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.

A low-lying country as The Netherland is prone to coastal flooding, and its risk may be enhanced by global-warming induced climate change. Sea level rise has been historically considered as the key factor in coastal retreat, but waves also play an important erosive role along the coast, which can also be affected by the changing climate. During the last years, important advances have been achieved in climate modeling, with a very detailed characterization of the different components of the climate system, for the present and for different future scenarios. However, characterization of future ocean waves is still a matter of discussion and ongoing research. In this thesis, a statistical downscaling methodology based on weather types has been chosen to model the present wave climate and explore potential changes in future waves. These changes are quantified in terms of the impact of these variations in the longshore sediment transport. The methodology is applied to Noordwijk, selected as a representative location of the central Dutch coast. The statistical downscaling methodology is based on a classification procedure of the predictor into similar atmospheric patterns over the wave generation areas, namely the weather types. Then, the wave data is grouped according to the occurrence of the weather types. The predictor is built from the sea level pressure fields (SLP) and the squared SLP gradients, while the predictant wave climate is characterized by significant wave height, wave peak period and mean wave direction of wind sea and swell components, resulting in unimodal or bimodal sea states. The chronology of the weather types is modeled using an autoregressive logistic model, which incorporates the seasonality, the interannual variability and the persistence observed from the historical data. For each weather type, wave parameters are modeled using the categorical distribution for the sea-state type, non-parametric kernel density functions for the central-mass regime and two generalized Pareto distributions for the lower and upper tails of wave height and wave period data. The statistical dependence between wave parameters for each sea state is included using a vine-copula approach, where the bivariate dependence of H_s and T_p is modeled using the AC skew t copula and the remaining relations are considered to be Gaussian. The effect of climate change is studied using SLP predictors from the global circulation model ACCESS1.0, under the RCP8.5 scenario and period 2070-2099. The statistical model is applied to identify changes in the occurrence probability of the weather types in the future. The importance of these changes are quantified in terms of the wave-induced longshore sediment transport using the process based model Unibest TC. The longshore sediment transport distribution for each weather type is computed and afterwards the changes in gross and net transports are estimated using the present and future probabilities of the weather types. In terms of the weather types, the results of this work suggest changes in the occurrence probability of the weather types in the future, with variations of ±60% with respect to present climate, and no relevant changes in the sequence of weather types, neither in terms of the transition probability matrix nor in terms of the persistence of each weather type. In terms of longshore sediment transport, significant changes are detected in the transport associated with some of the weather types. For the remaining weather types, the changes are in the order of the model uncertainty. Taking into account the contributions from all weather types, a net increase of the southward directed net longshore sediment transport with respect to the historical period is detected. This increase is driven by a decrease in northwards transport and a lower decrease in southwards transport. This result is in line with a poleward shift of the North Atlantic storm track reported in other studies. Most importantly, weather-type based climate classification has in this thesis been succesfully proven to be a reliable tool to analyze the wave climate at the location of interest. Furthermore, the statistical downscaling also provides a climate emulator that captures the climate dynamics at different time scales, which can be used for stochastic simulations of the atmospheric and wave climate, either for the recent past or future projections.

Predictions of coastal morphology evolution are necessary to assess engineering solutions as well as understand coastal systems behaviors. Among the tools used to predict morphological evolution are the process-based models that make use of physical laws and empirical knowledge. Such models account for a considerable range of coastal processes and are rather complex, hence demanding substantial computational time. Usually, when using complex process-based models, reducing the size of the input parameters, named input reduction, is made necessary in order to reduce the computational effort.
The scope of the present study is to understand the influence of reduced wave climate on simulated morphological evolution. Input reduction algorithms, sequencing methods, number of cases and duration of the reduced wave climate were investigated and evaluated with a 1D (cross-shore) brute force simulation of 3.3 years in Noordwijk, Netherlands. Noordwijk is a wave-dominated sandy beach characterized by a double sandbar system that has an inter-annual net offshore migration. The assessment of the methods was carried out through cumulative skill scores, temporal evolution of profile perturbations (bars and troughs) and profiles at the end of the simulation.
Furthermore, the findings on the Dutch coast were validated with a 1 year-long, 1D (cross-shore) brute force simulation in Anmok beach, South Korea. Anmok is a wave-dominated sandy beach with crescentic bars in the nearshore morphological evolution.
One of the conclusions of the present study is that input reduction methods that have more control on the definition of bins such as pre-definition of wave height and wave direction bins perform better than methods that are based fully on the statistical properties of the wave dataset. Also, the order of the wave cases in the reduced wave climate must resemble at its best the natural variability of the full wave climate. Finally, a less robust reduced wave climate (with less representative wave conditions) applied in a smaller timescale performs better in terms of morphology than a more robust reduced wave climate (with more representative wave conditions) applied in longer time-scales.

