Tidal surface currents

Currents in coastal areas are generally dominated by the tides. These currents can be significant: speeds of several metres per second are easily reached. Ship’s speeds are directly affected by currents and therefore naval safety and shipping efficiency benefit from a good knowledge of the local tidal currents. Also for search-and-rescue operations, or the monitoring of oil spills or dispersion of toxic material such knowledge is important.

Traditionally, currents are measured with buoys and drifters. Such surveys are expensive and results are scarce. Satellites provide a wealth of information about wind and waves, but a practical way to measure currents does not yet exist. Ship’s AIS (Automatic Identification System) data form a valuable complementary source of measurements from which valuable information about the surface currents can be derived.

AIS data are broadcasted frequently by almost all ships and contain information about their position, speed, and heading. When collected, individual ships can be tracked and variations in their speed can be monitored. As the currents affect this speed directly, variations in AIS speed data can be linked to variations in the surface current speeds in the direction of the ship’s heading.

To obtain reliable information about the currents, the AIS data from many ships must be analysed with robust statistical methods. HERMESS has developed techniques to assess tidal constants in shipping lanes from AIS data that can be used to forecast the surface currents. A proof-of-concept study to determine the potential of these techniques was carried out in the Straight of Gibraltar. An overview of the results are offered for publication to Hydro International (link when available). More details can be found in the graduation thesis of Feitse Leemburg.

The developed techniques, implemented in software routines, can be applied to obtain information about tidal surface currents in (busy) shipping lanes all over the world. The required AIS data can be freely collected or bought from several commercial providers.

This study is part of the research at HERMESS to develop innovative applications based on AIS data, such as tracking large ocean eddies. Assessed surface currents can help to improve the quality of hydrodynamic model forecasts. A dedicated tidal model, developed at HERMESS to assimilate satellite altimeter sea level measurements, is also well suited to assimilate tidal current constants assessed from AIS data. The potential of this approach will be investigated in further studies.