Bubble plume towed in salt-stratified water.

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Rob Uittenbogaard, Scott Socolofsky, Yann Friocourt, John Cornelisse

Thursday 2 july 2015

9:00 - 9:15h at Europe 2 (level 0)

Themes: (T) Managing deltas, (ST) Adaptive delta management

Parallel session: 10K. Managing deltas - Saline

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This paper reports on a part of our research on the usage of bubble plumes for reducing the intrusion of seawater in a 15 m deep, salt-stratified tidal channel (the Rotterdam Waterway in The Netherlands). For scaling laboratory experiments, we set the relevant prototype conditions at 15 m depth, 1.5 m/s maximum current, and 1 psu/m maximum salt-stratification with an assumed linear profile. The prime objective of this part of our research is to determine the efficiency of converting (air-compressor) energy into increased potential energy of a salt-stratified current as well as the required air (mass) flux for achieving a given upward salt (mass) transport under prototype conditions. In stagnant density-stratified water, the vertical transport of bubble plumes has been explored mostly through small-scale laboratory conditions; Zic et al. (1990), Lemckert & Imberger (1992) and Asaeda & Imberger (1993) present estimates for the energy efficiency of the bubble plume for upward transport and mixing of denser water in stagnant density-stratified water. For the first time we explore the essential properties of a bubble plume in a salt-stratified current by the equivalent case of a bubble plume towed through salt-stratified stagnant water (typically 10 psu/m) . We conducted laboratory experiments at 0.7 m water depth in a canal at Texas A&M University, USA, and at 2.5 m water depth in Deltares’ 30m long water-soil tank in Delft, The Netherlands; the bubble plume was towed up to 0.4 m/s. From the change in the density profile per tow run of the bubble plume we derive the vertical salt-mass transport of the bubble plume and the energy efficiency. In addition, by using 4 different camera viewing directions, with several different depth levels of dye injection in front of the plume, and by measuring the velocity field around the plume, we observed the plume rise angle, its non-circular cross section, and the general flow pattern. Destratification efficiencies are comparable to and can sometimes exceed values in stagnant stratification.