City of London Corporation is locked in a legal battle with campaigners at one of the capital's most popular green spaces over £15m of dam works that locals fear will disfigure the landscape.
Plans to strengthen three ponds on Hampstead Heath were challenged in a judicial review last week, with the campaigners arguing work should be blocked because it would damage the heath.
QC Stephen Tromans argued that the ponds should be regarded as a special case and not subject to the same safety standards as newly built dams. Campaigners including the Heath & Hampstead Society objected to City of London's proposal to strengthen the Model Boating Pond, Highgate No 1 Pond and the Highgate Men's Bathing Pond.
According to the Reservoirs Act 1975, landowners with large reservoirs are subject to statutory inspection by a Government-appointed engineer every six-to-10 years. City of London said a recent inspection concluded improvements are needed.
But the lobby group argued that the dams have withstood the weather for 250 years including a one-in-10,000-year storm in 1975. The watercourses are an exceptional case because of their history and importance to the community, it added.
The judicial review ended last week and a decision could take eight weeks. Heath & Hampstead Society chairman Marc Hutchinson said: "The case is continuing and we cannot speak with you until it's completed."
However, a City of London spokesman said: "The earth dams sit above an urban population and according to the act they need a high safety standard. The engineer has said if there's an extreme rainstorm the dams may not be strong enough.
"In an extreme scenario the dams could burst and we could have 75,000 cubic litres coming down - and because the heath's dams are in two chains we could have potential cascading and a domino effect if one burst.
"A less extreme scenario is 'overtopping', which happens regularly when a localised storm such as a flash flood or convection storm sends water into the ponds and it spills over the sides causing structural damage to the dam. This happened four years ago."
He rejected the claim that improvements would damage the heath, arguing landscaping and environmental works would form the "heart of the project". Water quality would be improved, the beds would be de-silted and new reed beds would be planted.
"We are talking about grass-topped earth dams, which blend seamlessly into the landscape, not big concrete dams."
The three man-made ponds are classed as "large raised reservoirs" under the Reservoirs Act 1975.
According to the act, a "raised reservoir" is designed to hold, or capable of holding, water above the natural level of any part of the land adjoining the reservoir.
A "large raised reservoir", meanwhile, is designed to hold, or capable of holding, more than 25,000cu m of water above that level.