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How smart control of the proposed Severn Barrage could prevent flooding

Civil Engineering researchers model the impact of a barrage on the Severn Estuary and surrounding area

Woodblock showing people and animals in river flood, with church partially submerged and some people clinging on to trees

Woodcut of the 1607 Bristol Channel Floods

A barrage is an artificial barrier across a river or estuary that can prevent flooding, provide irrigation, and supply energy from tidal power. However, barrages are expensive to build and can disrupt natural habitats for wildlife. As such, a new barrage faces many considerations before it can go ahead.

The proposed Severn Barrage, that would be located on the Severn Estuary between South Wales and Bristol, has been discussed since the 19th Century as a means for preventing flooding and for over 50 years as a possible method of generating clean and predictable power. The technology to build it is available, but the large cost and high environmental impact mean that plans have not yet been taken forward.

In a recent paper, researchers from the Environmental Fluid Mechanics group studied the impact the barrage would have on coastal flooding from storm surges. Storm surges are the result of wind blowing the ocean towards the land, often coupled with a low pressure system. In the UK, these are most common on the North Sea coast and rising sea levels make these a growing concern. Flooding is also possible on the Bristol Channel when a severe storm coincides with Spring tides. They have occurred throughout history causing substantial damage and flooding in the area. The most famous storm was in 1607 which, if it happened today, would cost the equivalent of between 7-13 billion pounds in damage.

Lead author and DPhil student Qian Mia elaborates,

“Building a Severn Barrage would make a huge impact on flooding in the region. We use barrages, such as the Thames barrage, to prevent flooding in London. The Severn Barrage would have a similar effect and could protect Bristol and Cardiff.”

The team ran numerical simulations based on real meteorological and tidal conditions of the ocean around the UK, and included a representation of the Severn Barrage. Amongst the cases considered was the worst storm in recent times (13 December 2000) that reached surge levels greater than 2m, which they combined with very high tides. They found that the barrage would provide protection to the land to the east of it but, it would increase water levels to the west; potentially leading to inundation of the areas such as the Somerset Levels.

However, this is a scenario where the barrage remains closed. Ma explains, “We found this negative impact could be mitigated by smart operation of the barrage. Our preferred operating strategy would be to close off the barrage at low tide when you forecast a surge is approaching. This creates a large void into which excess water can flow. When the storm surge approaches, the sluices in the barrage are partially reopened.”

This strategy would still prevent flooding east of the barrage but would not lead to water levels west of the barrage being higher than if the structure had never been built. This is would be a new step forward in the area, as no tidal barrages currently in operation are large enough to make smart control feasible.

Professor and co-author Thomas Adcock adds, “We found that building a Severn Barrage could completely remove the danger of coastal flooding to some areas whilst not increasing the risks of flooding elsewhere. This protection would be a significant secondary benefit to building the barrage.”

In terms of where this leads in the future, Ma concludes, “Ongoing research in this area looks at optimising the power production from barrages and improving our understanding of the consequences of extracting energy from the large-scale tidal system.”