Dam disasters of the 1920s made reservoirs safer—now the climate crisis is increasing risk again

A century ago, a catastrophic flood carrying huge boulders swept through part of the village of Dolgarrog in north Wales, destroying several houses, a bridge and the local chapel. Ten adults and six children died. The tragedy became widespread reported and King George V sent a message of condolence.

This was not a natural flood. It was caused by the failure of two dams covering the Eighiau and Coedty reservoirs on the Carneddau plateau, high above Dolgarrog, after a wet October. The Coedti Dam, overflowing with inflows from the Eighiau breach, failed catastrophically, causing a flood of approximately 1.7 million cubic meters. There was no time to warn the village.

The Dolgarrog disaster followed the collapse of a reservoir in FraudScotland, April 1925. Both brought attention to poor dam construction and maintenance practices and led directly to the passing of the Reservoirs (Safety) Act 1930.

The law was aimed at ensuring the structural safety of large reservoirs by introducing legal requirements for regular inspection and certification by qualified engineers. This was the first attempt in the UK to regulate the design, construction and maintenance of tanks through statutory safety measures.

Since Dolgarrog, the UK has made excellent progress in tank safety. But at the end of July 2019, the evacuation of more than 1,500 residents of Whaley Bridge was ordered downstream of Toddbrook Reservoir in Derbyshire, England. Toddbrook received a month's worth of rain in just two days.

Swollen flows of water overflowed the emergency spillway of the dam, undermining its concrete slabs. A large cavity appeared on the spillway, exposing the core of the dam, raising fears of a breach.

A Chinook helicopter dumped 400 tonnes of aggregate onto the Toddbrook spillway to shore up the damaged area, and fire services used high-flow pumps to lower water levels and relieve pressure on the dam. Within days, engineers declared Toddbrook Dam stable enough to lift the evacuation order.

The Toddbrook incident was one of the most serious dam failures in recent British history. It showed how extreme rainfall can threaten the safety of dams and communities living downstream. Gavin Tomlinsonfire chief, said: “We found ourselves in a situation where we had five times more water than we could handle. We are absolutely sure that it can fail. It was a very, very tense night.”

Following this panic, in April 2021 the UK government ordered independent verification to the safety of the tank. A ministerial leadership was issued to the owners of all large, raised bodies of water, which made the registration of an emergency flood plans a legal requirement to ensure that they are prepared for unforeseen circumstances that could result in an uncontrolled release of water.

Threat of climate change

How geomorphologists who work on river processes and landforms, we study the effects of such dam-breaking floods that change the landscape, and how topography can increase the risk to communities.

As the Dolgarrog disaster clearly showed, bodies of water that flow into steep and narrow mountain valleys pose a particular danger, especially where the flow speed increases and picks up destructive boulders. All aspects of the landscape should be part of flood planning.

Although Toddbrook Reservoir complied with existing legislation and was recently inspected, it suffered.unexpected and potentially critical damage that could lead to disaster” Questions were raised locals comment on how well it is maintained. The renovation was nearing completion at the end of 2025..

Most reservoirs in highland Britain were built in the 19th century under hydrological conditions that no longer persist. Embankment dams and old masonry dams may be particularly vulnerable to erosion, seepage, slope instability, or overtopping.

The most common cause of dam failure is when the spillway fails to cope with flood waters. Reservoir safety may also be compromised by rapid or sustained declines in water levels during drought. As pore pressure changes and the soil dries out and cracks, the stability of the embankment can be compromised.

Climate change is increasing the intensity of storms and droughts in many parts of the UK, posing a threat tank safety. Climate models tell us that by 2080, the likelihood of intense rainfall causing flash floods will increase fivefold. Steep, elevated catchments in hard, impervious rock are particularly vulnerable to flash floods and are where much of the UK's water storage infrastructure is located.

The Dolgarrog disaster was the latest death in the UK as a result of a dam failure. But if severe storms and prolonged droughts become the new normal for our climate, the risk of aging mountain water storage infrastructure will likely increase.

This article has been republished from Talk under Creative Commons license. Read original article. This article has been republished from Talk under Creative Commons license. Read original article.