Denham Culvert works completed

The culvert, a five arched brick tunnel, is over 200 years old, and is almost always entirely underwater, making both inspecting and maintaining the structure a challenging task.

Simon Bamford, general manager for British Waterways London, explains: "The original canal engineers followed the instructions of their funders – usually powerful local landowners. A number of mills operated in the Denham area at the time of the construction of the Grand Union Canal towards the end of the 18th century. The mill owners insisted that local rivers which powered their mills be unaffected by the new canal and so rivers like the Frays were diverted beneath the canal using a series of tunnels.

Essential historic structures are maintained

"It’s essential that these historic structures are maintained as part of our work to keep water in the canal. Fracturing of these old tunnels can lead to flooding. We regularly inspect all our structures and draw up a repair and maintenance programme which is focused on the winter months to minimise disruption to canal users.

"At Denham, our inspections revealed a series of brick arched tunnels, with wooden floors - a rare sight of a fascinating piece of Victorian engineering."

Working with their contractors, Morrison Construction, British Waterways gave the Victorian structure a detailed survey and inspection, before undertaking a series of works which included repairing cracks in the masonry walls and barrel arches, replacing old timber inverts and repointing the head wall of the culvert.

Ensure the works were sympathetic to the heritage of the area

British Waterways London, project manager, Antonia Zotali explains: "Although the culvert isn’t a listed structure, we still worked very hard to ensure that the works we were undertaking were sympathetic to the heritage of the area. We used traditional skills, such as applying lime mortar by hand to repoint the nearby lock and the head wall of the culvert.

"The culvert is in a unique position, sitting in the heart of the Denham Country Park, so we were keen to ensure that all works followed Environment Agency guidelines. The works also gave us opportunity to upgrade sections of the towpath and manage the waterside environment, clearing an overhanging and dangerous willow tree, and replanting the area with grasses and reeds which attract a variety of wildlife."

This large-scale project took eighteen months of planning, plus six months work on-site to complete, which included some extra time allowed for a moorhen that chose to nest and incubate her eggs on a ledge within one of the temporary coffer dams installed to help keep the culvert drained of water.

Aware of any impact work could have on local wildlife

British Waterways London, asset and programme manager, Peter Walker said: “The project to repair Denham Culvert has been a real test for the team, with some complex engineering and access solutions required to ensure this historic structure was repaired to a high standard.

"The team have also been keenly aware of any impact their work could have on local wildlife, ensuring that the works didn’t disrupt the flow of the river to protect fish spawning downstream. British Waterways would like to thank everyone, from boaters and businesses to walkers and wildlife enthusiasts for their patience as we carried out the works, and I’m glad to see the whole area re-open to the public, complete with improved towpath surfaces as well."