Publication Abstract

A recently compiled digital terrain model shows the western Channel to be dominated by a system of sub-aqueous dunes, starting abruptly at around 50o15’N, 2o45’W and extending westwards for more than 220 km, well beyond the landmasses of Cornwall and Brittany. We report on a more detailed characterisation survey of some of the larger dunes at the eastern edge (start) of the system which we surveyed in 2009 using acoustic, video and grab sampling techniques.

 

The dunes were up to 12 m high and 6 km long with wavelengths in the order of 1 to 2 km. They were barchanoid (crescent shaped) in form, having an asymmetrical cross-section with a gentle stoss slope facing east and a steeper, concave, lee slope facing west. Wave crests were generally aligned in a north-south orientation, perpendicular to the main tidal flow, but the northern horns of the barchans were typically longer than the southern horns and skewed westwards. The shape, length and location of the dunes matched almost precisely with features on the digital terrain model derived from earlier data.

 

Camera-sledge tows over the dunes revealed a gradation in sediment type, the coarse sandy gravel of the stoss slope changing to fine mobile sand at the dune crest, with coarse sand on the lee slopes changing abruptly at its base to consolidated cobble pavement with shell hash. This rapid local variation in sediment type is unusual and presents a range of niches for marine fauna to colonise. Epifauna were nearly absent, except on the cobble pavement where some large, sessile long-lived taxa were seen, suggesting these areas remained clear of sand for decades. A grab sample transect over the dunes revealed a variable infaunal community. This characterisation study will inform the management of this region, which is currently assessing the suitability of areas as Marine Conservation Zones.