The objective of this study is to get a new insight into in-situ characterization of fluid mud layers. Water Injection Dredging has been performed to create a fluid mud layer in the 8th Petroleumhaven at the Port of Rotterdam. Four measuring tools, conventional multi-beam echo-sounder, DensX, Graviprobe and Rheocable have been used to monitor the fluid mud layer properties over the time. The results suggest that the currently employed density-based nautical depth criterion has to be revised to ensure more efficient navigation in the vicinity of fluid mud layers@en