Mudflats are coastal features present in numerous locations around the planet. Depending on the latitude and local conditions, mudflats can be vegetated by salt marshes or by mangroves and play an important role in coastal evolution. Vegetated mudflats can create a remarkable sea defense and their high ecosystem value has been proven by various studies worldwide. In this study, the response of a vegetated mudflat under extreme hydrodynamic forcing is analysed using, as case study, a specific part of the Guyana coast at South America. The sediment dynamics of the Guyana coast are dominated by Amazon River plume that shifts northwards and travels along the coast of South America till the Orinoco River delta in Venezuela. Mudflats are quite wide reaching tens of kilometres in width because of the high supply of sediment by Amazon and the formation of mudbanks along the coast travelling northwards and then westwards. The hydrodynamic environment is relatively mild as Guyana is located far from the track of tropical hurricanes; however swell events can occur caused by the North Atlantic cyclones. Despite of its mild hydrodynamic environment, there are often cases of overtopping of the seawall along Guyana coast every year with a severe and characteristic one been held on October 2005. This was the motive for the mangrove restoration project of the study area in 2011 and for the initiation of this study. The purpose of this study is to analyze the response of a vegetated mudflat under extreme swell and storm events and investigate the impact of vegetation on this response. Furthermore, the impact of the extreme hydrodynamic event on probable damage of vegetation is tested presenting the complete interaction between hydrodynamics and vegetation. The study area is the area of Chateau Margot, southern of Georgetown, where a mangrove restoration project took place in 2011 by planting Avicennia Germinans mangroves and field measurements were applied in the end of 2019 by the researcher Üwe Best. Using the 1-D process based model Mflat, based on an open source Matlab code, a 3.5km portion of the mudflat extending offshore is modelled. Vegetation dynamics and its influence on the flow is part of the model, so a full interaction between hydrodynamics, sediment dynamics, vegetation and morphological evolution can be simulated, establishing a complete bio-geomorphological model. Using field measurements and literature data, the model is calibrated for hydrodynamic conditions and for the vegetation parameters giving a profile in equilibrium. A sensitivity analysis is applied to understand the behaviour of the model and the interaction between various hydrodynamic and sediment parameters and morphodynamics. After calibration, two extreme cases are tested in the model: an extreme storm that could be expected in Guyana coast and an extreme swell event based on the real case occured in October 2005. The Mflat model, after calibration, is able to reproduce a stable profile with mangrove vegetation similar to the observations. Vegetation appears to be quite effective in reducing the wave energy and thereby protecting the coast behind it during the extreme events, while in long term results in accretion of the landward part of the mudflat. The impact of the extreme hydrodynamic conditions on vegetation is difficult to be quantified, but for the extreme cases corresponding to the hydrodynamics of Guyana coast, vegetation is expected to be resilient enough having minor damages that could be restored in a short time period. The very mild slope of the coast is a factor that prevents the bed level of being highly dynamic contributing to the stability of the profile. ... Read more

The Wadden Sea is the largest system of tidal flats and barrier islands in the world extending from the northern Dutch coast to the coast of Denmark. Such a large natural area is of great importance to bothhuman beings and ecosystems. Over 10,000 species of flora and fauna are found in the Wadden Sea being a perfect habitat due to the relatively calm environment and high food availability. However, accelerating sea-level rise and human interventions, such as gas extraction, may induce an unwanted morphological change in theWadden Sea posing a threat to the natural habitats. This study investigates the morphological response of the Wadden Sea to SLR and subsidence induced by gas mining. A new hybridmodel whose aggregation level is between a process-based model (Delft3D) and a aggregated model (ASMITA) is applied in this thesis. The two-dimensional hybrid model applies depth integrated shallow water formulations and the advection diffusion equation for the transport of sediments like Delft3D, but calculates the exchange of sediment between bed and water column by means of an equilibrium bathymetry concept under scenarios disturbing these equilibrium conditions. The Ameland inlet is chosen as the research area because it is a relatively autonomous and undisturbed basin. The high robustness of the hybrid model makes it possible to apply a high morphological scale factor and a coarser grid compared to the process based model, which shortens the computation time by orders of magnitude. However, the equilibrium concept also fixes the shoal-channel structure and suppresses channel migrations. Three different scenarios of sea-level rise rate (4, 6, 8 mm/year) are applied over a simulation period of 100 years. The general morphological response simulated by the hybrid model shows that the channels and the ebb-tidal delta erode acting as the main source of sediment for accretion of the intertidal flats. The erosion/sedimentation is more pronounced with a higher sea-level rise rate. Sensitivity analysis shows a significant influence of the sediment diameter on the channel erosion and sediment supply to the intertidal flats. Diffusivity plays an important role in the horizontal sediment exchange between the channel and the flat but only slightly influences the sediment import. Global equilibrium concentration and power n are similar to diffusivity in affecting morphological activity. The adaptation time scale is inversely proportional to both of these two parameters. The possibility of using more than one sediment fraction is proved and it can reproduce a more realistic sediment distribution as the observation. The hybrid model is also applied to simulate the morphological response to local subsidence and the restoration after subsidence stops. The center of the subsidence circle do sen’t lower as much as the subsidence rate, which indicates that sediment is transported to the center. Sediment is supplied to the area of subsidence by the adjacent main channel. The result proves the sediment transport principle underlying the hybrid model that sediment is always transported along the gradient of the sediment demand. ... Read more

