Water – air bubble screens reducing salt intrusion through ship locks.
Rob Uittenbogaard
Thursday 2 july 2015
9:30 - 9:45h
at Europe 2 (level 0)
Themes: (T) Managing deltas, (ST) Saline and freshwater interaction
Parallel session: 10K. Managing deltas - Saline
A Dutch tradition is the application of air-bubble screens (Bulson, 1961) for reducing salt intrusion through ship locks (Abraham & Van den Burgh, 1962). We define the ratio between the hindered and unhindered salt intrusion (i.e. lock-exchange flow) as Salt-Leak Ratio (SLR). The original air-bubble screens were initiated by releasing compressed air through perforated pipes across the lock floor. Schematic prototype experiments in non-operational ship locks showed that air-bubble screens can reduce the SLR to about 25-30 %. The lack of control over the distribution of the air flux along the perforated pipes¸ marine fouling and neglect in maintenance appeared to increase the SLR to about 50% in operational ship locks. The increasing population and ship/cargo traffic in delta areas as well as the growing demand for more and longer storage of fresh water at lower river discharge invited the following research toward reducing the SLR of air screens to minimal levels. Firstly, across the ship lock, the air is equally distribution by an air duct and series of air regulators each releasing a constant air-mass flux. Secondly, circular diffusers produce air bubbles of optimal size that prevent clustering and undesired openings. Thirdly, the diffusers are mounted in staggered arrangement on two air ducts creating a thick uniformly distributed curtain of air bubbles. This design yielded low SLR in operating locks. Yet the small vertical momentum of water in the air screen’s toe makes that part penetrable for salt water by the baroclinic pressure. Further, unavoidably, air screens entrain salt water and thus mix salt to the fresh-water side. For reducing these salt leaks we added a fresh-water screen at the salt-water side of the air screen. The water screen acts as a sill and reduces the leakage driven by the baroclinic pressure. Finally, the fresh-water of the water screen is mixed into the air screen and partially replaces the entrainment of salt water. Based on observations in a well-instrumented working ship lock (145m length, 14 m width, 5m depth) we present evidence that this combined water - air bubble screen yield the lowest SLR of 0-15 % ever recorded.
