Morphodynamic Modeling of Tidal Basins: The Role of Sand-Mud Interaction

The morphology of tide-dominated systems is progressively influenced by human activities and climate change. Quantitative approaches aiming at understanding or forecasting the effects of interventions and climate change are often aggregated, thereby simplifying or schematizing the investigated area. In this work, we advance on the knowledge of sediment transport processes shaping tidal systems and on methodologies translating schematized model output into physically realistic variables. In terms of improved physics, we systematically evaluate the influence of sand-mud interaction processes. Most tidal systems are shaped by a mixture of sand and mud. Morphological models typically compute transport of sand and mud independently, despite studies clearly demonstrating that their physical behavior is mutually dependent. We investigate the effects of two interaction mechanisms (erosion interaction and roughness interaction, applied with varying mud erodibility) with a schematized process-based morphodynamic model. We convert model output into metrics that describe the meso-scale configuration of the modeled systems, allowing a quantitative comparison of scenarios. Modeled patterns and intertidal flat shape, size and composition widely vary with mud erodibility settings, but equally depend on the evaluated sand-mud interaction mechanisms (with erosion interaction having a larger effect than roughness interaction). Sand-mud interaction thus needs to be accounted for from a physical point of view, but also to improve predictions of tidal basin evolution models, particularly the (bimodally distributed) sediment composition of intertidal flats. Human interventions and climate change are progressively influencing the state of coastal systems. Their impact on coastal evolution takes place on decades to centuries and the present-day modeling tools have limitations to study these impacts. Models with highly schematized configurations have often been used in the past to increase our understanding of coastal evolution. We advance on this, by determining the effect of sand-mud interaction on the modeled long-term evolution of tidal basins. The bed of most tidal basins is composed of two sediment types: sand and mud. From previous research, it is known that sand and mud do not behave independently. We investigate the effects of two interaction mechanisms (erosion and roughness interaction) using a model with a schematized configuration in terms of topology and forcing, but including the most relevant physical processes. To quantitatively compare the output of different model scenarios, we translate the results into a number of metrics. Our results reveal that sand-mud interaction processes (particularly erosion interaction) influence tidal basin evolution. Including sand-mud interaction can significantly improve modeling results, especially the sediment composition of intertidal flats. We present guidelines that can be followed to improve models depending on the real-life system they should compare to. Sand-mud interaction largely impacts long-term morphodynamic evolution of intertidal areas in tidal systemsAccounting for sand-mud interaction in morphodynamic models is crucial to reproduce the observed bimodal distribution of the mud contentSchematized model results should be parameterized for validation and comparison with real systems