Dam safety |
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, said 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.
Above: The lake which is part of the Tredegar House estate in Wales
A significant heritage visitor attraction, the castle is
Above: Moss Eccles Tarn towards Scafell Pike in the English Lake District. It is one of the NationalTtrust’s 11 reservoirs located in World Heritage Sites
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 said, 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.”
12 | August 2024 |
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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.
References
The full meeting hosted by the British dam Society on 8 January 2024 can be found at
www.ice.org.uk/ events/past-events-and-recordings/recorded- lectures/historic-structures-and-dams
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