Coastal Wiki - Coastal Flooding

Storm Surges

One of the most common causes of coastal flooding are storm surges, which occur when strong winds and low atmospheric pressure cause sea levels to rise above normal. Storm surges can cause extensive flooding when they coincide with high tides, resulting in water levels breaching coastal defences or natural barriers.

In Wales, storm surges are often associated with winter storms, which can bring powerful winds and heavy rainfall, compounding the risk of both coastal and river flooding.

Sea-Level Rise

As a result of global climate change, sea levels are rising. This is a long-term trend that increases the baseline for coastal flooding. In Wales, sea-level rise is already noticeable, with projections suggesting further increases in the coming decades.

Even small increases in sea level can lead to more frequent and severe coastal flooding events, as higher baseline water levels mean less capacity for storm surges and high tides to be absorbed by the coast and any sea defences in place.

High Tides and Spring Tides

Tidal patterns can greatly influence the likelihood of coastal flooding. Wales experiences a significant tidal range, particularly in areas like the Bristol Channel.

During spring tides—when the sun and moon align to create higher-than-usual tides—the risk of flooding increases, especially if these high tides occur alongside stormy weather.

Heavy Rainfall and River Flooding

In addition to marine influences, heavy rainfall can exacerbate coastal flooding by swelling rivers that flow into the sea.

This is particularly a concern in Wales, where the steep, narrow valleys that characterize many coastal areas can funnel water into low-lying areas, leading to compounded flood risks at river mouths.

This image shows a river in the Vale of Glamorgan that frequently floods neighbouring fields during spring tides.

What is a storm surge?

A storm surge is an abnormal rise in sea level caused by strong winds and low atmospheric pressure during a storm. This rise in water level can result in flooding along coastlines, particularly when it coincides with high tides. Storm surges are a major cause of coastal flooding and can lead to extensive damage to coastal infrastructure and communities.

Storm surges occur when strong winds, typically from an intense storm, push large volumes of water towards the shore. The force of the wind causes the water to pile up, while the low pressure in the storm’s centre further contributes by allowing the sea surface to bulge upwards. The height of a storm surge can vary, depending on factors such as the intensity of the storm, the shape of the coastline, and the sea depth.

For more information and storm surge predictions for your nearest port, please click here.

How are the impacts of climate change affecting storm surges?

In Wales, we are likely to see an increase in the frequency and intensity of storms due to climate change. This means that due to the increase in the number of low pressure systems and the increase in average storm wind strengths we are likely to see an increase in the frequency and height of storm surges.

Additionally, as sea levels rise, the baseline water level is higher, meaning that even a moderate storm can cause significant coastal flooding. This allows storm surges to have a greater reach, affecting areas that previously might have been safe from flooding. Rising seas also reduce the natural buffer zones, like beaches and dunes, that protect coastal areas from storm surge impacts.

Impacts of Increased Storm Surges

The increase in storm surge frequency and intensity poses significant risks to coastal communities and ecosystems:

Increased Flooding: Higher storm surges, combined with rising sea levels, can flood low-lying coastal areas more frequently, causing more damage to homes, infrastructure, and businesses.
Coastal Erosion: Powerful storm surges can accelerate coastal erosion by removing large amounts of sand and sediment from beaches and dunes, undermining natural coastal defenses.
Economic Damage: Storm surges can disrupt ports, transportation, and industries located near the coast, leading to significant economic losses.
Ecosystem Disruption: Coastal habitats such as wetlands, mangroves, and estuaries are particularly vulnerable to storm surges. These areas provide important environmental services, such as carbon sequestration and flood protection, and their loss can have cascading effects on biodiversity and coastal resilience.

What is sea level rise?

Sea-level rise refers to the increase in the average level of the world’s oceans over time, primarily driven by climate change. This gradual rise in sea levels is caused by two main factors: the thermal expansion of seawater as it warms and the melting of land-based ice, such as glaciers and ice sheets. Sea-level rise has significant consequences for coastal regions, as it exacerbates coastal flooding, erosion, and the loss of natural habitats.

Causes of Sea-Level Rise

The primary drivers of sea-level rise are related to the global warming effect caused by increased greenhouse gas emissions. These include:

1. Thermal Expansion of Water

As the atmosphere and oceans warm, seawater expands. This process, known as thermal expansion, is responsible for about half of the observed sea-level rise. Since water expands as it heats up, even a small temperature increase can cause a significant rise in sea levels. The oceans absorb much of the heat from global warming, making thermal expansion a key contributor to long-term sea-level changes.

2. Melting of Glaciers and Ice Sheets

The melting of land-based ice, such as glaciers and the polar ice sheets in Greenland and Antarctica, contributes large volumes of water to the oceans. As global temperatures rise, glaciers around the world are retreating, and ice sheets are losing mass at an accelerating rate. This process is particularly concerning because once large ice sheets begin to destabilize, they can contribute to rapid and irreversible sea-level rise.

3. Melting of Polar Ice Caps

In addition to glaciers and ice sheets, the polar ice caps in the Arctic and Antarctic are melting at an alarming rate. While the melting of sea ice (floating ice) does not directly cause sea levels to rise, it does reduce the reflectivity of the Earth's surface, allowing more heat to be absorbed and accelerating the overall warming and melting of land-based ice.

