Nautical Depth Survey
Many ports in the world are suffering from the effects of fluid mud. Fluid mud is generated in areas where fresh water meets salt water. The result is the formation of a fluid mud layer where flocculation processes are taking place. Fluid mud is a low viscosity layer that changes in time and moves with tide and currents. The fluid mud layer is navigable but generates a number of problems with significant economical consequences.
Failure of echosounder and density measurements
The main problem is caused by the fact that echosounders are not capable of measuring the nautical depth in the presence of fluid mud. High echosounder frequencies (210 kHz) are reflected on top of the fluid mud layer and the low frequencies reflections (15 kHz, 33 kHz) are unstable, inconsistent and usually show depth levels well below the true nautical depth.
Density based measurements fail just as well as echosounders in reproducing realistic nautical depth levels. The reason for these failures is that the nautical depth defined as the transition from fluid to solid mud does not represent a clear discontinuity for the density parameter. Only viscosity based methods are known to be successful in measuring the transition from low viscosity fluid mud to high viscosity solid mud.
Worldwide applied nautical depth measurement methodologies using combinations of echosounder results with density probes are notoriously inconsistent and inaccurate.
The fact that nautical depths are poorly defined has several negative economical consequences for ports trying to cope with fluid mud.
- The first problem is the unability to optimise dredge costs. Poorly known nautical depth levels do not allow port officials to decide when to start a new dredging campaign and to estimate dredge volumes. As a consequence dredge costs and mud dumping costs generally are significantly higher than they should be, because large volumes of fluid mud are unnecesarily dredged.
- Poorly defined nautical depth levels can generate contractual issues between port and dredging contractor. Cases are known involving poorly defined nautical depth levels being higher after the dredging campaign as compared to the situation before dredging!
- Severe economical consequences may be caused by shipping companies loosing their confidence in the port due to poor quality bathymetric maps supplied by the port authorities. If shipping companies would decide to avoid sailing through fluid mud, many ports would see their port traffic significantly reduced and some even would have to close down.
Solution: the Rheocable
The rheocable method was developed by Demco NV and MDCE bvba to fullfill the need for a viscosity based continuous bathymetric method capable of handling fluid mud.
A lead weight is towed over the seabed behind a survey vessel. Next to the lead weight a pressure sensor is attached to the same towing cable with a short resistivity cable trailing behind it. The resistivity cable serves the purpose of verifying that the pressure sensor is situated on the solid/fluid mud interface. If the cable is located on the solid mud layer solid mud resistivity values are measured. If the cable is floating above the solid/fluid mud interface fluid mud and/or seawater resistivity values are measured.
If the survey vessel stops the lead weight slowly sinks into the solid mud. However, if the boat moves, the high viscosity of the solid mud generates significant drag forces on the weight resulting in the weighted pressure sensor being pulled out of the solid mud. The heavy lead weight ensures the weighted pressure sensor remains on top of the solid mud as soon as the device leaves the high viscosity solid mud. The resistivity cable results are used to verify if the cable indeed remains on the seabed (depending on survey velocity and umbilical cable length).
The pressure sensor is placed in a sealed pressure pod with 2 circulation tubes reaching above the fluid mud layer to ensure the a correct translation of pressure measurements into water depths based on the known density of seawater. The water density is continously measured using CTD probes (C: Conductivity, T: Temperature, D: Depth).
A thick rubber sleeve is used to protect and streamline the cable weight and pressure sensor.
The Rheocable method is a very practical and effective method to quickly and accurately map an entire port without the need for slow and discontinuous probe measurements. While the survey launch keeps going depth measurements are obtained every second, similar to a normal echosounder survey.
Rheocable systems are commercially offered for sale by THV Nautic. The system comes with acquisition software graphically showing the ship’s position, rheocable depth, high and low frequency depths and measured resistivity value. Processing software is included as well as a short personalised training session.
More in-depth information about the Rheocable can be downloaded here.