A large number of the UK’s dams were built in the 1800s and 1900s, when construction methods were both varied and evolving. And now many of their features are considered to be key heritage structures with a listed status or other significant designations. Typically, these dams can also be set in areas such as Registered Parks and Gardens, Sites of Special Scientific Interest, or Areas of Outstanding Natural Beauty.
Access to these dams can often be difficult with other structures close by also listed or considered ancient scheduled monuments. Therefore maintenance or capital works need to take these difficulties into account and ensure the correct approvals and licences are obtained, with works being undertaken sympathetically, whilst retaining key features and ensuring such heritage is retained for the benefit of all.
British Dam Society
To address the challenges of maintaining these dams and conserving historic structures in an uncertain future, the British Dam Society recently held a meeting to discuss the topic.
Dr Mark Roberts works as a consultant for the National Trust which preserves historic and green places across England, Wales and Northern Ireland. As he explained, the trust has facilities in some of the most iconic landscapes and many of these have dam related water features. Due to the visual appeal of the water body being closely associated with historic buildings such as mansions, these features are of great significance.
Across its portfolio the National Trust has over 200 reservoirs. Of these 47 are statutory reservoirs with 39 being of high risk. Over 150 reservoirs are smaller than 25,000m3, and 40 are between the 10,000-25,000m3 threshold. And most, if not all, are design landscaped features and play a significant part in the places the trust looks after.
Looking into more detail, Roberts states that 69 of the National Trust’s reservoirs are located in registered grade 1, 60 are in registered grade II* and 18 are in grade II landscapes. There are 11 reservoirs in World Heritage Sites, 141 in the setting of a mansion, with 39 in designated Sites of Special Scientific Interest.
The above numbers, Roberts says mean that you automatically can create tension between nature and heritage. However, the National Trust has learnt key lessons when trying to successfully address any safety or maintenance issues associated with such dam structures. These include understanding the significance of the project and involving stakeholders early on, as well as communicating any importance of the dam/water element. For example are reflections in the lake important? Does the lake or dam affect the setting of the area or house? Could there even be the potential for archaeological finds?
It’s also important to develop a project team as soon as possible with the appropriate specialists on board. Any loss or harm needs to be minimised and justified to the public, while gains should be designed in where possible.
“Through design,” Roberts adds, “heritage loss can be offset elsewhere.”
Climate change is also set to have an impact on heritage sites.
“We know climate change will impact reservoirs. Four in five sites in the National Heritage List for England will face high levels of risk by the second half of 21st century. There will be a requirement for more investment and a risk of more harm,” Roberts said. “Tough decisions are ahead which will result in the loss of some heritage features. And with changing hydrological records how do we also ensure we deal with risk but don’t overdesign?” he asks.
Developing support mechanisms and communicating change will deliver benefits for all. And where change needs to happen, Roberts urges, let’s make most of it.
Welsh experiences
Over in Wales, Cadw is the Welsh Government’s historic environment service which is responsible for the sustainable management of heritage assets such as scheduled monuments, listed buildings, registered historic parks and gardens, and outstanding and historic landscapes. Wales has 400 large raised reservoirs, and located within 150m of these are:
- 50 scheduled monuments.
- 268 listed buildings.
- 34 registered historic parks and gardens.
- 33 registered landscapes.
Dr Jonathan Berry, a senior inspector of ancient monuments and archaeology at Cadw, explains that Cadw would expect to be consulted on any proposals to change, decommission or demolish reservoirs that impact these heritage assets. Such assets do not necessarily have to be associated with the reservoirs and could, for example, be Second World War pillar boxes etc.
Highlighting that heritage assets are a finite, fragile and vulnerable resource, Berry says the highest levels of justification will be required to override national policy and demolish or alter a monument. Archaeological evidence cannot recover or regenerate, he adds. Once it’s lost, it is gone forever.
Cadw’s policies state all historic reservoirs and associated structures have intrinsic and recognised heritage values, whether they are designated or not. The justification for altering or decommissioning a historic reservoir should be set out in a heritage impact assessment, where the impact of the proposed works is assessed against the reservoir’s evidential, historical, aesthetic and communal values.
Indeed some historic reservoirs are legally designated as scheduled monuments and/or listed buildings. Heritage legislation is designed to protect the heritage asset for the future, irrespective of whether the asset is operational or not while the decision to designate does not take into account future maintenance costs.
Prior consent is required before undertaking works and Berry urges dam owners to contact them or local conservation officers for advice as unauthorised work/damage to designated heritage assets is a criminal offence.
Better understanding
Various examples of projects were given throughout the BDS presentation. They included several from Wales, such as the Beacons Reservoir Dam which was built in 1897 to supply water to Cardiff. Its listed buildings include the clay core of the dam, inner pitched masonry face, turfed outer face, spillway weir, cascade and overbridge and revetment walls. The structure was listed as it is seen as being an integral part of the historic water supply system for Cardiff, representing a major Welsh civic engineering scheme.
