Weir-d noises as Settle Hydro hits millionth milestone

Settle Hydro has marked the impressive milestone of having generated one million kWh of clean, green renewable energy. The community project, which was commissioned in January 2010, generates electricity by drawing water from the River Ribble through a sluice gate to power nearby homes, with surplus energy sold into the National Grid.

But the landmark achievement of the millionth kilowatt was nearly overshadowed by a strange and irritating noise which led to complaints from residents and mystified the Hydro team – especially when it continued even after they turned off the hydro generator.

At times when the river level is moderately low, a low frequency ‘rumble’ gradually increases in intensity and then stops. After approximately 30 seconds the rumble builds again and disappears as before, with the cycle being repeated over and over again. The subsequent disturbance for nearby residents is probably not unlike sleeping with a snoring partner!

Steve Amphlett, chairman of the board of volunteers that founded the pioneering hydro scheme, is naturally anxious to address the problem.

He explains: “We all assumed the noise was coming from the Archimedes screw which powers the turbine, so we decided to manually stop the hydro.  We were amazed when the noise continued and realised it seemed to be emanating from the river itself, which seemed very peculiar. We then pinpointed the sound to the weir upstream of the hydro.”

During the intermittent periods of rumbling, a series of horizontal lines could also be observed which appeared to dance and flutter across the face of the weir, which was a rather strange visual manifestation of the phenomenon.

Steve’s wife, Sue Amphlett, resorted to the internet and typed “strange rumbling noise coming from weir” into the search engine. To her surprise, she found several academic papers about a phenomenon known as ‘Nappe Vibration’ or ‘Nappe Oscillation’.

During 2021, a crowdfunding campaign had helped supplement the cost of essential repairs met by the residents of Bridge End Mill and Settle Hydro Ltd. to the weir boards, for the benefit of both the fish in the fish pass and the efficacy of the Archimedes screw. A permit was issued by the Environment Agency for the work, which was eventually carried out in September when the river was sufficiently low.

Ironically, the problem with the recurring mysterious noise appeared following the completed works. In successfully achieving a ‘perfect flow over the weir’ they had also created ideal conditions for a Nappe Vibration.

Steve explains “Having established the cause, we needed a way to mitigate the problem, as turning off the River Ribble was not really an option! The next step was to see if disturbing the flow over water over the weir would have an effect, so we tried temporarily placing several sandbags on the weir crest. Hey presto, the noise stopped.”

The hydro team are now in the process of designing a permanent solution which will involve attaching boards at intervals along the length of the weir boards, and protruding above the weir crest, to create a series of gaps in the nappe to disturb the conditions that create the vibrations. The work will be carried out when the water level is deemed sufficiently low for it to be safe.

Reflecting on the achievement of the hydro to date, Steve said “It is a wonderful feeling to have generated over one million kWh of clean electricity – it really is a special milestone which shows that even small communities can take action to help reduce the carbon emissions responsible for climate change. Its success is a tribute to the team of hard-working and visionary volunteers who worked tirelessly to install and maintain the hydro, which has also inspired many others across the globe.”