Mud density measurement saves dredging costs

Paul Boughton

In one outing, the Hydramotion MudBug mud density probe has saved Associated British Ports (ABP) over £70,000 in dredging costs.

The MudBug is used to determine the navigable depth in ports, harbours and estuaries by measuring the density of 'fluid mud'. It is an easy-to-use “plug-and-play” system consisting of a rugged, towable transducer connected by an umbilical cable to DataPod Connection Unit on the surface. 

The heavy-duty device is made from 316 stainless steel using all-welded construction, and is robust enough to withstand being towed through marine sediments. 

Importantly, the sensing element consists of a single short probe around which fluid can flow easily with no risk of material entrapment.  Unlike some other types of density meter the MudBug uses no radioactive materials, so there are no environmental hazards involved. A further advantage is that with no moving parts, maintenance is minimal.

Using the MudBug, an ABP survey team in the north-east of England was able quickly and accurately to detect the depth at which the mud reached a density of 1240 - 1250 kg/m³, generally regarded as the maximum density through which a ship can safely navigate. 

The MudBug was rigged on a davit and towed through the waters of the river Humber at Immingham Outer Harbour. The chart [shows a typical bathymetric survey produced by running the MudBug simultaneously with a standard 33 kHz echo sounder.  There was an average 0.5m difference between the results from the MudBug and those obtained from the echo sounder. 

"This run alone shows us that we can save around £35,000 of dredging," commented hydrographer Mike Abbey. "The run we did a few weeks ago already saved us about £71,000."

The MudBug also enabled the survey to be completed much more quickly than was possible with earlier density measurement methods.  Using the MudBug, it took no more two hours to gather the data for the chart and another two hours to process it.  "With our old method of density probes, it would have taken two days to collect that amount of data," said Abbey.

The MudBug's multifunctional transducer incorporates density, temperature and pressure (depth) sensors plus all interface electronics.  The unit is calibrated at the factory, so on-site calibration is not necessary.  Once at the desired location the transducer is deployed using a support line, and measurement can start as soon as the probe is submerged. The weight of the transducer ensures that it will easily sink into silt layers.  Readings are taken approximately five times a second and, as a large number of measurements can be made very quickly, it is possible to cover a wide area in a short time.

Density is measured in the range 800 to 1600 kg/m³ with an accuracy of ±1% at depths up to 100 metres. The MudBug uses its own pressure sensor to determine the depth of the transducer, thus allowing real-time density versus depth profiling.

Density, depth and temperature measurements are output by the MudBug as serial data which can be imported directly by third-party hydrographic surveying software such as HYPACK. Alternatively, the DataPod Connection Unit can be connected to a USB port on a PC or laptop, in which case no separate power supply is required as the instrument is powered simply through the USB connection. The proprietary NaviTrend software supplied with the system shows real-time measurements in both tabular and graphic form on the PC/laptop, while logged data can be exported to Microsoft® Excel™ for further analysis.

Easily integrated into existing survey equipment, the MudBug can be deployed in minutes and provides reliable data straightaway, with minimal setup and no risk to the environment.

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