The dam in question got good safety reviews right up until the day it failed. Toddbrook dam collapse | ‘Closing gap between compliance and safety’ must be priority.
First a review of what happened.
On 1 August 2019, a single slab of the dam’s spillway chute collapsed into a large void that had formed underneath, and a brown slurry could be seen discharging from under slabs (which had also failed and lifted) further down the spillway chute. During the day the void enlarged, and more slabs collapsed, risking the integrity of the dam. A full-scale emergency was declared, and, as a precaution, 1,500 people were evacuated from the town of Whaley Bridge immediately downstream of the 25,000m3 Toddbrook Reservoir.
I haven’t read the complete report, but the summary isn’t surprising. The design of the spillway, which was built 100 years ago, and redone in some way in the 1970s, wouldn’t meet design standards today. The concrete in the spillway was too thin, reinforcing was inadequate, there was no under-spillway drainage, etc.
But the problem – to my mind – is the maintenance. Even though they got good reports from the inspections, and had NEVER had a safety violation, it’s clear that something was wrong.
The reports adds: “Maintenance over the years had been intermittent with extensive plant growth in cracks and joints for prolonged periods, suggesting open passageways to the embankment beneath. Generally, the slab concrete remained sound but there was honeycombing and/or deterioration at some joints, some missing chute plums, some cracking and evidence of significant prior plant roots through joints and in some cases through slabs.”
For a more complete review of what happened, see It Isn’t Only The USA That Has Been Ignoring Infrastructure. There is a video from Blancoliro that covers details of the dam and the emergency response, which included a helicopter from the Royal Air Force.