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Plume Measuring Techniques for Dredging Works: the SiltProfiler

M. Bollen, M. Mathys, M. Sas

 

Acoustic backscatter information is widely used to assess suspended sediment concentrations in rivers and coastal waters.  As such, this technique is for more than a decade applied to determine the sediment concentration in plumes, created by dredging activities.  The disadvantage of acoustic methods is that they lack accuracy (and applicability) when air bubbles are present.  Therefore optical techniques are more appropriate to be used in such conditions.

The SiltProfiler was recently developed to assist port authorities and dredging contractors in understanding siltation problems in their harbor or the behavior of plumes.  The SiltProfiler is an instrument capable of rapidly profiling the suspended sediment concentration as well as the salinity structure of the water column.

As such, the SiltProfiler has successfully been used to assess the (environmental) impact of dredging works by allowing to quickly measure the suspended silt concentration in the plume (even in the propeller jet and near the bottom).

Equipped with 3 different turbidity sensors, a conductivity sensor, a temperature sensor and a pressure sensor, the SiltProfiler is capable of delivering very fast (a mere 90 seconds for a depth of 25 meters) and accurate measuring results through minimal effort when compared to other measuring instruments.

The SiltProfiler also boasts extremely accurate measurements, serving as a solid basis for meaningful data analysis.

Some recent examples of the successful use of the SiltProfiler are the measurement campaigns (in 2009 and in 2011) at the mouth of the Loire, where the intention was to closely monitor the behavior of the dredged material dumped at sea and the plumes generated by gravel dredging (France).

Other examples include a measurement campaign in Ada (Ghana, 2010-2011), the measurement of environmental conditions at the offshore Eldepasco wind farm (Belgium, 2010), a series of long term monitoring projects of the siltation rate in the tidal Deurgangdock (2006-2009, 2009-2012) or the measurement of the density currents in the locks of the Panama Canal.

The paper will illustrate the technical components of the instrument as well as a number of findings during the different measurement campaigns (in comparison with more classic instruments and techniques).