Since 2011, large amounts of seaweed, sargassum, have increasingly impacted the Caribbean, with 2025 having another record influx. The resulting sargassum inundation events cause severe ecological degradation, public health concerns and economic losses for coastal communities that rely heavily on tourism, like the area of Quintana Roo in Mexico.
The existing nearshore barriers often fail to intercept most of the sargassum due to poor coordination, unsuitable anchoring, poor design or excessive sargassum accumulation in “sacrificial zones.” If sargassum is allowed to stay still for more than 36 hours, it decomposes, releasing toxic gases, heavy metals and arsenic. The sacrificial zones are a problem area that all current onshore solutions share. To address this issue, this study features the design of a floating nearshore system that transforms these sacrificial zones into “opportunity zones”. It enables efficient interception and guidance, concentration, and extraction of sargassum before it reaches the shore. To attain the goal of the design, a multidisciplinary approach was applied; combining environmental and stakeholder analyses,the system design of a barrier with a collection point, and hydrodynamic simulations in Delft3D to optimize barrier orientation and performance. Different design concepts were developed and evaluated based on technical, operational, and environmental criteria. The resulting concept of the system features a modular floating barrier that guides sargassum towards a designated collection point, the opportunity zone, for extraction. This barrier is supported
by a buoyancy-stable floater, flexible mesh skirt, innovative connections and mooring system. The hydrodynamic analysis confirmed that the orientation of the barrier compared to the barrier significantly affects the flow efficiency of sargassum along the barrier. Through the hydrodynamic analysis, an optimal barrier angle was found to maximize redirection and minimize the length of the barrier. The collection point proposes a solution to collect clean sargassum, whilst minimizing the impact on the environment. This study shows that integrating civil engineering, design thinking, and local knowledge can yield a sustainable near-shore solution. It has to be noted that there are aspects of the design that need further consideration and detailing. This study forms a foundation for scalable sargassum
management. ... Read more

Over 30% of the world's coastline consists of permafrost and large sections of these coasts are subject to erosion. In the Arctic, unlike with temperate low-latitude coastlines, thermal processes affect erosion mechanisms. There is a lack of long-term predictive capability of morphodynamics for Arctic coastlines, which results in a knowledge gap regarding the coastal processes inducing permafrost erosion and how these processes will change under the effects of climate change. Previously developed parametric morphodynamic permafrost models lack generalizability because they either do not include all erosion mechanisms or were calibrated for specific eroding coastlines. Comprehensive morphodynamic models that include permafrost dynamics are too computationally expensive to perform long-term analysis and are thus applied to storm time scales only.

This study implements a novel method that integrates thermodynamics, hydrodynamics, and morphodynamics to predict the morphodynamic evolution of a permafrost-affected coastline. We developed and validated a process-based numerical model for Barter Island (North Slope, Alaska). This model showed skill in predicting the ground temperature distribution and the erosion of a permafrost bluff. Sensitivity analyses indicated that the environmental drivers affected by climate change (i.e., air and sea temperatures, water level) are expected to accelerate the erosion of permafrost-affected coastlines under the effects of climate change, confirming the findings of previous work. Rising temperatures will compound with diminishing sea ice to widen the annual window during which erosion can occur, which will increase the number of storm events that lead to erosion. Lower bluffs composed of finer sands are especially vulnerable.

The low computation costs mean that the model can be used to predict coastal erosion for larger regions, potentially benefiting strategic coastal management and policy-making.
Additionally, the developed model will improve global climate models. It can facilitate the mapping of permafrost degradation and organic carbon release, with the release of organic carbon through permafrost erosion being one of the greatest unknown drivers of global warming. Though further calibration is required, the developed model can be used as a tool to research the quantitative effects of climate change on the erosion of Arctic coastlines and gain a deeper understanding of how climate change affects the processes that ultimately lead to the erosion of permafrost bluffs. ... Read more

Under tidal currents and wave action, sediment particles show complex transport patterns for coastal systems like tidal inlets. To model the sediment transport in such coastal systems, models have been derived that can predict that transport using Eulerian (grid-based) or Lagrangian (particle-tracking) methods. The sediment transport model SedTRAILS uses a Lagrangian method to predict the pathways that sediment particles will follow. As a process-based model SedTRAILS is limited in how fast it can compute results. Here we show a methodology to make a faster probabilistic metamodel from the process-based model SedTRAILS by creating an Auto-regressive Hidden Markov Model (ARHMM). Creating a probabilistic model of the sediment transport in a coastal system using Markov chains is
a novel approach in that it builds further on the connectivity approach of SedTRAILS, which is still quite a new development in the field of sediment transport at present. For different versions of the model, correspondence with physical properties of the system were found, that the development to estimate sediment pathways accurately look promising. The method of describing sediment transport pathways of a coastal system as an ARHMM is a form of probabilistic Machine Learning allowing for faster computation times by describing the patterns the system can make. This is in contrast to a process-based model that has to compute all of the underlying processes. The autoregressive component in this approach is expected to be essential in describing any type of dynamical system that consists of trackable pathways. Taking into account the position of a particle at the current time step highly limits the position a particle can have at the next time step in favor of accurately estimating sediment pathways. The modelling approach discussed can find uses in engineering applications where particle pathways are of importance like the dispersal of nourishments, but also other dispersion events like dispersed goods from when a vessel loses a container, as long as there is a process-based model available to create the necessary Lagrangian data. ... Read more

As a result of growth in the offshore energy infrastructure, the connection between the offshore environment and the mainland is increasing in importance. This leads to an increased use of the seabed for cables and pipelines as part of our energy infrastructure. Cables or pipelines are placed in trenches and covered by sediment, to be protected from any damage from activity near the sea bed. During execution, the trenches will refill with sediment prior to the placement of the cable or pipeline, which is known as siltation. The siltation rates are increasing when entering the breaker zone near the shoreline. The predictability of the siltation rates are a crucial step in efficient and safe realisation of new connections between the offshore energy infrastructure and the mainland.

