Sint Eustatius is a Caribbean island located in the north of the Lesser Antilles. The coastline west of the capital, Oranjestad Bay, is of vital importance to the island as it is here where tourism, recreation, the harbour and historical ruins are located. On the other hand, Oranjestad Bay has a heavily fluctuating beach, which is undesirable. The Public Entity of St. Eustatius is investigating the possibility of a coastal solution that could preserve and extend this beach. However, there is insufficient knowledge of the hydrodynamic processes and morphology available to design a solution. By analysing the morphological system, this study aims to advise the Public Entity of St. Eustatius on the decision for a coastal solution to create a beach in Oranjestad Bay.

A literature study was performed to obtain basic knowledge of the coastal system of Oranjestad Bay. A two-dimensional numerical Delft3D model was then developed to gain more in-depth knowledge of the wave, current and sediment patterns. The result of both findings is combined in a conceptual model, where the cause of the beach fluctuations along Oranjestad Bay was analysed. Based on the system knowledge and stakeholders’ demands and regulations, three alternatives to create a beach at Oranjestad Bay are proposed. The alternatives are qualitatively verified and evaluated with a multi-criteria analysis and cost-value ratio.

The literature review shows that Oranjestad Bay has the characteristics of a wave-dominated coast. The seasonal wave climate has a strong influence on the beach fluctuations that occur in Oranjestad Bay. The influence of tides and wind on sediment dynamics is expected to be minimal. The modelling study concluded that the wave climate can be described by three main wave conditions, namely a sea condition, a storm condition and a swell condition. The different hydrodynamic and morphodynamic characteristics of these conditions provide a reasonable simulation of the coastal processes in Oranjestad Bay. The model results give an impression of the seasonal wave climate and a net northward sediment transport capacity is found. These results are consistent with the expected coastal morphodynamics found in the literature study. The effect of a storm is less well represented in the modelling study. This limitation is caused by the modelling approach. In addition, there are some uncertainties in the model due to the lack of detailed coastal data in the nearshore. The conceptual model indicates that the coast of Oranjestad Bay may experience seasonal gradients
in alongshore sediment transport leading to beach fluctuations. From April to September, beaches are likely to erode due to net northward sediment transport. In addition, beaches erode due to periodic storm conditions that occur from June to November. It has been found that the recovery of beaches after a storm varies along Oranjestad Bay. Scubaqua beach recovers much faster than the beach at Smoke Alley. This is due to the larger sediment transport capacity in the alongshore direction at Smoke Alley beach, the amount of larger cobbles on the shoreline of Smoke Alley and the lack of sediment north of Smoke Alley...

The Bollen van de Ooster, in this report referred to as the Ooster, is a sand bar in the outer delta of the Grevelingen. The Ooster is separated from the coast by a relatively deep channel, called the Schaar. Closure of the Grevelingen in 1971, with the construction of the Brouwersdam, initiated vast changes in morphology. Damming of the estuary mainly affected the tide-induced flow patterns and therefore the relative influence of the waves at the outer delta. In this study the main focus is on the morphological development near the coastline of Goeree-Overflakkee. During the past years an erosion trend of locally up to 27 m/year, just south of the Flaauwe Werk, led to concerns with Rijkswaterstaat. This erosion is caused by the migration of the Ooster along the coast. Out of precaution a beach nourishment has been planned in this area due to the potential danger for the Flaauwe Werk. However, uncertainty about the future morphological development and therefore the necessity to take measures remains. The aim of this study is to provide a better understanding in the changes that have occurred, bring more certainty about the future and therefore contribute to informed decision-making. The coast of Goeree-Overflakkee has had a long history of coastline erosion. Closure of the Grevelingen estuary led to an increase of this erosion trend at the Westhoofd due to a combination of increased tide-induced flow velocities and morphological development of the Ooster. The latter was characterised by an eastward migration which forced the Schaar into the coastline. This process initiated multiple nourishments in the period 1969-1985. The attachment of one of the shoals of the Ooster resolved this problem, resulting in a large accumulation of sediments on the beach. In the years thereafter these sediments have been eroded as the Ooster migrated along the coast and transported further north. The latter resulted in beach widening along the coast of Goeree-Overflakkee. Due to the rapid elongation of the Ooster the erosion problems of the 80’s returned. The Schaar shows a continuous decrease in depth and flow surface since 2003, according to data analysis. Model results show that the channel mainly plays a role during low tide when the Ooster is emerged. Significant tide-induced flow velocities occur in the channel. In general, the decreasing channel dimensions lead to an increase of the magnitude of the flow in the channel. However, this occurs for a limited time duration during the full tidal cycle. A striking observation is the small influence of the wave angle on the wave-induced flow velocities near the channel. Waves coming from the north are refracted considerably due to the extension of the Haringvliet outer delta. The result is a large net longshore sediment transport rate at the seaward side of the Ooster. This could explain the pace of the migration in eastern direction. Moreover these sediments are a source for the channel, contributing to decreasing the channel dimensions. In general the presence of wind decrease the magnitude of the flow in the channel. Based on these findings it can be concluded that the waves are the dominant forcing mechanism in shaping the morphology and attachment of the Ooster is expected to occur in the near future. Attachment of the Ooster implies the disappearing of the eroding currents. The duration at which this attachment can be expected cannot be deduced from this study. The erosion of the coastline will continue as long as the channel is present. Based on the width of the dune row and the presence of beach groynes the potential treat of the morphological development for the primary flood defences seems minor.

