The nautical bottom is defined as 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 the controllability and manoeuvrability of ships. The nautical bottom is usually assessed using the density of the bottom fluid mud layer as a criterion. However, for navigability purposes, rheological properties of bottom layers (and in particular yield stress), rather than density, are key parameters. The rheological properties of fluid mud depend on both the density and composition of mud and this composition can vary over time, owing to organic matter degradation.
In this study, the influence of organic matter degradation on the rheological properties of mud is investigated. A total of 129 samples from different locations and mud layers (depths) from the Port of Hamburg were analysed. They were degraded in the laboratory under aerobic and anaerobic conditions for 250 days. The rheological properties of these samples were analysed before and after degradation using a rotational rheometer (HAAKE MARS I).
The rheological properties of the samples before and after anaerobic degradation were significantly dissimilar. On average, the fluidic yield stress decreased by 26 %. This percentage had a strong positive correlation with the total organic carbon contents and degradabilities of the samples under consideration. In contrast, the seasonal variability of the samples did not show any correlation with the rheological properties.
After degradation, the decrease in total organic carbon content is small and could not be correlated to yield stress changes. This leads to conclude that the structural breakdown of organic matter and/or the breakdown of organic bridges between organic matter and clay particles are the reasons for the decrease in strength.
The fluidic yield stresses of the aerobically degraded samples increased by 2 %. The difference with their anaerobically degraded counterparts is suspected to be caused by oxidation, which could add cohesion to the mud. From this study, it can be concluded that intentional organic matter degradation during dredging operations could be very effective in making the mud navigable.