Hydrodynamics and Sediment Transport at the River-Ocean Interface: Analytical and Laboratory Investigations

Abstract

This study presents a combination of numerical and analytical investigations of fluid and sediment transport mechanics through the river-ocean interface. This region is defined broadly as the region within the river that is influenced by the receiving basin--both the salinity and the oscillating and mean basin heights--as well as the portion of the river influenced by the presence of a distinct fluvial buoyant water mass. More narrowly, we consider in this study an atidal salt wedge, the upstream hydraulic transition zone in the unstratified river, and the near-field river plume. The first main chapter presents a hydraulic model of the salt wedge estuary in sloped and landward-converging channels. It is found that the non-dimensionalized intrusion length is a function of the freshwater Froude number, as noted in previous studies, as well as new parameters describing the channel geometry. Further, it is found that the primary geometric influence on the intrusion length is the channel bottom slope. Comparison to field data is given indicating that the influence of nonzero bottom slope may account for the discrepancy between observation and the canonical flat estuary theory \citep{Schijf+Schonfeld1953}. Next, we link our hydraulic model of the salt wedge to a hydraulic model of the upstream river transition zone, which is influenced by the depth of the receiving basin and is not in normal flow. We add to this a parameterization of total sediment transport in the unstratified river \citep{Engelund+Hansen1967} and a newly developed hydraulic model of sediment transport in the salt wedge. The model retains the key mechanistic features of sediment transport in highly stratified estuaries and is ideal for morphodynamic applications. We find that the principle influence of the salt wedge is an increase in net deposition in the lower river and the introduction of a secondary maximum aggradation length scale in addition to the backwater length discussed in \citet{Chatanantavet_et_al2012}. Finally, we present experimental simulations of the steady state estuary and river plume. The results of the estuary experiments quantify the influence of bottom slope on the reduction of sensitivity of intrusion length to river discharge and confirm the results of the hydraulic model. The plume experiments indicate that the plume transitions to a jet-like outflow for sufficiently large values of $F_f$ in which the spreading rate is determined by lateral entrainment instead of the plume buoyancy and the liftoff is pushed far offshore. This transition is not gradual but rather step-like, being concentrated on one value (or a narrow band of values of) $F_f$. Both this critical value of $F_f$ and the jet spreading rate depend crucially on the plume inflow aspect ratio. This jet-like behavior is anticipated to have crucial implications for delta progradation processes and the magnitude of sediment erosion in the lower river during flood events.