Discontinuance or adaptation?10 May 2021
A recent webinar hosted by the British Dam Society discusses managing reservoir assets and options for either remedial works or discontinuance.
Like many organisations throughout 2020, the British Dam Society (BDS) had to cancel its annual conference due to the Covid-19 pandemic. However, in an effort to showcase some of the conference papers it had received, the BDS decided to host a number of webinars. On 28 September 2020, the theme covered Legacies – Discontinuance or Adaptation and focused on managing assets and options for either remedial works or discontinuance.
Hazel Durant is the Head of Flood and Coastal Policy at the Department of Environment, Food and Rural Affairs (DEFRA), and said that first of all she wanted to thank everyone for all the work they do in relation to reservoir safety.
“Can there be any more important a role in life than taking on a job that has the potential to prevent loss of human life and or damage to the environment, property and the economy?” she asked. “I hope everyone is proud of what they do in this respect.”
Durant went on to talk about emergency plans for dams and said that although most have them now, there are still a number that do not.
“You don’t have to wait for emergency plans to become mandatory,” she said. “It is good practice and should be in place for all assets such as reservoirs. They will provide you and your team with the knowledge and understanding what to do should an incident be occurring or in order to prevent an incident. I encourage you to work on those emergency plans now.”
Durant then discussed a Mott MacDonald study which was completed in 2020 and found that there were 1500 small, raised reservoirs in England and that 30-35% could be considered high risk (ie risk to life). Durant said that this “is not acceptable” and is something DEFRA is working on with the Environment Agency to move forward.
DEFRA commissioned the research study into small raised reservoirs in 2017. The findings will be used to assess the need for any possible changes to the legal framework, and whether there should be an extension of current regulations for reservoirs between 10,000m3 and 25,000m3 capacity.
DEFRA is also expecting that Part B of the government-commissioned, independent investigation of the 2019 Toddbrook Reservoir failure in England should be available soon. Professor Balmforth, who led the investigation, is expected to share his findings with DEFRA during February 2021 and publication should follow shortly afterwards.
However, Durant urged dam owners and engineers “not to wait for guidance to come out for some of the recommendations in Part A”.
“There are things that you as engineers and undertakers and others can take forward,” she said. “Such as holding meetings rapidly after inspections, getting inspection reports out urgently and undertaking the spillway inspections that Balmforth recommended.”
Tony Deakin, Manager of Reservoir Safety at the Environment Agency (EA) gave an overview of what the agency is currently working on. Focusing on the government review of the Toddbrook Reservoir incident, he said that three of the 22 recommendations in Part A have been completed with 14 new guidances and five processes (including legislation review) still required. To address this the EA is setting up groups to focus on guidance for spillways, undertaker and supervising engineers, inspecting engineers, plus regulatory issues.
Deakin also mentioned the importance of compliance, monitoring and enforcement. Looking at measures in the interest of safety (MIOS) he said that now if a new inspection is called for, and outstanding safety measures are continued into the new report, the inspecting engineer has to provide reasons to justify this.
He said that the above was bad practice and that engineers must also consider the risk they are exposing themselves too. Deakin wanted to remind engineers that continually extending deadline dates without good reason may lead to the EA taking legal action against the undertaker. He also urged that all inspection reports should be competed as soon as possible even though there is a six-month deadline.
In other areas, over past two years, the EA has been working on new reservoir flood maps to replace existing reservoir inundation maps. These will be available in early 2021 and changes include:
- Wet day (showing flooding impacts during an ongoing river or tidal event downstream) and dry day maps (showing flooding impacts on a sunny day).
- Better digital terrain models – the old map used a 20m grid but the new one has a 2m grid and provides significant improvement to the quality of outputs.
The maps will also be used for different purposes such as emergency planning, increasing risk awareness of members of the public to flood risk, local planning applications and strategic planning considerations, plus how undertakers may be affected by downstream development.
Deakin finished his presentation by saying that Covid-19 has impacted the EA’s work and although the agency has not been able to deliver on various things it has been able concentrate on others, such as the benefits of removing redundant reservoirs (discontinuance) and projects on improving PMP and PMF estimation. The agency has also recently published completed work on pipework, valves and associated equipment on dams, and the grouting of dams.