Inclusion of nearshore processes is missing in existing quick-assessment siltation tools. Complex process-based models, like Delft3D or XBeach, are capable of predicting siltation volumes in the nearshore environment accurately. However, these models demand large computation capacities. This makes them unsuitable for probabilistic modelling, requiring large numbers of calculations. Probabilistic modelling however is a crucial step in identifying and quantifying uncertainties and related risks in the execution. This research presents a quick-assessment tool that includes nearshore processes. Ensuring low complexity makes quick-assessment tools suitable for probabilistic modelling of siltation predictions, reducing and quantifying uncertainties within nearshore trench siltation.

This research presents an approach to include wave transformation and wave-driven currents into an existing siltation prediction tool (SedPit). The resulting SedPit Nearshore tool allows fast predictions of siltation volumes in the nearshore zone. The performance of the SedPit Nearshore tool is assessed by comparing it to data from a field case, and comparing the accuracy to the accuracy of the existing SedPit tool. The SedPit Nearshore tool gives accurate predictions on the total siltation volume, and gives good insights in the spatial distribution of siltation volumes. The potential of the SedPit Nearshore becomes most evident when comparing it to the existing SedPit tool. A great improvement compared to the existing SedPit tool is seen. For the test case, the SedPit Nearshore tool reduces the absolute error in redicting the total siltation volumes by 82% compared to the existing SedPit tool. The SedPit
Nearshore tool predicts the total siltation volume with an error margin of 7%, while the existing SedPit has an error margin of 41%. The largest improvements compared to the existing Sedpit are seen in the most onshore regions, as this is the zone where most wave-driven currents are generated. The computational speed of the tool has proven its applicability for analyses on model sensitivity and uncertainty quantification. Computation times are reduced by factor 9,000 when comparing it to XBeach, a complex process-based model. Bottom roughness ks and wave roller steepness β were identified as most influential free variables in driving nearshore siltation volumes. Calibrating the model to obtain likely values for a range of free variables has helped to reduce the 95% confidence interval of peak siltation rates by 36%.

The inclusion of wave-driven currents into existing siltation prediction tools has shown a great improvement in the accuracy of siltation predictions in the nearshore zone. Although the SedPit Nearshore tool is calibrated on one specific field case, the method and the workflow of the tool show potential to help as general prediction tool of nearshore trench siltation. The power of the SedPit Nearshore tool lays in its simplicity, making it a fast, efficient, and accurate tool, suited for probabilistic modelling. ... Read more

Low-lying reef-lined coasts are vulnerable to coastal flooding. While fringing reefs usually protect coastal communities from moderate storms by dissipating incoming wave energy, they can exacerbate flooding during strong events with energetic waves. Low-frequency waves (0.001-0.04 Hz) play an important role in coastal hazards, dominating nearshore reef flat wave patterns. Furthermore, low-frequency resonance on reef flats has been suggested to exacerbate coastal hazards. Despite expanding knowledge of wave dynamics on coral reefs, the transient characteristics of low-frequency resonant oscillations on reef flats have not been extensively researched, and no methods currently exist for analyzing this phenomenon in field observations.

This thesis introduces a new methodology for assessing transient characteristics of resonant and energetic low-frequency oscillations using the Hilbert-Huang transform. The study is based on a 5-month dataset from a cross-shore transect of a reef at Roi-Namur in the Marshall Islands, where the reef geomorphology corresponds to a natural frequency in the very low-frequency range (VLF, 0.001-0.005 Hz).

The findings indicate that resonance in the dataset typically had a short persistence, with a median and 90th duration of approximately 5 and 10 minutes respectively. Normalised with the period of the oscillations, which have a typical frequency of 0.0035 Hz, these durations are equal to approximately 1 and 2 wave oscillations. While these results are consistent with previous notions about the potentially short durations of resonance for this particular reef and dataset, they contradict suggestions that resonance is associated with a build-up over several wave periods. Comparison of resonance durations with instances of coastal flooding at the site shows that resonance occurred for about 1 hour within a 24-hour recording period during overwash and large runup events, but also at other times. The duration of energetic VLF oscillations, in general, was more closely linked to the severity of coastal flooding events. While resonance did not directly correspond to coastal flooding, it was observed that energetic VLF oscillations persisted for longer durations under (close to) resonant conditions.

These results suggest the critical role of energetic VLF waves in coastal hazards, while resonance may be of secondary importance. However, it is important to acknowledge the limitations of this study, which focused on a single dataset from a specific reef geometry and included short-duration recordings (34 minutes). Moreover, this study demonstrates the application of the Hilbert-Huang transform for analysing sea surface elevation signals recorded at a coral reef.

The insights gained from this research underscore the importance of considering both energetic VLF oscillations and resonance in coastal hazard assessment. Future investigations should explore factors influencing the persistence of such oscillations, like the characteristics of incoming wave forcing, to enhance coastal hazard prediction. Furthermore, the Hilbert-Huang transform offers a versatile framework for exploring more aspects regarding the time-varying and non-linear behaviour of waves on coral reefs.