4. Land Water Storage Changes

Changes in how water is stored on land, for example in reservoirs or with groundwater, can also contribute a small amount to sea-level rise. When groundwater is extracted or a reservoir is drained, it often ends up flowing into rivers and eventually into the ocean, adding to the rise in sea levels.

Impacts of Sea-Level Rise

Sea-level rise has far-reaching and often devastating impacts, particularly for low-lying coastal areas and small island nations. Some of the most critical impacts include:

1. Increased Coastal Flooding

As sea levels rise, coastal flooding becomes more frequent and severe. High tides, storm surges, and even minor storms can result in flooding that reaches further inland than in the past. In Wales, many coastal towns and cities are at risk, especially when storm surges coincide with higher base sea levels, leading to compound flooding events.

2. Coastal Erosion

Higher sea levels mean that waves and tides reach further inland, accelerating the process of coastal erosion. Beaches, cliffs, and dunes, which naturally protect the coast, are being eroded at a faster rate, leading to the loss of valuable land and habitats. In Wales, where much of the coastline consists of soft sedimentary rocks, this erosion poses a particular threat to communities and natural landscapes.

3. Saltwater Intrusion

As sea levels rise, saltwater can infiltrate freshwater systems, including rivers, groundwater, and wetlands. This saltwater intrusion can contaminate drinking water supplies, harm agricultural lands, and disrupt freshwater ecosystems. Coastal communities in Wales that rely on groundwater for agriculture or drinking water may be at risk from this process.

4. Loss of Coastal Habitats

Rising seas can drown important coastal ecosystems such as saltmarshes, mangroves, and estuaries. These habitats are critical for biodiversity, acting as breeding grounds for fish and other wildlife. They also provide natural flood protection by absorbing wave energy. As sea levels rise, these ecosystems may become inundated, leading to a loss of biodiversity and ecosystem services.

5. Displacement of Coastal Communities

Low-lying coastal areas are at increasing risk of permanent inundation, leading to the displacement of communities. In some cases, entire neighborhoods or towns may need to be relocated. In Wales, particularly vulnerable areas include parts of North and South Wales where urban development is close to the coastline.

Image Source: Ontario Beneath Our Feet: Rising Sea Level

Image Source: National Geographic - Image Credit Paul Nicklen

Image Source: ElanValley.org

The Role of Precipitation in Coastal Flooding

Precipitation can increase the severity of coastal flooding, especially when it occurs during storms or periods of high tides. Several factors make heavy rainfall a key contributor to coastal flood risks:

Impacts of Precipitation and River Flooding on Coastal Areas

When heavy precipitation and river flooding combine with coastal flooding, the impacts can be severe:

1. Increased Flood Damage

When coastal and river flooding occur together, the volume of water is significantly greater, leading to more extensive damage. Buildings, roads, and infrastructure are often designed to withstand flooding from one source (e.g., a river flood or storm surge), but compound flooding events overwhelm these defenses, causing greater destruction.

2. Health and Safety Risks

Floodwaters from rivers and coastal surges often carry debris, pollutants, and sewage, creating health risks for communities. In addition, the force of the water can make evacuation and rescue efforts more dangerous, especially when both river and sea water are rising simultaneously.

3. Disruption of Services

Flooding can disrupt transportation, electricity, and communication services, particularly in low-lying coastal towns and cities. When compound flooding occurs, these disruptions are often prolonged due to the higher volume of water and the difficulty of draining flooded areas.

4. Loss of Ecosystems

Coastal ecosystems, such as wetlands, saltmarshes, and estuaries, are vital for biodiversity and flood protection. However, compound flooding can overwhelm these natural barriers, leading to the loss of critical habitats. Repeated and prolonged flooding events may prevent these ecosystems from recovering.

Image Source: David Powell/North Wales Live

Climate Change and the Increasing Risk of Compound Flooding

Climate change is making these compound flooding events more frequent and severe. Several factors are contributing to this increased risk:

1. More Intense Rainfall

Warmer air holds more moisture, which means that as the climate warms, the likelihood of intense rainfall events increases. Heavy rainfall leads to flash flooding in rivers and can overwhelm drainage systems in urban areas, particularly near the coast. These more intense rainfall events lead to higher river levels, which can then combine with rising seas during coastal storms.

2. Sea-Level Rise

As sea levels rise, coastal flooding from high tides and storm surges will become more frequent. At the same time, sea-level rise reduces the capacity of coastal areas to drain floodwaters from rivers. Higher base sea levels make it easier for river flooding and storm surges to combine, increasing the risk of compound flooding. Even small increases in sea level can lead to significantly more frequent and severe compound flood events.

3. Increased Frequency of Storm Surges

Climate change is likely to increase the frequency and intensity of storms that cause storm surges. More powerful storms mean higher storm surges, which, when combined with heavy rainfall and river flooding, can overwhelm coastal defenses and drainage systems. As storms become more intense, the risk of extreme compound flooding events grows.

Image Source: PA Images/ITV Wales