Sand-Mud Morphodynamics

The world’s coasts and deltas offer a multitude of valuable ecosystem services, providing safety against flooding and economic benefits. Many of these systems are, however, under pressure by climate change and increasing human activities. Protecting these systems and preservation of their multiple functions requires a thorough understanding of their morphodynamic behaviour. The sediment bed in many coastal systems worldwide is composed of two sediment types: sand and mud. While most previous research focused on the individual sediment dynamics of sand and mud, little is still known about how combined sand-mud morphodynamics differs from the sum of individual sediment fractions. In order to assess the impacts of anthropogenic interventions and climate change, we thus need to better understand sand-mud morphodynamics.

This research aims to improve the understanding of large-scale morphodynamics in sandmud tidal systems. This is done by investigating processes related to long-term deposition, sediment supply, sand-mud interaction, and segregation of sand and mud. We focus on generic idealized cases, as well as on case studies in the Wadden Sea — an example of a heavily-impacted system whose existence is threatened by sea level rise (SLR). Unique long-term data sets of its hydrodynamics, bathymetry and sediment composition are available, making this an excellent area to study the morphological responses to human interventions in detail, and to improve our understanding of sand-mud morphodynamics.

Analysis of the morphological evolution after a closure in the Western Dutch Wadden Sea (Chapter 2) illustrates the importance of distinguishing between the response of sandy and muddy sediments when analyzing the morphodynamic impact of an intervention. Our findings reveal that sand and mud respond on different temporal and spatial scales. Moreover, the results show that the contribution of mud to the total infilling was much larger than the average mud content in the top layer of the bed, because mud preferentially deposits in areas with high net sedimentation rates. This demonstrates that the contribution of sediment types to morphological change is not necessarily reflected by the spatial bed composition.

Up to now, the availability of mud to the Wadden Sea was poorly known, while we know that this availability is crucial for predicting the response to future climate change. Therefore, a first system-wide mud budget of the Wadden Sea has been developed (Chapter 3), revealing a nearly closed balance between the sources and the sinks. This observation implies that disturbing the mud balance at one location will impact downdrift areas. Anthropogenic sediment extraction provides the second largest sink, even surpassing salt marsh deposition. Field data suggest that a mud deficit already exists in some areas of the Wadden Sea, which will only become more pronounced with increased SLR rates. Mud is thus a finite resource similar to sand, and should be treated as such in sediment management strategies. Furthermore, local interventions may have consequences in downdrift areas, stressing the need for a cross-bordering perspective.

The influence of small-scale sand-mud interaction on large-scale modeled morphodynamic development has been studied by implementing two abiotic interactions (erosion interaction and roughness interaction) in a process-based model (Chapter 4). Model output was converted into metrics that describe the macro-scale configuration of the modeled systems, allowing a quantitative comparison of scenarios. The results demonstrate that sand-mud interaction can significantly impact tidal basin evolution, especially having a large influence on the intertidal flat shape, size and composition.

Lastly, we have seen that the mud content of the sediment bed in tidal systems is often bimodally distributed, indicating a preferential sand-mud segregation (Chapter 5). Bimodality represents the existence of two stable equilibrium conditions, which result from sediment deposition processes (and not erosion processes), and can be expected for a large range of suspended sediment concentrations in sand-mud systems. In order to correctly reproduce this bimodal character in process-based models, and therefore correctly modeling the bed sediment composition, one must account for erosion interaction in the model set-up — despite the role of deposition as a driving mechanism.

In conclusion, this dissertation illustrates the importance of a sand-mud perspective in morphodynamic studies, considering the contribution of both sediment types to the morphodynamic development as well as their interactions. We have seen that advancing our understanding of sand-mud morphodynamics requires combined data-based and modeling approaches, adopting a system-wide perspective, and considering the interactions between the various spatial and temporal scales. Morphological metrics, such as the ones that have been presented, are essential for the evaluation and comparison of model results and coastal morphology worldwide. Enabling successful and sustainable management of coasts and deltas will require further increasing our understanding of sand-mud morphodynamics through additional measurements and modeling studies. Developing a system understanding should be at the heart of all of these studies.