The methodology framework proposed in this study facilitates the bulk analysis of extensive field data and longer sea surface elevation recordings, enabling further exploration of reef morphology and hydrodynamic controls on the persistence of energetic and resonant VLF oscillations. ... Read more

In 2019, a 5 million m³ sand nourishment was placed at the Ameland ebb-tidal delta. This pilot nourishment aimed to investigate the feasibility and effectiveness of ebb-tidal delta nourishments and to understand the dynamics in this system. To assess how this nourishment has dispersed through the ebb-tidal delta system, SedTRAILS was used. SedTRAILS is a particle-based sediment transport model currently in development. A new process for interaction with the bed and burial of sediment was recently added to the model. The impact of this process on dispersal and sediment pathways needed to be analysed, as not much is known about the effects of its inclusion on transport and dispersal.

Several schematisations of wave- and tidal forcing were tested. The wave forcing selection had little effect on the overall model results. Storm conditions included in the more detailed schematisation were underrepresented in the model results. The implementation of wave-driven bed velocity had little influence on result as well due to small magnitudes that were calculated for this. A significant difference was found between using a morphological tide and an artificial spring-neap tidal cycle however, requiring further investigation.

Investigation of the burial formulation gave insight into the response of the model to changes in the different parameters. Direct dependence of erosion probability on deposition probability presents a major limitation in the current implementation of burial. Additionally, deposition probability in its current form is positively dependent on the maximum bed shear stress. Inclusion of burial does provide significant room for calibration of results.

Through modelling these various configurations of SedTRAILS, results consistently showed a recirculation of sediment from the nourishment on the ebb-tidal delta together with bypassing around the edge of the ebb-tidal delta, as seen in Figure 1. However, the degree to which bypassing occurs depends heavily on the chosen burial configuration. The bypassing is largely a result of flow around the periphery of the ebb-tidal delta and differs from known bar migration and shoal attachment pathways. Transport into the Wadden Sea basin was not found, likely due to the underestimation of influence of storm conditions.

To improve dispersal modelling with SedTRAILS, the impact of storm conditions needs to be more accurately included in the model. An under-representation of energetic wave conditions leads to a consistent underestimation of wave-related transport. Additionally, more investigation into the effects of the spring-neap tidal cycle is required. The significant difference presented here is an early indication that this is an important factor for SedTRAILS. Validation of dispersal modelled with SedTRAILS can be achieved through quantification of transport, assigning a volume of sediment per particle. ... Read more

Coastal lagoons, encompassing nearly 13% of the world's coastline, represent a vital ecosystem for a considerable variety of flora and fauna. The appealing environment of coastal lagoons has resulted in the establishment of numerous communities around them. The intensification of extreme events due to climate change, the expansion of urban systems, and the release of more pollutants into surface water have altered the natural dynamics of the system. These alterations increase the challenges regarding water safety and quality. Therefore, it is genuinely important to ensure the long-term survival of coastal lagoons. Coastal lagoons are shallow waterbodies parallel to the coast, separated by a small barrier and connected to the sea or ocean by one or multiple inlets. Their behaviour is influenced by the tides, freshwater discharges, wind, geometry, and bathymetry. In a well-functioning lagoon, the water is of sufficient quality, and the surrounding area lives harmoniously with the lagoons, meaning fishing activities are balanced with the lagoon, and the surroundings can cope with floods and droughts.

The Ciénaga de la Virgen in Cartagena, Colombia, is a prime example of a coastal lagoon that faces problems such as pollution, contamination, floods, and droughts, which are common in the area. The consortium ConAgua had proposed multiple measures to enhance the natural dynamics of the Ciénaga, commissioned by the Dutch governmental organisation Rijksdienst voor Ondernemend Nederland.

This report analysed the influence of those measures on the water safety and quality of the Ciénaga.
As part of this, the lagoon and its behaviour were simulated using the numerical hydrodynamic model Delft3D. The measures were implemented in this program, and by altering the boundary conditions, the effects of each measure under different circumstances were evaluated.
The implemented baseline model represents a dry month, normal tides and current bathymetry. Modifications were adapted to evaluate the effect of the limited data and observe the impact of extreme rainfall, sea level rise and wind forcing. Combinations of measures and their behaviour were also simulated.
For the water quality, the flushing time was considered by means of a tracer. Inserting a passive tracer, which either indicated polluted water (1) or clean water (0) into the model, made it possible to analyse the reduction and/or mixing of the pollution. For water safety, the rise in water level at any observation point within the Ciénaga was compared to the baseline model without measures, and the maximum water levels within the month were considered.

The simulations showed a significant impact of tides, wind, and sea level rise on the behaviour of the lagoon. Incorporating wind showed an improved refreshment rate due to the enhanced mixing and altered the plume formation into the Caribbean Sea. However, there are windless days on which the advantage of wind may not be taken into account.
Other simulations, such as the simulation of the combined measures, showed that combinations are not merely the sum of the individual measures and their effect, but the total hydraulic behaviour is altered.
Furthermore, the importance of a clean discharge from the urban and rural side into the Ciénaga was demonstrated. Due to the low refreshment rate, polluted water lingers inside the Ciénaga for a long time, affecting the ecology and the health of the surrounding residents.

In short, the rate of improvement of the water quality by the measures is limited, because the effects remain local. Therefore, achieving water of proper quality is a challenge. The measures did not negatively influence the water levels inside the Ciénaga. For flood risk, it is important to prioritise other effects since it remains a problem in the area.


