Density versus viscosity ­ A comparative study of the nautical bottom
Type:
Presented during:
WODCON XVII: "Dredging in a Sensitive Environment" - 2004, Hamburg, Germany
Authors:
N. Greiser , R. Gamnitzer, J. Rupp, R. Wurpts
Abstract: In general thixotropic sediments and corresponding dredge material (silt, mud) lose their solid state when they are exposed to increasing shear stress. This liquefaction process can occurs as a result of stirring or carefully mixing the dredge material, without any input of water or other low-density substances. Although the density is not changed by this mechanical treatment, the flow curves of the sediment samples reveal that the viscosity may be reduced by about three orders of magnitude. This leads to the assumption that in situ measurements of density also cannot reveal the real stiffness of sediment layers that are to be dredged.
Nevertheless the 1.2 t/m3 density value is widely accepted as a reliable parameter for the determination of the nautical bottom. Undoubtedly this concept has been proven to guarantee safe ship passages in dredging areas, but still millions of cubic metres of liquefied navigable silt and mud that could have been left undisturbed without any negative effects on the manoeuvrability of ships, are dredged and relocated in landfills annually.
Now a reliable measuring technology is available to determine both: in situ density and viscosity and already this advanced modular ultrasound system (admodus) has been successfully used in several harbours and rivers as a pre- and post dredging monitoring instrument. Comparative results of sediment-composition related density and stiffness-related viscosity profiles, as well as the spatial distribution of the corresponding sediment layers are presented in this paper. For instance, investigations in the River Ems and in the in Port of Bremerhaven showed that sediments with density values greater than 1.2 t/m3 or considerably lower than 1.2 t/m3 may correspond well to the nautical bottom. It is proposed that the nautical bottom should be linked to that depth of the sediment body at which a specific shift of the impedance indicates the transition to truly solid sediment material. This link is a prerequisite for the derivation of reliable spatial information by echosounding. This particular stiffness-related shift of impedance can be detected in situ with the admodus-ultrasound sediment profiler (USP). Because density and viscosity-related attenuation are measured in parallel, certain additional density and/or viscosity values can be selected to define the nautical bottom for a particular dredging location.
Keywords: density, viscosity, maintenance dredging, nautical bottom, ultrasound measurements