Introduction
San Francisco Bay is one of the largest estuaries of the US Pacific Coast with an area of 4000 km2. It consists of two hydrologically distinctive sub-embayments North Bay and South Bay. South Bay is unique as it does not experience the freshwater flushing typical for estuaries. It experiences the largest freshwater input during a regime of reverse estuarine circulation during Winter, when the entire Bay becomes fresher following peak discharges from the northern rivers. Hence both saline and freshwater enter from Central Bay through the same entrance. South Bay is dealing with increased risk of inundation due to the combination of sea level rise and land subsidence and a deteriorating water quality. Understanding sediment pathways within South Bay and sediment exchange at its entrance can support the development of management strategies dealing with turbidity depended algae blooms and the development of salt marshes as a measure against sea level rise. Additionally, by understanding the current factors controlling fine sediment dynamics, future implications of climate change can be better predicted. Though San Francisco Bay is the topic of much research, a process-based model with a model domain covering the entire Bay is a novel approach to analyse fine sediment dynamics in South Bay. Methods
A Delft3D-DELWAQ buffer layer model was calibrated against a newly available combination of local high-frequency suspended sediment concentration (SSC) measurements and two-monthly, depth varying SSC measurements across the entire 145 km length of the Bay. Subsequently the method of Sediment Connectivity was applied to analyse sediment pathways and net sediment fluxes in South Bay. Sediment Connectivity is an approach that uses network analysis to quantify sediment fluxes based on a schematization of San Francisco Bay into 17 segments. Large data sets of spatial and temporal output were reduced to a 17x17 adjacency matrix permitting a more straightforward analysis and the application of different statistical metrics unavailable in more traditional approaches. Results and implications Calibration enabled the model to better capture seasonal and episodic variations in SSCs across the Bay. Connectivity analysis uniquely revealed key dominant pathways, which agree well with literature. Additionally, it unveiled intra-basin transport pathways, regions of erosion and sedimentation and an indication of the varying controlling forcings during the year. Finally, this study acts as a proof of concept for the Sediment Connectivity method that could be applied in other estuaries. ... Read more

Estuarine shoals are valuable areas with functions of nature, safety and navigation. It is of utmost importance to understand their underlying physical processes and long-term evolution to achieve protection. The first aim of this report is to determine the impact of wind-waves on long-term morphological development of estuarine shoals. The rapid sea level rise requires a valid model prediction of esturine shoal evolution in the future. Whether waves should be included for forecast needs to be evaluated. This rises the second aim to investigate wind-waves impact on estuarine shoals evolution under the sea level rise. A 2-D, process-based numerical model (Delft3D) is applied. This study sets up a large scale realistic model covering the whole Western Scheldt geometry and focuses on shoal Van Ossenisse by constructing high resolution grids and imposing wind-waves. Simulations are compared on a timescale of 50 years, with and without wave effects. Results revels that waves tend to slightly migrate shoals along their propagation direction. Waves erode sediment in the intertidal area, resulting in a lowering elevation of the shoal and high suspended sediment concentrations in water column. On the one hand, high SSC combined with wave asymmetry and wind-driven flow enhance sediment transport rate along wave propagation direction over top of the shoal, causing more sediment appearing in the lee side of the shoal. On the other hand, wave-induced suspended sediment follows tidal currents and transport to shoal edges where low bed shear stress exists. These cause shoal widening at lower intertidal area and upper subtidal area. Channel velocity hence increases with response to channel area reduction. This leads to erosion in the channel and channel deepening. In a longer timescale, waves impact do not lead to fundamental difference on estuarine autonomous behaviors. It reveals that waves impact may be sensitive to some parameters (eg. sediment grain size, tidal range, type of boundary conditions), especially in the inner channel. It is recommended to do more investigations. Sea level rise scenarios are carried out by imposing gradually sea level rise at the seaward boundary. Its timescale is 100 years. Both in wave and no wave case, sea level rise leads to elevation of shoal height and area lose in shoal edges, resulting in steeper slope in the intertidal area. Larger channel area and volume are presented with sea level rise. Waves maintain their function under the sea level rise. It lowers and widens the shoal, resulting in the increase of intertidal area and volume. Sea level rise does not change the tendency of waves impact. ... Read more