Directional characteristics of free infragravity waves in the North Sea
M. Tissier, J. Rutten, X. Zhang, A. Reniers, P. Van Wiechen, S. de Vries, D. Rijnsdorp

Infragravity waves (long surface waves of period ~20-200s) are key drivers for coastal erosion and thus need to be properly included in process-based modelling of coastal hazards. 
Uncertainties remain regarding the offshore boundary conditions for these long waves. Typically, only locally generated, bound, infragravity (IG) waves are imposed at the offshore 
boundary in wave models. This means that the possible contribution of free IG waves such as those radiated from distant coastlines is neglected. Recent studies however suggest 
that shoreward propagating free IG waves could be significant, particularly in semi-enclosed basins such as the North Sea where they could contribute to coastal hazards. 

In this contribution, we aim to determine the (directional) characteristics of bound and free IG waves from detailed measurements performed off the Dutch coast 
(deepest sensor at 14.5m depth) during an entire storm season. The analysis relies on a 5-month dataset (Nov 2021-Apr 2022) of continuous 4 Hz measurements of velocity, 
pressure and sea surface elevation collected as part of the Realdune/REFLEX field campaign. Directional spectra for both the free and bound IG waves are estimated from the data. 
This allows us to examine how the properties of the free and bound IG wave field change for different storm events and to identify the conditions during which significant 
shoreward propagating free infragravity waves are observed.