Balgray Reservoir in Glasgow, Scotland
In a presentation looking at the Balgray Reservoir in Scotland, J Sampson, TM Hewitt and P Lambert from Mott MacDonald, and J Malia from Scottish Water, discussed how outlet pipework condition assessment and rehabilitation became a major focus at the project.
As infrastructure ages there is an increasing need to develop protocol to determine condition and optimal repair strategy for assets such as pipework, the authors say. The age of pipework is often provided in records but determining its condition and whether it presents a risk of failure to the dam can be more complex.
Built in 1853-54 to supply water to the Gorbals area of Glasgow in Scotland, there is a main dam with five smaller embankments at Balgray Reservoir which are all earthfill structures with puddle clay core. The main embankment is 439m long, with a maximum height of 14.3m and an impounding capacity of 3.35Mm3.
The outlet pipes are believed to date from when the reservoir was constructed and are still used for reservoir drawdown. In 2013 a CCTV survey showed that the pipes had substantial encrustation and, as they run through the embankment, there was growing concern.
As investigations got underway, another CCTV survey in 2019 showed encrustation to be much the same as it had been six years earlier. Sampling and material testing took place even though access to the pipework was difficult.
The authors stress that this case study highlights how beneficial material testing is to qualify encrustation composition and gain understanding about the intrinsic risk to the pipework and dam. In this instance no capital work was required due to the presence of encrustation. Rather than leading to pipe degradation, the encrustation was a stable layer of deposits that was actually acting as an internal layer of the pipe lining.
Moving forward, the authors say the that challenges facing dam owners and engineers in relation to encrustation in pipework in embankment dams include:
- Determining if encrustation is a sign of degradation or just a build-up of deposits from passing water flow.
- Obtaining sufficient samples from encrusted pipework to provide for appropriate testing.
- Determining the most appropriate action to promote reservoir safety. It must be noted that pipe cleaning to improve capacity can in some cases lead to a greater risk to pipe integrity.
In 2006 pipe failure was found to be the cause of a sinkhole that had appeared on the left-hand shoulder of the embankment at Ten Acre Reservoir in Yorkshire. Although this was repaired, further investigations found that the risk of internal erosion was high and, as repairs were costly, discontinuation of the structure was considered the best solution.
Jade Toulson from Mott MacDonald Bentley presented the Ten Acre Reservoir Discontinuance Case Study at the BDS webinar. Sited in Beckwithshaw, the 152,000m3 capacity dam was 17m high and 200m long. It was built in 1875 to supply water to a local water treatment works but had not been used for water supply since the 1970s.Prior to discontinuation it was only used by local fishing club.
As part of the process towards discontinuation, it was decided that the reservoir volume should be reduced from 152,000m3 to 5000m3. In case of potential changes in reservoir legislation in England, it was agreed that the reservoir volume would be kept below 10,000m3. However, it wasn’t possible to fully discontinue the structure as a body of water needed to be maintained for a local bat population that feeds on the reservoir.
The construction solution involved excavating a V-notch in the embankment, placing the material on the downstream face and constructing a new lower level, stone lined channel to the downstream watercourse.
Several licences were required before the reservoir could be discontinued. These included:
- Consent to discharge as the reservoir needed to be drawn down significantly before construction work could begin.
- Planning permission from Harrogate Borough Council.
- Change to the impoundment licence. Ten Acre was built under an Act of Parliament so the appropriate part of the Act had to be located before applying for a new impounding licence.
The Ten Acre Reservoir site was demobilised in May 2019. Toulson said that the owner Yorkshire Water was happy about the quality of the work and success of the finished scheme. Over the next few years the grass will grow on the reprofiled embankment slope, and the discontinued reservoir basin will be left to naturally vegetate and allow new wildlife habitat to develop.
Jon Pratten from Arcadis spoke about the Discontinuance of Legacy Dams and the experience of Natural Resources Wales.
On 1 April 2016, the reduced threshold of 10,000m3 at which a Large Raised Reservoir can be designated in Wales came into force. This instigated a programme of works to deal with MIOS issues on a range of assets. In his presentation Pratten discussed three high risk dams which were discontinued.
The embankment dams – Ratcoed, Llaeron and Rhiw Bach – are located in Snowdonia National Park in Wales in isolated mountainous areas within forestry plantations. The reservoirs were originally created in the 1800s to support the slate quarry industry and are all now redundant with no beneficial purpose.
