There is a definite need to better define a uniform and exact nautical bottom within loose mud deposits, characterized by low shear strengths in their upper parts. This chapter presents a better approximation of a mud nautical bottom definition, based mainly on the rheological characteristics of the mud considered. From the results it will be clear that direct application of this theory in the field is possible. Maintenance dredging can be optimized in two ways, by introducing such a definition of the nautical bottom:

  • a better definition of the target depth for piloting the dredging works;

  • the possibility of better and more uniform production work.

Additionally, intensive mud survey allows a better understanding of the behaviour of the deposit and may form a programming tool for maintenance dredging; this will be illustrated by some practical examples.


In 1984 the Coastal Service of Ostend, of the Administration of Waterways (Ministry of Public Works, Belgium), ordered a research program wherein, amongst other items, special research is to be carried out on the definition and the detection of the so-called nautical bottom in mud. This chapter presents some of the research results obtained so far.

A nautical bottom as determined by an echo-sounder or a lead wire does not seem to form a basis for discussion, since the hydrographer want to define the bottom according to safe navigability and manoeuvring capacity, and will thus use for this bottom the level where the medium changes its behavious drastically, i.e. the water-sediment or water-rock interface. From there on, all calculations can be executed to determine the nautical depth (including keel clearance, sedimentation, tide reduction, etc.), for the programming of maintenance dredging works and for the description of the navigability and manoeuvrability of the access channel or waterway considered.

Loose mud deposits in waterways are characterized by:

  • a sharp water-mud interface;

  • very low mud shear strengths in the upper parts of the deposit.

These considerations lead to the conclusion that a nautical bottom in mud is not necessarily determined by the water-mud interface, but may be located within the deposit itself. The problem is what level to indicate and what unit to use for the characterization of the nautical bottom, and thus safe navigation also.

Previous work and investigations tend to define the nautical bottom as a density level within the mud deposit. Some relevant work can be summarized here. Extensive investigations have been made in Bangkok and along the coast of Surinam, where ships sail in mud with negative keel-clearance; in Bangkok a volume-mass value of 1.230 t/m3 of mud is still considered as a safe value in which vessels can sail. On the basis of these investigations and a literature search, it was found that densities up to 1.2 has only a slight influence on manoeuvrability (Kirby and Parker, 1979; Van Bochove, 1979; Nederlof, 1979). Actually in the Rotterdam Europoort, the so-called "nautical bottom" has been set at the 1.2 t/m3 density limit to be on the safe side (Nederlof, 1981).

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