16/10/2021 Unusual Phenomenon at Settle Hydro!!
Saturday, 16 October 2021 | Admin
The Settle Hydro team recently received a call from a resident of Bridge End Mill advising that an unusual loud (and irritating!) noise was coming from the rotating Archimedes Screw.
A team member went to the Hydro Site to investigate and was able to confirm that there was a low frequency 'rumble' that would gradually increase in intensity and then cease. After approximately 30 seconds the rumble would build again and disappear as before. The cycle kept being repeated.
As far as the team member could tell, the noise appeared to be coming from the Screw, although exactly why was difficult to discern. As the river level was low (approx. 75mm above the weir crest) and close to the level at which the Hydro would automatically stop it was decided manually stop the Hydro.
The team member left the site presuming that the noise would stop. However, soon after returning home a further call was received to say that the noise had started again and appeared to be coming from the weir!! A return visit to Bridge End confirmed the report and, indeed, it did seem that the noise emanated from the proximity of the weir. How strange!! How could the weir be making a noise??
The team member's wife decided to Google 'strange rumbling noise coming from weir'. To her surprise, the search found several academic papers about a phenomenon known as 'Nappe Vibration' or 'Nappe Oscillation'.
One of the academic papers describes the phenomenon as follows:
Free-overfall structures (such as weirs and crest gates) are commonly used as flow control structures for a variety of open channel/free surface flow applications including irrigation, water treatment, and dam safety. The gravity-driven free falling jet on the downstream side of these structures, called the nappe, may display a variety of behaviors and instabilities. In particular, nappe oscillations, also known as nappe vibrations, can occur under relatively low-head discharges and have been observed to occur with a variety of weirs (linear, labyrinth), crest gates and fountains.
Frequently identified as undesirable and potentially dangerous, this oscillating-instability phenomenon induces a disturbing noise with unmistakable acoustic energy resulting in low frequency noise that can be heard and felt in the vicinity of the structure.
The problem appeared following the recent work to replace the weir boards. It seems that in successfully achieving a 'perfect flow over the weir, ideal conditions had unwittingly been created for Nappe Vibration.
A further effect with the flow over the weir was observed which is also a feature of Nappe Vibration. During the fleeting period of rumbling, a series of horizontal lines could be observed which appeared to dance and flutter across the face of the weir. Very strange!
So, the cause of the problem has been identified, but what can be done to mitigate it? Turning off the River Ribble is not an option!!!
After some discussion it was postulated that if the flow over the weir could be disturbed, the conditions causing the phenomenon may be disturbed and the noise stopped. To test this theory, it was decided to disturb the flow over the weir with the temporary placement of several sandbags on the weir crest.
Sandbags were carefully put in place and, hey presto, the noise stopped.
A permanent solution has now been designed 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 and thus disturb the conditions that create the vibrations.
The proposed solution will be installed as soon as river conditions permit.
Nappe Vibration causes horizontal bands to appear in the water cascade.
Sandy braving the river to place sandbags on the weir crest
The sandbags successfully create a gap in the water cascade (the 'nappe')
The finished 'temporary' solution!