For the transportation of the produced energy at offshore windfarms to the onshore grid, export cables have to be installed in the sea bed. Usually, at first trenches are dredged, in which approximately one month later the cables are installed. During this period the trench should remain sufficiently open to install the cables, fulfilling the depth requirements. This thesis focusses on the influence of sand wave fields on the sedimentation rate. It is investigated how trench sedimentation can best be assessed: with a simple model in combination with an extensive probabilistic assessment, or with a more complex model in which a less detailed probabilistic assessment is possible. The research is based on a case study at the Borssele Wind Farm Zone, for which the sedimentation rate is determined during the month April.
The sedimentation is primarily caused by a reduction in (the perpendicular-directed) flow velocity. Shifting of shoals or banks does not occur during the short timescale. The same holds for the sand waves, which do not migrate significantly during this time. Under influence of bedload transport however, the largest sedimentation rates are found for trenches around the sand wave crest. Closer to the troughs, less sedimentation can be expected.
During the probabilistic assessment of the trench sedimentation in flat bathymetries, it was found that the uncertainty obtained by Latin Hypercube Sampling (LHS) with an 2DV-model is reduced compared to the Monte Carlo-analysis (MC-analysis) in SedPit: the median is for both cases 0.06m, the 5-95 percentiles are 0.03m-0.09m and 0.04m-0.13m respectively. Including (idealized) sand wave perturbations in the 2DV-model, increases the range of the P05-P95 values for in trench sedimentation to 0.01 and 0.13m.
Based on the results, it is concluded that SedPit can be used as first assessment tool for the trench sedimentation, since it covers the range obtained with the more complex 2DV-model. The main advantage is the limited computation time of around 1.5 hours, compared to around 48 hours with the sand wave model. On the other hand, the 2DV-model is able to produce more accurate results.
The siltation rate in trenches located at sand wave crests is underestimated by SedPit compared to the results obtained from the 2DV-model. The opposite holds for trenches located at sand wave troughs, in which more sedimentation is predicted by SedPit. Adjusting the calibration parameter of sand transport in SedPit within the ranges provided in Van Rijn (2013) (0.5 for a sand wave trough and 1.0 for a sand wave crest), can at most decrease the difference between the models. For trenches located near a sand wave crest or trough, it is therefore advised to use at least the 2DV-model to assess trench sedimentation.
Furthermore, the 2DV-model can only be used in the case of perpendicular flow with respect to the trench-axis. Inclusion of the sand wave processes, which act in the 2DV-direction, for non-perpendicular flows therefore requires 3D modelling.

In the field of coastal engineering computational models are a commonly used tool to gain insight in dynamics of a coastal system, to gain insight in or hindcast the historical development or can be used as engineering tool to assess the efficiency and consequences of a proposed coastal measure. A new coastline model, called ShorelineS, is under development which promises to be applicable for long term simulations on geometric complex coastlines or unstable beach regimes where, for example, spit formation may arise. This new model aims to overcome the gap between existing relatively simple coastline models and more advanced coastal area models. In this thesis the applicability of this new model on spit formation was assessed. A study case in Lobito (Angola), were a natural spit has formed, was used for this purpose. Three research objectives are of interest in this thesis: 1) getting insight in the relevant processes regarding spit formation (in general and for the Lobito spit) 2) validate the longshore sediment transport module of ShorelineS and 3) asses (and improve) the spit formation in ShorelineS. An inventory of existing spit formations around the world and a detailed modelling study (XBeach) on the transport rates around a spit showed that especially the bimodality of the wave climate is an important process influencing the final shape of the spit. Regarding the calculation of the longshore sediment transport it was found that for the modelling of the wave transformation a distinction should be made between the static fixed offshore depth contours and the dynamically changing nearshore depth contours for changing coastline orientations. This distinction is essential to consistently and correctly determine the breaking wave parameters which directly influences the longshore sediment transport calculations. This distinction was implemented in ShorelineS using the concept of a so called ‘dynamic boundary’. The spit formation in ShorelineS was assessed by means of hindcasting the historical spit formation of the Lobito spit using the ShorelineS model. Continuing on the previous finding it was found that the coastal orientation for which the longshore transport is maximum, which reflects the direction in which a spit migrates, should be derived on the actual, spatial and temporal variating, transport curves. This in contrast to the theoretical fixed angle as suggested by Ashton et al. which was previously used in ShorelineS. Both the implementation of the dynamic boundary and the routine to find the actual (variable) angle of maximum transport were needed to be able to correctly hindcast the observed spit formation. With the current, underlying, model assumptions it is not possible to correctly model the local decay of longshore sediment transport along the head of the spit. The width and local shape of the spit is therefore controlled by the so called ‘upwind correction’ routine in ShorelineS. This routine was updated to prevent the grid resolution dependency which originally directly controlled (forced) the resulting spit width. With this updated routine the spit width is based on an average (expected) spit width. This new routine should be considered as a proof-of-concept, further research is required to further develop this routine and find ways to include some kind of feedback mechanism in the model between the forcing, shape and the resulting LST over the head of the spit.

