Site Description
The River Usk supports of wide range of fish species with high conservation value such as migratory salmonids, shads and lampreys. During construction of the new abstraction pumping station at Prioress Mill, a bubble barrier needed to be installed. This bubble barrier would prevent fish entrainment during on-going abstraction operations. The site presented a range of challenges as the River Usk is a spate river. As such, it showed rapidly changing water levels (with increases of up to 5m in water depth) and flow regime. As such, the bubble barrier needed to address the downstream flows presented within the river channel. Our system also needed to address the lateral flow from large scale abstraction operations of over 300Ml/d.
The Solution
This project needed to be implemented in a very short timescale to meet the requirements of the statutory regulator. To meet these challenging conditions, a vigorous angled bubble barrier system was developed across the abstraction intake.
The photograph below shows the system running at around 60% of maximum air volume capacity and it was capable of delivering up to approximately 70Nm3/air/m, if required under full flood conditions.
Best practice design criteria were detailed in the reviews of fish screening and barrier techniques. (By Turnpenny et al. (1989) and Pavlov (1989)). The basic criteria recommended were:
- The bubble barrier is most effective as a behavioural barrier when placed at an angle of 11 – 12° to the river bank. This approach relies on a glancing contact of the fish to deflect them across the channel.
- Use of a 50mm main air feed pipe for bubble barrier greater than 3m in length.
- Use of a perforated pipe system with 2m holes at 25mm centres or 1mm holes at 6mm centres.
- Delivery of air at 3 l/s/m of barrier which equates to 10.8m3/air/hr/m.
However, it should be highlighted that a perforated pipe is not a particularly effective or efficient method. The bubble barrier is unlikely to be a viable proposition on a relatively fast flowing river with rapid fluctuations. The ISS-Flowthrough bubble barrier system is unique and of flexible design. This allows a bespoke barrier to be developed to suit specific site conditions. The use of our patent technology combined with the use of diffusers allows an effective barrier to be produced across a wide range of conditions. In said conditions, standard perforated pipe approaches are likely to be relatively ineffective.
High Volume Air Delivery
The use of high-volume air delivery is important to the success of bubble barriers. This is due to influence of both river currents and other currents generated by abstraction. Flow will tend to cause a degree of deflection within the barrier. Sufficient air needs to be delivered in order to ensure an effective barrier is generated. The degree of deflection is a function of water depth, flow velocity and bubble size.
Larger bubbles have a high rising velocity. For example, bubbles larger than 2mm-7.5mm diameter have a rising velocity of 0.28-0.3m/s. Therefore, they are less affected by deflection. Bubbles larger than 7.5mm have a rising velocity of between 0.3-0.45m/s. However, these are not usually stable and tend to sub-divide into smaller bubbles. No information was available on the river flow regime to estimate the degree of downstream deflection. In addition, the local flow regime was likely to have altered following recent bank reprofiling works and the removal of the intake wing walls.
An estimate of the degree of deflection due to abstraction flow may be calculated. This is based on the maximum daily abstraction rate of 318Ml/d. Based on the dimensions of the intake structure, a water depth of 1.8m the estimated abstraction intake water velocity would be 0.168m/s. Based on a bubble rising velocity of 0.3m/s the degree of deflection would be around 1m.
Therefore, our bubble barrier on the River Usk created a high air volume delivery and highly robust, low maintenance bubble barrier. Our system incorporates rubber membrane diffusers rather than perforated pipe. This provides a vigorous bubble barrier across a range of river and abstraction conditions.