Infragravity wave behaviour in a secondary tidal inlet, at the Slufter, the Netherlands

Abstract

In Autumn 2009, a field campaign was done in a secondary tidal inlet called the Slufter, located on the most southern Wadden island Texel, the Netherlands. A general definition of a Slufter, or secondary tidal inlet, is a salt or brackish dune valley, which is connected with the open sea and is flooded completely at least once a year. The aim of this research was to better understand infragravity wave behaviour at the Slufter during different energetic conditions and to investigate the influence of the supratidal beachplain on infragravity wave behaviour. The research is part of a larger research to better understand hydro- and morphodynamics of secondary tidal inlets. Infragravity, or low-frequency, wave height at the Slufter is related to offshore wave height and water depth. Low-frequency wave height remains constant in the shoaling and surf zone, whereas high-frequency wave height decreases. Low-frequency waves become more dominant over high-frequency waves in the inner surf and swash zone during normal, low-energetic, beach conditions. Low-frequency waves become increasingly skewed and asymmetric in the swash zone and break close to the. Low-frequency wave dissipation is caused by bed friction and low-frequency wave breaking and results in a cross-shore progressive infragravity wave pattern during low-energetic conditions, which contrasts with findings from other studies. During a storm event, the supratidal beachplain is flooded and low-frequency waves propagate over the beachplain. Low-frequency waves are dominant over high-frequency waves on top of the beachplain. However, the ratio is smaller than during normal beach conditions. Low-frequency waves become skewed and asymmetric while propagating over the beachplain, but do not break. During high-energetic conditions, low-frequency wave dissipation occurs at the landward side of the beachplain due to bed friction. Observations suggest that high-frequency wave-induced turbulence may also significantly contribute to low-frequency wave dissipation.