Evolution of coastline position under the influence of natural and anthropogenic processes is directly linked to the development of seaside societies. In the context of coastal zone management, process-based morphodynamic models are often used topredict coastline evolution and support the decision-making process for adaptation/mitigation strategies. Frequently, the processes driving themorphodynamic evolution transcend the applicability limits of a single model. In those cases, model ensembles can be used to estimate coastline change under the joint effect of the relevant processes. However, model output and in extent the aggregated result are characterised by uncertainty originating among others from forcing variability and parameter imprecision. The increasing exposure ofc oastal societies to coastal recession risks and emergence of risk-informed coastal zone management create the need for aggregated coastal recessionestimates with quantified uncertainty. This study investigates different statistical methods for forcing and parameter uncertainty quantification around coastline change estimates from process-based morphodynamic models. Subsequently a numerical convolution approach for the aggregation of the probabilistic coastal recession estimates from multiple models was formulated to account for the combined uncertain effect of processes acting on different timescales. The methods of this study were applied on Anmok beach, South Korea, a coastal stretch experiencing erosion caused by long, intermediate and short timescale processes. Available UNIBEST-CL+ and Delft3D model schematizations from theCoMIDAS research program, capable of simulating the relevant processes, were utilised. Following a literature review, two methods were considered applicable for process-based morphodynamic models: Standard Monte Carlo (SMC) and Latin Hypercube Sampling (LHS). The application of both methods on the UNIBEST-CL+model schematisation enabled the evaluation of their relative performance based on the precision of the different coastline change estimates achieved for the different sample sizes. Only LHS was applied on the Delft3D model schematisation due to computational demands limitations. Subsequently,the scenario-based approach currently used for the aggregation of multi-modelcoastline change outputs was extended to explicitly account for the uncertainties quantified in the individual model outputs. A numerical convolution approach, using Monte Carlo sampling, was suggested for linear super position of the contributing probability distribution functions. The advantages of this approach include speed, ease of implementation,comprehensibility and high resolution even at the tails of the aggregated distributions. Utilising this approach, the effect of alternative interventions (combinations of various breakwater designs with a small-scale nourishment) on the coastline change probabilities was quantified. The results showed that both methods (i.e., SMC and LHS) with adequate sampling can produce probability distribution outputs for coastline change when applied to the process-based models. SMC remains the most suitable method for coastline change uncertainty quantification for models with small simulation durations. The method gives quantified estimates of the precision, enabling the achievement specific target precisions, with the respective computational cost. For the smaller sample sizes used, LHS gave better precision results, proving more suitable for models with longer computational time. On the downside, without extra iterations of the procedure only upper estimates of the achieved precision for a specific sample size can be obtained. The probabilistic aggregation framework presented in this thesis has several advantages compared to the scenario-based approach currently used. It allowsfor quantified coastline change uncertainty estimates with the respective precision estimates and provides the distribution of the uncertainty across its range, information that could not be derived using the scenario-based approach. Different coastline change percentile estimates or confidence intervals with practical use to decision makers and the likelihood of any coastline change realisation of interest can be evaluated. The probabilistic uncertainty quantification and aggregation framework is believed to be useful for intervention assessment and comparison. It allows for the assessment of uncertainty around the morphological response under the combined effect of the processes acting on the coast with/without the intervention and thus the evaluation of the probabilistic intervention impact. Different interventions can be compared in terms of the probability of inducing desired/undesired morphodynamic realisations as well as in terms of the uncertainty range in the coastline change estimates.