Within the KustGenese 2.0 research project a 5Mm3 pilot nourishment is created on the ebb-tidal delta of Ameland. For this thesis I was involved in the parties; Van den Herik, the contractor who executed the work, Deltares and Rijkswaterstaat. A tool is created to quickly analyse new surveys in combination with the ships deposit data. The tool is applicable for any other dredging project.
The tool is applied on the 4-6 weekly surveys during the execution of the pilot nourishment. An analyse with the tool gives insight into the morphodynamics of the ebb tidal delta and the location of the pilot nourishment. The sediment transport on the nourishment location is wave dominated, as the sediment transport is limited during calm wave conditions. The sediment transport on the second ebb shield is however tide dominated. During the January 2019 storms an increase in sediment transport is seen on both tidal and wave dominated parts.

Considering scenarios of sea level rise, the sediment budget of the Wadden Sea is of interest for both coastal safety and for the maintenance of values of this unique environment. Sand nourishments are foreseen on the ebb-tidal deltas of the Wadden Sea to increase the sediment budget, which requires us to predict the fate of the nourishment. This study focusses on the exchange of water and sediment between the North Sea and the Wadden Sea through Ameland Inlet. Based on a combination of field observations from the extensive 40 days Kustgenese2.0/SEAWAD field campaign in fall 2017 and Delft3D modelling results, the flow conditions are explained and the contribution of different forcing mechanisms is unravelled. Residual flows and sediment transport at the ebb-tidal delta were found nonstationary due to the action of waves, but also due to local wind-shear stresses. The wind-driven exchange with neighbouring basins over the tidal watersheds leads to significant residual flows through Ameland Inlet, which increases the importance of wind for the system functioning. The many shallow areas and the orientation of the system in relation to the prevailing wind direction for strong winds are believed to make wind forcing as important as observed around Ameland Inlet.

With the implementation of new legal aspects for dune safety assessment in 2017, the current method, Duros+, is no longer feasible to represent the desired processes. This 1D deterministic, volume-balance model cannot simulate processes like inundation and overwash, which are crucial to fulfil the new requirements for dune safety assessment. Therefore, the implementation of a 2DH XBeach model for dune safety assessment is evaluated. A transformation from 1D to 2DH becomes necessary and several hydrodynamic effects come into play, which are not relevant for a simulation in 1D. Additionally, the definition of a new limit state is a challenging task in order to define failure properly: The legal definition needs to be transformed into a technically feasible one. In this thesis five different limit states have been defined in order to evaluate the influence of a limit state on its consequences. These five limit states are then applied in a case study on the Delfland coast and the consequences of the different definitions are evaluated. Additionally, the safety development of the Delfland coast over the last two decades was analysed. It was found that all nourishment strategies applied at this coastal stretch contributed to an increasing safety. This includes the placement of the Sand Engine and its on-going alongshore spreading.

