Sediment transport circulation pattern through mesotidal channels system

SILVIA SUSANA GINSBERG; SALVADOR ALIOTTA

Sediment Transport

Intech

Año: 2011; p. 275 - 294

The estuaries are transitory environments acting as highly effective sediment tramps. Any modification to the hydrodynamic parameters (tide, wave and river discharge), or sediment supply, influence the estuarine evolution and modify the achieved equilibrium in the system. The sediment transport patterns determine, to a large scale, the morphology and the distribution of sediment in this setting. Therefore, predicting the material transport pathways is significant because it can have large impacts on seabed stability, on the dispersal of particulate material, and on benthic habitat distribution. The erosion/deposition processes that long term act in this tidal environment can be determined through residual sediment transport. The net sediment transport is mainly dominated by the degree of asymmetry in tidal current, as well as demonstrated by experimental studies carried out by several researchers. This asymmetry can export (ebb dominant) or import (flood dominant) sediment from/to the area, although may also cause deposition and erosion due to convergence and divergence, respectively of bedload flow Considering that the tidal current intensity can change through time and in space, the processes that accompany the transport of materials will vary along the different morphological features (tidal channels and intertidal areas). Therefore, understand and predict in advance the movement of sediment is an important factor because the estuarine zone is generally associated with a port complex and the sediment dynamics could be cause negative effect to navigation. Knowledge and predicting the sediment transport in estuaries is some times limited due its complexity and because it involves a wide range of time and space scales. In particular, the net sediment transport is able to show different patterns of circulation if the estuary configuration consists of interconnected tidal channels or one single channel with few or not small channels. In adittion, the lack of a large sediments source or negligible from rivers discharging into the tidal area cause that tidal channels are responsible for controlling the dispersal of sediment into the system. When the estuary is formed by a network of interconnected tidal channels, at the confluence zone of the channels there are major changes which affect the hydraulic geometry due to complex hydrodynamic conditions. Sediment circulation pattern at this channel system, causing that the net sediment can be mainly transported throughout one of these channels. Thus one of these channels behaves as the principal pathway of sediment transport in estuarine system. The tidal channels systems often consist by an extensive network of different types and dimensions of channels. Plan-form, this system channels shows a very irregular configuration and exhibits a dendritic and meandering structure with different sizes of channels (large channel, creek and gullies). The movement of the sediment as bed load through interconnected system channels contributes to present morphology with establishment of dunes, point bar, small elongated banks at different sections of the channels. These large and small morphology are formed by the bidirectional currents and most of the deposition takes place during the ebb tide and the shoals in general not exposure at low tide. The point bar within the meanders is with a sigmoidal cross section across the channel-axis. In this chapter, attention is focused on the hydrodynamics and morphodynamics of an interconnected system tidal channel and your confluence area and about control residual circulation sediment. For sediment transport evaluation, each area identified within tidal channels system are characterised by differents hydrodynamics conditions, morphological features and sedimentary facies. Various mesotidal deposits developed in different sections of the channels can infer the direction of transport of sediment as bed load.Hence, the influence of tidal flows on bedload transport patterns is highly controlled by channel geometry. Spatially varying sediment transport pattern is revealed on the basis of hydrodynamic and morphologic data collected during diferent studies. There are several methods for predicting the sediments circulation as bedload. Among the different options can use direct (sediment sampling, diving, etc.) and indirect (viedeo camera, side-scan sonar, seismic equipment, etc.) observations by recording the structures with the shape and orientation of sand ripples, scours, erosive channels, shoal, point bar, etc. From this point, a model is established on residual sediment circulation in tidal channels confluence to verify the effect of differences in water depth and bottom morphology. Studies carry out at mesotidal channels systems characterized by an intricate network of channels and with morphological sections different (channel interconnected, confluence zone, straight sections or parts of the channels, meandering parts) showed that the duration of asymmetry governs the trends of erosion in the channel system, and the velocity asymmetry is more important in controlling the net sediment transport out of the system. Hence, the time/velocity asymmetry is instrumental in determining the morphological evolution of this system channels due to sediment transport.