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The ebb-tidal delta of the Ameland Inlet system is of great ecological importance since it hosts a variety of benthic habitats. In a pilot project, a (mega)nourishment of 5 million cubic meters of sand was carried out on the outer ebb-tidal delta. Since the strong correlation between habitat preference and sediment characteristics on the ebb-tidal delta, it is crucial to gain insight in the mechanisms governing sediment sorting in this area. To study the initial response of grain size distribution patterns, and subsequently potential changes to benthic habitats, process-based models are useful tools. Here, we present a hydro- morphodynamic model of the Ameland Inlet which is able to simulate grain size sorting patterns. Building upon previous modelling efforts, we specifically focus on the inclusion of graded sediment transport and bed stratigraphy. The simulations, here with the nourishment excluded, reveal that the model results on grain size distribution can be related well to the forcing in combination with the local depth. Particularly, it turns out that tidal motion is responsible for the sorting pattern in the deeper parts of the ebb-tidal delta, whereas the grain size distribution in the shallower, offshore parts is particularly shaped by wind and wave processes. Model sensitivity to several numerical parameters (thickness transport layer, MORFAC) is also investigated and seems relatively small, although further investigation is needed here. A preliminary comparison reveals that the model results compare qualitatively well to field data, although the shallower offshore areas seem to be predicted too coarse in the model

Al eeuwenlang stimuleren we kweldergroei voor landaanwinning, onder andere met behulp van kwelderwerken. Tegenwoordig is vanuit het oogpunt kustveiligheid een toenemende interesse in het gebruik van kwelders, waardoor men opnieuw kijkt naar de potentie van deze kwelderwerken als hulp bij hoogwaterbescherming van de kust.

Estuaries connect our rivers and oceans, and support densely populated regions worldwide. These estuaries are increasingly threatened by salt intrusion due to climate change and side effects of human interventions, limiting freshwater availability. Nature-based solutions (NBSs), e.g. sand waves or wetlands, show potential to increase mixing of fresh-and salt water and have shown to be adaptable to a changing climate. Hence, they may provide a resilient way to reduce salt intrusion, while also supporting important ecosystem services. An efficient modelling framework will be developed, that is able to evaluate NBSs to counteract salt intrusion, estuarine wide, including CC. It will be applied to establish a knowledge base around the interactions between estuarine ecomorphology, related to NBSs, and salt intrusion. This way, the proposed research will improve understanding and provide modelling tools on the interaction between NBSs and estuarine wide salt intrusion.

Observations from the field show that the spatial distribution of benthic organisms is strongly correlated to the morphological structure of tidal sand waves. In particular the troughs of sand waves are typified by a large benthic community, in contrast to the crest. In this paper, we present an idealised process-based model to study these patterns of biota and sand waves. Our model results agree with the observations that these phase-related patterns can arise on the seabed. Moreover, we show that local topography disturbances may lead to spatial patterns of both sand waves and biomass.

Living near estuaries comes with flood risks from riverine and coastal sources, nevertheless the population density is high and still growing. The population and assets are generally protected by conventional coastal measures, which are challenged by increased storm intensity and sea level rise (SLR). However, those solutions are static and do not adapt to a changing climate. Foreshores, consisting of a tidal flat and salt marsh can serve as add-on to conventional coastal protection. Such a hybrid solution is more sustainable, in that the salt marsh part can cope, to a certain extent, with SLR. Previous studies proved the wave attenuating capacity of foreshores. However, all studies focused on the relative small scale only; i.e. some isolated foreshores in space and at most up to several years in time. The key question addressed in this study is: to what extent can foreshores safely act as additional coastal protection over a decadal time scale? A unique bathymetrical dataset was analyzed, covering the whole of the Westerschelde estuary (The Netherlands) over a period of 65 years. A total of 36 transects (aligned with the dominant wind direction) were constructed over a selection of six foreshores. The wave attenuating capacity of the foreshores was assessed under daily conditions both with and without vegetation and under extreme conditions (i.e. events statistically occurring once every 10.000 years), using the numerical model SWAN (Simulating WAves Nearshore). By assessing the wave attenuating capacity over the evolving bathymetry over a period of 65 years, it was found that foreshores always contribute to wave attenuation. Under extreme conditions, foreshores sheltered from the prevailing wind direction were more efficient in wave attenuation (i.e. decrease of wave height per meter foreshore), than foreshores located at the exposed shores. Moreover, at all foreshores, the bare tidal flat caused a baseline wave attenuation, while a linear relation was observed between the width of the salt marsh and the wave attenuation; The longer the vegetated marsh, the larger the wave attenuation. Nevertheless, the relation was different per foreshore. The relation found, provides the knowledge needed to calculate the minimum width of the salt marsh to provide the desired wave attenuation under extreme conditions.

