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You are here : eLibrary : IAHR World Congress Proceedings : Managing deltas : Water c air bubble screens reducing salt intrusion through shipping locks
Water c air bubble screens reducing salt intrusion through shipping locks
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 water -
air bubble screen yields the lowest recorded Salt Transmission Ratio of just 0-15 % compared to the unprotected
File Size : 612,822 bytes
File Type : Adobe Acrobat Document
Chapter : IAHR World Congress Proceedings
Category : n/a
Article : Managing deltas
Date Published : 19/08/2015
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