Numerical process-based morphodynamic models are widespread in coastal engineering practice and have become the new standard when it comes to assessing the impact of natural or man-made structures on coastal environments. The most common practice among engineers is to focus on a single spatial and time scale, which means either neglecting certain processes under the assumption that they will average out, or performing detailed simulations for short time-spans in order to optimize the normally limited computational resources. Despite the efforts from several authors, at the moment there is a lack of a clear methodology which would allow incorporating the relevant physical phenomena only when required, hence optimizing the computational effort.

The above leads to the main research objective of this thesis: to gain insight in what is the added value of coupling process based morphodynamic models, regarding the morphological impacts near the beach. For this purpose two models that were originally conceived to resolve different timescales are selected; XBeach as a storm model, and the new suite from Deltares, Delft3D-Flexible Mesh (D3D-FM) as a long-term morphodynamic model. The area selected as study site is Anmok beach, located at the east coast of South Korea. The coastal erosion at this location is not yet well understood (mainly due to human interventions and storms) plus the micro-tidal wave-dominated environment makes this location ideal for this study. Recent researches on this site have found that there is a delicate balance between the stormy and calm periods, where the high energy wave events are the main drivers of local morphology.

One of the main findings in this thesis is that the coupling of independently calibrated models does not necessarily provide better morphodynamic results than the results obtained by running each model separately. Including different processes such as infragravity waves or Eulerian mass transport (which enhances the offshore sediment transport in the surf zone) during highly energetic events tend to generate large supratidal beach erosion. However, the post-storm recovery mechanisms present in long-term morphodynamic models are not sufficient to bring the sediment back to the beach. Therefore, it is recommended to include all the relevant physical processes (storm erosion and post storm recovery mechanisms) when following a coupling approach in order to have a coherent morphodynamic balance. Furthermore, the coupling of models can play an important role in identifying which processes are missing or are not fully represented by the different modelling packages.

The erosive effect of cumulative storms was shown to be relevant in the short to medium term and might become a key parameter when defining, for instance, the worst case scenario regarding shoreline retreat. Despite the fact that uncoupled long-term morphodynamic models produce better average results in the case of Anmok beach, the implementation of a coupled scheme was proven to be important when the erosion due to cumulative storm effects cannot be neglected.

Among the advantages of using D3D-FM for this work is the implementation of the Basic Model Interface, which meant an important reduction in bookkeeping efforts and the possibility to seamlessly couple D3D-FM with XBeach. This procedure allowed for the incorporation of more complex phenomena (such as infragravity waves) with an acceptable increase in computational time.

A research version of Delft3D (D3D) with specialized sediment transport equations in the swash zone was tested in an attempt to enhance the post-storm recovery mechanisms. The results obtained are promising in the sense that the accretion of the shoreline and lower dry beach was reasonably enhanced, especially when considering all the limitations involved in modelling the morphodynamics of the swash zone with a stationary wave model. Another important conclusion is that these models were capable of depositing the sediments at the lower backshore at best. Hence, there is still the need of a mechanism/process capable of transporting the sediments farther upslope into the dry beach or dunes, such as aeolian transport.

The decision to undergo with a coupled or uncoupled approach depends on case-by-case basis. For current practice, it is recommended to develop a coupled simulation in the medium-term, where both storms and calm periods have significant effects and where the intra seasonal variation could be a parameter of interest. For short-term simulations, an uncoupled storm model (e.g. XBeach) is recommended as is the most accurate for such a time span. For long-term simulations, the general recommendation is to run an uncoupled long-term model such as D3D. In this case the storm erosion and post-storm recovery processes are expected to average out, being the long-term model the most suitable package to obtain average morphological results.

For future work it is recommended to add an aeolian process-based model and incorporate the swash zone sediment transport module into D3D-FM as this would move us one step closer towards the development of a fully coupled model where all the relevant processes (storm erosion, post storm recovery and aeolian transport) are included.