Coastal Flooding

Causes of coastal flooding

There are many possible causes of flooding on or near the coast. The major factors are:

• The height of land above sea level.
• The degree of erosion and subsidence.
• Vegetation removal.
• Storm surges.

Causes of coastal flooding: Height above sea level

Any low-lying coastal areas are vulnerable to coastal flooding as seawater can easily be swept inland. An example of areas vulnerable to coastal flooding is the mega-deltas of Asia.

Pearl Delta, China, NordNordWest/Wikimedia

Causes of coastal flooding: Erosion and Subsidence

The degree of erosion or subsidence can affect coastal flooding. Let's split these up.

Erosion

Erosion is when materials are being worn away, for example, by waves and soft geology and transported elsewhere by natural forces such as wind or water. In other words, materials, such as earth or sand, are being taken away from their original place and deposited elsewhere. This erosion can lead to a weakening of the area or even remove it altogether.

An example is Holderness, in Yorkshire, England. Waves, storms and tidal surges constantly batter the coastline of Holderness. An estimated 2m every year erodes from Holderness; in other words, the sea is making that stretch of land smaller every year. This has led to a loss of property, farmland, damage and loss of infrastructure, and it poses a danger to tourism and coastal protection.

Subsidence

Subsidence is when underground material moves, causing the ground to sink. This can be due to natural causes, such as earthquakes or erosion, or it can be due to artificial causes, such as mineral resource mining or removing natural gas.

Low lying coastlines are subjected to natural subsidence through settling and compaction of recently deposited sediment. This subsidence is usually outpaced by fresh deposition. Human activities can also cause local subsidence through activities such as:

• The drainage of saturated sediment/soil or agriculture, e.g. Fens of East Anglia.
• The weight of coastal towns & cities and the built environment can also compress sediment, leading to subsidence, e Venice.
• Land reclamation, e.g. the Netherlands, IJsselmeer polders, is subject to subsidence due to water abstraction via crop evapotranspiration.

Major signs of subsidence (in buildings) are:

• Cracks in walls, which will usually run diagonally.
• The floor drops, creating an uneven floor surface.
• Doors and windows are difficult to open/close or unable to open/close at all due to the property being out of line.
• Extensions can show cracks where the extension is attached to the main building, which could indicate that the extension is pulling away.

Causes of coastal flooding: Vegetation removal

Coastal vegetation, including trees, intercepts the rainfall slowing down its movement, storing some whilst the rest evaporates. The vegetation also absorbs water from the soil allowing more significant infiltration into the ground, as a result reducing surface run-off.

When vegetation is removed, infiltration and interception are reduced and surface run-off increases. This to a greater risk of flooding as more water reaches the river channel.

The vegetation also stabilises existing sediment and traps new sediment, raising the height of the land above sea level. In addition, it absorbs wave energy, reducing wave impact and erosion, and reduces the distance waves travel onshore before their power is exhausted.

• A 100m belt of mangrove forest is estimated to reduce wave height by 40%.
• A 1km belt of mangrove forest minimises the size of a storm surge by 0.5 m.

Storm surges

A lot of coastal flooding is a result of storm surges. Storm surges are short-term changes in sea levels caused by events such as tsunamis and cyclones. A storm surge is only measured by the water level that exceeds the normal tidal level, excluding waves.

Several meteorological factors contribute to a storm surge and its severity:

• Water is pushed towards the coast over a long fetch by high-speed winds
• The shallowness and orientation of the body of water
• The timing of tides
• A drop in atmospheric pressure

Storm surges are exacerbated through a variety of factors such as:

• Land subsidence - through tectonic activity or post-glacial adjustment.
• Removing natural vegetation - As mentioned previously, mangroves protect against extreme weather events like cyclones.
• Global Warming - As the surface of oceans gets warmer, the frequency and intensity of storms will increase; As a result, the severity of storm surges and flooding will increase.

The effects of a storm surge

As bad as it may seem, we need to remember that these impacts will be short term. Sadly as a direct result of the storm, there will be some deaths and injuries through drowning or collapsing buildings.

