Nearshore sandbar rotation at single‐barred embayed beaches

Abstract

The location of a shore‐parallel nearshore sandbar derived from 7 years of video imagery data at the single‐barred embayed Tairua Beach (NZ) is investigated to assess the contribution of barline rotation to the overall morphodynamics of sandbars in embayed environments and to characterize the process of rotation in relation to external conditions. Rotation induces cross‐shore barline variations at the embayment extremities on the order of magnitude of those induced by alongshore uniform cross‐shore migration of the bar. Two semiempirical models have been developed to relate the barline cross‐shore migration and rotation to external wave forcing conditions. The rotation model is directly derived from the cross‐shore migration model. Therefore, its formulation advocates for a primary role of cross‐shore processes in the rotation of sandbars at embayed beaches. The orientation evolves toward an equilibrium angle directly related to the alongshore wave energy gradient due to two different mechanisms. Either the bar extremities migrate in opposite directions with no overall cross‐shore bar migration (pivotal rotation) or the rotation relates to an overall migration of the barline which is not uniform along the beach (migration‐driven rotation). Migration and rotation characteristic response times are similar, ranging from 10 to 30 days for mild and energetic wave conditions and above 200 days during very calm conditions or when the bar is located far offshore.

Observations on sandbar behaviour along a man‐made curved coast

“…In addition, Blossier et al. () and Ojeda et al. () reported on the rotation of a sandbar due to longshore differences in 2D behaviour, similar to beach rotation (e.g.…”

LiDAR Validation of a Video-Derived Beachface Topography on a Tidal Flat

“…Such beach dynamic is the result of both beach extremities showing contrasting behaviors: erosion on one side and advance on the other [71]. This rotation could be related to the storms that occurred on 22 October and from 12 to 16 November, the latter occurring two days before the second survey.…”

“…Instead of assuming empirical relationships to estimate the shoreline elevations, the similar approach applied here uses in-situ water levels and only small wave heights (<0.15 m). Similarly, Blossier et al [71] selected only wave heights less than 1 m to minimize errors associated with wave setup estimations. Errors obtained above are thus in the range of what was observed by Morris et al [45], whom by using DGPS-RTK for shoreline validations in a lagoon without wave effects, obtained vertical accuracy in the order of 0.05 m. The results presented here validate the video-derived elevation values with the LiDAR topographic surveys.…”

Wave breaking patterns control rip current flow regimes and surfzone retention

“…Quantifying these wave breaking patterns using in situ measurements is difficult. However, such patterns can be relatively easily identified using coastal imaging systems, such as Argus (Holman and Stanley, 2007), Cam-Era (Gallop et al, 2011;Blossier et al, 2016), and WavePack (Gal et al, 2014). These cameras typically take high frequency, half-hourly images of the beach and nearshore during daylight hours (Holman and Stanley, 2007).…”