Ystrad Mynach Bubble Tubing® trial observation notes – August 2016
Natural Resources Wales south east operations team recently completed de-shoaling and essential maintenance of the flood bank on the River Rhymney at Ystrad Mynach as part of the capital works programme.
Approximately 900-ton of shoal material had accumulated, funnelling flows down a narrow, deep channel parallel to the west bank, a flood defence structure. This was having a detrimental effect on the bank, eroding soils and exposing root systems to create an unstable vertical profile.
It was identified early in the project planning stage that silt laden, muddy water would be created and there was a need to minimise the impact of the work on potentially sensitive downstream receptors.
Dirty water released through essential flood maintenance and restoration activities on rivers may have a number of deleterious water quality and habitat effects.
Controlling silt suspended during in-channel operations in deep, high flow velocity environments where conventional physical silt control interventions aren’t appropriate is challenging.
Bubble Tubing®, a micro bubble linear diffuser was deployed on the River Rhymney at Ystrad Mynach to create a curtain of sub-surface air bubbles. Two lines of ½” self-sinking Bubble Tubing were positioned downstream of the work activities in an area of natural deposition with a depth of 1m. The intention is that the air barrier would reduce silt transfer downstream and encourage sedimentation.
The installation was quick and easy, with no moving parts. The flexible self-sinking lines lay flat on the bed with the two lines positioned approximately 20cm apart. Air was introduced via feeder lines from a compressor unit situated on the bank. No physical obstruction was created meaning that navigation remained open at all times.
Variable flow velocities were encountered across the channel. A reduction in turbidity was observed downstream of the bubble barriers in all flow conditions. At flow velocities of 0.2 m/s and lower, the bubble barriers also held back floating debris.
At flow velocities of 0.4 metres per second fine sediment was seen to pass through the bubble barriers. Further trials are planned to determine the optimal air pressure, tubing dimensions, position and number of lines in a variety of flow velocities and water depths.
As well as mitigating the movement of sediment, deployment of Bubble Tubing (R) delivers a number of ancillary benefits downstream of in channel works:
- The impact of noise and vibrations on aquatic fauna can be reduced
- Oxygen is forced into the water column during times of high oxygen demand
- Navigation can remain open at all times
Further tests are planned to determine the optimal air pressure, tubing dimensions, position and number of lines in a variety of flow velocities and water depths.
frog environmental provide technical support and guidance on the deployment of bubble barriers, please contact us for further information.