Infrastructure such as roads, railways, ports, and airports will be flooded or destroyed. There will be damaged water pipes, electricity transmission lines and sewage systems; as a result, there is likely to be no power or water. Homes will be destroyed, and homes on marginally low lying land (slums and shantytowns) will be more vulnerable.

Storm surges and the future

So what of the future with regards to storm surges and flood risk?

Records show an increase in the number of storms that form year on year. The average number of storms forming in the North Atlantic annually was 11; However, from 2000 to 2013, 16 storms formed per year, 8 of these were hurricane force. This increase relates to a rise in surface temperatures of the Atlantic ocean. As sea levels rise, damage from erosion and increasing storms will cause damage further and further inland.

Coastal flooding examples

Coastal flooding is something that can happen anywhere along a coastline. Especially the last few decades have proven to be significant as not only does it appear to happen more often, but coastal areas seem to attract more people, tourists and locals alike. The latter could potentially lead to more casualties when coastal flooding happens.

Coastal flooding does not only impact people directly, as in injured or dead, but it can also damage or destroy houses, businesses, infrastructure, and agriculture (including the death of livestock).

Examples of coastal flooding

Here are some examples of coastal flooding.

Examples of coastal flooding: The Netherlands

As a low-lying country, the Netherlands has had its fair share of floods. One of the biggest floods was the North Sea flood of 1953. With the Netherlands being such a low-lying country, especially in the north of the country, it relies heavily on defences such as levees.

The storm surge hit the Netherlands, and on the night of 31 January 1953, things took a turn for the worst. The storm surge, combined with an unfavourable tide at the same time, caused a storm so powerful that water not only flooded over the barriers it also damaged and destroyed a number of them. The water flooded whole islands and coastal areas, killing 1,836 people in the Netherlands.

The storm also hit the north of West Flanders (Belgium), killing 28 people; the English counties Lincolnshire, Norfolk, Suffolk and Essex, killing 307 people; east Scotland, killing 19. Furthermore, approximately 220 people were killed at sea.

Examples of coastal flooding: New Orleans

On 23 August 2005, Hurricane Katrina hit New Orleans, Louisiana (US), leaving behind a trail of destruction. The storm breached 53 levees, flooding much of the city, and it was later discovered that most of the levees broke due to fatal engineering flaws. Ultimately, 1,836 people died, and it caused a total of $125 billion worth of damage.

Flooding after Hurricane Katrina in New Orleans, Louisiana, Wikimedia

Examples of coastal flooding: Indian Ocean

On 26 December 2004, one of the deadliest natural disasters in recorded history happened: an all-powerful tsunami, caused by an undersea earthquake, hit the countries and islands in the Indian Ocean.

There are 184,167 confirmed deaths, but it is estimated that approximately 227,898 people lost their lives. Other impacts are:

• Economic impact - the tsunami had a major impact on the economies of the affected countries/islands. The 2 main areas that were impacted were tourism and fishing. For many of the countries/islands affected, either or both were the primary source of income.
• Environmental impact - the tsunami had a massive environmental impact. Not only did the tsunami cause contaminated lands, but it also damaged or destroyed entire ecosystems.

Countries/islands affected by the 2004-tsunami - MapChart (2022)

Comparison of Population Density with Elevation, Bangladesh, SEDACMaps/commons.wikimedia.org

Almost 37,500.00 million (a fourth of the total population of around 150 million in 2011) people live in the coastal areas of Bangladesh, where most of the people are affected (directly or indirectly) by coastal floods, storm surges, and riverbank erosion, tropical cyclones etc. Bangladesh could lose up to 15% of its land by an increase in sea level of just one meter, large areas will be under the seawater, and people living in the coastal areas of Bangladesh would become refugees.

Bangladesh is especially vulnerable to the impact of flooding from tropical cyclones because:

• As you can see from the above picture, most of the country is a low lying river delta.
• Incoming storm surges often meet the rivers' outgoing river discharges, resulting in river and coastal flooding.
• Intense rainfall as a result of the tropical storms contributes to the flooding.
• Most of the coastline comprises unconsolidated sediment from the deltas, which is easily eroded.
• The Bay of Bengal is located at the tip of the north Indian Ocean, where severe cyclonic storms and long tidal waves are frequently generated and hit the coastline with severe impacts because of the shallow and conical shape of the Bay near Bangladesh.

