Seasonal mobility of transverse finger bars within a mixed sand-gravel bay measured using X-band Radar

Abstract

Transverse finger bars have largely been associated with sandy coasts. Here we show that these features persist within a wider mixed sediment environment, adjacent to a shingle cuspate foreland, which has not been previously reported. Details of the bars' characteristics were gleaned from analysis of bathymetry data, whilst weekly migration rates were inferred from remote sensing of the sea surface roughness as a proxy of undulating bedforms, using X-band radar reflectance data. The bars were on average ~380 m long, had wavelengths of ~160 m, amplitudes of approximately 0.2 to 0.6 m and were orientated 30° to shore normal. They were found in water depths between −3.3 and −5.8 m Ordnance Datum. The bars migrated by approximately 150 m over the first ‘winter’ observation period (15/11/2020–02/04/2021) and 70 m in the following winter period (Sept 2021–Feb 2022) but showed virtually no signs of movement during the intervening summer months. Analysis of hydrodynamic conditions suggested the bar mobility was related to the dominant longshore currents resulting from high angle, south westerly waves. Low amplitude rhythmic bedforms were also found in the upper beach, migrating at a similar rate to the nearshore bars, which are thought to be driven by high-angle wave instability.