Understanding the abiotic factors governing salt marsh dynamics is crucial to incorporate such ecosystems in holistic coastal risk management (Bouma et al, 2014). Process-based modelling of these ecosystems can help to gain insight into their response to changing environmental conditions. Therefore, a new process-based approach was applied to model establishment and survival of salt marsh vegetation, bridging the gap between geophysical and ecological models. A salt marsh ecosystem representative for the Western Scheldt Estuary, the Netherlands, which is characterized by semi-diurnal tides, locally generated waves and transport of fine sediments, was simulated starting from a bare mudflat. Salt marsh development is simulated in a Python script based on two concepts: Windows of Opportunity, accounting for seedling establishment (Poppema et al, 2019), and Population Dynamics, describing the growth and decay of established salt marshes (Temmerman et al, 2007). This external script is coupled to a morphodynamic Delft3D Flexible Mesh model through Basic Model Interface (BMI). The model results show that the establishment of pioneer vegetation on bare tidal flats governs the of tidal channels. The rate of seedling establishment affects the geometry of tidal channels, leading to narrow and deep channels aligned with the direction of tidal flow for fast colonizers (e.g. Salicornia europaea; glasswort) and wider, meandering channels for slow colonizers (e.g. Spartina anglica; cordgrass). Moreover, the response of the salt marsh-tidal flat system to changing abiotic conditions (e.g. waves, sediment influx) is assessed, showing that although fast colonizing salt marsh species are less resistant to physical forcing, they can adjust more rapidly to changes in environmental conditions. Concluding, a new process-based approach for biogeomorphological modelling has helped to improve the understanding of salt marsh development under changing environmental conditions, bringing incorporation of these ecosystems into coastal management a step further.

To demonstrate that the approach works, the €30 million BwN innovation program (2008–2012) was initiated by two major Dutch dredging companies, Royal Boskalis Westminster and Van Oord. The program was carried out by the EcoShape consortium consisting of private companies, public authorities, universities (including the University of Twente) and applied research institutes like Deltares.

There is growing recognition of the importance of feedbacks between organisms and physical forces in landscape formation; a field labeled biogeomorphology. Biogeomorphological processes typically involve so-called ecosystem engineering species, which are organisms that modify the abiotic environment via their activity or physical structures and thereby create, modify and maintain habitats. Biogeomorphological processes are known to shape a broad range of landscapes. However, in the underwater landscape these interactions have received little attention, despite the high abundance of ecosystem engineering species in the bed of coastal seas. This thesis aims at understanding the interaction between ecosystem engineering species, hydrodynamics and sediment dynamics in the formation of the underwater landscape. The most dynamic large-scale seabed patterns are tidal sand waves, with wavelengths of several hundreds of meters, heights of several meters and migration rates up to tens of meters per year. Within this dynamic landscape, ecosystem engineering species have a large impact on both the hydrodynamics and the sediment dynamics. In order to understand this interaction, we followed a model approach. Additionally, field observations and flume experiments are executed to obtain input parameters and validation data for the model studies.

The coastal zone is the most densely populated area globally and economic activity is high. 70% of the world’s megacities is located in this area, and the population is still growing. However, coastal protection is a prerequisite to work and live in this area, especially with predicted sea level rise and extreme storms becoming more frequent and severe. Salt marshes can contribute to coastal protection by having the ability to adapt to a changing environment, whereas conventional infrastructure like dikes and seawalls remain fixed over time. So by combining conventional coastal infrastructure and ecosystems in nature based flood defenses, coastal protection becomes more sustainable. The contribution of salt marshes to coastal protection by attenuating waves, depends on the size of the salt marsh and thereby the location of the salt marsh edge. The key question of this thesis is: What is the long-term (50 year) variability of the location of the salt marsh edge and what is the corresponding variability of the wave attenuating capacity of salt marshes? The location of the marsh edge is restricted by two interacting factors: inundation time and bed level change. For vegetation establishment to withstand longer inundation stress, more stable bed levels are required so that plants are not disturbed. Vice versa, to withstand more dynamic bed levels that disturb plant growth, lower inundation stress is required, so that plants grow fast enough to recover from the stress. Moreover, the seaward growth and landward retreat of the marsh edge is driven by the magnitude of daily mild wave conditions and sediment availability. Periods of small waves and/or large sediment availability enable the marsh to grow seaward and vice versa. Short-term high waves affect establishing juvenile salt marshes, resulting in a retreating marsh edge. All processes resulting in retreat and expansion of the salt marsh edge, affect the wave attenuating capacity. Generally, the larger the cross-shore width of the salt marsh, the larger the wave attenuating capacity. Nevertheless, both artificial marshes and existing salt marshes contribute to the wave attenuation under a range of hydrodynamic conditions.

Sand waves are rhythmic features on the bottom of shallow seas. Their shape drives the habitat selection of benthic organisms and contributes to the redistribution of sediments. In turn, benthic activity and the sorting of sediments affect the dimensions and dynamics of sand waves. These complex interactions among sand waves, benthic organisms and sediment sorting processes are investigated in this thesis.