The spillway at Upper Cannop Ponds

Frequently asked questions

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

In accordance with the Reservoirs Act 1975, the dam(s) 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. Forestry England regularly monitors water levels in the reservoir and carries out maintenance and repairs as necessary.

Additionally, a periodic inspection is undertaken at least once every ten years, by an independent ‘all-reservoirs panel engineer’ who reports on the condition of the dams and any necessary maintenance actions. All annual visits and periodic inspections are reported to the Environment Agency who are responsible for enforcing the requirements of the Reservoirs Act. They ensure that the necessary inspection and maintenance tasks are completed.

The future of Cannop Ponds is being discussed as a result of the findings of the latest inspection.

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 carried out. Investigation of the spillway identified that large voids (holes) had formed beneath it. These were repaired in the late summer of 2021 and are the latest 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 weather events.

The original design of the reservoirs would never have anticipated the standards to which modern reservoirs are built.

We must now ensure that the reservoirs do meet current standards for public safety.

Why is action being taken on this issue 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.

If the reservoirs are to be safe for the future, they would require substantial works, such as:

  • raising the height of the dams.
  • buttressing and reinforcing their downslope faces.
  • increasing their existing spillway capacities.
  • increasing their resilience to storms.

At Lower Cannop, a complete replacement of the existing spillway is required, with a much larger and more substantially engineered structure. At Upper Cannop upgrades to the existing spillway, or construction of a secondary auxiliary spillway, may be necessary.

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. This is why Forestry England is considering options for the future of the reservoirs now. 

What other options are being considered?

A more natural approach to dealing with extreme and severe weather events is to restore the natural stream process in the valley, by slowing the flow and holding water in a myriad of small and medium sized pools and wetlands.

Removing the dam structures would also permanently remove the risk of flooding to downstream communities from a dam failure. A well designed scheme could hold more storm water back in wet periods, and hold water in the forest for longer during times of severe drought.

Options for removing the dams could allow for more natural, ecologically rich wetlands as well as encouraging natural stream processes. This has the potential to restore the habitat of, and secure the future of a range of native species, notably fish and aquatic and wetland plants and insects that live wild in the forest. 

What is the flooding risk?

Downstream flooding may occur due to the failure of the dam or dams or it may occur due to extreme rainfall. The worst case scenario would be when an extreme rainfall event triggers an uncontrolled release of water from the reservoir(s) due to a failure. 

Whilst there is no immediate threat of either dam breaching – that is water flowing over the top of the structures and causing a collapse - doing nothing is not an option, as the potential for a dam failure at Upper or Lower Cannop still remains and will worsen into the future.

Such a failure would result in the uncontrolled release of water from the reservoirs and must be avoided. A catastrophic collapse would cause property flooding in Parkend, Whitecroft and Lydney with the associated risk to residents and visitors. Essential infrastructure in those villages and the adjoining valley, for example roads, water and sewage pipes, could be badly damaged in a flood event. 

To assist with design work, and to comply with the periodic inspection findings, a new flood study was undertaken by independent consultants in January 2022. The study confirms that the current structure and spillway design for Lower Cannop is inadequate for a 1 in 150-year flood event.

To put a 1 in 150-year event in context it is worth highlighting that Forestry England commissioned a flood risk review after wide-spread property flooding in the Forest of Dean in December 2020. The consultants undertaking that review calculated that the rainfall event leading to that flood event was a 1 in 130-year event. At the time of the December 2020 floods the flow on Cannop Brook at the Parkend monitoring station hit its highest recorded level.

The reservoirs also help to moderate, or 'attenuate', the peak flows at times of heavy rainfall, with water falling in the woods upstream being intercepted and held for a time in the reservoirs, as only a certain amount of water can get out over the spillway at any time. This reduces the ‘peak’ flow for any storm event further downstream.

As part of the flood study undertaken in January 2022, the consultants looked at the amount of ‘attenuation’ provided by the Lower and Upper Cannop Reservoirs. They modelled eight different storm scenarios – duration and intensity of rain – and looked at the inflow to the reservoirs, and the outflow from the reservoirs and concluded very little storm water attenuation was provided. 

With climate change models predicting wetter winters and heavier summer storms, the frequency of rain events where the capacity of the spillway(s) is exceeded, and the dams are breached (water flows over the top of the dam), can be predicted to increase as well. This additional flow (even without a catastrophic collapse caused by the dam breaching) will worsen downstream flooding in Parkend, Whitecroft and Lydney.  

To address the predicted increase in rainfall, and thus predicted increases in peak flows on the Cannop Brook, the dams either need to be strengthened and raised with new, larger spillways or the opportunity taken to remove the structures and design improved storm water storage capacity in a naturalised stream and valley system.  The degree of flood mitigation afforded to downstream communities will be a key decision criteria for which option is eventually taken forward.

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. 

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 which may provide greater benefits to the forest, the forest’s community and the forest’s wildlife.

What is the environmental benefit of the existing reservoirs? 

The existing reservoirs are man-made and have existed for almost 200 years. They are artificially stocked with fish and have been managed over many years for the enjoyment of the angling community and to improve their value for fishing. The reservoirs are not natural features of the landscape and include non-native pond weed and fish species.

Whilst Cannop Ponds provides a perception of a natural ecosystem, this was not the valley’s original state, which has been significantly altered by humans. 

Forestry England will be undertaking 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 and wetland biodiversity, which is why restoring the stream and wetland system is being actively considered as an option.

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?

Irrespective of the solution taken forward, it will be necessary to lower the water levels in the reservoirs, either partially or completely. 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 translocated 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 translocation of aquatic wildlife and in collaboration with Yorkley Angling Club. 

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 an important part of the industrial heritage of the Forest of Dean. That is why we have 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.

We also understand that landscapes do not stand still and must evolve. Restoring the valley would provide a unique opportunity to tell the 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.

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.

Why are the public not being consulted on whether or not the dams are repaired or removed?

Forestry England have an obligation to address the short-comings identified in the design and capacities of the Lower and Upper Cannop Dams under the 1975 Reservoirs Act. We are accordingly 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. 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.  

Once we have received all the technical advice, survey results and views of our technical consultees, then we will develop our preferred option. The final details of that preferred option will be made available to the wider public for comment and we will receive views that will help shape the final scheme.  

Will you be publishing the Reservoir Engineer's report?

No. 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. 
 
 

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