Many small island developing states (SIDS) are among the most vulnerable to climate change (e.g. sea level rise) and seasonal to inter-annual climate variability, and subsequently experience flooding due to swell waves and wind waves, coastal erosion and salinisation of freshwater lenses. To counteract this, reef flat mining for sand and aggregate offers a possible solution to the material demand for coastal protection infrastructure and other engineering projects. However, the knowledge on the effects of these pits is limited to only two studies at a single reef: near-shore wave transformation based on measurements (Ford et al., 2013) and a numerical modelling study (Yao et al., 2016). This MSc thesis provides insights on the effects of pits, related to hydrodynamics and wave runup, on a large variety of fringing reefs, by using a 1D and 2DH process-based wave-resolving hydrodynamic model (XBeach non-hydrostatic+, “XBnh+”). Model results indicate that excavation pits cause a decrease in amplitude for waves in the infra-gravity (IG) frequency band. For the majority of the modelled reefs, this was mainly due to a modification of the longest natural frequency of the reef caused by the pit, resulting in a decrease in resonant amplification. Both pit width and cross-shore location have a strong influence on this mechanism. The observed changes in variance in the high frequency (HF) band can be partly explained by a decrease in wave dissipation, as well as a decrease in wave-wave (triad) interaction, both associated with (locally) increased water depth, which causes an increase of the HF peak and a decrease of the HF tail. Of all modelled reefs, there is a 15% chance of an increase in wave runup due to the presence of a pit. This probability is lowest for pits with narrow width and/or located close to the reef crest. The change in runup ranges from +10% to -20%. An increase in runup is caused by a combination of increased wave energy at peak frequencies, as well as smaller reductions in resonant amplification. Moreover, the effects of pits on mean water level near-shore of the excavation can cause circulation patterns that potentially result in coastal erosion and entrapment of sediments.

With the implementation of new legal aspects for dune safety assessment in 2017, the current method, Duros+, is no longer feasible to represent the desired processes. This 1D deterministic, volume-balance model cannot simulate processes like inundation and overwash, which are crucial to fulfil the new requirements for dune safety assessment. Therefore, the implementation of a 2DH XBeach model for dune safety assessment is evaluated. A transformation from 1D to 2DH becomes necessary and several hydrodynamic effects come into play, which are not relevant for a simulation in 1D. Additionally, the definition of a new limit state is a challenging task in order to define failure properly: The legal definition needs to be transformed into a technically feasible one. In this thesis five different limit states have been defined in order to evaluate the influence of a limit state on its consequences. These five limit states are then applied in a case study on the Delfland coast and the consequences of the different definitions are evaluated. Additionally, the safety development of the Delfland coast over the last two decades was analysed. It was found that all nourishment strategies applied at this coastal stretch contributed to an increasing safety. This includes the placement of the Sand Engine and its on-going alongshore spreading.