Cwm Clydach Dam was operational from 1840 and was built to supply water to Neth Abbey Ironworks. Viewed as an integral part of the industrial water supply system, its scheduling includes the dam core, inner and outer pitched masonry faces, spillway, culverts, road machinery and overbridge, and revetment walls.
Caerphilly Castle is operated and managed by Cadw in Wales. Located in a bowl surrounded by hills, the castle made use of a gravel ridge in an acre of marshy ground. This scheduled monument is extensively and legally protected from alteration and damage due to archaeological values above and below ground.
A significant heritage visitor attraction, the castle is the largest in Wales and second largest in the UK. It came into state care in 1950 and from then onwards and during the 1960s, a lot of restorative work was undertaken before it could be presented to the public. This included the creation of silt trap by the south lake; landscaping to create the south lake, inner moat and north lake as one contiguous water entity drained via an overflow shaft and culvert into the outer moat; along with the creation of a second overflow drop shaft in the outer moat feeding a culvert beneath Caerphilly town centre.
An inspection in 2017 identified various concerns such as the threat of erosion through the masonry or the foundations of the north and south dam platforms, along with deterioration of the 1960s culverts. Recommendations were made to retain reservoir integrity and avoid release of water and sediment into the town centre. It was also recommended that these were completed by October 2019 but it did take longer. As Berry says, there was quite a package of works that hadn’t been anticipated in Cadw’s budget.
The engineering recommendations included installing interceptor drains along the north dam platform, which meant digging into the ground which could be potentially harmful for archaeology, as well as monitoring erosion caused by resident geese and repairs to maintain the integrity of embankments. After an archaeological evaluation found no evidence of archaeological remains, the proposed scheme went ahead with 400mm deep rock armour installed and returfed on the moat protection
Cadw was obliged to balance the engineer’s recommendations with historic interests of the monument, needing to take both statutory requirements into account. So it complied with engineering report recommendations and developed appropriate schemes of work, tender contractors and obtained scheduled monument consent, following archaeological best practice.
Berry concluded that most of the proposals arising from the engineer’s recommendations benefited both the historic and natural environments. Addressing members of the dam industry, he said: “We need better understanding and awareness of each other’s roles, value and statutory responsibilities and consent regimes. Reservoir panel engineers need to be mindful of CAdw and Welsh government responsibilities for scheduled monuments and guardianship sites. Early pre-application discussion is needed to ensure positive outcomes for all.”
Berry added that it would be good work closely with the dam industry to “deconflict” any apparent conflicts at heritage assets with reservoirs. He said that patience, understanding, flexibility and seeing the bigger picture are required, while all successes should be celebrated, and colleagues educated.
Unexpected discoveries
When working on heritage reservoirs, Craig Goff, Technical Director for Dams and Reservoirs at HR Wallingford, says it’s wise to expect the unexpected
He gave the example of Buscot Park Lake, a National Trust property in Oxfordshire. This modest 4m high earth embankment dam was almost hidden completely from view of the house, but its heritage significance was the water view from the drives and pathways of estate.
When working on this scheme the team was mystified to discover a huge chamber under the dam. This was later identified as probably being an old pump chamber for drawing water from the reservoir and distributing around the estate.
Work carried out here included protecting this historic pump chamber, with safety measures around the auxiliary spillway and improved drawdown capacity. A modern RC high capacity spillway with brick façade to match other heritage structures on site was also built, wave protection on the upstream face was improved and the crest levelled while protecting historic trees situated there.
David Littlemore from Stillwater Associates also spoke about the work carried out at Tredegar House Lake, which is a National Trust property in South Wales. Constructed in 1790 to provide water supply to the estate, the dam is located in flood zones within a historic park and gardens. It also comes into contact with ancient woodland, a conservation area, as well as listed buildings within a designated amenity area.
Littlemore marvelled at his experience on this project and described the “staggering” length of time it took to undertake a relatively simple project that was completed in a heritage setting. He spoke about the benefits of having a full time National Trust project manager to help oversee the work, as well as early consultations with arboriculturists due to problematic large specimen trees located in close proximity to embankment. This led to constraints such as no machine or linear excavations, no filling above roots unless by an approved methodology, while cement also couldn’t be used. As a result, this also led to innovative measures such as using screw piles and plastic timber on the embankment, along with a cell web layer for tree root protection.
Reflecting on the lessons learnt, Littlemore’s advice was that contractors need to understand they are working on historic landscapes and that even access routes can affect the landscape. And as soon as an MIOS deadline is set, discussions need to start straight away with Cadw and Historic England.
