Any reservoir can outlive its original purpose or design life but redundant reservoirs, whether they are subject to the provisions of reservoir legislation or not, can still pose a risk to life for communities downstream, and have ongoing maintenance requirements that place a burden on owners.

As a recent lecture hosted by the British Dam Society highlighted, for these reasons reservoir owners often consider discontinuing redundant assets but such a decision isn’t always that straight-forward. This is because reservoirs can deliver amenity value for local communities as well as contributing to a desirable environmental habitat, and so careful consideration should be given to balancing the advantages and disadvantages when deciding whether to discontinue them or not.

The lecture discussed the processes to be followed leading up to discontinuance, and outlined lessons learned such as:

• Identifying and mitigating environmental aspects.
• Findings from public consultations.
• How to approach whole life cost estimates.
• The benefits or otherwise of carrying out ground investigations.
• Dealing with sediment issues.
• Managing flood risk during removal of the reservoir.
• Managing downstream flood risk following removal.

Working with stakeholders

The importance of identifying and working with stakeholders from an early stage was highlighted by the example of one project where the local angling association feared for its future, amid concerns the plug would be pulled on the town’s reservoir. Local MPs became involved and public pressure meant the discontinuance wasn’t taken forward. It’s paramount that those who could voice their opinion about discontinuations are identified as soon as possible and can include local authorities, angling clubs, local residents, the Royal Society for the Protection of Birds and other such organisations, plus national and regional parks.
Frank conversations with third parties need to be carried out in the early stages and engagement with enforcement authorities, environment permitting teams, key stakeholders is encouraged as quite often stakeholders, even within the same organisation, can have competing interests.

There also needs to be an understanding of legal implications for post-discontinued reservoirs such as land ownership, legal constraints, plus compensation to the watercourse and its reliability. Other factors to consider during the detailed design are whether any water needs to be retained in the reservoir, as this can provide benefits for the local environment, ecology, and wetland birds etc, as well as helping to dampen the peak of floods. Fish passage and the increased risk of flooding both during construction and afterwards also need to be considered as downstream risks will be changing.

There also need to be plans in place for excavated material. Is there space for it or issues with land ownership or change of land use? Are material management plans required and can vehicle movements be minimised during the project? The export of material usually has substantial costs and causes disturbances to surrounding third parties and may trigger an environmental impact assessment.

Scottish studies

Case studies were given from Scotland where Scottish Water has 250 reservoirs and 73 are no longer used for supply. Sixty-six of these are deemed as high risk and need ongoing maintenance, pose a risk to public and are impacting river restoration.

Many lessons were learned from the experience gained at Pundeavon Reservoir which was the first major dam to be discontinued by Scottish Water in 2016. Originally built for public water supply, this 15m high earth embankment dam with a reservoir surface area of 40,000m2 and volume of 268,000m3, was no longer operational. There were potential concerns about the condition of the outlet pipe and lack of available freeboard.

Discontinuanace was formed with a V-notch through the embankment and less than 10,000m3 of water was retained in the reservoir basin. In order to control inflows during construction a new by-wash channel was formed around the north side of the project, tying into the spillway channel. During construction there was a major silt release in the first attempt at discontinuance in 2015, and so consequently Scottish Water implemented a drawdown protocol to be followed.

There was a lot of public support for discontinuing Mill Glen Reservoir in 2021. This 10.7m high earth embankment with clay cores was built for public water supply, with a 50,000m2 reservoir surface area and 150,000m3 of volume, but was no longer operational. It had a history of spillway erosion, vegetation issues, problems with outlet structure and two fatalities had also occurred there in 2013.

Discontinuance of Helensburgh No.2 Reservoir is still underway. This 8.5m high earth embankment with a clay core was once again built for public water supply (50,000m2 surface area and 230,000m3 volume) but is no longer operational. A valve tower bridge collapsed in 2012 and the reservoir been empty since then, and the basin rewilded.