Overflow leads to lock wall collapse
The recent storms that have swept the country have affected the towpath and locks at Dudbridge on the Stroudwater Navigation. The canal has overflowed and scoured away the newly laid towpath surface leading to the undermining and collapse of three metres of lock side coping stones and lock wall.
Cause & effect
Stroud District Council (SDC) Canal Partnership Manager Dave Marshall explains…
“To put this all into context it is important to understand that we are not dealing with a ‘normal’ canal here.
"This section of the Stroudwater Navigation is actually classed as ‘main river’. Back in the 1950s a flood alleviation scheme channelled the Painswick Stream and Slad Brook into the course of the canal above Dudbridge Locks.
"Now, in 2012/13, putting the locks back into working order provided the opportunity to utilise the flow of river water to power the hydro-electric generation scheme installed by The Stroud Valleys Canal Company (SVCC) and Cotswold Canals Trust (CCT).
“The hydro scheme takes in water from above the top lock whilst the main flow of river water passes around both locks via a separate open channel that leads into a 1.8 metre diameter culvert.
"The culvert, or by-wash, is designed to cope with a ‘once in one hundred years’ flood. For safety reasons the entrance to the culvert is protected by a metal safety grille. See the top picture.
“The issue is at times of high winds and heavy rainfall large amounts of floating debris is washed down from the Painswick and Slad Brooks. This then becomes lodged against the safety grille.
“This creates a dam that prevents the excess water from passing around the locks in the bywash. The water backs up and flows over the top of the canal edge and into the channel between the locks and out onto the towpath.
"SDC monitors weather forecasts. During the recent spate of winter storms SDC & CCT Volunteers ensured that the safety grille was clear of debris prior to the onset of the storm.
"The problem is that the debris only gets deposited once the storm has set in and water levels and flow increase. When this occurs it is unsafe for anyone to work on site. However, volunteers cleared the debris from the safety grille on Christmas Eve and Boxing Day and New Year’s Day.
"The long term solution to this situation calls for the design and fabrication of a system whereby debris can be caught and deflected from the safety grille that protects the bywash culvert.
Short term solution
In the short term the top and bottom lock gates on the offside of the lower lock will be left open at times of expected heavy flow. This will allow excess storm water to flow from the channel, through the open lock and then out into the canal below.
Click on the pictures to view a larger version
Dave Marshall concludes…
"Whilst we have had some horrible weather over the holiday period there have also been some fine days for walking in that period. This has led to many people exploring the towpath. It will remain open over the coming weekend but we ask that people take care and stay away from any area of water covered towpath."
Repair work will commence today 7th January 2014.
The towpath will be closed for a period of three weeks.