Dynamics and stratigraphy of a tidal sand ridge in the Bristol Channel (Nash Sands banner bank) from repeated high‐resolution multibeam echo‐sounder surveys

Abstract

AbstractRepeated multibeam echo‐sounder surveys can provide information on developing stratigraphy over large areas and during periods when environmental conditions are known. In this study, analysis encompasses 13 time‐separated multibeam echo‐sounder surveys between 2002 and 2010 of a tidal sand ridge in a macrotidal estuary: Nash Sands, a banner bank in the Bristol Channel (UK). Over the surveyed period, Nash Sands was S‐shaped in plan‐view, with twoen échelonsegments separated by a channel (swatchway). Migration of these inflections along the ridge led to deposition of clinoforms up to 5° to 7° steep and 12 to 14 m tall. The clinoforms downlapped onto their substrates or onto cross‐sets formed by dunes migrating clockwise around the lower flanks of the ridge. Clinoform topsets were removed or truncated by repeated erosion and/or dune migrations over the ridge crest and replaced with packages of near‐horizontal laterally discontinuous irregular beds. Flank dune crests were oriented obliquely to nearly perpendicularly to the clinoforms in all three sets, so the clinoforms developed by oblique−lateral accretion locally. Although one area had strongly eroded in the prior decade during elevated wave conditions, the 2002 to 2010 stratigraphic development revealed a different relationship with extreme wave heights; Nash Sands generally accumulated sand during times of more extreme waves and lost sand during more quiescent conditions. Using also single‐beam survey data from 1991 to 2002 to study the ridge morphology over 19 years to 2010, the swatchway was absent in 1991 to 1992 and progressively developed as the ridge sinuosity became more accentuated. Dunes found migrating north‐west through the swatchway are potential evidence of a current caused by tidal height differences across the ridge during ebb conditions. The study illustrates how repeated sonar measurements reveal the processes and timescales that lead to the deposition of stratigraphic units.