Observations of swash under highly dissipative conditions

Video measurements of swash were made at the low‐sloping beach of the multiple bar system at Terschelling, Netherlands. The majority of the measurements were conducted under highly dissipative conditions with Iribarren numbers ξ 0 (the ratio of beach slope to the square root of offshore wave steepness) less than 0.2. Infragravity (0.004–0.05 Hz) waves dominated the swash with an average ratio of infragravity and total swash height R ig / R of 0.85. Using linear regression we investigated the dependence of swash parameters on environmental conditions such as short‐wave height, period, and local beach slope. On average, R ig was about 30% of the offshore wave height H 0 ; the slope in the linear H 0 dependence of R ig amounted to only 0.18, considerably smaller than that observed on steeper beaches. The data set shows evidence for saturation of the higher infragravity frequencies for ξ 0 less than, roughly, 0.27. In our opinion, this saturation caused the constant of proportionality in the linear relationship between R ig / H 0 and ξ 0 to be significantly larger than that observed under higher Iribarren number regimes. The saturated tails of the swash spectra had an approximate f −3 roll‐off (where f is frequency), whereas, in general, the nonsaturated parts were white. This lack of significant peaks casts doubt on the causality between infragravity waves and nearshore bars.