The report shows that preserving healthy lagoons worldwide is important. To maintain proper water quality, the input of clean freshwater and sufficient mixing due to tidal (ensured by inlets) and/or wind forcing is important. Likewise, the effect of floods and droughts should be limited since many people live near the lagoon. However, water safety cannot be assessed solely by the water levels inside the lagoon. Models can help to assess challenging problems regarding water quality and safety in complex lagoon systems. In future steps, it is recommended that more data be gathered through measurements and observations to assess the quality of the lagoon with more certainty.
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Worldwide, estuaries are centres of life. Their connection with the open seas while providing fertile lands and freshwater has resulted in these regions becoming major hubs of human settlement and activities, which is expected to continue growing in magnitude. However economically useful, the open connection to the seas also poses a challenge: freshwater availability. Via this open connection saline water can enter the estuary and contaminate the freshwater reserves - so-called (estuarine) salt intrusion. This may cause freshwater shortages and damage to all living beings that depend on this invaluable resource.

This research aims to develop mitigation measures to this estuarine salt intrusion following the Building with Nature-philosophy. Thus, the goal of this research is to develop nature-based solutions to mitigate salt intrusion. This is achieved by (1) exploring the potential of estuary-scale interventions to affect salt intrusion; (2) conceptualising various nature-based solutions that mitigate salt intrusion; (3) evaluating nature-based solutions in a multidisciplinary context; and, to conclude, (4) reflecting on the role of nature-based solutions.

The exploration of potential estuary-scale modifications is a computationally expensive endeavour for which a novel simulation strategy is developed. This strategy is presented in Chapter 2 and proposes the use of machine learning techniques to determine the input space - i.e., which model simulations to execute, and which to exclude. The aim of the strategy is to put more focus on exploring the output space instead of exploring the input space.

Subsequently, the model simulations are analysed in Chapter 3. Thus, Chapters 2 and 3 present respectively the method and the results of an extensive sensitivity analysis of estuarine salt intrusion to estuary-scale modifications. The end-result of Chapter 3 includes a shortlist of potential nature-based solutions to mitigate salt intrusion, including a ranking based on the sensitivity analysis.

The conceptualisation of nature-based solutions focuses mainly on two potential options: (1) a (temporary) sill, or submerged dam (Ch. 4); and (2) enhancement of intertidal area (Ch. 5). In addition to these novel nature-based solutions, Chapter 6 evaluates a third nature-based solution: shallowing of an estuary. From a physical perspective, this mitigation measure is well-known but mainly poses challenges in the socio-economic domain, which is why it is not extensively covered in the conceptualisation-phase of this dissertation but the evaluation-phase instead.

Both the sill and the intertidal area show a dependency on estuary class in how effectively the mitigation measures are. For the sill it holds that the weaker the tide, the more effective the sill mitigates landward salt transport - i.e., salt intrusion. When the tide is limited, salt intrusion is largely (if not fully) driven by gravitational circulation, for which the sill functions as a wall beyond which the formed salt wedge can hardly penetrate. However, with tidal energy increasing, the tidal momentum to push the saline water over the sill also increases resulting in more salt intrusion - the sill functions more like a speed-bump than a wall. All in all, a sill is most effective for estuaries with little tidal influence.

Enhancement of intertidal area increases the mixing in the estuary. When the dominant salt transport mechanism is related to the estuarine circulation, this enhanced mixing by increasing the intertidal area reduces the salt intrusion. However, the opposite holds for estuaries in which the salt transport is dominated by the tidal oscillation. This means that in case of salt wedge and partially mixed estuaries, enhancement of the intertidal area reduces salt intrusion; and in case of well-mixed estuaries, the intertidal area increase promotes salt intrusion. Thus, the additional (vertical) mixing caused by the intertidal areas reduces salt intrusion as long as there is something to mix - i.e., as long as there is a (sufficient) vertical salinity gradient.

The evaluation of nature-based solutions is inherently multidisciplinary. In this dissertation,
two different perspectives are considered: (1) socio-economy (Ch. 6), and (2) socio-ecology (Ch. 7). In both cases, the Rhine-Meuse Delta is considered as case study.

The socio-economic evaluation addresses the effect of water depth on two major stakeholders in an estuary: a port, and water boards. These stakeholders have opposing interests regarding the water depth: a port benefits from enhanced water depth to facilitate larger vessels, but the resulting contamination of freshwater reserves via salt intrusion is negatively affecting water intakes, and everyone that depends on them. Chapter 6 presents a multidisciplinary evaluation method based on Pareto-fronts, which are to inform decision- and policy-makers. The Pareto-front in Chapter 6 shows that the port performance remains relatively unaffected for shallowing until two metres with the current bed levels, while the freshwater availability improves. However, beyond this level of shallowing, port performance drops substantially against limited gains in freshwater availability.

The socio-ecological evaluation focuses on the effects of reopening a closed-off estuary on two opposing interests as well: freshwater availability, and estuarine ecosystem functioning. Reopening is beneficial for the ecological diversity in the former estuary, but comes at the costs of returning saline influences into a freshwater lake - i.e., freshwater availability is hampered. The ecological implications are mapped by translating hydrodynamic (model) data to an ecotopes-potential map due to which the ecological impact of interventions can be quantified. As in Chapter 6, Pareto-fronts are used as method to inform decision- and policy-makers. The Pareto-front in Chapter 7 shows an increased ecological diversity in the former-estuary without impeding the freshwater availability when partially opening the gates. However, once the salt intrusion reaches the most western water intakes, the freshwater availability drops without major gains in diversity.

