North Sea Infragravity Wave Observations

Coastal safety assessments with wave-resolving storm impact models require a proper offshore description for the incoming infragravity (IG) waves. This boundary condition is generally obtained by assuming local equilibrium between directionally-spread incident sea-swell wave forcing and bound IG The contribution of free waves thus ignored. Here, in-situ observations periods 100 s 200 at three measurement stations in North Sea water depths O(30) m are analyzed to explore potential total height period from 2010 2018. computed theory Hasselmann using measured frequency-directional spectra as input. largest observed open sea maximum significant O(0.3) 32 depth during Xaver (December 2013) concurrent excess 9 m. Along northern part Dutch coast, this has reduced O(0.2) 28 7 O(0.1) most southern location 34 5 These appreciable heights represent lower expected given fact that present analysis only fraction full frequency range considered. Comparisons predicted show these can contribute substantially conditions. ratio bound- variance ranges 10% 100% depending on timing storm. typically high peak both onset waning There spatial variability stations. It shown differences directional spreading play role this. Furthermore, along decreased south compared be partly related changes mean period. For coast corresponds an increased difference assumed although it not clear how much IG-energy onshore directed barring more sophisticated and/or modeling.