Jeremy Stern reports on the newly restored dam at Blenheim
A major restoration scheme costing a "seven figure" sum has been completed on the dam at Blenheim Palace.
The dam was constructed by the renowned Georgian landscape architect Lancelot Capability Brown in 1760.
It formed the centre of his landscape, the Blenheim Lake, and has held back the waters of the Glyme ever since.
But the years have taken their toll and to conform with the Safety Reservoir Act 1975 a 20-week project had to commence to restore it.
The dam is built to withstand a 1 in 10,000-year flood event
"Over the last few years we've been noticing an increasing number of leaks in the dam and depressions appearing along the crest as water was getting through," explains Roger File, the property director.
"The dam is constructed of a clay and rubble core and over the last two centuries water has seeped through.
"Those channels are getting increasingly bigger. Last year we had the back wheel of a tractor disappear down a hole which was nearly two thirds of a metre deep."
It is not the first time the dam has been repaired.
"It's been a headache for many dukes since the 4th Duke and Brown constructed the lake," says Jeri Bapasola, author of The Finest View in England: The Landscape and Gardens of Blenheim Palace.
"Two generations on and already it was beginning to leak and the 6th Duke had the water of the lake drawn out. He reconstructed the dam and parts of the cascade as well.
"That was a major lot of work that took three years in the 1840s. Subsequent dukes have done minor repairs to it but the 11th Duke is doing the first major repair job since then."
According to the Environment Agency, Blenheim Dam must be of a design to hold back the water levels that would result in a 1 in 1,000-year flood and withstand over-topping in a 1 in 10,000-year flood event.
The first stones were put into place in July
Roger puts this in context: "10,000 years ago we were in the middle of the Ice Age. I think a glacier would probably be able to remove it but in terms of an extraordinary flood event the water should be able to come over the top of the dam without the dam failing."
"The engineers who model these scenarios say if the dam had failed at any time you'd be looking at a significant volume of water heading all the way down towards the River Thames at Cassington.
"The lake holds back nearly 600,000m³ of water which is not an insignificant volume.
"To make sure that water isn't unleashed downstream in an uncontrolled manner it is important that the integrity of the dam is the best it can be."
Restoration work began on the dam in mid-May 2009. Leaks in the core were repaired by digging a trench along the top and filling it with bentonite, a liquid clay which sets when poured into the ground.
To withstand the flood event the downstream face of the dam has been stripped and reinforced with 6000m² of interlocking concrete blocks.
"If water ever comes over the top the dam it will wash the top 75mm of surface soil off," explains Roger, "but then these blocks will channel the water in a controlled manner into the lower lake rather than run the risk of the dam being eroded out by the flowing water."
In addition the finished dam proves more of a viewing destination point. A rerouted path leads to the top of the cascade with a view down the valley.
Lancelot Capability Brown was employed by George Spencer, 4th Duke of Marlborough to work on Blenheim Palace at the height of his career. "He completely reconfigured the landscape," says Jeri. "One of the first things he did was to create the lake and in order to create the lake he had to build the dam."
Capability Brown's contract at Blenheim Palace was worth £21,500
Brown's genius was in great demand at the time.
"The whole style of what was considered fashionable in gardening had changed.
"By the mid 18th century it was no longer fashionable to have formal gardens, it was fashionable to have these romantic gardens that looked as though they were natural when in fact they were quite contrived to look natural.
"So it was a garden made to imitate nature and one of the key components in that was the creation of this lake in the valley. Prior to that there had been a system of canals under Vanbrugh's bridge which had terminated at almost the exact point where the dam now is."
Brown was born in Northumberland, but it was when he moved to Stowe that he started to develop his landscaping ideas. He set up independently after the death of his employer and was soon moving around the country improving estates.
His work is evident across the country's most celebrated stately homes but much of his handiwork can be seen in and around the Oxfordshire area, such as Adderbury, Fawley Court, Kiddington, Kirtlington, Nuneham Courtenay, Wootton Place and Stowe Landcape Gardens.
His contract at Blenheim Palace was worth £21,500; after paying his workers he was left with a profit of £4000.
"He made these naturalistic valleys and vistas, planting trees to draw your eye either to the house or to water which he often brought in where there was none," Jeri continues. "He was considered an absolute master of his trade.
"He was a gentlemen but wasn't part of their social scene. He was definitely sought by the aristocrats of the time as the only person who could actually improve their grounds to make it look as though nature had left them with this wonderful landscape."
The 9th Duke commissioned photos like this one in 1890
With the works on the dam completed, a two and a half century old piece of history lives on, but the work was expensive. Out of the 27 world heritage sites in the UK Blenheim Palace is the only one that doesn't receive lottery funding.
"The reason we don't receive any is because it is also the only privately owned world heritage site in the country," says Roger. "We don't have access to anything like the heritage lottery funding which means we get no help with projects such as this that run into seven figures."
The palace is lobbying for change, but will have to rely on visitor numbers through the door as their main income for projects in the meantime. With the aesthetic as well as the practical improvements to the dam, there's one more reason for visitors to do just that.