Estuaries are located where freshwater river flow meets ocean saltwater in an environment of channels, shoals, intertidal flats and vegetated area, like salt marshes or mangrove belts. They comprise both unique ecosystems and economic values such as navigational gateways to ports. Estuaries face significant challenges from morphological changes caused by natural and human activities such as land reclamation and dredging. In addition, rising sea levels threaten tidal flats and biodiversity. This research investigates centennial timescale morphodynamic developments in the Western Scheldt by means of a process-based model (FINEL2d). The model demonstrates good skill in hindcasting morphodynamic changes uniquely observed between 1860 and 1970. Analysis of sub-surface sediment composition and bathymetric data reveals a trend of historic sand export and mud import with the estuary narrowing and deepening over time supported by model results. Model simulations further suggest that, on a centennial timescale, morphodynamic developments evolve towards equilibrium.
This study shows that dredging and historic land reclamation have significantly altered morphodynamic evolution of the Western Scheldt estuary. It demonstrates both the significant impact of human interferences and the potential for designing sustainable management strategies to preserve a combination of vital ecosystems and economic values in the face of ongoing climate change.

Estuaries are dynamic environments where riverine fresh water meets tidally forced marine saline water. They encompass unique ecosystems and provide natural buffers that protect economic livelihoods. Many estuaries face continuous pressure from urbanization, altered river flows and sediment supply, subsidence, and sea level rise (SLR).

Estuarine beds are dynamic, influenced by marine (waves, tides, salinity, sea level) and fluvial (discharges, sediment load) forces. Morphological development is key to sustainable estuarine ecosystems and human activities. SLR will trigger a long-term (century-and-beyond scale) morphodynamic adaptation that will significantly impact the future state of estuarine systems. There is an urgent need for an enhanced understanding of relevant morphodynamic processes and the development of skillful forecast tools to assess SLR impact.

This research aims to assess SLR impact on the long-term morphological development of the estuarine environment, focusing on intertidal area by applying a process-based, numerical model (Delft3D) to predict morphodynamic behaviour. Fundamental studies on idealized estuaries are performed to determine governing processes and model parameter settings. In particular, these include wind wave dynamics, sediment properties, and grid resolution. Further, the modeling approach is applied in two case studies, namely San Pablo Bay (USA) and the Western Scheldt Estuary (Netherlands). Hindcasts are validated against unique datasets covering a period from decades to 1.5 centuries, while forecasts cover a century timescale under various SLR scenarios.

Model results show that morphodynamic adaptation lags behind SLR. Despite accretion under SLR, intertidal areas decay. The adaptation time lag and intertidal area loss mainly depend on SLR rate and magnitude along with sediment supply and sediment properties. Extreme SLR scenarios (1.67 m and 3.02 m over a century) can result in a 91% and 54% loss of intertidal area for San Pablo Bay and the Western Scheldt, respectively.

The results of this study advance understanding of the SLR impact on the morphological evolution of estuaries. It also shows that, despite their complexity, process-based models are reliable and valuable tools for performing morphological forecasts. Delft3D-type models can be used in future studies to investigate potential adaptation measures or to determine and quantify parameters for more aggregated models.

Estuarine intertidal flats comprise valuable ecosystems and act as an important sediment source for the adjacent salt marsh systems. However, accelerating sea-level rise threatens the mudflats and associated ecosystems, where the mudflat accretion lag behind sea level rise. A reliable forecast on the morphological developments of the mudflat under sea-level rise scenarios is of vital importance to assess sea level rise impact on the estuarine system.

Different tools exist that can predict the long-term evolution of the mudflats, viz. Delft3D, ASMITA and the hybrid model (Delft3D-ASMITA). Since the hybrid model is newly developed, the comparison among the various approaches has not yet been available. However, it is significant to know if they can produce the same results.

The research aims to compare the three modeling approaches (Delft3D, ASMITA, and the hybrid model) based on a case study in South San Francisco Bay. This comparison will reveal the strengths and weaknesses of the three approaches as well as indications where the approaches may strengthen each other.

The research is conducted in three main phases. Phase 1 consists of the sensitivity analyses in Delft3D for the case of South San Francisco Bay; Phase 2 contains the calibration of one-element and multi-element ASMITA models to reproduce the Delft3D model; Phase 3 focuses on the potential to improve the simulation efficiency of Delft3D in the hybrid model.

Model result comparison shows that, after the calibration, the ASMITA and hybrid model can efficiently simulate the same cases as in Delft3D. However, the upper part (landwards end) of the mudflat is more sensitive to the water level changes in the hybrid model due to the different sediment transport computation modules. The power indicating the relation between the equilibrium and actual morphology as well as the reference level is the important calibration coefficient to adjust the steepness of a mudflat. It can be concluded that the Delft3D model is used as the foundation to calibrate ASMITA and the hybrid model, both of which can improve the simulation efficiency with simplifications, especially in the long-term morphological development.

