Infragravity waves on the North Sea

Abstract

Recent analysis on measurements in the North Sea has shown a large amount of wave energy ranging in infragravity bands in the North Sea during storm events. To better understand the generation and propagation of free infragravity waves, the SWAN model is used to inspect the infragravity wave pattern, the infragravity wave origins, and the corresponding contribution from different coastlines in the North Sea. The local equilibrium between sea swell wave and infragravity wavebands is applied to parameterize the source infragravity wave along the coastlines bordering the North Sea following the approach of Arduin et. al. (2014). The empirical parameter α1 representing the radiating energy level from each coastline and bottom friction coefficient χ representing the dissipation strength are combined to calibrate the model. 10 storm events with an observed infragravity wave height over 10cm are chosen to gain detailed insight into the evolution of free infragravity waves under extreme conditions in SWAN. A correlation between α1 and χ is found. A relatively good explanation between measured and predicted infragravity wave height can be obtained as long as a large α1 and a large χ are implemented in the model or vice versa, showing an accuracy of up to 75% in the open sea. Based on a proper combination of two variables, the contribution of infragravity energy from each coastline varies spatially in predicted results. The Danish coastline is the dominant region radiating infragravity waves to the open sea and the reflected free infragravity waves could reach the adjacent coastlines such as Netherlands and UK. Behind the peak of an extreme storm event, the excitation of free infragravity energy is observed along the south of the UK coast originating from Denmark. The north of Holland coasts also radiate large amounts of free IG energy but are unable to have a prominent impact in the center of the North Sea due to energy dissipation. These results imply that the radiated infragravity waves can be influential in distant coastlines in the North Sea during storm events and the changes of local coastal dynamics can be attributed to a different generation region due to the storm characteristics.