This dissertation has shown the complexity of developing future-proof nature-based solutions to mitigate salt intrusion. Building on the lessons learned, the reflection of this dissertation proposes a next step for nature-based solutions: DARE (diverse, adaptive, and robust engineering; Ch. 8). DARE is based on nature's own approach to dealing with uncertainties. The three axes of DARE show how to deal with uncertainties in the three dimensions of engineering: (1) a diverse set of solutions to deal with uncertainty in forcing conditions; (2) an adaptive approach with room to change course in response to the uncertainty that comes with time; and (3) robust - or even antifragile - solutions regarding the uncertainty in performance. With DARE, the focus shifts from the input to the output: Which part of the output is desirable, and what are the options to get there?

The nature-based solutions presented in this dissertation form a starting point for further explorations, with work in this dissertation already being used as a stepping stone for other studies. Besides further exploring the opportunities of nature-based solutions to mitigate salt intrusion, next steps also include bringing these findings into practice. This includes reassessing past estuarine modifications from a new perspective, one that is less susceptible to the challenges that the future may have in store.
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Threatened by rising sea levels and other climate change induced hazards, there is an increased need for coastal flood protection, such as storm surge barriers. However, the construction of such barriers may lead to unwanted coastal changes. This thesis examines the effect of the Eastern Scheldt storm surge barrier on sediment pathways and the implications for coastal erosion. In addition, possible sediment-based interventions to redirect the current with the aim of reducing coastal erosion were investigated. First, a literature study was performed to provide historical context on the natural and human induced development of the ebb tidal delta. A data analysis on the sediment budget of the ebb tidal delta was performed, which showed that from 1960 to 1987 the ebb tidal delta gained sediment, from 1987 to 2010 it lost sediment, and from 2010 to 2019 it gained sediment again. A numerical model in Delft3D FM was used to produce sediment transport vector fields over a morphologically representative tidal cycle, which were then visualized using SedTRAILS. The research found that the barrier decreased the strength of the ebb current on the entire ebb tidal delta. The Roompot Zuid has a predominantly ebb-dominant character before the barrier in 1976 but changes to a flood-dominant character due to the construction of the barrier, which weakened the ebb-current. In 2019, part of the Roompot Zuid regained an ebb-dominant character. The Schaar van Onrust also became flood-dominant due to the barrier, eliminating the sediment-retaining effect the ebb dominated current in this channel. This shift indicates that sediments that previously stayed in front of the coast of Noord-Beveland are now redistributed further along the coast in south western direction, which led to a sediment deficit and erosion at the coast. The tested interventions seem to induce little structural changes in the tidal currents. The removal of sediment causes the sediment transport patterns to converge into the dredged area, suggesting deposition. Therefore the tested interventions do not seem effective from a coastal management perspective. The approach used here may also be useful for the assessment of sediment transport impacts at other sites where the construction of similar barriers is being considered. ... Read more

Mangrove forests are important wetland ecosystems that inhabit intertidal zones within tropical and sub-tropical latitudes. They offer a diverse array of ecosystem services, including coastal protection and carbon sequestration. Despite their significance, global mangrove cover has declined in recent decades, prompting various restoration initiatives, most of which often fail due to a lack of system understanding of the local mangrove habitat and associated physical and biological processes. A method gaining prominence involves the use of permeable dams to restore the sediment balance along eroding coastlines, thereby facilitating the restoration of mangroves in the area. These dams help dissipate the approaching waves and reduce current magnitudes, thus creating a low energy environment behind them, ideal for sediment deposition. However, a comprehensive understanding of the system-wide implications of utilisation of permeable dams remains largely unexplored.

The study is focused on examining the response of a coastal system to structural presence, based on currents, waves and sediment and propagule pathways, to optimise restoration strategies. The coastline of Demak in Indonesia is chosen as the system to be modelled due to the persistent erosion problem in the region and the ongoing use of permeable and impermeable structures to mitigate the problem. A nested model was set-up in Delft3D-4 to estimate the flow hydrodynamics resulting from river discharge, tide and wind forcing in the area. Wave propagation in the region was also modelled using Delft3D-4 with a standalone nested wave model. Output from the hydrodynamic models served as input for the Lagrangian particle tracking model, SedTRAILS, to compute the sediment and propagule pathways.

The hydrodynamic models demonstrate a decrease in both current magnitude and wave activity behind the structures, with a greater reduction observed in the case of impermeable structures. However, the radius of influence is constrained to less than a kilometre from the structures. The sediment pathways reveal reduced sediment movement behind the structures. However, the sediment imported due to the structures are sourced from within the intertidal basin which could trigger coastline retreat in the region. Additional tests uncover impermeable structures with larger length to opening size ratio, placed closer to the mangrove fringe to be able to retain sediment with comparatively higher efficiency. In case of propagules, the trapping behaviour of the structures is less apparent, with dependencies on the relative location of the structure to the sources and the direction of wind. In Demak, when structures are located relatively offshore to the propagule sources, they are able to trap the propagules, thereby creating a possibility for mangrove restoration. The study concludes that coastal structures enhance sediment and propagule retention, with varying efficacy depending on the type, location and length of the structure, suggesting their potential utility as a valuable tool for mangrove restoration. ... Read more

