Sediment transport directions by infragravity waves in the surf zone at the Sandmotor

Abstract

Numerous studies have analyzed the sediment transport by infragravity waves and their relative importance to sea-swell wave induced sediment transport. Sometimes, suspended sediment transport by infragravity-waves can dominate the net sediment transport while, at other times, suspended sediment transport by infragravity waves is insignificant. Furthermore, there is no general consensus on the direction of sediment transport by infragravity waves at a particular location in the surf zone. This study aims to explain the direction of sediment transport by infragravity waves along a cross-shore transect. Four frames were deployed at a relatively steep-sloping (1:30) beach along a transect during a six week field campaign at the Sandmotor in The Netherlands. Each frame was equipped with Optical Backscatter Sensors (OBS) to measure suspended sediment concentration, a Pressure Transducer (PT) to estimate the sea-surface elevation from pressure below the waves and an ElectroMagnetic Flow meter (EMF) to measure the flow velocity in both cross-shore and alongshore direction. Fifteen-minute averaged suspended sediment transport calculations are used to determine the direction and relative importance of suspended sediment transport by infragravity, sea-swell and mean transport components. Co-spectra between suspended sediment concentrations and cross-shore velocity are used to examine the frequency dependence of the cross-shore suspended sediment transport rate. In this study, suspended sediment transport by infragravity waves has the same order of magnitude as sea-swell waves while the mean offshore directed transport is dominant. We observe that the r0 parameter, which describes correlation between sea-swell waves crest/trough and the infragravity-wave crest/trough, is a useful tool to estimate the direction of transport by infragravity waves. In addition it pinpoints and where peaks in suspended sediment concentrations can be expected relative to the infragravity-wave along the transect. When r0 has a negative (positive) value which is predominantly in the outer (inner) part of the surf zone, the largest sea-swell waves predominantly occur in the trough (crest) of the infragravity wave and suspended sediment transport by infragravity waves is directed offshore (onshore). When r0 is negative, intense suspension events predominantly occur during negative infragravity-wave velocities (trough phase). When r0 becomes increasingly positive towards the shoreline, intense sediment suspension events occur during positive infragravity-wave velocities.