Ratcoed and Llaeron are located deep in the forest with no direct access. Ratcoed has a theoretical capacity of 90,000m3 and failed in 1936. Pratten says there were no signs of site remediation, but the structure was informally breached shortly afterwards. Llaeron has a theoretical capacity of 450,000m3. It posed some past instability issues and was informally breached in the 1960s. Although both of these structures were empty the existing channel sides were unstable. The concern was that should they fail then the reservoir would start to fill again.
Rhiw Bach is located 2.5km away from the nearest highway up in the hills. It has a theoretical capacity of 17,000m3 (above natural ground level). Most of the water body is a flooded quarry excavation.
Pratten discussed the various difficulties encountered when undertaking discontinuance of the schemes. Technically he said that the works to discontinue the dams were simple, but the following factors were a challenge:
The access works were potentially more complex and costly then the discontinuance works and were largely undertaken as advance works in early 2018. Working in a tree planation is dangerous with weak rooted trees collapsing, especially in high winds. Using a contractor familiar with such hazards was highly beneficial.
Ecological and archaeological issues had the potential to delay and frustrate the projects. However, advance studies ahead of the design were able to ensure that matters were dealt with but still consumed a significant amount of time as a variety of licences and checks were needed. All the sites needed screening under the Water Framework Directive. At Rhiw Bach the access route was bracketed in structural remains which, although were not listed, were of historical importance. The area is now in the process of becoming a World Heritage Site.
The design of all three discontinuances began in October 2018 with Llaeron and Ratcoed being completed in the summer of 2019. Rhiw Bach was delayed for a year as licences were more complex to obtain and when they were available the works would have started in the winter of 2019. Work was postponed and executed in the dry months of the summer of 2020.
Pratten says that the lessons to be learnt from these projects is that although they were technically simple, successful execution was really achieved through effective management of:
- Access issues.
- Ecological assessments.
- Buildability discussions.
- Advance planning for all licences such as impoundment, planning permissions, water course consents and material handling issues.
Natural Resources Wales was applauded for its successful management of the programme. The discontinuance works were designed by Arcadis and supported by staff from Ove Arup, Black and Veatch and others. The principal contractor for all three schemes was William Hughes Civil Engineering, with Dr Andy Hughes as the Qualified Civil Engineer.
Panorama view of South Dublin, Ireland. To ensure a secure supply of drinking water to the region, Irish Water planned to replace the Stillorgan Reservoirs with a single new 160 ML covered reservoir
Stillorgan Reservoir comprises a series of open reservoirs which store approximately 400 – 450 ML of treated drinking water. Operated by Dublin City Council on behalf of Irish Water, the reservoir comprises three discrete cells; the Gray, Upper and Lower Reservoirs built between 1862 and 1885. To ensure a secure supply of drinking water to south Dublin, Irish Water plan to replace the Stillorgan Reservoirs with a single new 160 ML covered reservoir constructed within the larger basin of Gray Reservoir.
Murphy International was appointed by Irish Water to design and construct the new covered reservoir whilst Atkins was engaged to act as the independent geotechnical engineer – carrying out assessments of the shared Gray/Upper reservoir embankment during construction.
In their paper, S Battye and M Hughes from Atkins, and M Quinn from Murphy International, looked at how to maintain stability during decommissioning and for new construction. They discussed the analysis and methodology used during construction to safely and completely drawdown Gray reservoir for the first time since its construction, whilst both the Upper and Lower Reservoirs remained operational.
The authors gave details about how piezometric data and stability modelling were used to establish initial safe drawdown rates and to manage embankment stability until drawdown was completed. Once the reservoir was emptied, vibration monitoring and surface markers were installed to monitor the embankment during preparation of the basin (including rock breaking) and during construction of the service reservoir.
The authors say that full drawdown of Gray Reservoir was safely and effectively achieved by:
- Establishing a baseline for the initial drawdown rate.
- Critical analysis and calibration of monitoring data.
- Ongoing review of the stability model.
- Adjusting the drawdown rate to suit.
In addition, construction of the new reservoir (including rock breaking) was achieved by:
- Refinement of the specific vibration limits.
- Implementation of thorough surveillance and monitoring measures.
- Preparation of a well thought out emergency plan.