Salt marshes or sea walls? Preventing coastal flooding in the UK

A man and his dog stand at the end of a footbridge over a flooded car park. Sign reads 'Long Stay'.
A man walking his dog are caught on a bridge, unable to pass the floodwaters. (Credit Keith Williams, flickr CC BY 2.0)

Nick Reynard, Lizzie Ellison and Amy Wilson, Research Postgraduates on the Science and Solutions for a Changing Planet DTP, consider how nature-based solutions can help boost flood defences and tackle climate change.

Climate change is causing vast glaciers to melt, a drastic rise in global sea levels and more extreme weather events. A major consequence of these changes is the increased frequency and severity of coastal flooding across the world.

As an island nation with one of the longest coastlines in Europe, the United Kingdom is highly vulnerable to sea level rise and storm surges. Currently, over half a million properties are in areas of significant coastal flood risk, with this number expected to treble by 2080 if no flood management strategies are put in place.

The UK Environmental Agency has considered many strategies to reduce the impact on communities, including building flood defences, abandoning towns that are at high risk and building homes that dry out quickly after being flooded. At the moment, the UK’s approach to installing flood defences is generally through ‘hard engineering – for example, building concrete seawalls and installing boulder barriers. However, these installations are often expensive, regarded as ugly and destroy the natural habitat of marine and bird species. An alternative approach to flood defence is the use of nature-based solutions to prevent coastal flooding.

A natural solution to a human-made problem

Nature-based solutions involve the management or use of natural resources, such as plants and ecosystems, to resolve challenges like flooding. A nature-based solution for flooding could involve managing a coastal landscape, or planting ecosystems that thrive on the coastline and protect against coastal flooding and erosion. Sand dunes, mudflats and saltmarshes, which are native to the UK, are all effective at protecting the coast from flooding and erosion. They do this by decreasing the ocean’s wave energy near the coast and on land, and absorbing excess rainwater during times of heavy rain. The UK’s extensive salt marshes have been shown to reduce wave height by up to 80%, and prevent soil erosion. Yet, despite their important role in coastal protection, salt marshes are disappearing across the UK because humans are building homes and infrastructure close to the coast and land is being lost to rising sea levels caused by climate change.

Although not found in the UK, mangrove forests, seagrass meadows and coral reefs provide nature-based solutions to flooding in other parts of the world. In Da Loc, Vietnam, for example, 250 hectares of mangroves were successfully planted after Typhoon Damrey to protect from future coastal flooding. As well as protecting the coast, these ecosystems are also home to many of the world’s most fascinating marine species, such as dugongs and lemon sharks, and this project also led to a ten-times increase in the harvest of local edible molluscs. The ecosystems therefore have several benefits that span coastal protection, increased numbers of marine animals, and the economic benefits brought by tourists who come for these unique ecosystems.

Underwater picture of a sea cow, (dugong, one of the manatee family) grazing on plants on the sea bed, with puffs of sand around it.
Dennis the Friendly Dugong eats in a seagrass meadow (Credit: Rutger Geerling / flickr CC BY 2.0)

Could natural marine-based solutions help slow down climate change?

In addition to solving the problems associated with climate change, nature-based solutions have been may help to resolve the cause of climate change. This is because coastal and ocean plants remove greenhouse gases from the atmosphere, storing the carbon in sediments and biomass, much like trees do on land.

This is a way of offsetting some of the carbon dioxide (CO2) released into the atmosphere by the burning of fossil fuels and other human activities like agriculture. Once stored, this carbon is referred to as ‘blue carbon (ocean-based storage of carbon). Mangroves, seagrass meadows and our more familiar salt marshes, get the name blue carbon ecosystems (BCE) because they are coastal ecosystems that can store blue carbon. These BCEs are distributed all over the world and can hold onto vast amounts of greenhouse gases. They cover less than 2% of the ocean’s surface but store as much carbon as all the sediment in the rest of the ocean.

However, BCEs are not a silver bullet solution to climate change. Despite their depleted state, they currently capture 0.07 gigatonnes per year of carbon across the globe. Even by growing back to their recorded historic levels, the maximum amout of carbon captured would only be 0.12 gigatonnes per year (see the soon-to-be-released blue carbon briefing paper). There is considerable variation across the globe, but the overall potential for carbon uptake in these ecosystems is likely to be equivalent to less than 1% of total global emissions by human activities. Furthermore, the carbon cycle of these systems isn’t yet fully understood, which makes it difficult to accurately assess their potential to capture and store carbon.

Why should we increase the coverage of blue carbon ecosystems?

Increasing the coverage of BCEs around the world will not solve the root cause of sea level rise – climate change. But these ecosystems can contribute to absorbing carbon dioxide from the atmosphere, and bring a host of important local benefits to the regions where they grow. Furthermore, the cost of restoring and protecting these ecosystems is often comparable to that of hard engineering solutions – making BCEs an attractive option when it comes to protecting our coastlines from flooding and erosion.

You can find out more about the co-benefits of blue carbon ecosystems in an upcoming Grantham Institute briefing, due to be released soon.

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