A. Colina Alonso1*, D.S. van Maren1,2, Z.B. Wang1,2, P.M.J. Herman1,2
1 Delft University of Technology, 2 Deltares
Deltas are under pressure by climate change and increasing human activities, resulting in changes in abiotic and biotic factors. Especially when these pressures exceed certain thresholds, these changes may be abrupt or even irreversible (i.e., regime shifts). This research focuses on large-scale regime shifts in the morphology of the Dutch Wadden Sea. Its diverse morphological features were initially formed under a temperate climate and sea level rise, strongly influenced by human activities later on. The significance of anthropogenic interferences for the large-scale morphological development of the Wadden Sea compared to its natural sediment dynamics is still largely unknown.
Previous studies have determined short- and long-term effects on the sedimentation and erosion patterns after the closure of the Zuiderzee. However, little is known about which sediment fractions caused the changes in the sediment budget. In this research, we therefore investigate and link the morphological evolution of the Western Wadden Sea (WWS) in terms of bed level changes to changes in the distribution of sand and mud in the sediment.
Analysis of long-term field data reveals that the human interventions led to large sedimentation rates in parts of the WWS. Surprisingly, the distribution of sand and mud has not changed substantially. An exception is found in the channels in front of the Afsluitdijk: they used to be predominantly sandy, but the closure triggered a rapid siltation leading to a large accumulation of mud (see Figure 1). Mud-dominated areas tend to coincide with areas where net deposition rates are large. A first estimate of the mud contribution to overall sedimentation suggests that 24% of the total sediment deposition volume since the closure consists of mud. Besides, it is striking that the ratio of the gross mud deposition volume to the gross mud erosion volume is consistently larger than the same ratio applied to the sand fraction.
Burning questions for further research
Former sediment budget studies have suggested a balance between the large-scale sedimentation of the Wadden Sea basins and erosion of sand along the Dutch coast. Our results, indicating a significant contribution of mud to the sedimentation, reject this and seem to reveal a lack of balance. Establishing a realistic sediment balance for both sand and mud in the Wadden Sea area requires a better understanding of the sand-mud patterns and their transport pathways. We plan to further research these patterns and investigate the processes that determine the mud content by combining analysis of field data with idealized models and detailed process-based numerical simulations.
Figure 1: Evolution of the sediment types in the Western Wadden Sea. The colours indicate the sand fraction.
Funded by the Royal Netherlands Academy of Arts and Sciences (KNAW) within the framework of the Programme Strategic Scientific Alliances between China and the Netherlands.