Dossier Océan et énergie - Énergie Thermique des Mers

Sommaire IOA News Letters

Overview of Mixing and the Marine Environment(U.S. Overview, Part 1*)

Dept. of Civil and Environmental Engineering
Massachusetts Institute of Technology
Cambridge, Massachusetts, USA

An understanding of oceanic mixing is important in a number of contexts to environmental engineers.

Global Climate is influenced by near surface processes--modulated by wind, waves, temperature and salinity differences--which control the flux of heat and chemicals (e.g. CO2) across the airwater interface. Large scale transport processes redistribute these constituents affecting energy and chemical budgets, and hence global climate. Recent field measurements involving natural and introduced tracers combined with 3-D computer models are aiding our understanding of these transport mechanisms.

A number of proposals have also been discussed recently to dispose of CO2 directly to the ocean in an attempt to shave the peak in atmospheric CO2 which is anticipated over the next century or two. These proposals have come principally from Japan, Europe and the United States and involve discharge of CO2 variously as a gas, a positively buoyant liquid, a negatively buoyant liquid (a dense CO2 - seawater solution, or liquid CO2 discharged to a seafloor depression to form a "lake") or a dense solid (dry ice or CO2 hydrates). The ability to successfully sequester CO2 will require an understanding of near and far field mixing processes (e.g., bubble and droplet plumes, density currents, meso-and global scale circulation), combined with knowledge of mass transfer rates, ocean carbonate dynamics, etc. Marine environmental impacts (e.g., lowered pH in the vicinity of the discharge) also require study. Mathematical models of these processes are being developed with the aid of high pressure laboratory experiments (to study chemical processes such as hydrate formation). Results have been presented at several recent international conferences including the Int'l Conference on Carbon Dioxide Removal, held in Amsterdam, 1992 and Kyoto, 1994; and to be hosted by MIT in 1996.

Renewable ocean energy technologies including ocean current, waves, hydroelectric and ocean thermal (OTEC) are expected to play an increasing role in our future energy mix. The success of OTEC, in particular, requires an understanding of ocean circulation and transport to assure the availability of warm (near surface) and cold (deep) sea water used by the plant boiler and condenser (i.e., to prevent recirculation between discharges and intakes). Important processes include jet mixing, gravitational circulation, and selective withdrawal. Despite the vast size of the ocean, design of plant intake and discharge structures to avoid recirculation is not trivial owing to the size of the flows: the small temperature differences (and hence low thermodynamic efficiency) of these plants dictate volumetric flow rates that are two orders of magnitude larger than corresponding condenser flowrates used by conventional steam electric power plants employing once through cooling. In addition to the concern over recirculation, there is also the potential for environmental impact (positive or negative) associated with the artificial upwelling of deep nutrient rich waters. OTEC research was active in the late-70s and early 80s, and involved a combination of theoretical and laboratory experimental data; there is currently a pilot scale open cycle OTEC plant on the west coast of the large island of Hawaii.

Ocean disposal continues to be practiced for a variety of wastes including treated and untreated wastewater (sewage), waste heat from electric power plants, chemical wastes, dredged spoils, etc. An understanding of mixing, coupled with a description of biogeochemical fate processes, is important in evaluating whether these wastes can be assimilated by the ocean without causing excessive environmental impact. A similar understanding is required to deal with accidental releases--e.g., those resulting from oil spills or the rupture of waste containment vessels--and the runoff of natural and anthropogenic contaminants from shorelines.

Common to each of the above problems is a need to couple our physical understanding of ocean transport and mixing with an understanding of biogeochemical fate processes. This coupling must, in principle, bridge 3-D space scales ranging from centimeters (describing jet mixing) to 100s of kilometers (the computational domain of a large basin) and time scales ranging from minutes (typical computational time step) to years (time constant for some sediment processes).

Only very recently have comprehensive long term simulations of coupled hydrodynamics and water quality begun to be possible, with three studies from the east coast of the US being Chesapeake Bay, Long Island Sound and Massachusetts/Cape Cod Bay. Even with the continued acceleration in computer hardware, important questions remain concerning how best to couple numerical models of hydrodynamics and water quality: resolution of sub-grid scale mixing (e.g., how to represent near field mixing from a sewage outfall in a hydrodynamic model encompassing Massachusetts Bay), the desirability of using different space and time steps for hydrodynamics and water quality, the need to resolve intratidal flow, treatment of open boundaries, etc. Finally, as we improve our ability to resolve processes computationally, we must strive to improve our theoretical understanding and experimental validation of these processes as well.