Reservoir safety management in Wales


Natural Resources Wales (NRW) is the undertaker of over 40 reservoirs registered under the Reservoirs Act 1975 (as amended by the Flood and Water Management Act 2010). In Wales, this Act applies to large raised reservoirs capable of holding
10,000 m3 or more of water above ground level. Many of NRW’s reservoirs are inherited from predecessor organisations and have a long, complex history. Some are over 150 years old, built originally to supply water to former mining areas such as Gwydyr Forest near Betws-y-Coed.

Each reservoir has a qualified Supervising Engineer appointed to assist the owner with ensuring the reservoir is safe. They are also inspected at least every ten years by a member of the UK All Reservoirs Engineering Panel. As part of the recent round of ‘Section 10’ inspections, a suite of Measures to be taken in the Interests of Safety (MIOS) has been identified, which NRW must act on.


In March 2018, NRW awarded us a two-year call-off contract to assist with the implementation of these MIOS. The contract includes the provision of reservoir compliance modelling and topographical services.

We are assisting currently with the MIOS for 16 of NRW’s reservoirs across Wales, from the mountains of Snowdonia to the urban regions along the south coast. We are providing four main types of service:

  • Topographic and bathymetric surveys of the reservoirs and surrounding environments. Where disused mines and shafts pose a risk to health and safety, we are employing state-of-the-art drone technology to assess hazardous areas. We are working with Infomap Surveys and Mapping Limited and DTM Technologies Limited, as well as our own in-house team of survey specialists on these aspects.
  • Using the topographic and bathymetric data to undertake ‘flood routing’ to determine whether the reservoirs can pass extreme flood flows safely. The flood studies are being carried out in accordance with the fourth edition of the Institution of Civil Engineer’s Floods and Reservoir Safety guide.
  • Assessing the areas at risk of flooding in the event of a dam or reservoir failure and sudden uncontrolled escape of water. For this, we are drawing on the experience and expertise we have gained in the delivery of over 1,000 reservoir flood maps for risk management authorities throughout the UK.
  • Our team of emergency planning specialists is preparing on-site pumping plans to help NRW respond to emergency incidents relating to their reservoirs. The plans include useful information about the dam’s location, construction, capacity, and the operation of its valves and other key equipment. Also included are response escalation procedures and actions to be taken.

The two-year call-off contract is being led by our Newport (South Wales) office, with support from expert staff across our other offices including Warrington, Skipton and Exeter. We have also built strong collaborative relationships with NRW’s reservoir engineering team, helping ensure that we meet or exceed their expectations.


Over the past year, we have helped NRW complete 21 MIOS and, thereby, fulfil its obligations under the amended Reservoirs Act 1975. Some of the reservoirs have even been found to hold less than 10,000 m3 and can, therefore, be taken off the register of large raised reservoirs.

We have also uncovered some interesting historical information about the reservoirs along the way, such as probable mines drainage patterns in the Gwydyr Forest. This history is helping NRW to better understand the operating context of its reservoirs and dams.

“Public safety around reservoirs is of paramount importance and we are delighted to be able to use our expertise in reservoir management to help NRW fulfil its safety obligations under the amended Reservoirs Act 1975,” commented Jeremy Benn, Project Director and member of the Supervising Engineer Panel.

He added, “The Act is a vitally important piece of legislation, there having been no fatalities from dam failures in Wales since the introduction of reservoir safety legislation after the Dolgarrog Disaster in 1925. We are now looking forward to continuing to support NRW in the year ahead.”

Want to know more?

E-mail Claire French or Jeremy Benn for more information on this project, and the services we can provide in relation to reservoir safety management. You can also visit our dams and reservoirs web page.

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