Tree roots exposed alongside water

Frequently asked questions

Cannop Ponds are surely ponds, not reservoirs? 

Whilst commonly known as ‘ponds’, the Upper and Lower Cannop bodies of water have been created by damming Cannop Brook and collectively hold around 100,000m3 of water above ‘normal’ ground level. As such, they are properly defined, registered and managed as reservoirs.

Many would ask ‘what’s in a name?’, but the reality is what makes a body of water a pond, a lake, or a reservoir is pre-determined and, as such, certain rules and regulations apply.

All reservoirs over 25,000m3 are required to be registered under the Reservoirs Act 1975. Until recently, Upper Cannop was not registered as a separate entity under the Reservoirs Act 1975 but as part of Lower Cannop, as it was thought its stored water volume was below 25,000m3. In early 2022, depth surveys confirmed the stored water volume to be circa 28,000m3 and, therefore, it has subsequently been independently registered with the Environment Agency. This does not mean Upper Cannop was not managed or inspected prior to its registration, but that it is now subject to the specific legal requirements of the Reservoirs Act 1975.

Why are options for the future of the reservoirs being considered?

In accordance with the Reservoirs Act 1975, the dams are under the supervision of an external and independent engineer who undertakes annual visits and provides a written statement on works undertaken and required to maintain the reservoirs. This is nothing new, since the first inspection of Lower Cannop in November 1976, we have always closely monitored the water levels and invested in necessary maintenance and works to keep the dams and reservoirs compliant.

As a result of the latest inspection – an ‘S10’ inspection which is undertaken once every ten years, by an independent ‘all-reservoirs panel engineer’, who report to the Environment Agency – much more significant concerns, repairs, and replacements were identified. On the realisation that what was required this time meant more than a significant cost, it meant draining the reservoirs and replacing some of the notable heritage features like the spillway with much more imposing modern alternatives – simply to make the site safe, with no additional environmental benefits – we felt it was our responsibility to explore further options.

What has been done to date?

In 2021, the annual supervising engineer’s visit identified a small sink hole adjacent to the spillway at the top of the dam and raised other issues, such as the condition of the masonry spillway. As a result, an early periodic inspection was instructed and undertaken in May 2021.

The periodic inspection required several legally enforceable actions to be taken. These included:

  • removing unauthorised modifications which had been made to the spillway.
  • further investigation of the condition.
  • maintenance of the spillway.
  • undertaking a flood risk assessment.
  • removal of trees to the left-hand side of the approach to the weir.

In addition, the report required a longer-term action to replace the spillway.

In autumn 2021, the short-term actions from the report were all completed.

Further investigation of the spillway identified that large voids (holes) had formed beneath it. These were repaired in the late summer of 2021 by pumping liquid concrete under the spillway slab. These are the latest repairs in a history of voiding within the dam over the past 50 years. Voiding within the Lower Cannop Dam had been identified and addressed in 1978, 1986, 1992-1995, 2003 and 2020, in addition to that observed and addressed in 2021.

In late 2021, a new flood study was completed. This showed that both Upper and Lower Cannop reservoirs do not meet the standards for the flows of water during extreme rainfall events.

As a category ‘B’ reservoir, Upper Cannop is required to safely pass a 1 in 1,000-year rainfall event without any water flowing over the dam, and a 1 in 10,000-year event with only minor flows allowed. As a category ‘C’ reservoir, Lower Cannop is only required to safely pass a 1 in 150-year and 1 in 1,000-year event respectively. The reason Upper Cannop has a higher classification is because, if it fails, it poses a terminal risk to Lower Cannop, which would be overwhelmed by the downstream deluge.

The flood study for Upper Cannop shows that the spillway is too small and cannot accommodate the 1 in 1,000- year flood flow. Should a rainfall event of that magnitude occur, the dam will over-top and a catastrophic failure could result. This is a greater risk due to the uneven, earthen nature of the dam at Upper Cannop.

The position at Lower Cannop is less clear as the spillway considerations are made more complicated by the volume of water that is diverted to the hydro-electric plant. However, the overall assessment is that Lower Cannop does not reliably contain the 1 in 150-year storm flow without risk of water flowing over the dam.

The original design of the reservoirs would never have anticipated the standards to which modern reservoirs are built. It is also very unlikely that the dams would have been designed with any particular calculation of likely storm flow and need to accommodate those flows. The inadequacies of the two spillways could, ordinarily, be met by widening or installing supplementary spillways. However, this significant alteration of the existing idyll begs the question of whether there is a better alternative.

Why is this being done now?

The issues identified in the last periodic inspection mean that, under the Reservoirs Act 1975, Forestry England has both a legal and a moral obligation to act, and we are given dates that actions have to be completed by.

