The importance of free infragravity waves in the North Sea: Insights from field observations and unstructured SWAN modelling

This study examines the importance of free infragravity (FIG) waves in the North Sea using a recent collection of wave measurements and a newly developed unstructured SWAN model. The measurements include new observations of infragravity waves at offshore (30-40 m water depth) and nearshore (10-20 m water depth) locations in the southern North Sea. These observations serve as the basis for model optimization and verification. Good agreement is obtained between model predictions and measurements during two recent storm periods, including severe storms with unusual wind directions and high wind speeds (e.g., “Storm Babet”). Model investigation along the coasts of Belgium and The Netherlands demonstrated a strong dependence between nearshore FIG conditions (i.e., energy intensity and sources) and storm characteristics (i.e., alongshore wind pattern and storm track). Specifically, several storms have demonstrated significant contributions of FIG energy originating from remote sources (e.g., the coasts of UK and Denmark). This suggests that nearshore FIG conditions in the North Sea cannot be determined based on the local sea-swell conditions alone and may be significantly underestimated if non-local contributions are ignored. Finally, modelled and measured results at nearshore locations along the Dutch coast revealed that under storm conditions FIG energy can be an order of magnitude higher than energy due to bound infragravity (BIG) waves. This result, augmented with estimated ratios of free and forced infragravity energy at the shoreline, emphasises the necessity of considering the FIG waves as an integral part of coastal safety assessments along the coasts of the North Sea. • An unstructured SWAN model is developed to predict free IG waves in the North Sea. • Model results closely match offshore and nearshore free IG wave observations. • Free IG energy dominates IG signals during storms, as shown by field observations. • Nearshore free IG conditions depend on storm tracks and alongshore wind patterns. • During storms, free IG energy often exceeds bound IG energy near Dutch coasts.