Effects of internal waves on mixing and transport by gravity currents

Stanford University, Stanford CA

Investigators

Abstract

Because of increased global water scarcity governments are increasingly considering the contribution made by desalination as a primary source of drinking water. Thus, the fate of dense brine effluent from coastal discharges is becoming an increasingly acute issue. The areal extent over which brine plumes from desalination facilities impact the coastal ocean remains a major uncertainty, particularly because there are energetic internal wave fields which can cause significant mixing in many coastal oceans where desalination facilities are present or proposed. Fundamental understanding of the mixing of the plume with the ambient receiving waters is thus critical to predicting the concentrations of brine and chemical additives in the plume and surrounding waters. In particular, determining the pathways and mechanisms in which the mixing and dilution of dense gravity-current underflows down slopes are influenced by oncoming internal waves is an outstanding question. Although our understanding of how waves on slopes break and how gravity currents descend through stratified environments have independently moved forward considerably in recent years, surprisingly little work has been done on the increasingly important coupling problem. This project will address this need using laboratory experimentation in combination with complementary numerical simulations and theoretical modeling. The experiments and theoretical approach extends gravity current analysis to the important setting of breaking interfacial waves in the environment. Understanding the influence of this kind of ambient motion on dispersion in gravity currents is important to many industrial and scientific fields. For example, the results of this work will enable industry and other stakeholders to make better assessments of the environmental impacts of desalination design alternatives, including the potential creation of GIS-based tools. Summer research students will be engaged through the Department of Civil and Environmental Engineering?s Research Experience for Undergraduates Program and the Stanford Summer Engineering Academy run by the School of Engineering. This project will also directly support the training and education of a post-doctoral researcher and one graduate student, who will both be involved in teaching undergraduates. The proposed laboratory experiments and numerical simulations will establish what happens to a gravity current that descends down a slope through an ambient that has a two-layer density profile and internal gravity waves. Numerical simulations of the same experimental setup will be validated against the laboratory observations, and then used to further investigate parameter regimes unattainable in the laboratory. A theoretical framework based on the potential energy required to change the density profile within the current will be created to understand the detrainment observations from first principles. The investigation will be performed using internal wave tank and numerical modeling facilities, and will examine the physics of how interfacial waves influence inclined gravity currents. Shoaling internal waves have different regimes of breaking, including non-breaking seiches on steep slopes; coherent boluses; less-coherent and more turbulent boluses; and turbulent surges. Gravity currents descending through two-layer ambients can either form intrusions at the pycnocline; form underflows which penetrate beneath the lower layer; or split into an intruding and an under flowing part. When gravity currents and internal waves are present at the same time, however, much less is known. This project will lead to substantial new insight in two ways: by identifying and systematically cataloging the ways that internal waves can influence the physics of gravity currents, and by building and validating mathematical models of the mechanisms by which they do so. Three key parameters (the wave Froude number, the Iribarren number, and the density Richardson number of the current) will be used to define the parameter space of the investigation.