The world’s coasts and deltas are progressively threatened by climate change and human activities. The degree at which coastlines can adapt to these changes strongly depends on the sediment availability. The availability of muddy sediments is however poorly known. This study aims at developing a mud budget for the world’s largest system of uninterrupted tidal flats: the Wadden Sea. The resulting mud budget is nearly closed: ~ 12 million ton/year enters the system on its western end, ~ 1.5 million ton/year is added by local rivers, while ~ 12 million ton annually deposits or is extracted by anthropogenic activities. A mud deficit already exists in the downdrift areas, which will only become more pronounced with increased sea level rise rates. Mud is thus a finite resource similar to sand, and should be treated as such in sediment management strategies. Resolving future challenges will therefore require a cross-border perspective on sediment management.@en

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. @en

Many estuaries and tidal basins are strongly influenced by various human interventions (land reclamations, infrastructure development, channel deepening, dredging and disposal of sediments). Such interventions lead to a range of hydrodynamic and morphological responses (a changing channel depth, tidal amplitude and/or suspended sediment concentration). The response time of a system to interventions is determined by the processes driving this change, the size of the system, and the magnitude of the intervention. A quantitative understanding of the response time to an intervention therefore provides important insight into the processes driving the response. In this paper we develop and apply a methodology to estimate the response timescales of human interventions using available morphological and hydraulic data. Fitting an exponential decay function to data with sufficient temporal resolution yields an adaptation timescale (and equilibrium value) of the tidal range and deposited sediment volumes. The method has been applied in the Dutch Wadden Sea, where two large basins were reclaimed and where long-term and detailed bathymetric maps are available. Exponential fitting the morphological data revealed that closure of a very large part of a tidal basin in the Wadden Sea initially led to internal redistribution and import of coarse and fine sediments, and was followed by a phase of extensive redistribution while only fine-grained sediments are imported. Closure of a smaller part of a smaller basin led to shorter response timescales, and these response timescales are also more sensitive to rising mean sea levels or high waters. The method has also been applied to tidal water level observations in the Scheldt and Ems estuaries. Exponential fits to tidal data reveal that adaptation timescales are shortest at the landward limit of dredging. The adaptation time increases in the landward direction because of retrogressive erosion (Scheldt) or lowering of the hydraulic roughness (Ems). The seaward increase in adaptation time is related to the seaward widening of both systems.@en

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.

@en

The sediment composition of the seabed governs its mobility, hence determining sediment transport and morphological evolution of estuaries and tidal basins. Bed sediments often consist of mixtures of sand and mud, with spatial gradients in the sand/mud content. This study aims at increasing the understanding of processes driving the sediment composition in tidal basins, focusing on depositional processes. We show that bed sediments in the Wadden Sea tend to be either mud-dominated or sand-dominated, resulting in a bimodal distribution of the mud content where the two modes represent equilibrium conditions. The equilibria depend primarily on the sediment deposition fluxes, with bimodality originating from the dependence of suspended sand/mud concentrations on the local bed composition. Our analysis shows that bimodality is a phenomenon that is not only specific for the Wadden Sea; it can be expected for a wide range of suspended sediment concentrations and thus also in other systems worldwide.

@en

Human interventions and climate change can heavily influence the large-scale morphological development of tidal basins. This has implications on sediment management strategies, as well as ecological and recreational purposes. Examples of heavily impacted tidal basins are those in the Western Dutch Wadden Sea. The closure of a large sub-basin in 1932 triggered a shift in the sediment budgets of the remaining basins, leading to sediment infilling that is still ongoing. This paper presents a quantitative analysis of the post-closure sediment volumes, differentiating between sand and mud. Analysis of historical sediment composition data combined with bathymetry data revealed that the intervention caused a redistribution of sand and mud sedimentation. The responses of both sediment types differ spatially and temporally. The total infilling of the basins over the last century was substantially caused by mud (~32%, which is much larger than the average mud content in the bed). Initially, large mud volumes accreted in abandoned channels. At present, mud sedimentation along the mainland coast is still ongoing with nearly constant sedimentation rates over the past century, while the net import of sand significantly decreased over time and has been fluctuating around 0 over the last two decades. This research shows the importance of distinguishing between the response of sandy and muddy sediments when analysing the morphodynamic impact of an intervention, since they operate on different time and spatial scales. Sea level rise is currently a major threat for the existence of the Wadden Sea; its future fate will depend on whether the tidal flats are able to keep pace. Our results show that the supply of mud is sufficient to keep pace with the current sea level rise rates.

@en