Graduation of Kevin Hoogeveen

Climate Adaptation of Rubble Mound Breakwaters

• Professor of graduation: Prof. dr. ir. M.R.A. van Gent (TU Delft/ Deltares)

• Supervisors: Dr. ir. A. Antonini (TU Delft), Dr. ing. M.Z. Voorendt (TU Delft), Ir. J.P. den Bieman (Deltares)

 

Rubble mound structures are often constructed to prevent severe wave damage to ships. By constructing the crest at a certain level, the waves are reduced such that safety is ensured. However, a large increase in sea level rise is expected in the next century due to climate change. Because of this, multiple adaptations might be necessary to meet the required lifetime of a rubble mound structure. In current guidelines, the influence of these adaptations is already included. However, for a combination of solutions the empirical equations are not always accurate enough (van Gent, 2019). Accurate guidelines are necessary to correctly predict the overtopping rates for a combination of solutions. In this research, the accuracy of current guidelines is investigated.

The research performed in this thesis is divided into two parts. First, multiple solutions are derived to adapt a breakwater and ensure safety for a rising sea level based on existing empirical equations. Safety is ensured if the mean overtopping rate remains less than 50 l/s/m. Beyond this point, the ships behind the structure might become prone to large wave attacks. Secondly, the accuracy of current guidelines (i.e. the empirical overtopping equations) is tested in an OpenFOAM model. OpenFOAM is a so­called open­source Computational Fluid Dynamics (CFD) software that can solve complex fluid flows. 

Several solutions are possible to ensure safety against severe wave overtopping. The four solutions applied in this thesis are the addition of a berm, the addition of a crest wall, an increased foreshore and the implementation of a low­crested structure. By combining these solutions, the overtopping rate remains below the maximum of 50 l/s/m. The combination of solutions forms a path, all paths together form a pathway. The adaptation pathways are a guideline for the moment in time at which a certain solution should be implemented. Therefore, a structure is not unnecessarily expensive and can be managed easily. The paths are rated based on the implementation costs of the combination of solutions.

In total two empirical equations are applied to derive an adaptation pathway. The first pathway is based on the overtopping equation proposed by the TAW (2002). The TAW is a Dutch advisory committee on flood defences. Based on the applied theory, the economically most attractive solution consists of a low­crested structure, a foreshore and a berm. As this equation does not account for the influence of a berm in non­breaking waves, an adapted TAW equation is applied as well. The adapted equation was proposed by Krom (2012) and includes the influence of a berm. Based on the adapted equation, the economically most attractive solution consists of a foreshore, a crest wall and a berm.

Once the economically most attractive solution is derived, the accuracy of current guidelines is reviewed in a phase­resolving model. It is found that, there is a large discrepancy between the results calculated with the empirical equations and the results from the model. As no physical data is applied
in this research it is hard to interpret and analyze the exact numbers. Therefore, the relative effect of an adaptation is compared. It is found that in contrast to the TAW overtopping equation for non­breaking waves, a berm decreases the overtopping rate by at least 30% for the case study applied. Furthermore,
the current method to account for a crest wall proposed by the TAW overestimates the reduction (73% compared to 40% in OpenFOAM). Finally, it is concluded that the addition of a low­crested structure decreases the overtopping rate by a larger value than based on the applied guidelines (74% in OpenFOAM compared to 35% in theory).

Based on the performed research a realistic combination of the adaptation measures consists of a combination of a berm, a crest wall and a shallow foreshore. Therefore, it is advised to focus further research on the combination of these measures. It is necessary to improve the guidelines for combinations of these adaptation measures since the existing ones seem to be either incorrect (TAW, 2002) or require a better validation (Krom, 2012).