Cwm Ebol Reservoir Dam Breach (2012)

A landslip cracked the dam wall of Cwm Ebol quarry's old mill reservoir and blocked its sluice gates, causing water to overtop the structure and prompting the precautionary evacuation of the nearby village of Pennal.

Cause

A landslip near the reservoir -- built over a century earlier to supply water pressure for Cwm Ebol quarry's mill machinery -- opened a crack in the dam wall and blocked its sluice gates, so that water began flowing over the top of the stone wall into the old quarry workings below, through which the Afon Pennal runs.

What happened

On 10 June 2012, North Wales Police evacuated residents from the village of Pennal, near Machynlleth, after a breach was discovered in the dam of the disused reservoir that had once served Cwm Ebol quarry, about a mile away. Police superintendent Andy Jenks-Gilbert explained that a crack in the reservoir's stone wall, thought to have been triggered by a landslip, was allowing a small but worsening amount of water to escape; sluice gates in the dam had become blocked, adding to the pressure. 'The water is flowing over the top of the reservoir stone wall into a quarry below which has the Afon Pennal running through it,' Jenks-Gilbert said, warning that a complete breach would send the reservoir's contents directly into the river running through the village itself, around 800 metres away. The evacuation, covering roughly a fifth of the village's population, was precautionary rather than compulsory; evacuated residents were moved to a nearby hotel and a leisure centre in Machynlleth. Staff from the local councils, the water board, North Wales Fire and Rescue Service and North Wales Police worked together to stabilise the reservoir, which fire crews later described as under 'immense' pressure. Over the following days, fire officers carried out a controlled release of the reservoir -- reported at around 18,000 cubic metres of water, lowering the level by roughly two metres -- averting a full breach. Residents were subsequently given the all-clear to return home.

Investigation & inquiry

No formal public inquiry is recorded; the incident was managed as an emergency response by North Wales Police, North Wales Fire and Rescue Service, the local water board and local councils, acting jointly.

Legal & regulatory aftermath

Not established in available sources; the reservoir, a disused former industrial structure, presumably came under closer subsequent inspection as a result, but no specific regulatory changes or reservoir-safety follow-up have been identified.

This account describes a post-closure incident in abandoned workings. It is not an access guide, and deliberately omits route detail beyond what's needed to understand what happened.

Recorded claims

Individual factual claims, each attached to the source(s) that support it below. Where sources disagree, both are kept.

• On 10 June 2012 a landslip cracked the dam wall of Cwm Ebol quarry's old mill reservoir and blocked its sluice gates; North Wales Police evacuated around a fifth of the village of Pennal, roughly 800 metres downstream, as a precaution. ¹
• Fire officers carried out a controlled release of around 18,000 cubic metres of water, lowering the reservoir's level by roughly two metres and averting a full breach; residents were then given the all-clear to return home. ²

Related terminology

Mining terms this disaster is structurally linked to, with the nature of the relationship.

• Level (contextual relationship)
  Named in this record’s narrative: “…-- reported at around 18,000 cubic metres of water, lowering the level by roughly two metres -- averting a full breach. Residents were…”

Sources

1. (secondary) Welsh village evacuated amid dam breach fears — New Civil Engineer
2. (secondary) Cwm Ebol quarry — Wikipedia

Record created: 30 August 2026 · Last researched: 30 August 2026