Instead of maintenance dredging, an alternative option for port authorities is to adapt the PIANC's nautical bottom approach. For practical purposes, the nautical bottom is defined as the level at which the fluid mud reaches either a critical density or a critical yield stress (the shear strength). These values generally correspond to a level at which the mud undergoes a so-called "rheological transition", where the density and strength of the mud increase rapidly over a short distance. Below this level, the mud becomes more and more like solid ground and is therefore no longer navigable.
Recently, new scientific and practical research has been conducted in order to gain additional knowledge on navigability in ports with fluid mud layers. In particular, a systematic rheological analysis was conducted to determine the critical limits of the yield stresses and density of fluid mud. Furthermore, a Computational Fluid Dynamics (CFD) model was developed to numerically investigate the ship-mud interaction. The model was applied to study the effects of muddy bottoms on the full-scale resistance of a modern oil tanker at speeds between 3 and 9 knots. It was confirmed that not only the density but also the yield stress of the fluid mud should be considered in the practical application of the nautical bottom. Finally, the paper discussed how the standard maintenance dredging methods can be used for producing navigable fluid mud layers.@en

For future strategies in water depth maintenance in the Port of Hamburg, determining the navigability limit (i.e., the safe nautical depth) is of major importance. For this purpose, the project Nautical Depth was set up at the Hamburg Port Authority. The aim of the project is to measure a safe nautical depth under various boundary conditions and to identify limits for a safe passage in port areas with fluid mud. Among other things, the project is conducted in close collaboration with the Port of Antwerp Bruges, the Port of Rotterdam, and TU Delft within the research platform MUDNET (www.tudelft.nl/mudnet).

To gain the necessary acceptance for a reassessment of the nautical depth, it is important to determine the rheological properties of the sediment in-situ. Several existing survey devices for monitoring nautical relevant rheological sediment properties have been tested. However, these devices cannot be used for spatial determination of rheological properties as they only provide cross-sectional measurements and depth profiles. Therefore, new evaluation algorithms were developed to correlate echo sounding data with in-situ rheological properties to ensure spatial coverage of a safe nautical depth.

As a first step, hydrographic surveys and sampling campaigns were carried out to provide system knowledge of the water column and the riverbed. Basic scientific research on the rheological behaviour of the fluid mud was also carried out. In a second step, the safe conditions for adapting the fluid mud layers at the berths for the nautical bottom concept were investigated and new nautical depth approaches were introduced at different locations in the Port of Hamburg. The next step was to analyse the effect of the fluid mud on the manoeuvrability of the ships. To this end, existing Computational Fluid Dynamics (CFD) models were extended to determine the non-linear effects and forces of non-Newton fluids on ship hull, rudder, and propeller efficiency. The results of the CFD model have been and are being validated with hydraulic model tests (scale 1:60) and in-situ measurements in the Port of Hamburg. In a final step, the results of the CFD model will be used to adapt the software of existing ship handling simulators. These simulations will be used to define the safe boundary conditions and limits for a new nautical depth for vessels to navigate through the fluid mud.@en

The nautical bottom (i.e., the level at which contact with a ship’s keel causes either damage or unacceptable effects on controllability and manoeuvrability of a ship) should be associated to a measurable physical characteristic. Bulk density is typically used as a criterion for nautical bottom by many ports worldwide. However, the rheological properties particularly the yield stress of mud are eventually more suitable parameters for defining a criterion for nautical bottom due to their strong correlation with the flow properties of mud and navigability. The density-yield stress correlation depends significantly on different parameters of mud such as organic matter type and content, clay type and content, particle size distribution and salinity. Therefore, a single critical value of density cannot be chosen for the nautical bottom criterion, where the above-mentioned parameters are varying from one location to another in the port. This justifies the need for a study of the rheological properties (yield stress) of mud. The main objective of this review article is to provide (i) an extensive overview of the rheological
properties (particularly yield stress) of mud from different sources, (ii) factors affecting the rheology of mud, and (iii) defining a nautical bottom for berthing areas in the port of Hamburg using a combination of yield stress and density.@en

This paper provides a new insight into the surveying strategies that are used (or can be potentially used) in ports and waterways with fluid mud layers. The combination of acoustic methods with other methods, which are based on the density or on the shear strength measurements, is discussed. In particular, the measurements by the DensX, the Rheotune, the Graviprobe and the Rheocable are presented Due to the complexity of mud, the strength of mud exhibits a non-linear relationship with the density due to the thixotropic behaviour (deformation, history and time dependence). Therefore, sampling and measuring procedures, followed by data processing and interpretation of the measurements, have to be standardized by means of recognized practical protocols. In-situ measuring tools are available for characterizing the behaviour of fluid mud. Based on our observations, we can conclude that the new surveying strategies can be potentially developed in the ports and waterways with fluid mud layers.@en