The St Lucia Estuary is Africa’s largest estuarine system and part of the iSimangaliso Wetland Park, which was listed as South Africa’s first World Heritage Site in December 1999. The St Lucia Estuary is connected upstream to Lake St Lucia and downstream it flows into the Indian Ocean. The St Lucia Estuary is located at a wave dominated coast and experiences a micro-tidal regime. Along the coast of South Africa about 70% of the estuaries do not have a permanently open link to the sea, i.e. they are temporarily open/closed estuaries, which are mainly regulated by the amount of river flow entering the estuary. Anthropogenic impacts and fresh water reductions have altered the natural system. The mouth of the St Lucia Estuary is unstable and has been closed for longer periods in the last century. The drought period ranging from 2002 has drastically affected the St Lucia System as a whole with the desiccation of 90% of the lakes, reaching the all-time low. The management authorities had to take action and research supported by the Global Environment Facility (GEF) which started in 2009 showed re-linkage of the Mfolozi River is the preferred solution to bring fresh water to the St Lucia System. This MSc study was initiated to investigate the effects of re-linkage of the Mfolozi River and dredge spoil removal from the estuary on the tidal inlet. The main focus is on the closure times of the inlet, the transition between an open – and closed inlet, and the multi-year morphological development of the inlet. The process-based model Delft3D is used to simulate various scenarios. The model is calibrated quantitatively on hydrodynamics and qualitatively regarding morphodynamics. The results from this study show that the re-linkage of the Mfolozi River leads to an inlet that remains open for majority of the time leading to a connection between the St Lucia Estuary and the Indian Ocean. This can make the St Lucia Estuary and Lake System a breeding ground for many marine species as it was before. The removal of dredge spoil however does not seem to be very effective. The closure times of the inlet barely change and the inlet re-imports most of the sediments. It is advised to limit the amount of dredge spoil removal, because the estuary will return to its equilibrium state.

Sea level rise poses challenges to the tidal inlet systems of the Wadden Sea, including the barrier islands. If no measures are taken, the shorelines of the islands will retreat and the basins may drown. In the Ameland Inlet, the Boschplaat at the eastern tip of Ameland and Bornrif at the western tip of Terschelling are currently shrinking.
This thesis evaluates different nourishment strategies to counteract the coastal retreat, with the ultimate goal of improving our understanding of how nourishment sand of different grain sizes is shared between the different morphological units of the Ameland Inlet.
The process-based model Delft3D is used as a means of predicting the transport pathways of nourishment sand on a time scale of 10 years. In this thesis an existing model of the Ameland Inlet is improved, in order to make it suitable for medium term nourishment calculations. The Mormerge (parallel online) approach is applied in combination with a phase shift in the tidal signal for the different wave conditions. As a result the computational time is significantly reduced. The bathymetry is updated based on the 2016 Vaklodingen and Bed Composition Generation is used to create a suitable bottom composition. The composition of the nourishments is based on the particle size distribution of the borrow area.
The model results suggest that different objectives, such as feeding the Terschelling or Ameland coast, feeding the basin, or reinforcing the ebb tidal delta, can be met by varying the nourishment location and composition.
The main findings regarding the transport pathways within the inlet system are as follows: Near the Terschelling beach import via Boschgat is dominant. Further offshore, bypassing via the ebb tidal delta is the most important transport pathway. Boschgat feeds the western part of the basin and Borndiep feeds the eastern part, but interaction between the two is limited. The model predicts some transport from Boschgat to Borndiep via Westgat under the influence of the tide, or driven by waves over the shallow area in between the channels. There is no connection in the other direction. Borndiep connects the basin to the ebb tidal delta, resulting in transport in and out of the system. Bornrif is the only location where a net westward transport into the inlet is observed.
Finer fractions are imported further into the basin by the tidal flow. When bypassing the inlet, they are transported further offshore, since they are more influenced by the tidal currents flowing out of the inlet. As a result coarser nourishment fractions end up closer to the Ameland shoreline.
The model has several limitations, mainly since insufficient data is currently available for a sound calibration and validation. However important phenomena observed in the field are reproduced by the model, such as the coastal retreat at Boschplaat and the bar migration at Bornrif. This gives confidence in the model performance regarding the predictions of the sediment transport patterns.