Physical Oceanographic Studies in Hong Kong
Introduction: Oceanographic information is indispensable in many activities including astronomical tides for maritime operations, storm surge levels to warn of possible sea flooding and for reclamation projects, and wave heights for shore protection, sea wall design and offshore operations.
The Hong Kong Observatory carries out work in physical oceanography to:
(a) give technical support to the Central Forecasting Office in connection with storm surge advisory services during tropical cyclone passage and with the provision of marine meteorological services;
(b) provide a tidal prediction service;
(c) provide consultative services for coastal engineering projects;
(d) answer enquiries related to tide, waves and other aspects of ocean hydrodynamics.
Tide Measurements: The Hong Kong Observatory now operates six tide gauges at locations shown in Figure1. They provide information for tidal prediction, storm surge advisories or mean sea level analysis.
Preparation of Tide Tables: The Hong Kong Observatory prepares tidal predictions for Hong Kong waters every year to meet the requirements of users ranging from ferry operators, coastal engineers to anglers. Based on the analysis of past tide records, predictions of astronomical tides for the following year are made and published in "Tide Tables for Hong Kong" and are also available on Observatory's homepage ((http://www.weather.gov.hk/tide/estation_select.htm).
Consultative Services: The Hong Kong Observatory has long provided consultative services on oceanographic matters to other government departments and to commercial companies for engineering design. Examples range from oceanographic advice on Sha Tin New Town development to the airport project at Chek Lap Kok. A fee on a cost-recovery basis will usually be charged for non-government projects.
Research and development: Storm surge: On 2 September 1937, villages along the coast of Tolo Harbour were flooded by a storm surge brought about by a typhoon. Thousands of lives were lost and about six kilometres of embankment between Sha Tin and Tai Po were destroyed. The protection of population and property from sea flooding through the provision of storm surge information is therefore an important activity of the Hong Kong Observatory.
The Observatory adapts and runs storm surge models. There are generally two categories of numerical storm surge models. One is the 'open coast' model and the other the 'bay' model. The main purpose of the first model is to give indications of the magnitude of storm surges in a place when tropical cyclones are still far from land. It estimates storm surges from the forecast intensities and movements of approaching tropical cyclones so that advice on storm surges can be issued to the public in time when necessary.
As tropical cyclones can cover distances of hundreds of kilometres, the area covered by the open coast model has to be large and the grids used in the model correspondingly coarse. The disadvantage is that small variations in coastlines such as bays and inlets cannot be resolved. The 'bay' model complements the first by using a very fine grid to resolve those geographic features such as bays and inlets at which storm surge observations are absent and where storm surge information is required as design parameters for coastal structures such as embankments.
The Observatory presently uses a single storm surge model operating from finer grids in bays and near coasts but coarser grids to the open sea. It achieves the function of the 'open coast' and 'bay' models at the same time.
Waves: One of the Hong Kong Observatory's responsibilities under its marine meteorological services is the forecasting of wave heights or sea states for shipping. Conventional nomograms representing parametric relationships among wave height, wind, fetch and duration are used in operational forecasting of sea state. The Hong Kong Observatory also runs wave models to give forecasters additional guidance. An example of the wave model output is shown in Figure2.