The required changes to make the reservoirs safe for the future, in terms of their ability to retain the current volumes of water, plus the growing volumes predicted in light of our changing climate, would have a significant impact on the existing site:

  • increasing the spillway capacity by widening the existing, or more likely building new wider spillways.
  • raising the height and levelling of the dams so that, if over-topped, the water level is equally spread across the dam top and not focused into a low point.
  • buttressing and reinforcing their downslope faces.
  • rebuilding the structures to stop water ingress, and thus prevent voids from forming due to erosion.
  • draining the reservoirs to complete works.

Considering the potential scarring of a much-loved site, we are acting now to ensure we have the time to find a solution that leaves us in a better position than we find ourselves. It is our duty to look at all options for the future of the reservoirs before committing substantial resources to developing and implementing a preferred solution.

We must ensure Cannop meets with current standards for public safety, but we’d like to do it in a way that brings as many benefits for the community and the future of the forest as humanly possible.

What is the time frame for this project?

The original deadline for completion of the works required to address the issues identified with the reservoirs was November 2022. We have written to the Environment Agency requesting a formal extension, highlighting the significance of the works and the importance of finding the right solution for the community and the future of the forest.

In the meantime, we have commissioned surveys, including those to document the wildlife and the heritage, and will be carrying out further flood modelling work. We are also talking to professional stakeholders to take on a wider range of professional views. These will take time to complete and then the information will need to be analysed. We do not anticipate having sufficient information until spring 2023 to make a decision on a preferred way forward.

A decision on the preferred way forward, and its associated Planning Application, is therefore not expected before autumn 2023. Whichever option is taken forward for implementation will require various legal approvals, licences and consents. At this time the earliest start date for engineering work would be spring/summer 2024.

What options are being considered?

We are exploring numerous options in respect to both Lower and Upper Cannop Reservoirs. With multiple possibilities and variations across ‘repair and replace’ and ‘re-naturalise’, or an ‘intermediate’ option that blends aspects of both, at this stage we are gathering data and opinions of experts to ensure that the ultimate decision is made for the right reasons.

Is Forestry England trying to save money by not repairing the dam?

No. We are at the early stages of thinking about the future of Cannop Ponds and we do not have firm costs for any of the options. We do know, however, that all options will involve very significant costs.

We have always invested money into the maintenance of the structures at Cannop, because the ageing structures simply weren’t designed and can’t be expected to perform this long and with modern pressures in mind.

Forestry England is a public body and we must consider any expenditure carefully. It would be irresponsible and short-sighted to upgrade the existing dams without considering alternative options that might provide greater longterm benefits to the community and the future of Forest. We’d like to reassure everyone that, although it’s our duty to be financially responsible, the future of Cannop Ponds isn’t being decided with commercial benefits in mind.

What is the environmental benefit of the existing reservoirs? 

The picturesque, tranquil aesthetic of the reservoirs as they are might well paint the picture of a natural ecosystem, but what exists is far from the valley’s natural state. The reservoirs represent man’s historic disregard for natural processes in favour of rapid industrial development. Although the reservoirs have been a constant presence for 200 years, they are an industrial intervention that stopped the natural course of the river, and have since been artificially stocked with invasive, non-native species for their fishing value.

Beyond what we already know, it’s important to gain a greater knowledge about the natural efficiencies and deficiencies of the site as it is. We will continue to commission further environmental surveys and water sampling to establish the range of species within the reservoirs and to assess their biodiversity value.

We would like Cannop valley to perform better for wildlife and particularly aquatic biodiversity, which is why restoring the stream and wetland system is being actively considered as an option. At this stage it is not possible to determine whether the restoration of the stream system will deliver the range of anticipated ecological benefits. Nor can we be sure that the ponds aren’t performing an essential role in support of other native species. This is why we have commissioned a range of further ecological surveys and assessments so we can reach an informed decision.

How would restoring Cannop Brook affect biodiversity?

The option of restoring the existing stream system would aim to reinstate naturally functioning freshwater and wetland ecosystems. This promotes opportunities to re-naturalise both habitat provision and species presence, thus providing a nature-based solution to tackle the decline of the natural environment.

This approach is in line with building more ecological resilience into the way nature is conserved, restoring the multiple ecosystem services that flow from naturally functioning ecosystems, and adapting to climate change.

As part of the decision-making process, any biodiversity gains of a potential stream restoration will be assessed against the value of the existing reservoirs to quantify the benefits which will be achieved.

What impact will works to the reservoirs have on bats?

Whichever option is progressed for the future of the reservoirs, the potential impacts and benefits for bats will be thoroughly assessed. We will engage with local bat groups to ensure every opportunity is taken to maximise benefits for bats. 

