Sediment Transport and Morphodynamics at an Estuary Mouth: a Study using Coupled Remote Sensing and Numerical Modelling

Abstract

The balance of the physical processes that drive the morphodynamics of a complex inlet system is investigated in this work. For this purpose, an innovative technique using coupled video imaging and numerical modelling has been used to study the relative importance of the driving forces that control the sandbar dynamics at the Teignmouth inlet system. The sandbars that form the ebb tidal delta are highly dynamic, leading to a cyclic morphological behaviour. Application of the numerical model (MIKE21 HD, NSW, ST) served two separate functions. The hydrodynamic model has been used for the image processing and, combined with the sediment transport module, the full model has been used to understand the relative importance of the driving forces at the region. The iterative application of the hydrodynamic model and the video images, with the modelled water levels used as input to the image processing, provides the video-based intertidal morphology that is used in further modelling experiments. This loop is repeated several times during the three-year study period that covered a complete morphological cycle. This results in a quantitative assessment of the relative influence of the key processes that control the environment and in initial steps towards the prediction of its evolution. In order to assess the relative importance of the driving forces a series of modelling experiments were designed to include a variety of forcing conditions. These include the tidal range, wave conditions and river discharge values. The relative importance of each of the physical processes on the sediment transport and consequent morphodynamics varies across the region. The main inlet channel is dominated by tidal action that directs the sediment transport as a consequence of the varying tidal flow asymmetry, resulting in net offshore transport. Sediment transport over the shoals and secondary channels at both sides of the main channel is dominated by wave related processes, displacing sediment onshore. The interaction between waves and tide generated currents controls the transport over the submerged sandbar that defines the channel's seaward extent. High river discharge events are also proven to be important in this region as they can change sediment transport patterns across the area. Waves play a major role in the sandbar morphodynamics. Despite the relative low frequency of high wave energy events that reach the region they are responsible for large amounts of sediment displacement, catalysing some dramatic morphological changes. Therefore, the temporal distribution of storms defines the cyclic behaviour of such environments, making the system more dynamically active over the winter months. It is also during this period that river discharge values reach high peaks, increasing the capacity of the ebbing tidal flows and interacting with the opposing waves. The opposite occurs during summer periods, when less energetic conditions lead to slower morphological changes. The application of an initial sedimentation/erosion model proved to be useful in giving qualitative predictions of the morphological evolution of such a complex sandbar system, reflecting the initial morphological changes for different forcing conditions. Qualitative comparisons between the modelled sedimentation/erosion patterns and the video based observations of the changes at the dynamic offshore sandbar show that the model is able to reproduce its overall evolutionary tendency. The morphological adjustment of the system to the forcing conditions shows the progression towards the next morphological stage, allowing the initial steps towards predicting the evolution to be taken. The technique applied, coupling the numerical model with the video images, has been shown to be of great value in providing a better understanding of the processes that control the dynamics of inlet systems. At short time-scales, quantitative information about the acting processes and how they interact has been gained by the modelling experiments, and at medium time scales, the combined application resulted in qualitative predictions of the evolution of most regions of the system.