Some pounds 1.5m is needed to stabilise the embankment and re-line the channel but British Waterways is facing a crisis, with a backlog of pounds 100m of repairs urgently needed to the country's network threatening significant closures.
Not just the canals and pleasure boaters are at risk. Any failure of Sutton Weir on the Weaver navigation in Cheshire, where "unexplained voids" have been detected beneath the sandstone barrier, could severely disrupt rail and road traffic, including on the M56. Repairs costing more than pounds 1m are "imperative".
The one-and-a-half-mile stretch hugging the Blorenge at Llanfoist, near Abergavenny, has a long history of instability. In 1975 the bank was breached and mud flowed round homes and blocked the valley road. The canal remained closed for six years before a concrete lining was put in.
But cracking was detected two years ago and the canal is now kept open by a temporary artificial channel just wide enough for pleasure craft to pass through. Constructed of steel frames and plywood with a plastic membrane, the 400ft flume is expected to last another 12 months but extensive work cannot be long delayed. Just before Christmas water was discovered leaking down the hillside.
"We have now reached a stage where continued under-funding poses a threat to the safety of people on or near the waterways," David Fletcher, British Waterways' chief executive, said.
Government grant to British Waterways has been reduced in real terms by one-third over 12 years. This year's pounds 51m is a shortfall of pounds 7m on the minimum believed necessary to operate and maintain the late 18th- and early 19th-century network. Urgent talks are underway with the Department of Environment about funding.
The 33-mile-long Monmouthshire and Brecon canal was opened in 1812 and carried coal, limestone and iron to Newport. Today it is popular with pleasure boaters. There are22,000 privately-owned craft using Britain's canals and another 1,500 holiday hire boats. A typical pleasure-boat licence costs pounds 360 a year.
Only about 250 commercial boats still ply the canals and even this traffic is threatened by the repairs crisis. More than pounds 350,000 worth of work needs to be done on a busy length of the Aire and Calder Navigation in West Yorkshire. British Waterways generates an income of about pounds 45m in addition to its grant, including pounds 16m from property and pounds 10.4m from leisure uses. It estimates 10 million people visit canals each year, including anglers and towpath walkers.