Collaborative Research: The dynamics of sediment-laden river plume and initial deposition off small mountainous rivers

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Abstract This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5). This new award is for a multi-scale modeling study of the dynamics and sediment-transport processes related to the highly turbid outflows from small montane rivers. The project will contribute to the 3D wave-averaged Community Sediment Transport Modeling System (CSTMS, a suite of models based on a coupled ROMS/SWAN framework). The enhancements will address critical intra-wave processes and are primarily two-fold: a new formulation on the interaction of surface waves and stratified shear flow, and parameterizations of the influence of convection on vertical sediment flux that may significantly alter the distribution of sediment deposition. The theoretical formulation for wave-current interaction is based on a multiple scale expansion that allows arbitrary vertical current variation. The resulting formulation represents a significant modification to the numerical coupling between ROMS and SWAN. A 2DV wave-resolving RANS model which is capable of resolving sediment-laden plume dynamics, such as hyperpycnal flows and convective instability, will resolve the interaction between a sediment-laden plume and surface waves, then tested against the theoretical formulation and enhanced CSTMS in idealized simulations. A 3D turbulence-resolving simulation tool for fine sediment transport will be extended to study instabilities, coherent structures and turbulence-salinity-sediment interactions at fine scales in order to reveal physical mechanisms responsible for the occurrence of convective instability and to improve the turbulence closure of the RANS model. The enhanced CSTMS will be utilized to carry out domain-scale (up to 5 km) scenario studies using typical physical settings of small montane rivers in order to determine how the small-scale convective, turbulent, density-driven and wave-driven processes interact at the scales of actual river outflows and affect sediment deposition. The results of this study will lay the groundwork for the design of future field programs and more comprehensive model-data comparisons. The research represents a combination of fundamental fluid dynamics (stratified hydraulics, instability, wave dynamics, turbulence-sediment interaction) and sediment-transport processes (frontal trapping, hyperpycnal flows, wave-supported gravity-flows), in context with a geophysical regime of global significance. While the importance of the contribution of small mountainous rivers to the total sediment discharge into the global oceans has been recognized since the early 90's, the dynamics of sediment-laden river plumes and the corresponding sediment transport processes remain poorly understood. The contributions from this research will thus occur both in the elucidation of the individual mechanisms and in their integrated impact on the cross-margin transport of sediment. Broader Impacts: The proposed research will impact various disciplines such as sedimentary geology, earth-surface processes research, and coastal engineering. The proposed modeling activity will be incorporated as an element of the Community Surface Dynamics Modeling System, a comprehensive suite of models quantifying the processes that modify the earth's landscape. These modeling efforts will also benefit several ongoing international research activities on field studies of hyperpycnal flow of small montane rivers in New Zealand and Taiwan. Hands-on laboratory experiments on sediment-laden plume will be developed by undergraduate scholars from underrepresented groups in collaboration with RISE program at University of Delaware (UD). Undergraduate scholars will also participate in outreach programs at UD to introduce the importance of sediment source-to-sink in our everyday life to middle/high school students and teachers. Two graduate students will be supported by this project for their PhD studies at UD. A post-doctoral investigator at WHOI will gain valuable experience as a participant in this project.