Dudbridge hydropower station spring clean
Dudbridge hydropower station spring clean
by: David M.
The hydroelectric generator at Dudbridge is powered by river section flow from Wallbridge to Ebley, using the 5m drop of the two locks at Dudbridge. Its earnings come from the feed-in-tariff index linked from 2013 for 20 years plus export income too, and it feeds into the electricity grid above the bridge on Dudbridge Road. Build of the station was supported by Summerfield Charitable Trust and it is maintained by CCT volunteers.
The generator uses a crossflow turbine situated 3m below the level of the upper pound. Water leaving the turbine falls down a rectangular draft tube into a sump in the outflow culvert below, and its vacuum effect makes up the remaining 2m of ‘head’. During normal flow conditions the system uses up to 700 litres of water per second to keep the upper pound level about 50mm below the top of the top weir. However, during high rainfall periods in recent years the flow can exceed 5,000 litres (5 tons) per sec. Floodwater overtopping the weir runs through the bypass channel, and it has brought down logs which have damaged the grille protecting the bypass channel pipe.
During the present low-flow period an inspection of the outflow culvert and sump was required to check for damage and remaining debris. Project Manager Keith Tibbitts contacted the Gloucestershire Speleological Society (GSS) who kindly agreed to share their time and resources. Planning activities over the last few months led to the inspection itself taking place on 8th May.
Preparation included the fitting of mooring rings at the outflow under the bridge. One condition of sending people into the culvert was that the turbine is shut down, which normally causes the upper level to rise and run water down the bypass channel. As this was also to be avoided, our team of lockkeepers was mobilised at Wallbridge and Dudbridge locks to control the flow and ensure that water would be diverted through Dudbridge locks while preventing the pounds from dropping too low. The view of the upper weir shows the fish pass on the far right. The pound level was lowered overnight to reduce fish pass flow from 70 to around 40 l/s, and on the day the level was dropped a further 150mm which virtually stopped all culvert flow.
The GSS team of four embarked on workboat Jasper for the short trip up to the outflow culvert. Depth to the concrete floor is only about 0.5m at the outlet, sloping down to 1.2m in the sump. Two of the team went in the water, the others remaining in the boat for safety line handling and communications. External lighting had been arranged, though the team battery lights were most effective.
The first GSS activities were to check for air quality. No dangerous gases were detected, and checks had been made in advance for wildlife which might be disturbed, none were found.
The possible build-up of debris in front of the draft tube and its supports was not found, the assembly was in generally good condition after 9 years operation. A log was taken out, along with some debris and silt build-up in the sump. A tube used for vacuum measurement had bent due to high flow or large debris, and this was restored. All went to plan with excellent collaboration between the various teams involved.
