D.Y.Y. Teng

Master thesis (1)

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Sea level rise adaptation of rubble mound breakwaters

An adaptation pathway approach including sea level rise uncertainty and numerical overtopping modelling

Master thesis (2023) - D.Y.Y. Teng (author), M.R.A. van Gent (mentor), Marcel van Gent (mentor), M. R.A. van Gent (mentor), Marcel R.A. van Gent (mentor), M.Z. Voorendt (graduation committee member), Marisol Irías Mata (graduation committee member)

If sea level rises faster than anticipated in the initial design of rubble mound breakwaters, a serious threat is posed to their functionality. To limit wave overtopping, breakwaters must be adapted to the rising sea level and subsequent increase in wave loading due to reduced depth-induced wave breaking. However, the projections of sea level rise are highly uncertain. To deal with this uncertainty and avoid unnecessary costs, the method of adaptation pathways can be applied.

The thesis aims to incorporate changes in depth-induced breaking uncertainty of sea level rise in the creation and selection of adaptation pathways for rubble mound breakwaters.

As a first step to reach the objective, methods are proposed based on existing concepts from literature, modified to be applicable to adaptation pathways for breakwaters.
To consider changes in depth-induced breaking when determining wave loading on breakwaters, two empirical estimates are proposed. The maximum significant wave height at the toe of the breakwater is assumed equal to half the water depth at the toe. The spectral period at the toe is assumed to be equal to the deep-water spectral period for shallow foreshores.
To account for sea level rise uncertainty in the selection of pathways based on cost, methods for model uncertainty and for scenario uncertainty are proposed. The first method uses an approximated probability based on model uncertainty to estimate the expected cost of the adaptation pathway. The second method deals with scenario uncertainty by computing the weighted average of the cost of pathways for all considered scenarios.

The applicability of the proposed methods is tested on a case study for the location of IJmuiden (the Netherlands). For the case study, five adaptation measures are considered: placing a low-crested structure, adding a berm, raising the foreshore bed, adding a crest wall, and raising the armour crest level. The last three mainly form the optimal pathways in the case study.

Lastly, the empirical estimates and formulae used to create adaptation pathways are validated with an XBeach model and an OpenFOAM model. The estimates of the significant wave height and spectral period have a maximum deviation of 21% and 15%, respectively, compared to the numerical results. Moreover, the comparison with the numerical model indicates that the overtopping expressions of Van Gent et al. (2022) can predict overtopping results with reasonable accuracy, even for conditions which fall outside the range of validity.

Based on the case study it is concluded that the method to incorporate sea level rise uncertainty in the selection of optimal pathways gives insight into the preferred measures and the likelihood of measures being applied in the lifetime of the structure. The results of the case study also indicate that the preferred pathways do not vary between different sea level rise scenarios. Based on the numerical validation it is concluded that the method to incorporate depth-induced breaking in adaptation pathways can be used as a first estimate but more detailed calculation methods such as numerical models are necessary to accurately create adaptation pathways.