What would be the impact on the fish in Cannop Ponds?

Yorkley Angling Club currently has rights to fish in the reservoirs. This lease came to an end on 31 December 2022 and Forestry England will work with the angling club to extend the lease for as long as possible before works begin. No angling will be possible during the works.

Regardless of whether a decision is made to replace the dams and spillway or to remove them, the ponds will need to be either fully or partially drained to allow the work to be carried out. This is to ensure that works can be completed safely by reducing water pressures on the dams and to provide a buffer to prevent the work area being inundated with water from a storm event.

It is therefore likely that some or all the fish will need to be moved to another suitable location(s), following appropriate checks for fish health. Fish introduced to the reservoirs through fish stocking will likely not be suitable for release into the water course below the dams as the stream is not sufficient to support the density, size and species of fish stocked in the reservoirs.

The exact details of how the fish will be managed during the works period will be developed by experts in the management and movement of aquatic wildlife and in collaboration with Yorkley Angling Club and the Environment Agency.

Cannop Ponds is a well-loved focus for visitors – will you be restricting access?

Cannop Ponds is a popular destination accessed through well-used walking and cycling trails as well as a car park.  Whichever option is pursued, providing a quality network of public paths so that people of all abilities can continue to enjoy this location into the future will be a key consideration.

The scale of this scheme gives us an opportunity to think about what we can provide to help people move around the area, sit and relax and connect with nature and the industrial heritage of the forest. However, exactly what is provided, and where, will be dependent on which option is taken forward. 

Whichever option is pursued, the scale of the engineering works will be very significant so during the build period there will inevitably be a lengthy period where some paths are closed or diverted to allow the construction work to be undertaken safely.

What is the heritage value of the existing reservoirs? If the valley is restored, what will the impact be?

The reservoirs are part of the industrial heritage of the Forest of Dean. That is why we commissioned a heritage impact assessment to look at their value and the impacts of any potential options for the reservoirs, and will make sure we take responsible actions based on specialist advice.

The heritage value of the reservoirs, and especially the structures such as the spillways, would be affected significantly by the ‘repair and replace’ option, so it’s important we understand the impact on these elements for all options.

We also recognise that landscapes do not stand still and must evolve – as the people building the dams on top of pre-existing industrial heritage of the area did. Restoring the valley would provide a unique opportunity to tell a deeper story of the rich industrial heritage of the area and how this has evolved over time. We would seek to do this through retention and preservation of important features of the reservoirs and through the provision of high-quality interpretation with the help of experts.

Is Forestry England considering a similar approach at other water bodies in the Forest of Dean?

No. Each lake and dam structure is inspected, assessed and maintained on its own merits.  

Mallards Pike Lake was built relatively recently and as such we know how it was constructed and have continuing confidence in its design. The spillway was damaged in recent flood events and has been repaired. We may need to carry out further repairs, and potentially some re-sizing work on the spillway to safeguard it for the future but, no, we don’t envisage any significant problems with this dam structure.

Woorgreens lake makes use of a shallow depression, and thus the amount of water impounded by the low dam is relatively limited and its (unlikely) breach will not cause any significant issues.

In a similar way, any breach of the shallow dam at Speech House Lake will cause a peak flow that will dissipate before it reaches Mallards Pike Lake.

Will you be publishing the Reservoir Engineer's report?

Forestry England is a public body and as such is bound by the ‘National Protocol for the Handling, Transmission and Storage of Reservoir Information and Flood Maps’. This protocol restricts the information relating to reservoirs that can be published. However, we have endeavoured to share the information from the independent reports that set out why it is necessary to act in the ‘What has been done to date?' section above. 

Will the public get a chance to feed back their views?

Forestry England has an obligation to address the shortcomings identified in the design and capacities of the Lower and Upper Cannop dams under the Reservoirs Act 1975. Accordingly, we are looking at the feasible options available with a range of professional and technical consultants and consultees.

We have sought to ensure the community is made aware of the very significant issues that are impacting Cannop Ponds. Going public at this early stage, without all the answers, was a conscious decision on our part to keep the future of Cannop Ponds an open and frank discussion. We know what it means to people, and that getting this right for the community and for the future of the Forest is of the utmost importance.

We will be holding public engagement events, where we will be able to share more information and insight based on the results of the surveys and stakeholder and expert engagement. We are still organising these events but expect them to take place in the first half of 2023 and will share details with the public in due course.

We are listening to what people are saying as we consider the future. The key point is that doing nothing is not an option as at some point the dams will fail, and that is not an eventuality that can be allowed to occur.

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Welcome to the Cannop Ponds news page, where you can keep up to date with all of the news from the Cannop Ponds project. All press releases will be uploaded here, so keep checking back for further updates.