Many coastlines feature a sediment transport in a dominant direction, possibly burying a traditional breakwater in sand in several years. The sandbar breakwater concept uses this sediment supply to its benefit to make the use of vast amounts of rocks abundant. During this research the morphological development of the worlds first sandbar breakwater built in Lekki, Nigeria, is analysed: The minimal amount of sand used for construction of the Lekki breakwater is placed such that the combination of the adaptation of the initial profile and the natural supply of sediment results in a smooth coastline at the end of the first year. This development is in turn used to setup a calibrated model to study the influence of the wave climate on the development of the sandbar breakwater concept. The wave climate at Lekki is characterised by a single dominant wave direction (called unidirectional) and has a narrow direction bandwidth (yearly standard deviation of the wave direction is about 5 degrees). This ideal wave climate is altered to research the influence of the mean wave direction, directional variation and the sequence of waves (seasonality). The study showed the sandbar breakwater concept to be mainly influenced by the mean wave direction and to lesser extent by the directional variation and the seasonality. These results can be used to make a first assessment on the possibility of applying a sandbar breakwater for a wave climate somewhere around the world.

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.

To exploit its socio-economic functions, engineering measures are regularly applied in estuaries. Estuaries are, however, known to be very complex systems. Stemming from this complexity is the generation of a so-called estuarine turbidity maximum (ETM), which poses great siltation problems to the engineering measures. An engineering measure, which is looked upon in this thesis, is a trench accommodating for the construction of a submerged tunnel. Despite the complexity of estuaries, trench siltation rates are predicted in practice by simple empirical engineering tools. However, a lot of uncertainty is associated with the predicted trench siltation rates, as such engineering tools do not capture the complex estuarine mechanisms. These mechanisms are found to dominate the sediment supply to the trench and the subsequent trapping of sediment in the trench. Therefore, in this thesis, it is investigated to which degree of certainty trench siltation rates in estuaries can be predicted, based on a process-oriented and engineering-oriented viewpoint. This is researched according to the implementation of a detailed process-based numerical model, which is considered to be highly accurate. For this purpose, the following case study is adopted: a trench near the ETM of the well-mixed Scheldt Estuary, at Oosterweel, Belgium. The uncertainty of the numerical model is estimated based on a sensitivity analysis, which maps the epistemic uncertainty, and a scenario analysis, which approximates the intrinsic uncertainty. For verification purposes, modelling results are compared with the state-of-the-art theory on trench siltation mechanisms, and estuarine sediment transport and trapping mechanisms. These are thoroughly analyzed in this thesis based on an extensive literature study. Additionally, a link to practice is made by comparison of the degree of certainty and practicality of the numerical model with the engineering tools. It could be estimated, based on the parameter uncertainty, that the epistemic uncertainty of the numerical model equals approximately 1.5 times the expected siltation volumes. A similar uncertainty due to intrinsic uncertainty was estimated, as the trench siltation rates showed a strong dependency on the variation in forcing regarding tide, river and storms at sea. In total, this led to a quantifiable uncertainty in the order of 2.5 times the expected siltation volumes. Regardless of this uncertainty, great confidence is put in the performance of the numerical model, as in accordance with existing literature of the study area and theory on well-mixed estuary, important mechanisms in the supply of sediment to the trench were found to be: salinity-induced circulation, tidal rectification, Stokes’ drift and river discharge. Additionally, the governing sediment trapping mechanisms of the trench, found by the model, are in line with state-of-the-art literature. In contrast, yet unidentified by literature, the longitudinal salinity gradient over the estuary also seem to dominantly influence the trapping efficiency of the trench, in particular during flood in which it induces a strong decrease in sediment trapping. In comparison, it is believed that engineering tools, applied for trenches in estuaries, are prone to very high epistemic uncertainty caused by model inadequacy. This is because stand-alone application of the engineering tool on the problem gave a significant over-estimation of the siltation volumes, as predicted by the numerical model. Furthermore, the engineering tool was found to behave differently within a tidal cycle, and on changing environmental conditions. Above epistemic uncertainties due to model inadequacy could, however, not be quantifiably supported, as the total quantifiable uncertainty was in the same order of the numerical model. In conclusion, the degree of certainty of the trench siltation rates is believed to be improved significantly using a detailed numerical model instead of engineering tools. However, a huge drawback of the application of detailed numerical models, is the complexity and the impracticality of the numerical model. Therefore, this thesis opts for the development/use of a more sophisticated semi-empirical tool for engineering measures in estuaries. Though, more research is recommended on trenches in both similar and different type of estuaries in order to generalize and confirm the findings of this thesis.