Author(s): Rob Uittenbogaard; John Cornelisse; Ken O&Apos; Hara
Linked Author(s):
Keywords: Shipping lock; Salt Transmission Ratio; Bubble screen; Water screen; Sill
Abstract: The increasing population and ship/cargo traffic in delta areas, as well as the growing demand for more and longer storage of freshwater at lower river discharge, has led to the following research towards reducing the salt intrusion through shipping locks. In addition, to improve the poor ecology and anoxia of the closed-off sea arm Volkerak-Zoom Lake, now a freshwater reservoir, plans are being made to increase its salinity to about 20 g/kg for the removal of the freshwater cyanobacterium (blue-green algae) Microcystis. This major step would necessitate the suppression of salt intrusion through the three large Volkerak shipping locks into the freshwater reservoirs Hollandsch Diep and Haringvliet. A Dutch practice is the application of air-bubble screens for reducing salt intrusion through shipping locks. Originally bubble screens were created by releasing compressed air through perforated pipes on the lock floor and across the lock width. The lack of control over the distribution of the air flux along the perforated pipes¸marine fouling and neglect in maintenance decreased the effectiveness of the bubble screens to a reduction in salt transport of about 50%. In this paper we present our new design, and its performance, of bubble screens supported by a freshwater screen or a solid sill. Firstly, across the shipping lock, the air is equally distributed by an air duct and by a series of air regulators each releasing a constant air-mass flux. Secondly, we added diffusers, producing air bubbles of optimal size that prevent clustering into larger bubbles which would yield smaller entrainment. Thirdly, the diffusers are mounted in a staggered arrangement on two air ducts creating a thick uniformly distributed curtain of air bubbles avoiding gaps. However the small vertical momentum of water at the base of the bubble screen makes that part penetrable for salt water by the baroclinic pressure. Further, unavoidably, bubble screens entrain and thus mix salt to the freshwater side. To reduce this salt transmission we added a freshwater screen at the seaward side of the bubble screen. The water screen then acts as a penetrable sill and reduces the salt transport driven by the baroclinic pressure. Finally, the freshwater from the water screen is mixed into the bubble screen and this freshwater partially replaces the entrainment and mixing of salt water. Based on observations in a well-instrumented and operating shipping lock (148m length, 14 m width, 4. 7m depth) we present evidence that this combined waterair bubble screen yields the lowest recorded Salt Transmission Ratio of just 0-15%compared to the unprotected lock.
Year: 2015