Van Sickle Island, located in the Suisun Marsh in the San Francisco Bay Area, has experienced multiple levee breaches in the past. Due to the large social and economic implications of flooding, questions have arisen about whether the current management of the island is still feasible. This project, therefore, aimed to find the most financially favorable management plan for the upcoming 50 years. First, an understanding of the system was obtained through a site investigation, a multivariate analysis, and a hydrodynamic model. It was concluded that discharge, tide, air pressure, and wind setup all contribute to the water level. These variables were then used as input for a 1-dimensional hydrodynamical model, which quantifies their effect on the water level. Subsequently, four management plans were considered: status quo, raising the levees, conversion into an estuarine wetland, and abandoning the island. To allow for comparison between these management plans, a cost-benefit analysis and a net present value calculation were performed for every plan. One of the major contributors to the costs is risk. Risk is defined as the product of the probability of failure and the consequence of that failure. The failure mechanism assessed is overflow, as this is the most relevant one. To quantify this failure probability, a statistical model was developed. This model includes an extreme value analysis and an event tree. Due to large uncertainties in the behavior of both the levee and the water level over time, the original 50-year lifetime of the management plan was deemed too ambitious. Therefore the lifetime was reduced to 10 years. The conclusion of the cost-benefit analysis and the net present value was to convert Van Sickle Island into an estuarine wetland. This is because it was the only management plan that was profitable after 10 years. The net present value was found to be equal to $14,924,048. ... Read more

A large part of the Dutch coast, the barrier islands in the Wadden Sea included, would be eroding if the deficit in the total sediment budget was not compensated for through nourishments. Ebb-tidal deltas have an important function within the coastal system that make them of interest for coastal defence as a source and transport path for sand to the back-barrier basins and the island coastlines. As part of the Kustgenese 2.0 project, a pilot nourishment of 5.5 million m3 has been placed on the Ameland ebb-tidal delta. Improved understanding of sediment bypassing processes and the interaction with a nourishment is needed for strategic placement of nourishments in the future. In this thesis we analyse the high-frequency bathymetric dataset available at Ameland inlet between 2005 and 2020 and model the tide- and wave-driven transport pathways using to SedTRAILS to determine how sediment bypassing works under natural circumstances and under the influence of a nourishment. A new series of conceptual models has been proposed. These show that the formation and growth of a series of ebb-shields on the western side of the ebb-tidal delta plays an important role in sediment bypassing. Their development gradually results in transport pathways connecting the western side of the ebb-tidal delta to the major transport pathway along the ebb-delta front, which forms a direct connection to the Ameland coast in 2017. The influence of the pilot nourishment on sediment bypassing processes is limited, adding volume to the system but not altering the existing transport pathways. Transport pathways show that sediment from the nourishment eventually reaches the Ameland coast and is unlikely to feed the Terschelling coast or the tidal basin. This is valuable knowledge for the future sustainable coastal management of Ameland inlet, which can also be extended to other inlets. ... Read more

Burrard Inlet (Vancouver, Canada) has been the home of the Tsleil-Waututh Nation (TWN) for thousands of years. Over the past decades, ongoing erosion has been observed along the shores of Burrard Inlet and the TWN reserve specifically. This leads to loss of land for the TWN community, damage to infrastructure, and exposure of historic sites with cultural value. Currently, there is insufficient knowledge concerning both the governing processes for sediment transport and transport pathways into, within, and out of Burrard Inlet. This knowledge is needed to propose and evaluate effective measures to prevent further erosion. This study aims to investigate the transport pathways in Burrard Inlet and give more insight into the mechanisms governing sediment transport in this inlet.

For this purpose, a Delft3D FM model of the area is set up and calibrated. This model is used to analyze sediment transport in the inlet under various forcing conditions. Transport pathways are visualized using SedTRAILS.

The model shows that flows and sediment transport in Burrard Inlet are tide-dominated and governed by the topography. Flows are strongly accelerated in constricted areas (First Narrows and Second Narrows), which leads to large velocity differences. Following the velocity field, sediment transport patterns are correspondingly dominated by these topographical restrictions. In the wider basins, flows slow down and form eddies. The model results suggest that these eddies act as sediment sinks. Additionally, sediment is lost into Indian Arm, a deep fjord with low flow velocities at the eastern end of Burrard Inlet. The possible pathways for sediment originating from the eroding shorelines at the TWN reserve are visualized. As soon as sediment from these banks is mobilized, it tends to move away from the shore with a final destination either in one of the eddies or in Indian Arm. The impact of wind and waves on the sediment transport patterns is limited.

Since first European contact in 1792, the shoreline of Burrard Inlet has changed significantly due to dredging activities, land developments, and industrial development as the city of Vancouver was built. Reconstructed historic shorelines are implemented in the model to assess the consequences of these shoreline changes on the sediment transport. Model results show that the tidal prism and the velocities in the Narrows have decreased since 1792, while the tidal range has increased. Moreover, sediment mobilized along the eroding shorelines showed greater potential for deposition along these same shores in 1792, compared to the present-day situation.
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Coastal aeolian sediment transport is influenced by supply-limiting factors such as sediment sorting by grain size. Sorting processes can cause a coarsening of the bed surface and influence the formation of aeolian ripples. However, it is not fully understood what influence sorting processes have on the magnitude of this transport. In this study, sorting processes of different grain sizes when interacting with wind and their influence on the magnitude of aeolian transport are investigated by performing a tracer study and developing a conceptual model. Sand grains were painted in different colors according to particle size and placed on Noordwijk beach, Netherlands. Several experiments were conducted with varying wind speeds. Surface sampling and cameras tracked the sand color movement on the bed surface, and wind speed and direction were measured. The tracer experiments show that for moderate wind conditions, ripples developed. Once the ripples have formed, the supply of finer tracer grains in downwind direction decreased over time, while the supply of coarser grains remained constant. A strong linear relationship between ripple speed and wind speed was found. For higher wind velocities, no ripples or differences in transport of grain size fractions were observed. Alternating phases of erosion and deposition of upwind sand were observed which could not be related to local variations in wind speed. Based on these results and literature, a conceptual model for an Active Bed Surface Layer (ABSL) with two regimes corresponding to moderate (I), and high (II) wind speeds was developed. The concept enables to estimate the magnitude of aeolian sediment transport as a function of wind speed, bed characteristics, and upwind sediment supply. For Regime I, a linear relationship between sediment transport and wind speed is suggested and for Regime II a third power relationship in combination with a process-based model accounting for supply limitations is suggested. ... Read more

