Sea level rise adaptation of rubble mound breakwaters
An adaptation pathway approach including sea level rise uncertainty and numerical overtopping modelling
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Abstract
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.