Bristol invests in new hydrographic system

Survey & GPS - March 16, 2001

The Port of Bristol in the UK has revealed details of new hydrographic survey systems it has invested in to tackle the problem it has with fluid mud.

Port of Bristol has a longstanding problem with fluid mud. In fact, Bristol is one of only a few ports in the British Isles where fluid mud is a characteristic of the seabed.

It is a feature that Bristol shares with a small number of other European ports including Rotterdam in the Netherlands and Wilhelmshavn in Germany, and it requires frequent surveys to determine both its depth and the density of the material.

The fluid mud at Bristol forms a layer several metres thick at times and becomes impassable to shipping if it exceeds a navigable density of 1.2 tonnes/m3. Large quantities of silt in suspension in the Severn Estuary combined with the effect of a high tidal range can result in mud conditions changing by 0.1m or more a week at the entrances to Bristol's Avonmouth and Portbury docks. As a consequence the company operates three suction cutter dredgers to maintain access for the 4,000 ship movements that typically occur every year.

In trials, multibeam echosounders proved the least suitable for surveying the fluid mud, and the Bristol hydrographers found that the majority of the single beam echosounders they tested only performed well with normal seabed conditions.

As part of a wide-ranging upgrade of its survey equipment, Bristol Port Company invested in a Navitronics DS017 duel frequency single beam echosounder last year, along with a package of related equipment supplied by Del Norte Technology, with a Compact Motion Sensor, for motion compensation, from TSS (UK) Ltd.

The dual frequency Navitronics unit - a licence-produced Atlas DESO 17 - has been integrated with a non-nuclear density meter developed by the port to differentiate between the mud and the harder layer underneath and to maintain lock of the digitised trace.

The echosounder is installed on board the port's 20m survey launch, and was delivered as part of an equipment package that also included five complete computer systems with supporting hydrographic/dredging survey software, plotters and printers. Each ofthe port's dredgers was also fitted with a high intensity flat screen Nokia displays to reduce the lighting and reflection problems that conventional computer monitors on vessel bridges experience.

According to Giles Stimson, Conservancy Manager for the port's Marine Department, the new equipment will has made a significant difference to Bristol Port's survey capabilities.

"The new equipment has improved survey turnaround and has provided a vast increase in the amount of data received and processed in a day. This has brought important benefits to the port as enables us to extend the tidal window and provide access to the port for more deep draft ships and optimise the dredging programme," explained Stimson.