Net sediment transport and morphological change in the swash zone of a high-energy sandy beach from swash event to tidal cycle time scales

Abstract

A field experiment was conducted on a high energy sandy beach (Truc Vert, France) to examine swash zone processes and shoreline erosion–accretion in response to changing wave and tide conditions. Beachface morphological changes were measured at the swash-by-swash timescale using an array of ultrasonic bed-level sensors and at the tidal timescale through total station surveys. Hydrodynamic and sediment dynamic data were collected using arrays of pressure transducers, electromagnetic current meters, optical backscatter sensors and total load sediment traps. Data were collected for 16 consecutive days (33 tides) over a spring–neap–spring tidal cycle with offshore significant wave heights of 1–4 m and periods of 5–12 s. The swash motion was predominantly at infragravity wave frequencies. Over the measurement period, the upper beach experienced phases of accretion, erosion and stability. Trapped sediment loads for the uprush and backwash were predicted reasonably well by cross-shore velocity skewness, but the prediction of net transport over individual swash events by velocity-based models was more problematic, particularly lower in the swash zone where swash interactions were more prevalent. Net sediment fluxes over individual swash events often exceeded 100 kg per unit meter beach width and such rates were only one order of magnitude less than the net transport typically experienced across the high tide swash zone over complete tidal cycles. Upper beach erosion–accretion at the tidal timescale was reasonably well predicted by the Dean criterion based on offshore wave steepness and dimensionless fall velocity, but the beach gradient also played a significant role in controlling the occurrence of erosion and accretion.