Bubble screens are applied at shipping locks between salt and fresh water bodies, in order to reduce the amount of salt intrusion as a result of the locking process. The importance of limiting this salt intrusion is found in the required quality of the fresh water, which is given by ecological reasons or due to its use for agriculture and/or drinking water.
Recent shipping traffic developments contribute negatively to salt intrusion unless proper mitigating measures are taken. Increasing shipping intensity demands more frequent lockages, with every lockage adding to the amount of salt intrusion through the lock. Furthermore, the enormous size of modern locks means they contain a large amount of salt water that potentially flows towards the inland water bodies. These developments have called for a renewed assessment of the potential of bubble screens to mitigate salt intrusion.
Compared to the application of a freshwater flushing discharge through the lock, the primary alternative, bubble screens are obviously more expensive as the required flow rate and pressure demand a significant compressor size. An important question therefore is to optimize the design or use of the bubble screen such that the required salt intrusion reduction is met at a minimum use of energy. An additional question is how the bubble screen technology scales up towards the larger depths of modern locks.
Various measurements have been performed in the past decades to address the application of bubble screens in shipping locks. Recent work has focused on the application of various research methods to contribute to the further understanding of the bubble screen technology.
Examples of in situ measurements are found in Uittenbogaard et al. (2015), who present the application an innovative combination of bubble and water screens at the Stevin Lock (The Netherlands). A similar design of bubble screen was tested subsequently at the Krammer recreational lock (The Netherlands) by Weiler et al. (2015). This study also evaluated the expected costs of operation of a bubble screen in the larger commercial locks of the Krammer lock complex.
Scale model research regarding bubble screens considers various application areas and approaches. Van der Ven and Wieleman (2016) present small scale experiments of a bubble screen in a flume of 1 m depth and 1 m width. They make use of practically unscaled bubbles. Another important simplification is that the performance of the bubble screen is evaluated only on the induced water motion, as use is made of homogeneous water. The measurements are compared to available in-situ measurements from Bulson (1961) and scale model measurements of comparable scale by Riess and Fanneløp (1998).
Recently scale model measurements have been performed at Deltares that provide data of high accuracy and at a high spatial resolution. These comprise PIV measurements of the flow and the quantitative recording of the mixing induced by the bubble screen by applying a dye to the salt water compartment. The quantitatively high quality data help to accurately establish the performance of bubble screens as salt intrusion mitigating measures and may show room to improve the bubble screen operation or design.
A third research method with great potential is the application of computational fluid dynamics (CFD). An example is found in Meerkerk et al (2015). More recent computational studies show a promising increase of numerical stability which helps in the application of these methods in advisory or research projects.
The various research methods have their specific drawbacks and benefits. This presentation gives an overview of these methods and their applicability and accuracy when assessing the performance of bubble screens as a means to mitigate salt intrusion through locks. The presentation includes results from ongoing research currently being conducted at Deltares.
References
Bulson, P.S., Currents Produced by an Air Curtain in Deep Water – Report on Recent Experiments at Southampton, Dock and Harbour Authority, 1961, Vol. 42, Nr. 287, pp. 15-22
Meerkerk, M. van, O’Mahoney, T., Twerda, A., Development of a CFD Model of an Air Curtain for Saltwater Intrusion Prevention, 36th IAHR World Congress, 28 June – 3 July 2015, The Hague, The Netherlands, 2015
Riess, I.R., and Fanneløp, T.K., Recirculating flow generated by line-source bubble plumes, Journal of Hydraulic Engineering, 1998.124:932-940
Uittenbogaard, R., Cornelisse, J. and O’Hara, K., Water – Air Bubble Screens Reducing Salt Intrusion through Shipping Locks, 36th IAHR World Congress, 28 June – 3 July 2015, The Hague, The Netherlands, 2015
Van der Ven, P.P.D. and Wieleman, V.A., The use of small scale experiments for a shipping lock’s bubble screen, 4th International Symposium on Shallow Flows (ISSF), 26 – 28 June 2017, Eindhoven, The Netherlands, 2017
Weiler, O., Van de Kerk, A.J. and Meeuse, K.J., Preventing Salt Intrusion through Shipping Locks: Recent Innovations and Results from a Pilot Setup, 36th IAHR World Congress, 28 June – 3 July 2015, The Hague, The Netherlands, 2015
Interest to conference attendees
Salt intrusion is difficult to measure, to predict and to mitigate. This presentation gives an overview of the available methods to provide such a prediction when a bubble screen is appleid and the applicability and accuracy of these methods. This is interesting in particular to lock operators and lock designers. The presentation provides recent and state-of-the art methods. Further, the subject relates to the conference topic Environment (e.g. Fresh water availability, Multiple purpose water reource systems) and as such is interesting for various parties at the conference.