IntroductionNavigation in ports and waterways must be safeguarded by maintenance dredging, which removes sediments deposited by tide, river flows and currents. The volumes of dredged sediment have been substantially increased in the Port of Rotterdam (PoR) over the last five years (see Figure 1). In order to keep ports and waterways accessible, more than 11 million m3 of deposited sediment were dredged in 2017. The dredged volumes are almost doubled in comparison with the volumes dredged in 2011. The sediment depositions in these areas consist mainly of fine cohesive minerals forming mud layers, which are periodically dredged by a Trailer Suction Hopper Dredger (TSHD). As maintenance dredging and consequent relocation of mud can be highly expensive, port authorities seek for tailormade solutions that can help to reduce the maintenance costs as well as CO2 emissions and at the same time guarantee safe navigation in the port.
There are various measures that can potentially help to reduce maintenance costs and CO2 emissions. For instance, revising the intervention protocols can bring additional short- and long-term benefits. In the long-term instead of removing sediment, its presence As regular maintenance and relocation of sediment deposits are highly expensive, Port authorities seek more efficient solutions for reducing the costs and CO2 emissions of maintenance dredging. One solution, water injection dredging (WID), is carried out for maintaining the sediment deposits which predominantly consist of clay and silt. WID has been proven to be a cheaper solution by leaving the sediment in place, eliminating substantial costs for relocation of the dredged sediment. may be accepted since ships are – under certain conditions – able to sail through fluid mud.
Local sediment conditioning may be required to avoid that mud layers become consolidated to allow sailing through them. A conventional way for estimating the navigability in ports and waterways with fluid mud layers is done through the calculation of the nautical depth. This approach ensures that vessels can safely navigate through areas, where thick layers of fluid mud are detected. For practical reasons, a critical density (1200 kg/m3) is typically used for estimating the nautical depth within PIANC’s nautical bottom approach (PIANC, 2014). However, it has long been recognised, that a practical definition of the nautical depth should be based on considerations not only of density but should also include the so-called rheological properties of the water-sediment mixture (Wurpts and Torn, 2005, Kirichek et al., 2018b). Currently, a rheological parameter (100 Pa yield stress) serves as a criterium for estimating the nautical depth in the Port of Emden.