In some harbors and their approach channels, the bottom is covered with a layer of fluid mud. This not only complicates bottom survey, but also affects a ship's propulsive performance and maneuvering due to the specific rheological properties of the mud layer and to the generation of internal waves in the interface between water and mud. Finally, different types of hydrodynamic interaction between ships may take place in restricted waters, due to encountering and overtaking maneuvers, the effect of passing ships on moored vessels, lightering operations, and ship–tug interactions.