Toddbrook Reservoir
A helicopter helps shore up the dam at Toddbrook Reservoir, UK.
The town of Whaley Bridge in the UK has had to be evacuated after damage to a dam built in 1831. The Toddbrook Reservoir is just one of many ageing dams worldwide not designed for ever more extreme rainfall as the planet warms.

Dams are typically designed to cope with a so-called 1-in-100-year flood event. But as the world warms the odds of extreme rainfall are changing, meaning the risk of failure is far greater. Engineers have been warning for years that many old dams around the world are already unsafe and need upgrading or dismantling.

"The 1-in-100-year event is perhaps happening every five years," says Roderick Smith at Imperial College London. "I'm absolutely convinced that it is due to climate change."

What is happening at Toddbrook Reservoir, where 1500 people have had to evacuate, is very similar to what happened at the Oroville Dam in California in February 2017. Both are earthen dams where excess water flows over the top of the dam and down a concrete-lined spillway.

If this concrete is damaged, the water flowing down the spillway can rapidly erode the earth underneath, and there is a risk of the entire dam wall collapsing.

There is a much greater chance of this happening when extreme rainfall or melting of snow leads to very high water flows into already full dams. A 2018 study concluded that climate change exacerbated the high water flows that led to the erosion of the Oroville Dam spillway, where 190,000 people had to evacuate and repairs cost $1.1 billion.

It's not yet clear to what extent the Whaley Bridge situation is due to climate change, though politicians in the UK have already explicitly blamed it. "The collapse of the Whaley Bridge dam is unprecedented and must act as a wake-up call to the government on the urgency of preparing for the inevitable impacts of climate change," said Labour's shadow minister for flooding, Luke Pollard.

Comment: We should prepare for extreme weather, but tying it to climate change is a mistake
...attributing extreme weather to manmade climate change can keep us from understanding the variability of extreme weather events and reducing our vulnerability to them. Tying these events to climate change can lead us to adopt inappropriate policies and ignore practical approaches that would lessen personal injury and the destruction of physical assets.

We have an opportunity to be proactive in preparing for weather disasters, reducing our vulnerability and increasing our survivability. Doing so means strengthening our infrastructure, as well as changing our policies and practices.

Engineers have now managed to cut off the flow of water down the damaged section of the Toddbrook spillway, averting the immediate danger. Another spillway is being used to lower water levels

"Provided that the side spillway can cope with the amount of water entering the reservoir, the situation should not get worse," Chris Binnie of University of Exeter told the UK's Science Media Centre.

There have not been any serious dam collapses in the UK since the Eigiau and Coedty dam failed in 1925, killing 16. After that, regular inspections by qualified engineers became mandatory.

Despite this, there have been some close calls, such as when the overflow of Ulley dam in Yorkshire was damaged during floods in 2007.

Not all countries have such regular inspection regimes. In the US, it varies widely from state to state. California has a relatively strict regime but astoundingly Alabama doesn't require inspections of its 2300 dams.

Smith thinks the UK has one of the best inspection systems in the world, but even it may not be good enough. "As climate change continues, the frequency of inspections may need to be greater," he says.

Globally there have been at least 40 dam failures since 2000, resulting in hundreds of deaths. The most recent was the failure of the Tiware dam in India on 2 July after heavy rains, killing at least 19 people. The worst was in 1975, when an estimated 170,000 died after the failure of the Banqiao dam in China.