Collaborative Research: The interactions of waves, tidal currents and river outflows and their effects on the delivery and resuspension of sediments in the near field

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This study will focus on the mechanisms for sediment resuspension and removal in the near field of a river mouth plume characterized by a combination of strong freshwater discharge, significant vertical shear resulting from stratification, and strong surface wave conditions. It is hypothesized that the dispersal pathway of river-borne sediment is possibly more often governed by resuspension events in the region of sediment trapping, as opposed to only being an a priori property of the riverine discharge. Further, surface waves are hypothesized to be an important component of the overall physical mechanism of bottom stress and sediment resuspension in river mouths and coastal inlets, leading to greatly enhanced potential for sediment transport. In order to address these hypotheses, the researchers will further develop and utilize a nonhydrostatic model (NHWAVE) with capabilities to handle salinity stratification and suspended sediment in both wave-resolving and wave-averaged forms, using, in particular, results from ongoing NSF-funded work on river plume dynamics. The model will be validated against a high-resolution data set for instabilities in sheared buoyant plume flow in order to establish the accuracy of the formulation at high resolution, and against a larger scale transect of data from the Mouth of the Columbia River (MCR) to verify basic wave and circulation processes in the model. The model will be applied to the MCR in order to study the interplay of tidally pulsed river plume and waves and the resulting removal and resuspension of sediments in the vicinity of ebb-tidal plume liftoff, and will be further used in interpreting results from the ONR-sponsored RIVET-II experiment, conducted in June 2013.Estuaries and river mouths provide the pathway for riverine supply of sediments to the coast and the distribution of sediments to the shelf and deep ocean. This project will study the influence and coupling of tidal currents, river discharges and surface waves in determining the sediment transport regime in coastal inlets. The first task will be to develop and test both wave-resolving and wave averaged versions of a flexible, parallelized nonhydrostatic model NHWAVE, which has recently been extended to incorporate density stratification and the presence of a suspended sediment load, thus providing the basic tools needed for high resolution process studies. The further development of the model is important for several reasons. First, the strong vertical components of motion associated with the tidal genesis of fronts in surface advected plumes, and the occurrence of strong resuspension events near regions of plume liftoff, require the application of fully nonhydrostatic models. In addition, wave processes in strongly sheared flows associated with surface-advected plumes are not adequately described by either existing wave resolving models or by theory for wave-averaged flows in weakly sheared environments as presently used in standard coastal ocean models, thus requiring improved model formulations.This research will advance the understanding of sedimentation processes at the intersection of river and coastal environments. The model development effort will further the recent trend towards use of nonhydrostatic formulations in coastal ocean models by incorporating wave-resolving capabilities, leading to the ability to represent a full range of wave, current, stratification, turbulence and sediment processes in a common computational framework utilized at realistic scales. The resulting model will greatly enhance the community's ability to assess the effect of complex, irregular wave processes on bottom stress and sediment resuspension, leading to a better understanding of the pathways for sediment motion from river to shelf. The resources from the grant will partially support three doctoral graduate researchers, and undergraduate summer fellows through an existing REU program. The PIs will continue their efforts to promote science and research to a K-12 population through continued and expanded participation in University of Delaware programs such as Coast Day, Engineering Cool Stuff Camp and Young Engineers Camp.