Author: Raul Flores Audibert

Publisher:

Published: 2018

Total Pages: 166

ISBN-13:

River plumes are critical to the exchange of suspended particulate matter (SPM) between nearshore regions and shelf seas. These exchanges contribute fundamentally to the health and function of the coastal ecosystem, and have important implications for the overall sediment and biomass budgets. The physical processes that determine these exchanges depend on the dynamical characteristics of the river plume, which are largely affected by the freshwater input, water depth and the tidal flow, among many other factors. Here, we present field measurements and numerical modeling of hydrodynamics and sediment transport in order to investigate the dynamics of cross-shore exchange processes along a shallow river plume system. Measurements from moorings and bottom frames along the Rhine region of freshwater influence (ROFI) are used to evaluate the importance of baroclinic (stratified) and barotropic (unstratified) dynamics on the cross-shore transport of fine sediments, in a site located 10 km north of the river mouth. These measurements allowed for the evaluation of sediment transport dynamics over a wide range of stratification, wind, wave and tidal conditions. Both barotropic and baroclinic processes are found to be relevant for the cross-shore transport of fines at depth. The observations suggest that wind and wave-driven transport during storms tends to move fine sediment offshore, while calmer, more stratified conditions move it back onshore. Data collected during an intense 2-day storm are used to document the occurrence of a wave-supported gravity flow (WSGF) in a shallow inner shelf site along the Rhine ROFI. These observations are the first to document a WSGF on a predominantly sandy environment; previous observations had been restricted to muddy shelf deposits with very fine particle size distributions. The observed WSGF dynamics support previous studies regarding the use of a friction-buoyancy balance, and suggest that the same balance can be used on sandy seabeds. The offshore transport associated with the WSGF was much higher than other modes of transport, such as the suspended transport in the upper water column or the bedload, highlighting the importance of these events to cross-shelf transport and morphological evolution on inner-shelf regions. The occurrence of a WSGF under conditions unique from previous observations suggest that WSGF may occur in a much wider range of shelf locations than previously thought. In the Rhine, the tidal flow interacts with the cross-shore density gradients to generate tidal straining. The influence of tidal straining in the generation of a turbidity maximum zone (TMZ) along the Rhine ROFI is investigated using idealized numerical simulations. Tidal straining leads to cross-shore sediment convergence and the formation of a nearshore TMZ, that is detached from the coastline and confined to the near-bed region. Subtidal landward sediment fluxes are created by asymmetries in vertical mixing between the stratifying and de-stratifying phases of the tidal cycle. Model simulations show the development of a coastal TMZ for a wide range of horizontal density gradients, latitudes and settling velocity of bed sediments, suggesting that these phenomena are not limited to the Rhine ROFI. A parameter space for the occurrence of tidal straining and a coastal turbidity maximum is proposed in terms of a horizontal Richardson number and a Stokes number. Lastly, near-bed turbulence and bedform measurements are used to investigate the influence of bed roughness in the estimation of bed shear stress under the influence of large waves and currents. Direct measurements of the near-bed Reynolds stresses are compared to bed stress estimates obtained from a 1D bottom boundary layer model that accounts for wave-current interaction. Model-derived bed stresses compare well with measurements when field measurements of bedform dimensions are incorporated into the model calculations. The use of standard bedform predictors based on bulk wave properties results in a severe overestimation of the bed stresses at this particular site. We find that the combination of a time-dependent bedform evolution model and a 1D wave-current boundary layer model provides a simple approach that allows improved bed stress estimates in cases where bedform data is not available. Estimates of bed stresses and measured vertical turbulent sediment fluxes are then incorporated into a linear erosion formulation to obtain field-based estimates of the critical stress for erosion and resuspension constant.