There is not a lot that Bangladesh can do about the physical factors which make it prone to flooding; However, human actions are increasing the risk of coastal flooding through:

• Subsidence - Some of Bangladesh's estuarine islands have sunk by as much as 1.5m. Human actions have prevented the natural deposition of sediment used to maintain the island's height. As a result, these islands are fast submerging, and millions of people living on them are susceptible to flooding if the embankments give way. Approximately 30 million people live in a coastal flooding danger zone.
• Removal of vegetation - forests are being cleared to make room for rice fields. This vegetation is key in stabilising the coastline against erosion, collecting nutrient-rich sediments, providing protection from extreme weather events, and absorbing and dispersing tidal surges. Recent satellite images show that 71% of these mangrove forests are now retreating by as much as 200 meters a year. This causes erosion, rising sea levels and human activities. In addition, converting the mangrove forests into shrimp farms now accounts for 25% of their loss.

Three major cyclones have hit Bangladesh since 1970. The death toll from these has fallen over time through a better warning system, but most of the flooding was caused by a failure of the extensive embankment system forcing millions of people from their homes and farms. The 2007 cyclone, Sidr, had a storm surge of 3 meters with accompanying wind speeds of 20kmph (Max 1 minute sustained wind speed), causing a death toll of 15,000 and an estimated US 1.7 billion.

Coastal flooding climate change

We know that sea levels are rising due to global warming, but how significant is this rise in terms of coastal flooding and erosion? Depressions and cyclones will continue to occur without global warming and sea level rises.

There is reason to believe that global warming will increase the risk to coasts. A summary of the IPCC, The UN's Intergovernmental Panel on Climate Change, 2014, stated that:

• Sea levels - There is a high degree of certainty that sea levels will rise by between 28 - 98cm by 2100, with the most likely rise being 55cm by 2100.
• Delta flooding - There is a high degree of certainty that the world's significant deltas at risk of coastal flooding are likely to increase by 50 per cent.
• Wind and waves - There is a medium degree of certainty that there is evidence of increased wind speed and more giant waves.
• Coastal erosion - There is a medium degree of certainty that coastal erosion will increase due to the combined effects of changes in weather systems and sea levels.
• Tropical cyclones - There is a low degree of certainty that their frequency is likely to be unchanged, but there are likely more significant storms.
• Storm surges - there is a low degree of certainty that storm surges linked to depressions are more common.

Global mean sea level rise projections, Parris et al./Wikimedia

Solutions to coastal flooding

The risks highlighted above create an uncertain future, and we will need to mitigate and adapt against them.

Some predictions on the impacts of coastal flooding associated with global warming are more confident than others. Even in the IPCC summary, its forecasts ranged from high to low confidence. It also made a fascinating statement about coast change which might be blamed on global warming.

It is important to remember that coasts are a very complex system that can be affected by many factors. Therefore, blaming it on any 1 aspect will misrepresent the many factors that affect the level of risk on the coast.

There are two possible approaches to dealing with the risk.

Adaptation Adaptation is vital as making changes lessens the impact of flooding. This can be done by:

• Building sea walls, e.g. on the North Norfolk coast and 3m sea wall on Malé.
• Constructing storm-surge barriers, e.g. The Thames barrier and the Eastern Scheldt, the Netherlands.
• Building earth dams, like the bunds in Bangladesh.
• Through the restoration of mangrove forests, e.g. Sri Lanka. As a result of the 2004 tsunami, 6,0000 were killed in one village alone where the mangroves had been removed compared to only two deaths in an adjacent village protected by a mangrove forest.

Mitigation

Reducing greenhouse emissions to limit global warming would mitigate sea level rise and cyclone intensity.

For future information on how coastal flooding could be managed, please see the following StudySmarter article.

Managing coastlines - Engineering Management Approaches and Governance Approaches.

References/sources:

1. Pearl Delta, China A weblink to the original file is provided: //commons.wikimedia.org/wiki/File:China_Guangdong_location_map.svg https://creativecommons.org/licenses/by-sa/3.0/de/legalcode
2. Figure 2: Map created by the author on MapChart
3. Fetch definition: https://forecast.weather.gov/glossary.php?word=fetch