Coastal flood

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 inundate the land via 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.

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 can cause flooding through storm surges which are waves significantly larger than normal and if a storm event corresponds with the high astronomical tide extensive flooding can occur.Storm surges occur during storm events, including hurricanes and tropical cyclones due to 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 set up. Low atmospheric pressure is associated with storm systems and this tends to increase the surface sea level, this is barometric set up. Finally increased wave break height results in a higher water level in the surf zone which is wave set up. Theses three processes interact to create waves that can overtop natural and engineered coastal protection structures thus penetrating seawater further inland than normal.

The Intergovernmental Panel on Climate Change (IPCC) estimate global mean sea-level rise from 1990 to 2100 to be between nine and eighty eight centimetres. It is also predicted that with climate change there will be an increase in the intensity and frequency of storm events such as hurricanes. This suggests that coastal flooding from storm surges will become more frequent with sea level rise. A rise in sea level alone threatens increased levels of flooding and permanent inundation of low-lying land as sea level simply may exceed the land elevation. This therefore indicates that coastal flooding associated with sea level rise will become a significant issue into the next 100 years especially as human populations continue to grow and occupy the coastal zone.

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