Forestry England is today releasing a Dam Safety Assessment Report which sets out the facts about the engineering concerns about Cannop Ponds.
The report has been written in response to frustrations from the public about national protocol that required us to redact details of the independent reservoir inspection reports. The new report provides the facts, while omitting the sensitive information Forestry England was unable to share.
The report sets out the context of the situation, the history of repairs at Cannop Ponds, and the latest findings. It shares the results of the 2021 inspection report, which triggered the current concerns about Cannop. It also shares the results of subsequent investigations. While the Dam Safety Assessment sets out full details, in brief, there are two major problems with the dams at Cannop.
Firstly, both spillways have structural weaknesses. Whilst Upper Cannop’s can most likely be repaired, Lower Cannop’s spillway needs to be completely removed down to foundation level and rebuilt.
The second problem is that neither spillway can contain sufficient flood flows, with the risk that in a severe rainstorm they could ‘over-top’, triggering a catastrophic collapse. The failure of one or both dams could lead to loss of life, would result in significant damage in the villages of Parkend and Whitecroft, and could cause property flooding in Lydney.
The Reservoirs Act sets out the scale of a flood event that a dam must be able to safely cope with. Upper Cannop is a category ‘B’ reservoir, which means it must be able to withstand a 1 in 1,000-year flood. Lower Cannop is a category ‘C’ reservoir, and so must withstand a 1 in 150-year flood. These flood events do not represent timeframes but are ways of measuring the severity, with the higher the number being the greater severity flood. With our changing climate these severe storm events are predicted to get more frequent.
Neither spillway could currently cope with a 1 in a 150-year event. To address this, both dams require new, larger, stronger and far wider spillways to be constructed. The construction of the new spillways will require the ponds to be completely or at least partially drained.
Kevin Stannard, deputy surveyor of the Forest of Dean, said:
“The Forest of Dean experienced a 1 in a 130-year event in December 2020, which led to the highest flow recorded on the Cannop Brook. With our changing climate, it isn’t a question of if we will experience a 1 in a 150-year event, but when. We cannot allow the dams at Cannop to fail.”
Ground Investigation Works
As part of the ongoing survey work, this spring, ground investigations will take place at Cannop Ponds.
Kevin Stannard added:
“The ground investigation work will be carried out with a range of techniques and equipment, including some very large drilling rigs. I need to reassure the community that this is not the start of works to remove or replace the dams.
The investigations will provide more accurate and detailed information on how the dams and spillways were built, what materials were used, and what condition the structures are in. This will allow us to better understand the history and make-up of the dams. Our current knowledge is based on visual inspection of the surfaces and known history of previous collapses and repairs. We are now carrying out this necessary work to test our assumptions and increase our knowledge of the structures.”
The Ponds were created around 200 years ago to provide a reliable supply of water to the ironworks at Parkend and they represent an important part of the Forest’s industrial heritage. Over the decades the built structures have been repaired, modified and replaced. Sadly, any work to rebuild, or strengthen the dams and spillways is likely to lead to further loss of that original built heritage. The ground investigation work should help us understand how the original dams were built, and how much of those early structures remain. This is particularly true of Lower Cannop, where the dam was built by altering an earlier embankment and bridge for the Bixslade Tramway. This was a horse-drawn tramway that was constructed to bring stone from the quarries above the Cannop Valley down to meet the new railway at Bixslade Wharf.
The nature of the works means, for a time, access across the dams at Upper and Lower Cannop may be restricted for safety reasons. Full details of any diversions or footpath closures will be shared nearer the time.