The enormous quantity of suspended sediment within dock approaches often results in ships of up to 130,000 dwt sailing through what is effectively liquid mud which at times can form a layer several metres thick. Three port cutter suction dredgers are deployed whenever surveys indicate that a safe navigable density of 1.2 cubic million tons has been exceeded. Consequently, the Hydrographic Department's remit includes not only routine determination of seabed depths and contours but also assessing densities of suspended mud.

The remit has entailed a major investment in new equipment under a package assembled and supplied by Del Norte Technology following a year-long programme of research and trials.

A key element of the assembly is a TSS (UK) Type 25 compact motion sensor (CMS) interfaced to an Atlas DESO 17 dual-frequency singlebeam echosounder aboard the Department's 20m survey launch, Isambard Brunel. Tests of the two units have shown that a singlebeam sounder is much more effective than a multibeam system in determining seabed levels when used with the CMS, which can measure dynamic pitch, roll and heave to accuracies of better than 0.25 degrees and so provide higher levels of precision not otherwise possible.

Moreover, it can do so in difficult seas also encountered at Bristol, allowing surveys to be completed in any conditions with yet more quality data.

An added benefit of an independent motion sensor, says Giles Stimson, is that it can also be used as a backup for existing RTK (real-time kinematic) DGPS positioning facilities which may be considered adequate for motion sensing by some surveyors, but not so at Bristol, he adds.

Other elements of the new equipment package include five computer systems and associated plotters and printers together with Navitronics' Navisoft Survey and Dredge software modules. These are installed on all dredgers and Isambard Brunel together with high-intensity Nokia screens for displaying 3D seabed models and track plots used during dredging operations.

Dredging activities are also supported by a novel densitometer for determining mud densities, Dolphin 1. Developed by Blackdown Consultants in association with Bristol survey staff, Dolphin 1 is deployed aboard Isambard Brunel and is used to target densities typically around 1.2 tons per cubic metre.

Depths and locations are recorded for subsequent 3D density modelling of water columns which can then be used by dredger crews for selective targeting and removal of mud accumulations.

The levels of accuracy, says Giles Stimson, have led to significant improvements in dredging efficiencies and throughputs such that an operation which took up to 20 days can now be completed in just 6-8 days. A more sophisticated Dolphin 2 system is presently being developed based on practical experiences gained in the Bristol Channel but unlike its forerunner, it will be the first instrument of its type to become commercially available.