At the end of July the national media reported that Bristol Port's hydrographic survey had found the wreck of the Brunswick. Because the seabed within the Bristol Channel is highly mobile, the wreck was buried by shifting sands which have hidden it from detection for much, if not all, of the past 117 years.

At the end of July the national media reported that Bristol Port's hydrographic survey had found the wreck of the Brunswick, a British screw steamer which sank on Christmas Eve 1900.

The Brunswick ran aground on a sand bank in the Bristol Channel, resulting in the loss of seven lives. Because the seabed within the Bristol Channel is highly mobile, the wreck was buried by shifting sands which have hidden it from detection for much, if not all, of the past 117 years.

This example of a highly dynamic seabed is not unusual for estuarine and shallow coastal settings, where tidal currents and waves regularly mobilise and transport the loose sediment which covers the seabed in most places.

The movement of sediment can lead to progressive or episodic accumulation or erosion of sediment cover from an area. Change can occur quickly, with seabed level changes of 1 m or more being possible in a matter of weeks under the right conditions.

Over the past 20 years or so, considerable technological advances have enabled the seabed to be mapped in unprecedented levels of detail. In particular, use of multibeam echo sounder surveys (such as that which detected the Brunswick) has greatly improved our knowledge of how and why the seabed changes over time.

Understanding the extent to which the seabed in a particular area could be mobile and susceptible to change may be of interest to a variety of marine users. For instance, in shallow coastal and estuarine settings seabed mobility has critical implications for safe navigation and may necessitate costly dredging by port operators to ensure adequate under keel clearance.

Larger sediment bedforms (such as sandwaves) which actively migrate across the seabed also have the potential to expose or bury seabed assets such as electricity and telecommunications cables, affecting the risk profile for asset failure.

By Tony Brooks, Sediment Mobility Specialist