The Tsleil-Wautuh Nation (TWN) reserve, Sleil-Waututh, located at the north shore of the Burrard Inlet in Vancouver (British Columbia, Canada) is strongly influenced by climate change. Sea level rise, coastal flooding and shoreline erosion are contributing to loss of land, damages to infrastructure, ecosystem changes and exposure of historic sites with cultural value. The TWN are a First Nation, a recognized group of aboriginal people in Canada, and have lived in harmony on the lands and waters of the Burrard Inlet since time out of mind. As TWN has a sacred obligation to be caretakers of the land, they retained Kerr Wood Leidal (KWL) to conduct a climate change hazard and vulnerability assessment and to design a ten year climate change adaptation action plan. The existing conditions in the area are investigated from a technical, environmental and sociological point of view, including a study of the community context of the TWN. Climate change exposes the project area to hazards such as sea level rise, acidification and water temperature changes among others. After conducting a hazard assessment, the following climate change induced hazards are evaluated: Coastal flooding, coastal erosion, intertidal area change, ocean acidification, harmful algae blooms and other ocean conditions (water temperature, e.g.). The impact of waves and rising sea levels are assessed through an Xbeach model. The impact of harmful algae blooms and other ocean conditions are evaluated though literature research. The potential of four different approaches, varying from traditional to building with nature-based solutions, to mitigate the identified hazards are discussed: a rip rap, a nourishment, a salt marsh and a clam garden. They are evaluated based on technical, environmental, economic and social feasibility. For each alternative a trade-off exists between protection against the identified hazards – mainly between the ability of each of the solutions to prevent or mitigate coastal flooding and erosion while preserving the local ecosystem and intertidal area. All alternatives help the TWN in their own way and although further research has to be done, this report provides an insight in four possible alternatives that could support the process of developing a satisfactory solution for the coastal hazards that cause problems for the TWN people and their reserve. ... Read more

Small Island Developing States, including many low-lying atoll islands, are among the most vulnerable countries to natural hazards and climate change disproportionately amplifies this vulnerability. Hence, there is a strong need for disaster risk reduction and risk management. Further development and implementation of methodologies for flood hazard assessment of atoll islands contributes to this. The methodology proposed in this thesis was applied to Majuro, an atoll island and capital of the Republic of the Marshall Islands. More specifically, the flood hazard related to different flood drivers, and including compound events (i.e. the combination of coastal flooding and precipitation) was assessed for the densely populated Delap, Uliga, and Djarrit region, in the east of Majuro Atoll. Main flood drivers are waves during typhoon events and distantly generated (swell) waves, but precipitation and high water levels (mainly tide) are important as well. To include all possible combinations of these flood drivers, and to include the spatial variation in events, 1000 years of synthetic events was generated based on data for historical events. Accurate simulation of inundation depths was computationally unfeasible for all synthetic events. Hence, a method was developed to reduce the number of model simulations, without losing information on the probability of occurrence of each event. The main steps of this method seem applicable to many other study areas where many scenarios are needed to include all (combinations of) drivers. Main steps are: (1) Selection of representative events – by Maximum Dissimilarity Algorithm, based on parameters that characterize the events. Hereby, the most extreme events are included as well. (2) Simulation of inundation depths for the representative events – by use of Delft3D, SWAN, and XBeach models. The XBeach model included a module for rainfall (first application). (3) Weighted interpolation to obtain the inundation depths for the synthetic events – based on the same parameters as in step 1. Based on the inundation depths for 1000 years of synthetic events, flood maps for different return periods and flood drivers were derived. These provide insight in the flood hazard for the DUD region due to the different flood drivers and for different return periods. The importance of different flood drivers varies significantly per area. Generally speaking, flooding related to (swell) waves in combination with high water levels is more frequent, while infrequent typhoons lead to the most severe flooding. Precipitation is an important flood driver as well, and exclusion would lead to underestimation of the flood hazard. Analysis of inundation depths suggests that for many areas these are limited to a maximum, where after excess water drains to the ocean – mainly into the lagoon. The derived flood maps could be used as a base for assessment of flood risk, climate change impacts, and closely related freshwater availability. In relation to the latter, more in-depth understanding of the contributions of precipitation and coastal flooding to the total flood hazard is needed, as on the long term infiltration of precipitation seems favourable, while that of oceanic water is not. ... 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

