Coastal flooding occurs when dry and low-lying land is submerged (flooded) by seawater. The range of a coastal flooding is a result of the elevation of floodwater that penetrates the inland which is controlled by the topography of the coastal land exposed to flooding. The seawater can flood the land via several different paths: direct flooding, overtopping of a barrier, or breaching of a barrier. Coastal flooding is largely a natural event. Due to the effects of climate change (e.g. sea level rise and an increase in extreme weather events) and an increase in the population living in coastal areas, the damage caused by coastal flood events has intensified and more people are being affected.
Coastal areas are sometimes flooded by unusually high tides, such as spring tides, especially when compounded by high winds and storm surges. This was the cause of the North Sea flood of 1953 which flooded large swathes of the Netherlands and the East coast of England.
Human influence on the coastal environment can exacerbate coastal flooding. Extraction of water from groundwater reservoirs in the coastal zone can instigate subsidence of the land, thus 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.
Types
The seawater can flood the land via 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 swelling conditions during storms 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. 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 occurs when the barrier is broken down or destroyed by waves allowing the seawater to extend inland and flood the areas
Causes
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 tsunamis.
Storms and storm surges
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:
wind setup
barometric setup
wave setup
Wind 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 a 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.
Sea level rise
Tidal flooding
Tsunami waves
Coastal areas can be significantly flooded as the result of tsunami waves which propagate through the ocean as the result of the displacement of a significant body of water through earthquakes, landslides, volcanic eruptions, and glacier calvings. There is also evidence to suggest that significant tsunami have been caused in the past by meteor impact into the ocean. Tsunami waves are so destructive due to the velocity of the approaching waves, the height of the waves when they reach land, and the debris the water entrains as it flows over land can cause further damage.
Depending on the magnitude of the tsunami waves and floods, it could cause severe injuries which call for precautionary interventions that prevent overwhelming aftermaths. It was reported that more than 200,000 people were killed in the earthquake and subsequent tsunami that hit the Indian Ocean, on December 26, 2004. Not to mention, several diseases are a result of floods ranging from hypertension to chronic obstructive pulmonary diseases.