The study provides clear insights into the comparisons among different modeling approaches in the case of the long-term morphological developments of mudflats by the impacts of sea-level rise. It is recommended to do further research on the configuration of a 2D model and the conduction of the combination of different modeling approaches in other similar cases to confirm the validation.

San Francisco Bay is the largest estuary on the west coast, and it supports a surrounding population of over 7 million people. Human developments have reduced the amount of fine sediments delivered to the Bay-Delta system in recent years, which threatens habitat restoration initiatives and suitable water quality in the southern reach of the Bay (South Bay), where an ample supply of fine sediments is needed to support tidal flat development and to maintain conditions that limit the excessive growth of algae populations. A better understanding of how sediments enter and are redistributed within South Bay is needed to anticipate how the dynamics and supply of fine sediments will change under future conditions.  

In this study, a pair of process-based numerical models was applied to investigate fine sediment pathways in San Francisco Bay. An offline coupling between a DELWAQ water quality model and a Delft3D FM hydrodynamics model provides an efficient method to study suspended sediment transport in large, complex systems.   The DELWAQ model was applied to various hydrodynamic scenarios to better understand the effects of wind, density-driven flows, and freshwater discharge on sediment dynamics in San Francisco Bay. The results suggest that sediment import and export at the mouth of South Bay is linked to density-driven circulation induced by horizontal salinity gradients between South Bay and Central Bay. During years with particularly high volumes of freshwater discharge from the Sacramento and San Joaquin Rivers, South Bay is more likely to import sediments, and extreme pulses of freshwater were shown to enhance sediment connectivity between North Bay and South Bay. Sediment exchange between the channel and the flats in South Bay is facilitated by the tide, and wind wave resuspension influences the magnitude of sediment fluxes into and within the southern reach.  

This study advances our understanding of sediment dynamics in the complex San Francisco Bay estuary. With further validation, the DELWAQ model developed in this research can be used as a tool to predict system response to future conditions brought on by climate change and human-induced alterations to the watershed. Large-scale management decisions regarding the Bay Area and the Central Valley could alter the long-term trajectory of the estuary and determine whether it will maintain ecological viability while adapting to changing hydrodynamic and environmental conditions. Insights learned from sediment transport modelling can adequately capacitate the Bay's leadership as well as countless population-dense estuaries around the world confronting analogous challenges.

This study aims for a better understanding of the sediment pathways in San Francisco South Bay (South Bay). Many issues relevant to the Bay Area community such as shipping, recreational and commercial fishing, habitat restoration, human health, and environmental water quality are reliant on understanding sediment pathways (McKee et al. [2006]). Existing theories suggest that the Sacramento-San Joaquin Delta deliver sediment to South Bay during periods of high river flows. Different hydrodynamic forces, such as the tide, wind and baroclinic flows, redistribute these sediments around South Bay. In this research, trends are analysed using a new set of data (WY2015-WY2017) and sediment pathways in time and space are investigated using a 3D numerical model. In this model, the pathways of three sediment classes with different settling velocities are traced from their source throughout the Bay Area. No such study has yet been undertaken in the South Bay Area. There is a high Delta sediment input to South Bay during periods of high river flow. The local tributaries contribute only marginally to the suspended sediment concentration of South Bay. With a decreasing sediment load from the Delta, the importance of the local tributaries as a sediment source for South Bay could increase. In this research, three different types of sediment exchange between Central Bay and South Bay are observed. The first type is observed during periods of low river flows (Q < 800m3/s). A seaward directed residual flow is found in the channel at Bay Bridge. Sediments are slowly transported out of South Bay. The second type is observed after a period of moderate river flow (Q > 800m3/s). A pulsed sediment flux from the Delta increases the turbidity of Central Bay, resulting in a horizontal spatial variation in SSC from Central Bay to South Bay. The diffusive character of the tide transports the sediments slowly from the turbid Central Bay to the relatively clearer South Bay. The third sediment exchange type is observed after an extreme Delta river flow (QDel ta È 5000m3/s)). The extreme Delta flow refreshes a large part of Central Bay and South Bay. The salinity of Central Bay increases slowly, resulting in a substantially more saline Central Bay than South Bay for a couple of months. The resulting baroclinic flow transports the sediments from Central Bay into South Bay through the landward directed bottom current. Two dominant pathways with opposite transport direction characterise the sediment transport in South Bay. One pathway is located in the channel and is directed southward during periods of high river runoff. The second pathway is located on the extensive east flat and is directed northward during periods of high river runoff. The transport in the channel is dominant during the wet period, resulting in a net transport landward. Besides these two dominant pathways, four re-circulations cells are found, facilitating the exchange between the channel and the shoals. The next step in gaining more insight into the sediment physics is a model studywhich includes bed-interaction with the suspended sediments. A study like this could confirmthe hypotheses opted in this research.