In recent years, there has been an increasing need for solutions against the threats that SLR and climate change represent for coastal systems, especially in low lying areas like the SW Dutch coast. Simultaneously, awareness of the impacts that human activities have on natural environments has surged. Integrated and sustainable solutions are necessary. The Delta21 plan proposes an integrated plan for flood-protection, energy storage, and natural ecosystem restoration for The Netherlands, specifically for the Haringvliet estuary. This large-scale intervention will produce large disturbances in the system. Despite the resilient character of tidal basin systems \parencite{Wang2009}, the evaluation of the intervention's effects, in terms of the initial response and the variations in the mechanisms driving morphological changes is compulsory. Assessing the bed level changes is especially important, considering the valuable intertidal and subtidal ecosystems present in the Haringvliet mouth. It is essential to guarantee their preservation despite the large disturbances. The evaluation of the Haringvliet mouth response is performed employing a long-term morphodynamic 2DH computational simulation to forecast the response of the system. The models implemented for the simulation are the Delft3D-FLOW and Delft3D-WAVE. To reduce computational times, the modeling approach applies input reduction and acceleration techniques. The hydraulic forcing is schematized (morphological tide, waves, and discharges) while maintaining the seasonal character of the input. A variable morphological acceleration factor (MF), depending on the wave condition is applied. The variable MF allows us to employ low values during strong-episodic conditions and larger during more regular conditions. The approach is especially beneficial in the presence of oscillating forcings that are also enhanced by the MF. Finally, the impact of the D21 plan on the Haringvliet mouth is assessed by studying the alterations to the hydrodynamic regime, the related net sediment transport pathways, and the observed bed level changes. The analysis of the resulting morphological evolution focuses on the variations of the intertidal and subtidal zones and the mechanisms behind them. We showed that the tidal regime shifts again towards a long-basin regime once the D21 is implemented. The intervention also causes the emergence of new subtidal and intertidal areas, mostly within the Tidal Lake, resulting from the redistribution of the material from the main channel. The original intertidal and subtidal areas, formed by the Hinderplaat and Slikken van Voorne, are almost undisturbed. This behavior guarantees the preservation of the intertidal and subtidal ecosystems. The results also show that despite the initial dredging activity necessary for the D21 plan implementation, which decreases the extent of shallow areas, the emergence of intertidal and subtidal features balances the negative effect as long as no further dredging is performed. The TL shows a net sediment export of material at both control cross-sections (the Haringvliet Dam and the new TL inlet). The findings of the morphological development agree with the known morphology and processes of mixed-energy tidal inlet systems, despite the large-scale intervention. This study provided the opportunity to evaluate the emergence of typical tidal features in a highly disturbed system, where the tidal inlet is shifted further offshore. To achieve an accurate forecast, anthropogenic forcing signals also had to be considered and extrapolated to long-term morphodynamic simulations. The forecasting of large-scale anthropogenic interventions in sensible systems, such as tidal estuaries, is performed successfully by adapting an existing model. The development of this type of study will be increasingly relevant in the future for the evaluation of measures against SLR during conceptual project stages. ... Read more

Coral reefs are degrading at an alarming rate, affecting not only the precarious coral ecosystem but also human habitat. The combination of coral degradation, sea level rise and its exacerbated effect in the tropics, and the possible storm intensification increases the flood vulnerability of low-lying tropical islands. To protect reef fronted coasts against flooding, coral restoration is put forward as measure to increase coastal safety. This study addresses the effects and efficiency of coral restoration in four steps. First, a simple reef hydrodynamic model is formulated to identify important drivers of runup at a reef fronted beach, to be used as a quick scan tool before using more intensive process-based models. Next, 30.000 US reef profiles are classified to gain insight into the prevalence of different reef shapes. Four dominant reef profiles are then used in the subsequent modeling step, in which the hydrodynamics and flood vulnerability of different reef shapes are investigated using the XBeach process-based model. With the knowledge gained, the hydrodynamic effects and runup reduction potential of different restoration configurations at various reef morphologies can be assessed. ... Read more

Every year hundreds of thousands of people are affected by natural disasters that occur due to various physical phenomenon. They include earthquakes, tsunamis, volcanic eruptions, and hurricanes. In this research the focus is on hurricane events and the impact they have on a community. The ability for a community to bounce back is defined by their disaster resilience which, in turn, depends on their ability to identify vulnerabilities and prepare for the inevitable. Within this line of research, one of the main challenges is defining local risk due to hurricane events, especially when considering more than one hurricane-induced hazard. Furthermore, the challenge becomes even greater when analysing risk in locations around the world where data availability is scarce. This study aims to clarify and improve the existing methodology to define multi-hazard risk due to hurricanes. This methodology is verified by applying it to the case study of St. Martin, in order to delineate hurricane risk on the island. St. Martin was chosen to investigate, as it is a Small Island Developing State in the Caribbean that recently suffered immeasurable damages during Hurricane Irma (September 2017), and is still struggling to recover. The two hazards that were considered, in the application of the methodology, were hurricane-induced winds and hurricane-induced coastal flooding. In the case of St. Martin, hurricane-induced winds were found to contribute to 98% of damages due to Hurricane Irma, when compared to the coastal flood damages. Coastal flooding was found to be due to both increased storm surge levels and wave-induced flooding, showing that neither one is negligible for a reef island like St. Martin. Storm surge variation around the island was found to be minimal due to the scale of the island, and the fact that storm surge was predominantly pressure driven. Validation of the models to simulate hazards and impact on St. Martin proved to be challenging. An unconventional data source was used to validate the flooding model, which included analysis of Twitter data of images posted during Hurricane Irma. This is an example of a solution of how to deal with data scarcity in hazard modelling. The risk assessment of St. Martin involved simulating synthetic hurricane track scenarios and determining their respective wind and flood damages on the island. Combining the respective damages was done by including a damage threshold to ensure combined damages did not exceed 100%. The applied framework resulted in a hurricane risk map of St. Martin indicating Expected Annual Damages per community. The intention of the improved methodology is to apply it to a hurricane risk prone region, like St. Martin, and to use the outcome to delineate hurricane risk. This indicates hot-spots in the region of interest and improvements to disaster resilience can be discussed. The approach of Build Back Better is highlighted in this research to show how this risk map can be interpreted and what the results mean. Three branches are discussed, namely building back stronger, which involves ensuring infrastructure can resist more extreme events in the future. ... Read more