Many coral reef islands are low-lying, which in combination with population growth, sea level rise and possibly more frequent extreme weather events is likely to result in increased coastal risk (e.g. Storlazzi et al., 2015). On smaller scales of O(10 km) wave-driven coastal inundation can be accurately predicted with advanced models such as XBeach (Roelvink et al., 2009), at already high computational costs. For larger scales, larger number of islands, for scenario modelling, and for implementation in early warning systems, computationally faster methods are needed. Reduced physics models, which neglect some of the processes (e.g. non-hydrostatic pressure gradient term and viscosity), are a potential solution. However, their accuracy and the best method to force them has not been established. In this research we propose a new methodology to model wave-driven flooding on coral reef-lined coasts. A look-up-table (LUT), composed of XBeach model runs, is combined with a reduced-physics model, SFINCS (Leijnse et al., 2021), to achieve high accuracy predictions at limited computational expense. The LUT consists of pre-run 1D XBeach simulations for several reef profiles from Scott et al. (2020), forced with different offshore wave and water level conditions. Wave conditions close to the shore as predicted by the LUT are used to force SFINCS which then simulates the wave runup, overtopping and flooding. These are forced in SFINCS using random wave timeseries from an interpolated parameterized wave spectrum following Athif (2020). The accuracy of the method is investigated for 6 distinctive cross-shore profiles from Scott et al. (2020), for two wave scenarios (gentle swell and stormy conditions). Results of complete XBeach simulations are compared to LUT-SFINCS simulations with different boundary forcing locations. The sensitivity analysis shows that the most optimal boundary location to initialize the SFINCS model is at a water depth of 0.5 m, shore-ward of the reef edge. Interpolation of the forcing conditions at the boundary is investigated with 9 different interpolation methods. Results reveal that the most accurate method to interpolate spectral parameters (the amount of high frequency wave energy, the amount of low frequency wave energy and the frequency peak of high frequency part of the spectrum) and wave setup at the boundary is the Inverse Distance Weighting method with a power of -2. Errors introduced by the interpolated parameterized boundary conditions lead to runup estimation errors of 10-20% on average with the maximal errors up to 60%. Simulating wave runup with XBeach LUT – SFINCS couple leads to about 50-times higher computational speed compared to the XBeach model simulating hydrodynamic processes along the entire reef profile.

In highly dynamic and vulnerable tidal systems such as the Wadden Sea, the importance of understanding natural processes and how they are hampered by anthropogenic pressure is highly demanding. Within these processes the sediment transport is one of the most challenging movements to be monitored. With this in mind, suspended particulate matter (SPM) transport in the Marsdiep inlet, the southeastern most tidal inlet in the Dutch Wadden Sea, is monitored with high frequency acoustic backscattering measurements obtained with acoustic Doppler current profiler (ADCP) on Texels Eigen Stoomboot Onderneming (TESO) ferry. The calibration of ADCP measurements is practiced with another device - optical backscatter sensor (OBS). In order to obtain reliable suspended particulate matter concentration (SPMC) measurements, the first step is to calibrate OBS output with high precision. Based on the studies done in the past, the calibration needs to be done locally and regularly as the OBS is sensitive to the variability of SPM properties. The objective of the present study is to formulate an improved OBS calibration method with in situ water samples taken from the Royal Netherlands Institute for Sea Research (NIOZ) jetty. This was achieved by applying pumping suction method to collect the water samples while measuring optical backscattering signal with Campbell Scientific OBS3+ device. Subsampling of the water samples was tested and the results revealed that subsampling leads to undesirable outcome. Procedural control filters that were applied to the laboratory procedure showed filter mass loss that needs to be taken into the account, and the analysis of salt retention showed 1.06 mg of salt remaining on the filters after filtration procedure. Moreover, loss on ignition (LOI) technique revealed the amount of organic content of SPMC which is linearly correlated to full SPMC. The analysis of spring-neap tidal cycle showed that during neap tide there was 0:5 mg l-1 more organic SPMC compared to the one during spring tide. Finally, the sources of uncertainties were identified and the guidance for further research was suggested.