Clyde flood risk grows with sea rise: University of Glasgow

Managed realignment less effective in large estuaries

A University of Glasgow study finds sea level rise will increase tidal flood risks for large estuaries like the Clyde, requiring tailored strategies for effective coastal flood management.

A study by the University of Glasgow is warning that large estuaries like the Clyde, spanning 120 sq km, will require much greater and more targetted flood management strategies as climate change accelerates sea level rise and extreme weather events.

In a press statement, University of Glasgow says that the research, published in Environmental Research Letters, uses advanced hydrodynamic modelling to predict how tidal flooding will impact the Clyde Estuary, with significant implications for coastal cities worldwide.

The statement adds that the study, led by the University’s School of Geographical and Earth Sciences, highlights that while sea level rise has been accelerating since 2000, it will continue for centuries, even if global net-zero targets are met by 2050.

It adds that this persistent rise means that urban areas along major estuaries face heightened risks of devastating tidal flooding, especially during storm surges. The researchers found that during extreme weather, vast volumes of seawater are pushed onto land along the Clyde, threatening homes, businesses and infrastructure.

According to the statement, the team’s computer modelling tested various flood management strategies, focussing on managed realignment, a method that creates natural buffers like saltmarshes to absorb floodwaters instead of relying solely on traditional sea walls. The simulations covered key areas including Renfrewshire, Newshot Island, Longhaugh and the Erskine Bridge, assessing the effectiveness of managed realignment zones ranging from 0.15 to 3.3 sq km. The results were sobering: even with these interventions, the reduction in flood extent was modest.

Additionally, in present-day scenarios, tidal floodwaters covered 13.6 sq km without managed realignment; by 2100, under high-emissions projections, this could rise to 19.2 sq km. Managed realignment reduced the flooded area by only 0.32 pc to 9.76 pc.

“We were surprised by the results, which showed that managed realignment measures along the Clyde Estuary produced smaller reductions in tidal flood extent than we might have anticipated even when more than three sq km of space is devoted to tidal flood management,” says Octria Prasojo, corresponding author.

“For many cities built next to river systems, the volume of water which can contribute to flooding is limited by the amount of water which comes from the upstream river flow or rain. In the case of cities like Glasgow, however, where rivers are connected to the sea, the volume of water which can contribute to tidal flooding is vastly increased,” says Martin Hurst, co-author of the paper.

The university says that the research also analysed data from eight other UK estuaries, each smaller than the Clyde. It found a clear pattern that the smaller the estuary, the more effective managed realignment becomes. For every 20 pc increase in the size of realignment zones, floodwaters could be reduced by around 14 pc.

However, for large estuaries like the Clyde, the effectiveness of such measures is limited, suggesting that larger, more strategic interventions are necessary.

“What we have found in this study is that managed realignment works very well when appropriately-sized locations are chosen to deal with tidal flood waters, and it works particularly well in smaller estuaries. However, there comes a point where that alleviation strategy starts to require larger and larger areas of land to be devoted to managed realignment, which may not be practical in more densely-populated areas. Effectiveness depends on where and how large managed realignment is allocated relative to the size of the estuary,” says Prasojo.

“We hope the results will help lead to better-informed urban planning to mitigate the effects of climate change as sea levels rise and extreme weather events become increasingly common,” says Professor Larissa Naylor, a co-author of the paper.

This work is part of the Glasgow as a Living Lab Accelerating Novel Transformation (GALLANT) project, a EUR 10.5 million initiative funded by the Natural Environment Research Council.

“What this study shows clearly is that cities built on estuaries will need to carefully consider how they plan to mitigate the worst effects of climate change-induced tidal flooding in the years to come. Adopting a ‘right size, right place’ approach to managed realignment will be key to ensuring that infrastructure damage is minimised, along with the associated costs of repair. For many cities, that will mean making informed decisions about new structural developments and land uses that are close to the water, and long-term planning that considers future flood risks appropriately within the wider context of conflicting land-use priorities and societal needs,” says Professor Jaime Toney, Principal Investigator, GALLANT.