Since 1990, the Dutch coastline is maintained within the ‘Dynamic Preservation’ program, according to which the coastline is maintained seawards from a reference line, mainly by applying nourishments. Research into the maintenance of the Dutch coast is continuous and causes the content of the coastline preservation program to change constantly since the initiation in 1990. In recent years, the switch was made from yearly nourishment programs to the use of multiannual nourishment programs, in which an interim nourishment planning is included for 4 years. Next to the nourishments following the ‘Dynamic Preservation’ program, ten large reinforcements were applied along the Dutch coast in the past decade according to the ‘Zwakke Schakel’ project. After reinforcement, the coastline at the ‘Zwakke Schakel’ locations needs continuous maintenance to remain at the desired position.

The combination of the long term maintenance at the ‘Zwakke Schakel’ locations and the multiannual nourishment program, leads to a more or less fixed character of the nourishments program with recurring maintenance nourishments in each period. The question is to what extent also the adjacent coast is maintained by sediment transported from these recurring maintenance nourishments. A situation in which the adjacent coast can be sufficiently maintained by long term application of nourishments at the ‘Zwakke Schakel’ locations, would lead to an even more fixed character of the nourishment program. At this moment, knowledge on the contribution of sediment transported from beach nourishments to the maintenance of the adjacent coast is insufficient.

The research presented in this thesis focusses on one case study. Along the coastal stretch between Scheveningen and IJmuiden, three distinct ‘Zwakke Schakel’ reinforcement nourishments were applied at Scheveningen, Katwijk and Noordwijk. At all locations the coastline was migrated seawards, with varying distances of 60 to 100 meters. The coastal stretch between Scheveningen and IJmuiden is part of the Holland coast and bounded by the breakwaters of the Scheveningen and IJmuiden harbours. Along the Holland coast, sediment transport is dominated by wave related processes wherein longshore transport is the most important sediment transport process. Gradients in longshore sediment transport are therefore an important cause of erosion and accretion.
Results of the yearly measurements done along the entire Dutch coast already show a positive effect of the maintenance nourishments in the area. With a refined version of an existing Unibest-CL+ model the effect of the recurring maintenance is further assessed for the long term. In the model, the longshore sediment transport volumes and resulting coastline evolution are modelled for a timescale of 55 years, starting in 2006 before application of the ‘Zwakke Schakel’ reinforcement nourishments and including the effect of possible sea level rise of 0.2 to 1.5 cm per year. The model is validated by comparing transport quantities (volumes and gradients) and coastline development with real measurement results and results from earlier research.

In order to maintain a positive coastline position along the adjacent coast, the autonomous erosion needs to be sufficiently compensated by the accretion related to the long term maintenance. At several locations in the area of interest initial erosion is expected, after which the erosional trend switches into a seaward migrating trend on the long term, partly under influence of the maintenance nourishments. This process is expected to occur at both Wassenaar (between Scheveningen and Katwijk) and Noordwijkerhout (north of Noordwijk) in the upcoming decades, although the inclusion of some uncertainty in amongst others sea level rise shows that it is unsure whether a positive development at Noordwijkerhout will really occur. The erosional trend at Bloemendaal and Zandvoort, close to IJmuiden, cannot be compensated by the sediment transported from maintenance nourishments. On the time scale of 55 years, the region of influence of the maintenance nourishments does not reach Bloemendaal and Zandvoort. The regions of influence of all ‘Zwakke Schakel’ maintenance nourishments are expected to cover the area from Scheveningen up to around 10 kilometres northwards from Noordwijk in 2060. Individual regions of influences are expected to reach a size of 15 to 24 kilometres up to 2060.

Although in most cases the trends of coastline development within the regions of influence are expected to become positive on the long term, the coastline position itself may be located too much landwards due to the initial erosion. In order to solve this problem, additional (shoreface) nourishments need to be applied at Wassenaar and Noordwijkerhout. At both locations shoreface nourishments are already applied in the past decades, which supports the outcome of the model results. At Bloemendaal and Zandvoort, additional (shoreface) nourishments will surely be needed in order to maintain the coastline.