HYDRO 2016 Paper 10A1
The concept of nautical depth means to let vessels sail through fluid mud that obeys accepted criteria for a safe manoeuvring regardless higher density and viscosity values compared to water. Whereas density will be unaffected by a ship's movement, the viscosity of fluid mud does not stay at the same level when exposed to shear stress or pressure so that even (fluid) mud layers represented by density interfaces of <1.20 g/cm3 may become navigable by shear thinning.
The presence of fluid mud may cause complex sediment density and viscosity stratifications getting unclear transition interfaces to the consolidated nautical bottom and by fathometer surveys alone it is difficult to decide which frequency will give the best estimation of the nautical depth.
Rheological investigations on fluid mud samples from a broad range of European harbours and the U.S. Atchafalaya, Calcasieu, and Gulfport navigation channels have shown that in general fluid mud will become navigable if it is exposed to shear stress. Field tests in Husum (Germany) and other European dredging locations have proven that conditioning of fluid mud to safe density and viscosity levels can be successfully done also in situ and accompanied by a surveying strategy to generate reliable nautical depth navigation charts.