Transport Processes in Inland and Coastal Waters

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The transports of heat, contaminants and sediments in inland and coastal waters are unique and complex processes involving turbulence over a wide range of length scales. The horizontal length scale of turbulence is one to several orders of magnitude greater than is the depth of the flow. Buoyancy determines vertical exchanges, as interfacial friction and the radiation of internal waves suppress the horizontal shear instabilities. For motion of sufficiently large scales of horizontal length, the fictitious force of the Coriolis is also a deciding factor. The purpose of the research programme is to attain a fundamental understanding of these highly anisotropic processes, guided by field observations. Attention will be given to the intensification of fronts in climate-change scenarios. One long-term goal of the research programme is to assist government and industry to prepare for the impacts of climate change through "adaptation" – i.e., by planning to accommodate the changes that are expected to occur. Our research has produced highly cited publications that are fundamental to understanding the transport processes. The Scopus citations to the applicant’s 2007, 2008, 2009, 2010, 2011, 2012 publications are 10, 8, 20, 32, 20 and 32, respectively. A laboratory study of horizontal turbulence in shallow open-channel flow by Chu and Babarutsi (1988; 55 Scopus citations), a field observation by Ingram and Chu (1987; 67 Scopus citations) and a hydrodynamic stability analysis on the shallow flow by Chu, Wu and Khayat (1991; 57 Scopus citations) are now classics, and have inspired much subsequent research reported in the first, second and third International Symposium on Shallow Flow (ISSF 2003, ISSF2008 and ISSF2012). The applicant has made many advances since, including his recent notable discovery of a wave radiation on horizontal turbulence. Through direct numerical simulations, Pinilla and Chu (2008) and Chu (2010) have determined the effect of the wave radiation, and correlated the effect on shear instabilities with Froude and Rossby numbers. Most significant of all is our discovery of a window of instability related to the Earth’s rotation. In fully nonlinear models of the Gulf Stream and of the atmosphere, intense shear instability is observed to occur briefly in a narrow window centered on a critical Rossby number (Pinilla & Chu 2009; Chu 2014). The shear instability across the window is intense, brief in duration, and closely associated with the intensification of the fronts and the increase in the frequency of the severe storms that are likely related to climate change. The study of horizontal turbulence would not have been possible without the parallel development of accurate and reliable numerical methods: in the past six years, this has been the focus of research supervised by the applicant, and is reported in a series of recent journal publications by Pinilla et al. (2010), Tan & Chu (2008, 2012, 2014), Chu & Altai (2012), Chu & Gao (2013), and Ghannadi & Chu (2014). Our research in the next five years will focus on refining the simulation of the steep gradients across the fronts. We will develop quasi-three-dimensional length-scale-partition model to simulate the interaction between the waves and turbulence. Laboratory and field investigations will study: (a) the role of wave radiation; (b) the development of the coastal boundary layer; and (c) the heat and mass transfer process through gravity-stratified interfaces. The experiments will be conducted in a new flow-visualization facility using a video-imaging method to study the waves and turbulence interaction. The brief and intense steepening of the fronts is to be correlated to the climate-change scenarios.