Hydrodynamic Influences on Fluid Mud Distribution in the Amazon Subaqueous Delta

Abstract

Transport and distribution of sediments in the Amazon Subaqueous Delta and the Amazon Shelf (ASD; AS) depend upon of the loads in the Amazon River Basin and on the hydrodynamics aspects. The latter, on the other hand, reacts to the distribution of sediment patches due to decreasing of the bottom stress parameter on finer sediments and fluid mud regions, mostly located on the inner shelf. The Amazon River discharge, tides and stratification are the dominant forcing for currents and related phenomena on the inner AS. In order to study the physical aspects related to sediment transport in the ASD, we applied the Estuarine and Coastal Ocean Model and Sediment Transport (ECOMSED). This model is capable of simulating both hydrodynamics and sediment dynamics processes in coastal regions. In this chapter, we present results from hydrodynamic modeling experiments, taking into account the complexity of the AS dynamics, including the river plume fate and shape, as well as the frontal zone positioning. The North Brazil Current and other oceanic processes have not been considered in this study, since their main influences occur in the outer shelf, far beyond the river mouth. The Amazon River Estuary (ARE) does not fit into a classical definition of an estuary, once the mixing zone is not constrained by its margins, appearing in the open shelf. The haline front develops further ahead from the river mouth, preserving most of its characteristics, without being in an estuarine channel. The ASD consists of reworked sediment deposits located seward of the river mouth, on the inner continental shelf. Kineke et al. (1996) defined as fluid mud, the extensive regions of dense nearbed suspensions of sediments where concentrations are above 10 g L−1 . Thickly patches of fluid mud layers affect circulation by decreasing the bottom stress coefficient and enhancing tidal currents and the sea level oscillations. Model calibration considered a variable bottom stress distributed according to the ASD, accommodating the reworked sediments and fluid mud parameterizations. Values for these parameters ranged from 2.0 10−5, in fluid mud regions, to 3.2 10−3, in the reworked sediments background. The patches of fluid mud and reworked sediments define, on their vicinities, regions of strong bottom stress gradients capable of promoting residual vorticity and residual circulation. Residual flows in marine environments can be generated by wind stress variability, by horizontal density gradients, by barotropic effects due to remote processes or by nonlinear tide 12