[1] The southern Brazilian shelf (SBS) is a region influenced by fresh water. The initial deposition of suspended sediments carried by this fresh water presents important ecological consequences for the area. The aim of this study was to investigate the contribution of the Patos Lagoon coastal plume to the deposition pattern observed along the inner continental shelf by providing estimates of estuarine-shelf suspended sediment exchange. The study was carried out through three-dimensional numeric modeling experiments on coupled hydrodynamic and morphodynamic processes. Results were analyzed using the dynamic method proposed by Garvine (1995) to study persistent plumes. The results were also analyzed using wavelet analysis. Results showed that the Patos Lagoon coastal plume behaves as a hypopycnal plume covering the first meters of the water column. The Patos Lagoon presents a dominant ebb condition with a mean discharge of 2088 m 3 s −1 and an exportation rate of suspended matter of approximately 1.3 × 10 7 t yr −1 . In the adjacent coastal region deposition prevails in sheltered regions up to 10 m deep, with enrichment in silt reaching up to 10% in the deceleration region of the Patos Lagoon coastal plume. The dynamic balance indicates a behavior typical of small to average-sized plumes, with a well-developed displacement along the coast that is mainly controlled by the alongshore wind component. The Coriolis force and bed shear stress present significant contributions during periods of moderate to high freshwater discharge when large-scale plumes are formed. Transversally to the coast the force balance is associated with the Coriolis effect and wind influence, with a less important contribution from bed shear stress. The inner continental shelf adjacent to the Patos Lagoon entrance is dominantly influenced by plume occurrence. In this region northeasterly winds induce the formation of southwestward currents near the coast. These currents deflect around jetties and generate recirculation zones to the south. The formation of a cyclonic eddy occurs in this area and creates convergence zones that intensify the vertical velocities promoting the deposition of suspended sediments.
A numerical model was developed in order to estimate the residence time of a sediment plume generated by potential deep‐sea mining activities, with special attention to discharges in the bottom boundary layer. The site of the Disturbance and Recolonization Experiment (DISCOL) in the Peru Basin in the southeast Pacific Ocean was chosen as a case study. The model includes the actual bathymetry, as well as the characteristic flow patterns of this region. Various aspects affecting the transport and sedimentation of the plume, such as stratification, flocculation in a sediment‐laden water column, and the hydrodynamics are discussed in conjunction with field data and studied with the overall aim of providing a reliable risk assessment of deep‐sea mining environmental impacts.
In this paper, a three-dimensional isopycnal approach is presented to simulate the dynamics of fluid mud covering the formation, development, transport, and disappearance of fluid mud. The basic assumption is the assignment of the fluid's density as the indicating parameter for the rheological behavior. Considering stable stratification, as is usually the case for fluid mud, layers of constant density discretize the vertical domain. The non-Newtonian dynamics of fluid mud is simulated by solving the Cauchy equations for general continuum dynamics. Instead of using a turbulent viscosity approach, the viscosity is allowed to vary according to the rheological behavior of mud suspensions. This apparent viscosity can be determined for different rheological formulations in dependence of the volume solid fraction and the shear rate. An existing threedimensional isopycnal hydrodynamic model was extended for vertical mass transport processes and was applied on a schematic system with hindered settling. For including the rheological behavior of fluid mud, the Worrall-Tuliani approach was parameterized and implemented. The resulting flow behavior is shown on a model application of fluid mud layers moving down an inclined plane. With these changes, it is demonstrated that the isopycnal model is capable of simulating fluid mud dynamics.
For the investigation of turbulence and particles, interaction measurement systems are required, which are able to measure velocity and concentration fluctuations simultaneously. Acoustic Doppler Velocimeters (ADV) are widely used for velocity and turbulence measurements in natural and artificial flows. Based on the acoustic sonar theory, a model is presented that correlates the ADV’s Signal-to-Noise Ratio (SNR) and the suspended solids concentration of several natural (Ems Estuary, Lake Eixendorf, Lake Altmühl) and artificial sediments (Chinafill, quartz powder, bentonite, metakaolin) for the range 0.001 g/L–50 g/L. Within the presented method, the sound absorption in water and on particles is considered in a continuous approach for sampling frequencies up to 100 Hz. The widely-used log-linear relation between the SNR and the concentration, which is only valid for low concentrations, was extended for the high concentration regime. Measurement results show a similar behavior of the SNR with respect to varying suspended solid concentrations for different sediments. However, the analysis of the fit parameters shows systematic differences depending on the type of sediment. It is concluded that the proposed model is applicable as well for laboratory use as for measurements in rivers and estuaries. Finally, we discuss the reliability of the results and the methodology with regard to measurements in rivers, lakes, and estuaries.
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