Characteristics and dynamics of surfzone transverse finger bars

Abstract

Patches of transverse finger bars have been identified in the surf zone of Noordwijk beach (Netherlands). They consisted of three to nine elongated accumulations of sand attached to the low-tide shoreline. The bars extended up to 50 m into the inner surf zone, had an oblique orientation with respect to the shore-normal, and were quasiregularly spaced in the alongshore direction. We analyzed nearly 6 years of video data and observed a significant presence of finger bars (14% of the time with good data). Bars were visible on 193 days, gathered in 44 events that persisted from 2 days to 2 months. Obliquely incident waves of intermediate and approximately constant height were dominant during finger bar presence. Shore-normal incident or more energetic wave fields destroyed the bar patches. The underlying bathymetry affected finger bar formation: inner surfzone troughs with cross-shore areas of 100 m2 and inner surfzone slopes of 0.02 were more conducive to their growth. The mean alongshore wavelength of the finger bar patches was 39 m, ranging from 21 to 75 m. Bar crests deviated up to 40 degrees from the shore-normal against the alongshore current direction ("up-current orientation") and bar patches migrated at rates up to 22 m/day in the direction of the alongshore current. We used these observations to test existing theoretical self-organization mechanisms for transverse bar formation. The "bed-flow mechanism" was the most viable explanation for the generation and persistence of Noordwijk finger bars. Our observations were consistent with most of the predictions of two models that included this interaction, but migration rates differed by 1 order of magnitude.