O Dan y Dŵr - Hidden Seascapes of Wales

Liverpool Bay

man-made crater zone

This false colour projection of multibeam sonar data, where colours represent different depths below the surface (blue representing the deepest and red the shallowest) illustrates the depth and physical composition of approximately three square miles of seabed located twenty miles northwest of Llandudno, to the north of the Dee and Mersey estuaries in Liverpool Bay where average water depths range between 25m-30m.



This "North-up" image illustrates the variable nature of the seabed which is dominated by large asymmetric sand waves up to 3m higher than the surrounding seabed. They are orientated perpendicular to the prevailing tidal flow which is northwest-southeast. They are asymmetrical (here the eastern face is steeper than the western face) as more sediment is moved eastwards than westwards because the flood tidal currents (which move towards the southeast) are stronger than the ebb tidal currents (which move towards the northwest. Due to the tidal currents and nature of mobile sediment available some of these sand waves have similar small-scale patterns (10cm-20cm) of sediment superimposed on their surface.



The deeper south and southwest region of this site is characterised by several hundred small circular 'crater-like' features between 20m-50m in diameter and ranging between 1m-2m in height. These features represent separate deposits of sediment that has been dredged from within local estuaries such as the Mersey and Dee to maintain their integrity as navigable channels. Over time these deposits are reworked through the prevailing tidal current regime and much of the dumped material is then re-suspended and transported back into the estuaries or onto adjacent coastlines.

(Cratered area)

(Close-up of crater)