones, rising sea levels due to climate change and by tsunamis.
Storms, including hurricanes and tropical cyclones, can cause flooding through storm surges which are waves significantly larger than normal. If a storm event coincides with the high astronomical tide, extensive flooding can occur. Storm surges involve three processes:
Winds blowing in an onshore direction (from the sea towards the land) can cause the water to 'pile up' against the coast; this is known as wind setup. Low atmospheric pressure is associated with storm systems and this tends to increase the surface sea level; this is barometric setup. Finally increased wave breaking height results in a higher water level in the surf zone, which is wave setup. These three processes interact to create waves that can overtop natural and engineered coastal protection structures thus penetrating seawater further inland than normal.