The coastal safety of the dunes are of high importance for low laying countries (Chen et al., 2017). This is especially true for the Netherlands as it is used officially as a safety measure against flooding from extreme weather since 1984 (Rijkswaterstaat, 1989). As of current 1D modelling of the waves is an acceptable method for the Dutch coast to assess the safety of the dunes (Den Heijer, 2013). For assessing the dune coastal safety the 98t h percentile of the runup R2% (Stockdon et al., 2006) is used for defining the wave response during a storm on the dunes. The current dune coastal safety studies consider bound waves (infragravity and sea- swell waves) propagating from offshore to nearshore. However, due to the Dutch coast being located at a very large semi-enclosed basin additional free infragravity waves can be present during a storm next to the bound waves that are reflected from other coasts (Daouk, 2020) as leaky waves (Herbers et al., 1995), and potentially increasing the R2% magnitude. Here we show that the R2% with the additional free infragravity waves in the wave domain can have a significant response on the dune coastal safety. The additional free infragravity waves can have tens of centimetres of R2% magnitude increase that is dependent on the slope of the domain. For the additional free infragravity waves a higher R2% is observed for steeper slopes, while a larger R2% growth is observed at the milder slopes when the free infragravity waves are initially introduced to the slope. The dune coastal safety assessments conducted thus far do not consider the effects from leaky waves from other coasts, therefore underestimating the safety provided from the dunes against flooding. With the research conducted by considering additional free infragravity waves with the traditionally bound waves originating in a storm, a better insight can be provided for dune coastal safety. As a result, companies and governmental bodies such as Rijkswaterstaat in the Netherlands that assess the coastal safety of the dunes can benefit by having a more complete representation of the coastal hydrodynamics that can as a result prevent flooding or provide dune nourishment after the episodic event (Sallenger, 2000).

Infragravity (IG) waves are long waves whose frequency ranges between 0.005 Hz to 0.04 Hz. The directional properties of IG waves are essential in various engineering fields, but few spectra reconstruction methods have been tested to be robust for the last forty years. In this study, artificial wave signals were created to check the accuracy and reliability of several wave spectra reconstruction methods. These data were designed as if produced from the REFLEX field measurements. Three commonly used conventional directional wave spectra reconstruction methods, EMEP, IMLM and BDM, are applied and compared over Sea-swell (SS) wave field and IG+SS wave field.

Though the existence of bound IG waves weakens the accuracy and reliability of these methods, they may be used to reconstruct the IG wave spectra with reasonable results. It is found that the EMEP outperforms the BDM and the IMLM since it yields accurate results with low sensitivity to noise in most cases. The BDM is reliable in moderate wave conditions, but would fail in case of high wave height or narrow directional spreading. The IMLM tends to produce less accurate results than the EMEP. The EMEP is recommended because of its overall reliability and low sensitivity.

Estimating wave runup is of crucial importance during a coastal risk assessment. Currently, runup levels are usually calculated using empirical formula derived from experiments conducted at beaches all over the world, and during different beach states. During this research, conditions are analyzed where runup reaches the dune toe and above. The main objective of this research is to examine the changes in the runup characteristics, which are the wave setup, the infragravity swash motions (low frequency wave motion) and the incident swash motions (high frequency wave motion). In order to examine these conditions, a fieldwork experiment has be conducted at the Sand engine near Kijkduin (South Holland). a dune of 150 (m) in width and 5.5 (m) above NAP (Nieuw Amsterdams Peil) have be constructed near the waterline. Instruments have been installed which have monitored the incoming wave conditions and the behavior of the hydrodynamic conditions at the waterline. One of these instruments was an instrument new to measurements conducted at the coast. This was a LLC (Line scanning Low Cost) LiDAR, which was used to track the runup during extreme wave conditions.
The LLC LiDAR was evaluated in terms of measuring the nearshore hydrodynamics by comparing the water level measured by the LLC LiDAR to the water level measured by a pressure sensor. The water levels from both instruments compared well with an 𝑅2 ranging from 𝑅2 = −0.86. The match in the peaks of the water levels were lower, showing 𝑅2 in the range of 0.34 − 0.78.
Data from the Fieldwork Experiment showed an increase in incident swash wave height during collision conditions when compared to swash conditions. Although the nearshore wave field showed similar conditions between the swash and collision conditions, the swash height showed an increase. Several parameters have been investigated to find the dependence of runup height to the offshore wave or beach characteristics. A best fit is found between runup and the beach slope (𝑅2 = 0.86), also indicating that swash motions are significant. Runup heights during these conditions are also compared to an empirical runup formula. This comparison indicated that runup elevations could still be predicted during conditions where runup reaches above the dune toe elevation.