Climate Change

The environment of the Severn Estuary is one of the most dynamic in the UK, with changes in sea level, waves and storms all playing their part in shaping this coastline. Historical records show that severe storms have caused much coastal erosion and flooding, however, these extreme natural events are becoming more and more frequent. One of the biggest threats to the Estuary is from rising sea levels which, coupled with significant storm events, exacerbate the effects of coastal erosion and flooding. This causes significant issues for coastal planners, engineers, and local communities. The Severn Estuary coastline already boasts considerable defences, especially around the Wentlooge, Caldicot levels and Somerset Levels, with several small-scale localised defences existing elsewhere.

Marine Temperature Increase

Global temperatures are rising due to climate change¹ (See Figure 1), which has a variety of consequences for the Severn Estuary. Sea surface temperatures around the UK have been rising in recent years, and in 2014 were the warmest on record². Modelling shows that sea surface temperatures have been increasing by around 0.25°C – 0.4°C per decade².

1- Global temperature anomaly since 1880, showing a dramatic increase in temperature (NASA, 2021)

Increasing marine temperature changes the habitable area for many different organisms. Plankton and fish species such as the with affinities for cool water will be pushed northwards and may be unable to inhabit the Estuary in the future. However, species which prefer warmer temperatures, such as the honeycomb worm, may be able to extend their range further north as cooler waters warm³. As such, they may start to become more common in the Severn Estuary and build more reef structures.

Sea Level Rise and Coastal Squeeze

Warming temperatures leads to rising sea levels, which pose a variety of challenges. Coastal squeeze occurs when both rising sea level and human activities (such as buildings, flood defences and roads) ‘squash’ the coastal habitat between the sea and the built environment. This can mean that, over time, coastal flora and fauna have a smaller and smaller area to inhabit. Habitats which are affected by coastal squeeze include beaches, seagrass and reed beds, saltmarshes, mudflats, and sand dunes².

The sea level is rising at a current rate of 3.2 mm per year⁴, up from 1.7mm per year for 1901-2010⁵. This means that as time goes on, more of the coastline will become inundated with water. This can cause increased coastal erosion rates, as well as flooding areas which were previously land. This increase in sea level is due to a number of factors, including melting of terrestrial (land-based) ice caps and thermal expansion of the ocean. Thermal expansion is estimated to contribute up to 40% of the increase in sea level. Upon excess heating, the ocean gains energy and expands (see Figure 2).

Various man-made structures enhance the phenomenon of coastal squeeze². The building of barrages reduces the sediment supply from fluvial (river) sources, which will reduce the amount of sediment at the coast and increase vulnerability to erosion. Building of groynes across beaches can interrupt the flow of longshore drift, a process moving sand across a beach, which also increases the vulnerability of the coast to erosion. Sea walls can prevent sand dunes from moving towards land in order to compensate for sea level rise. Eventually, the sand dunes may have nowhere left to retreat, causing them to disappear. This also leaves the coast more vulnerable to storm damage, as sand dunes act as protection from high-energy storm waves. Figure 3 illustrates the problems associated with coastal squeeze.

3- An illustration of coastal squeeze and the processes involved (Environment Agency, 2021).

Storm Surges

Storm surges are a natural phenomenon, where areas of very low-pressure cause the sea to shift upwards. Similar to a ‘vacuum effect’ the ocean has less force pushing it down in these low-pressure areas, causing an area of the ocean to bulge upwards. When combined with high winds and stormy weather, storm surges create high energy waves and devastating floods.

Famous storm surges date far back into history. In 1607, a huge flooding event described as a ‘tsunami’ swept up the Severn Estuary. At the time, it flooded through Somerset and Bristol up to Monmouthshire and Cardiff. Most key information at the time was recorded by churches, with their records detailing a flood “six or seven feet deep”with strong south-westerly winds and stormy weather⁶. Up to 2000 people, and many sheep and cattle, lost their lives in the dangerous waters⁶. Such events are rare, though the distinct ‘funnel’ shape of the Severn Estuary acts to make storm surges much more powerful. As the weather becomes more unpredictable, natural events such as storm surges will become more and more common within the Estuary.

Impacts on the Ecosystem

The warming climate is strongly linked to changes in ecology and ecosystems. Different flora and fauna may have different ways to adapt to climate change, and those that are unable to adapt may become extinct or migrate to different areas. Increased freshwater runoff in recent years from melting ice and increasing rainfall alters the salinity of the ocean, making it less salty. In the Severn Estuary, the seagrass species Zostera noltii prefers a low salinity environment for germination. As such, Welsh seagrass beds have seen increased productivity in recent years⁷. Changes in migration patterns for birds such as the Bewick’s swan may occur as they venture into a warming Severn Estuary. Those at the limit of their southern ranges may disappear from the Estuary altogether and seek out colder climates further north.

Impacts on the Human Environment

The human environment consists of the places we live, work, and use for resources. From the countryside of North Somerset to the busy cities of Bristol and Cardiff, many human environments across the Severn Estuary will see different impacts due to climate change, as illustrated in Figure 4. In terms of socioeconomic impacts, significant urban development will be needed in the form of flood defences to cope with sea level rise. This can be in the form of ‘hard protection’, such as sea walls and flood barriers, or ‘soft protection’ which uses solutions working with nature, such as establishment of saltmarshes or replenishing beach sands.

4- Direct and indirect impacts of climate change on the Severn Estuary, including impacts within the human environment⁸

Cardiff is a particularly vulnerable city to sea level rise within the Estuary, and as such, the council has established a flood defence strategy. Part of these adaptations includes Sustainable Drainage Systems (SUDS)⁹. SUDS work on a community level to establish green spaces within cities, which imitates the natural drainage process. Construction materials such as paved and concrete surfaces effectively ‘block’ water from draining into the ground, causing flooding. With SUDS, water will be able to run into green spaces and infiltrate through the ground, as well as being absorbed by plants and directed into rivers. Rills and channels can direct surface water away from homes and infrastructure to these green spaces. Additionally, the green spaces can be used for recreation, exercise and to boost well-being! The roles of coastal planners and local councils across the Severn Estuary will become increasingly important for establishing strategies like SUDS to combat increases in storms, flooding, and sea-level rise.