Coastal flooding
Coastal flooding occurs when dry and low-lying land is submerged 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 or breaching of a barrier. Coastal flooding is largely a natural event. Due to the effects of climate change 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.
When humans modify the coastal environment this can make coastal flooding worse. 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. This can lead to erosion on adjacent stretches of the coast which also increases the risk of flooding.
Reduction and control of coastal flooding is carried out using structural methods to hold back or redirect flood waters. Non-structural methods include coastal management, behavioral and institutional response to adapt to the processes. Natural defenses include physical features like gravel bars and sand dune systems, but also ecosystems such as salt marshes, seagrass and mangrove forests which have a buffering function. Mangroves, wetlands and seagrass meadows are often considered to provide significant protection against storm waves, tsunamis, and shoreline erosion through their ability to attenuate wave energy. To protect the coastal zone from flooding, the natural defenses should, therefore, be protected and maintained in for example Marine Protected Areas.
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 or human-engineered, 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
Storms and storm surges
, 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
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.
Impacts
Social and economic impacts
The coastal zone is home to a large and growing proportion of the global population. Over 50 percent of the global population and 65 percent of cities with populations over five million people are in the coastal zone. In addition to the significant number of people at risk of coastal flooding, these coastal urban centres are producing a considerable amount of the global Gross Domestic Product.People's lives, homes, businesses, and city infrastructure like roads, railways, and industrial plants are all at risk of coastal flooding with massive potential social and economic costs. The recent earthquakes and tsunami in Indonesia in 2004 and in Japan in March 2011 clearly illustrate the devastation coastal flooding can produce. Indirect economic costs can be incurred if economically important sandy beaches are eroded resulting in a loss of tourism in areas dependent on the attractiveness of those beaches.
Environmental impacts
Coastal flooding can result in a wide variety of environmental impacts on different spatial and temporal scales. Flooding can destroy coastal habitats such as coastal wetlands and estuaries and can erode dune systems. These places are characterized by their high biological diversity therefore coastal flooding can cause significant biodiversity loss and potentially species extinctions. In addition to this, these coastal features are the coasts natural buffering system against storm waves; consistent coastal flooding and sea-level rise can cause this natural protection to be reduced allowing waves to penetrate greater distances inland exacerbating erosion and furthering coastal flooding. "By 2050, "moderate" flooding is expected to occur, on average, more than 10 times as often as it does today, and can be intensified by local factors."Prolonged inundation of seawater after flooding can also cause salination of agriculturally productive soils thus resulting in a loss of productivity for long periods of time. The effects of the soil salinization, which is brought on by sea levels rising and differed precipitation patterns, impacts agricultural production, ultimately leading towards food and water shortages. Food crops and forests can be completely killed off by salination of soils or wiped out by the movement of floodwaters. Coastal freshwater bodies including lakes, lagoons, and coastal freshwater aquifers can also be affected by saltwater intrusion. This can destroy these water bodies as habitats for freshwater organisms and sources of drinking water for towns and cities.
Reduction and control
Flood control
Non-structural mechanisms
If human systems are affected by flooding, an adaption to how that system operates on the coast through behavioral and institutional changes is required, these changes are the so-called non-structural mechanisms of coastal flooding response.Building regulations, coastal hazard zoning, urban development planning, spreading the risk through insurance, and enhancing public awareness are some ways of achieving this. Adapting to the risk of flood occurrence can be the best option if the cost of building defense structures outweighs any benefits or if the natural processes in that stretch of coastline add to its natural character and attractiveness.
A more extreme and often difficult to accept the response to coastal flooding is abandoning the area prone to flooding. This however raises issues for where the people and infrastructure affected would go and what sort of compensation should/could be paid.