Fluid mud density measurement saves harbour dredging costs

Paul Boughton
In just one outing, a mud density probe has saved Associated British Ports (ABP) over £70000 in dredging costs.

It is easy to determine nautical depth where the bottom of the water area is bedrock. In most cases, however, the 'bottom' is composed of soft material such as silt or fine sediments which gradually increase in density with depth.

Tidal currents or storms continually erode cohesive sediments and carry them into navigation channels. As the wave energy dissipates, initially mobile suspensions stagnate to form denser static suspensions, often called 'fluid mud', or sometimes 'slib' or 'sling mud'. Such static suspensions can appear very suddenly after a storm. They may have two or more layers and may reach up to three metres in depth, which can take them significantly higher than the channel datum.

Experience in the Netherlands and elsewhere indicates that although manoeuvring characteristics may change somewhat, ships can still navigate safely through fluid mud containing up to about 15 per cent by volume of dry sediment, corresponding to a density of around 1200kg/m3. The challenge is to detect the depth at which the fluid mud reaches this critical density. A standard leadline will always indicate the greatest depth, usually at a mud density of over 1300kg/m3, while an echo sounder with a 210 kHz transducer will indicate the least depth, where the density is around 1060km/m3. This appears to be the case regardless of the consolidation of the silt.

It is difficult to obtain a useful profile of fluid mud density by acoustic methods. Echo sounders respond to both the density and the acoustic velocity gradients of the medium. Any abrupt change in density, eg at the interface between 'dirty water' and freshly settled fluid mud, will give a signal, but progressive density changes do not register.As a result, an echo sounder will often return at least two signals: a 'hard' echo from the denser, well-consolidated mud of the seabed, and a 'soft' echo from the interface between the seawater and the top of the fluid mud. With a 3.0m static suspension, the first echo would come from a depth of 19.5m below chart datum in a channel maintained at -22.5m through which tankers drawing 20.7m regularly pass. Thus the true navigable depth of a channel may be significantly deeper than the depth indicated by the first reflection of the echo trace. This is an important consideration when deciding whether or not dredging is required.

Density measurements based on core or grab samples are slow and unreliable, especially in unconsolidated sediments where the sample is unlikely to be representative of the bulk material by the time it is tested. Over the years, therefore, various techniques have been developed for measuring fluid mud density in situ.

These techniques depend on: the degree to which the mud absorbs radioactivity; the degree to which the mud attenuates an ultrasonic signal; or the variation in frequency of vibration of a 'tuning-fork' style sensor.

Unfortunately, all these approaches have disadvantages.

The first 'nuclear' or 'nucleonic' density gauges were originally developed by the UK Atomic Energy Authority at Harwell in the early 1970s. They contain a radioactive source and a detector. The degree to which the radiation is absorbed depends on the density of the material around the probe. Two types of probe are in use: (1) the 'transmission' type, an H-shaped instrument with the radioactive source in one leg and the detector in the other; and (2) the 'backscatter' type, where both source and detector are contained in a single probe.

These nuclear density gauges have serious drawbacks. Since hazardous radioactive materials are involved they must be handled with care, and their use raises significant environmental and marine safety concerns. They are usually very expensive and unwieldy, and require periodic recalibration. What is more, accuracy may be seriously affected by pollutants in the silt, including heavy metals, organic materials or dissolved gases.

The ultrasonic method has been found to be unreliable in the presence of air bubbles, and gives varying results depending on the composition of the mud. In field tests, for example, it was found that organic materials increased the attenuation of the signal.

In the 'tuning fork' device, the density of the intervening medium is derived from the vibration frequency of the exposed prongs or 'tines' of the fork. However, granular material can easily become trapped between the tines, producing errors in the readings.

The Hydramotion 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. 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 1240-1250kg/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 (Fig. 1) shows a typical bathymetric survey produced by running the MudBug simultaneously with a standard 33kHz 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 £35000 of dredging," commented hydrographer Mike Abbey. "The run we did a few weeks ago already saved us about £71000."

The MudBug (Fig. 2) 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.

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 1600kg/m3 with an accuracy of +/-1 per cent at depths up to 100 metres.

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. 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.

Hydramotion Ltd is based in Malton, York, UK. www.hydramotion.com

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