Long-term morphodynamic evolution and energy dissipation in a coastal plain, tidal embayment

The morphodynamic system in alluvial, coastal plain estuaries is complex and characterized by various timescales and spatial scales. The current research aims to investigate the interaction between these different scales as well as the estuarine morphodynamic evolution. Use is made of a process-based, numerical model describing 2-D shallow water equations and a straightforward formulation of the sediment transport and the bed level update. This was done for an embayment with a length of 80 km on a timescale of 3200 years, with and without bank erosion effects. Special emphasis is put on analyzing the results in terms of energy dissipation. Model results show that the basins under consideration evolve toward a state of less morphodynamic activity, which is reflected by (among others) relatively stable morphologic patterns and decreasing deepening and widening of the basins. Closer analysis of the tidal wave shows standing wave behavior with resonant characteristics. Under these conditions, results suggest that the basins aim for a balance between the effect of storage and the effect of fluctuating water level on wave celerity with a negligible effect of friction. Evaluating the model results in terms of energy dissipation reflects the major processes and their timescales (pattern formation, widening, and deepening). On the longer term the basin-wide energy dissipation decreases at a decreasingly lower rate and becomes more uniformly distributed along the basin. Analysis by an entropy-based approach suggests that the forced geometry of the configurations prevents the basins from evolving toward a most probable state.Hydraulic EngineeringCivil Engineering and Geoscience

Modeling of multilayer cohesive bank erosion with a coupled bank stability and mobile-bed model

Streambank erosion can be an important form of channel change in unstable alluvial environments. It should be accounted for in geomorphic studies, river restoration, dam removal, and channel maintenance projects. Recently, one-dimensional and two-dimensional flows and mobile-bed numerical models have become useful tools for predicting morphological responses to stream modifications. Most, however, either ignore bank failure mechanisms or implement only simple ad hoc methods. In this study, a coupled model is developed that incorporates a process-based bank stability model within a recently developed two-dimensional mobile-bed model to predict bank retreat. A coupling procedure that emphasizes solution robustness as well as ease-of-use is developed and described. The coupled model is then verified and validated by applying it to multilayer cohesive bank retreat at a bend of Goodwin Creek, Mississippi. Comparisons are made between the predicted and measured data, as well as results of a previous modeling study. On one hand, the study demonstrates that the use of two-dimensional mobile-bed models leads to promising improvements over that of one-dimensional models. It therefore encourages the use of multidimensional models in bank erosion predictions. On the other hand, the study also identifies future research needs in order to improve numerical modeling of complex streams. The developed model is shown to be robust and easy to apply; it may be used as a practical tool to predict bank erosion caused by fluvial and geotechnical processes