Effect of pressure oscillations and hydrodynamics on
sediment-water exchange processes.
In shallow water, sedimentary bubble volume can change significantly with pressure oscillations caused by tidal and wave fluxes. It has been reported that bubbles 1m deep in sediment can be affected by a relative pressure change of only 3.5%. A conservative measure showed that at a pressure oscillation frequency of 0.5 Hz, bubble volume change rate has a similar pumping effect of that achieved by bottom fauna, which has shown to have a strong impact of biogeochemistry and subsequent sediment mechanical characteristics (e.g. Forster et al. 1999; Gehlen et al. 1995).
Although we are able to gather quantifiable data on sediment-water solute fluxes using in-situ measurement chambers, an in-situ quantification of the effect of bubble volume change (due to pressure changes caused by waves or tides) is not possible due to the complexity arising from the overlap of several different transport processes (i.e. bioirrigation, current-induced pore water exchange, lateral sediment transport and wave pumping). Therefore, I will investigate this process in the laboratory using a specially designed water column that will allow me to exclude the other natural sediment-porewater exchange processes, and focus on the behavior of bubbles in artificial and natural sandy sediments under the influence of sinusoidal pressure oscillations (waves).
I plan to measure bubble and water flow under the effects of pressure oscillations using PIV (Particle Image Velocimetry) and shadow imaging techniques. With the help of a high-speed camera, a strobe light and a laser, I can capture sequential images to map the flow of water and bubble velocity under the effects of pressure oscillations. The setup allows to create a variety of pressure rhythms and can therefore simulate real-world conditions as needed. For laboratory experiments the tank will be filled with water and seeding particles (tiny, neutrally buoyant plastic particles) will be added to the water to aid with PIV measurements. Bubbles will then be injected into the pressurized tank from a needle that extends through a rubber-stopper at the bottom of the tank, underneath the sediment. As bubbles rise out of the sand, I can capture their velocity with shadow imaging, determine pressurization effects, and as the water around the bubbles is dragged along, I can capture that water flow with PIV images.
Otherwise, my research will undoubtedly become affected by the recent oil spill disaster, and I perceive it will gravitate accordingly, possibly testing the effects of pressurization on biodegradation and other biogeochemical processes.