Revision of the nautical bottom concept in Zeebrugge based on the manoeuvrability of deep-drafted container ships

In the harbour of Zeebrugge, Belgium, the bottom is covered with a mud layer with physical characteristics (density, viscosity, yield stress) gradually increasing with depth. The bottom is difficult to define with echo-sounding, so that the nautical bottom concept has been introduced: the level where physical characteristics of the bottom reach a critical limit beyond which contact with a ship's keel causes either damage or unacceptable effects on controllability and manoeuvrability (PIANC, 1997). The upper mud layer can be considered as black water while at a lower level a transition between loose and solid mud occurs. In Zeebrugge, in-situ measurements in the 1980s revealed that this transition corresponded with a density of 1.15 ton/m3 or higher; this density was selected as the critical limit. More recent measurements showed an increase of the layer thickness and a shift of the rheological transition level to higher densities. An adaptation of the density criterion would offer the advantage of an optimisation of the maintenance dredging efforts without affecting the accessibility of the harbour. However, contact between the mud and the keel of deep-drafted ships would be unavoidable, so that additional research on the controllability of vessels navigating above or through mud layers was required. An extensive research program, consisting of captive model tests and full bridge manoeuvring simulations with a 6000 TEU container carrier was carried out at Flanders Hydraulics Research, Antwerp, scientifically supported by Ghent University, on behalf of the Maritime Access Department (Ministry of Flanders) and TV Noordzee & Kust (Ostend). During the simulations, Zeebrugge pilots experienced that a ship in contact with lower density mud layers is still controllable. Eventually a new critical density of 1.20 ton/m3 can be adopted. Nevertheless sufficient tug capacity must be available to guarantee safe and economic manoeuvres