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. ... Read more

Since 1990, the Dutch coastline is maintained within the ‘Dynamic Preservation’ program, according to which the coastline is maintained seawards from a reference line, mainly by applying nourishments. Research into the maintenance of the Dutch coast is continuous and causes the content of the coastline preservation program to change constantly since the initiation in 1990. In recent years, the switch was made from yearly nourishment programs to the use of multiannual nourishment programs, in which an interim nourishment planning is included for 4 years. Next to the nourishments following the ‘Dynamic Preservation’ program, ten large reinforcements were applied along the Dutch coast in the past decade according to the ‘Zwakke Schakel’ project. After reinforcement, the coastline at the ‘Zwakke Schakel’ locations needs continuous maintenance to remain at the desired position.

The combination of the long term maintenance at the ‘Zwakke Schakel’ locations and the multiannual nourishment program, leads to a more or less fixed character of the nourishments program with recurring maintenance nourishments in each period. The question is to what extent also the adjacent coast is maintained by sediment transported from these recurring maintenance nourishments. A situation in which the adjacent coast can be sufficiently maintained by long term application of nourishments at the ‘Zwakke Schakel’ locations, would lead to an even more fixed character of the nourishment program. At this moment, knowledge on the contribution of sediment transported from beach nourishments to the maintenance of the adjacent coast is insufficient.

The research presented in this thesis focusses on one case study. Along the coastal stretch between Scheveningen and IJmuiden, three distinct ‘Zwakke Schakel’ reinforcement nourishments were applied at Scheveningen, Katwijk and Noordwijk. At all locations the coastline was migrated seawards, with varying distances of 60 to 100 meters. The coastal stretch between Scheveningen and IJmuiden is part of the Holland coast and bounded by the breakwaters of the Scheveningen and IJmuiden harbours. Along the Holland coast, sediment transport is dominated by wave related processes wherein longshore transport is the most important sediment transport process. Gradients in longshore sediment transport are therefore an important cause of erosion and accretion.
Results of the yearly measurements done along the entire Dutch coast already show a positive effect of the maintenance nourishments in the area. With a refined version of an existing Unibest-CL+ model the effect of the recurring maintenance is further assessed for the long term. In the model, the longshore sediment transport volumes and resulting coastline evolution are modelled for a timescale of 55 years, starting in 2006 before application of the ‘Zwakke Schakel’ reinforcement nourishments and including the effect of possible sea level rise of 0.2 to 1.5 cm per year. The model is validated by comparing transport quantities (volumes and gradients) and coastline development with real measurement results and results from earlier research.

In order to maintain a positive coastline position along the adjacent coast, the autonomous erosion needs to be sufficiently compensated by the accretion related to the long term maintenance. At several locations in the area of interest initial erosion is expected, after which the erosional trend switches into a seaward migrating trend on the long term, partly under influence of the maintenance nourishments. This process is expected to occur at both Wassenaar (between Scheveningen and Katwijk) and Noordwijkerhout (north of Noordwijk) in the upcoming decades, although the inclusion of some uncertainty in amongst others sea level rise shows that it is unsure whether a positive development at Noordwijkerhout will really occur. The erosional trend at Bloemendaal and Zandvoort, close to IJmuiden, cannot be compensated by the sediment transported from maintenance nourishments. On the time scale of 55 years, the region of influence of the maintenance nourishments does not reach Bloemendaal and Zandvoort. The regions of influence of all ‘Zwakke Schakel’ maintenance nourishments are expected to cover the area from Scheveningen up to around 10 kilometres northwards from Noordwijk in 2060. Individual regions of influences are expected to reach a size of 15 to 24 kilometres up to 2060.

Although in most cases the trends of coastline development within the regions of influence are expected to become positive on the long term, the coastline position itself may be located too much landwards due to the initial erosion. In order to solve this problem, additional (shoreface) nourishments need to be applied at Wassenaar and Noordwijkerhout. At both locations shoreface nourishments are already applied in the past decades, which supports the outcome of the model results. At Bloemendaal and Zandvoort, additional (shoreface) nourishments will surely be needed in order to maintain the coastline.
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