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Why did the Whaley Bridge dam fail?

Unless you were away for the entire summer, you will almost certainly have heard about the crisis that unfolded in Whaley Bridge at the start of August.

Residents were evacuated after it became apparent that the dam above the town was beginning to collapse. With the reservoir’s capacity of 300-million gallons, a collapse would not just have flooded the town. It would have wiped it out.

What caused this crisis?

Let’s start at the beginning. The town’s vicar, Father Jamie Mcleod, told the BBC that he alerted the authorities. The Toddbrook Reservoir is at the bottom of his driveway, and at first he wasn’t troubled by the water cascading down the spillway. Overflow is normal after heavy rains.

But the flow kept getting heavier, and after he saw water appearing through the ground beside the reservoir he started to worry. He decided to take a closer look, and he saw water coming through the concrete wall rather than over it. That’s when he raised the alarm.

With the concrete cracking and slipping down the dam, the threat to the town of Whaley Bridge was significant, and 85% of the 6,500 residents – those living in the areas that would be hit if it collapsed – were evacuated.

With 1.3 million tonnes of water poised to engulf the town, this evacuation was the most sensible option.

If you watched this drama unfold, you’ll know that an RAF Chinook helicopter helped to secure the dam, by dropping sandbags very precisely onto the damaged spillway.

In addition, the fire service pumped water out of the Toddbrook Reservoir to reduce the pressure on the structure.

What type of dam is holding back the water in Whaley Bridge?

Victorians tended to use clay as the impermeable material at the core of their dams. It’s the same material used to construct canals. Earth is then piled up either side, to keep the core in place, and this compacts over time making the dam more stable. A concrete spillway goes on top of the earth and helps to drain away water when levels are too high.

It is similar to this picture below, although this is not an image of the Whaley bridge dam.

The dam at Toddbrook Reservoir was built in 1831. When heavy rainfall in 2019 led to water levels rising and flowing down the emergency spillway, at this point the concrete failed. Then the water flowing down the spillway began to further erode the structure.

Speaking to the BBC, Alan Warren, Chairman of the British Dam Society speculated on the possible causes. He said, “We don’t know whether the concrete was inadequate or whether there was some problem underneath those concrete slabs which means the slabs fell into a void that had been forming underneath”.

“Maybe the joints in the slabs weren’t properly sealed, and water was getting in through the joints.”

Luckily, it does appear that the clay core has remained intact.

Why did the dam fail?

Dams are designed so that when they are too full, excess water flows down the spillway. However, in this instance it appears the increased water flow caused damage to the concrete. Dr Nicholas Howden, a hydrologist and hydrogeologist at the University of Bristol spoke to Wired magazine, saying, “I can’t be certain, but it looks like what’s happened is that some of that water has got underneath some of the concrete flaps and started to wear away at the material”.

We won’t know the full details until an investigation has been completed, but it seems likely that engineering work will be required to ensure the future safety of both the town of Whaley Bridge and the dam itself. A similar failure at Ulley Dam in South Yorkshire in 2007 resulted in three years of engineering works to replace the spillway, costing £3.8 million. It will be interesting to see what work is ultimately carried out at Toddbrook Reservoir.

In the meantime, if you need assistance with the structural elements of an upcoming project, please do get in touch.

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