Introduction

The Hydrographic Office (HO) of the Marine Department operates a computer-based Tidal Stream Prediction System (TSPS) to deliver tidal stream predictions across roughly 1,200 points in Hong Kong waters. The system calculates tidal stream speed and direction at 15-minute intervals for both surface and averaged water flows. The prediction period covers an immediate past year, current day and a year thereafter. All data is freely accessible to the public for online viewing and downloading.

How Predictions Work

The TSPS predictions are generated by a hydrodynamic numerical model driven by tidal components at the offshore boundary. The tidal components consist of the amplitude and phase of 34 tidal constituents. Users must note that predictions reflect purely astronomically driven flow. Observed field conditions will deviate due to unmodelled temporal and localised meteorological effects and non-tidal flow variations. Model accuracy is further governed by the chosen tidal constituent set, accuracy of the underlying model, length of the simulation and location of the data points within the model, etc.

Accuracy & Operational Considerations

Comparative analysis against actual field observed datasets indicates an average difference in peak current magnitude ranging between 0.06 and 0.41 knots, with an average phase difference in peak velocity of approximately 17 minutes. Users must note that the predictions should be utilized strictly as a supplementary planning tool. They do not replace real-time watchkeeping, localized observation, or active monitoring of dynamic sea conditions.