RIASSUNTO
ABSTRACT
In coastal waters, algal blooms (the rapid growth of microscopic phytoplankton) are often observed. The resulting oxygen depletion and even poisoning make local fisheries suffer severe damages. The dynamics of harmful algal blooms (HABs) are poorly understood. Novel models applying unstructured grids, e.g. Delft3D Flexible Mesh (Delft3D-FM), offer a good tool for a better representation of the hydrodynamics of coastal waters. In this paper, the performance of Delft3D-FM in hydrodynamic simulation of coastal waters was examined by a series of benchmark tests. At the end, the simulated tidal flushing processes in Hong Kong harbours were compared with previous studies.
INTRODUCTION
In subtropical coastal waters around Hong Kong of China, algal blooms are often observed. Harmful effects caused by algal blooms include dissolved oxygen depletion, fish kills, shellfish poisoning, and beach closures. In April 1998, a devastating red tide initiated in Mirs Bay resulted in the worst fish kill in Hong Kong's history - over 80% (3,400 tones) of fish stocks in HKSAR were wiped out, with an estimated loss of over HK$312 million. More recently, a severe algal bloom event started in Tolo Harbour in December 2015 and lasted for two months, resulting in fish kills and threats of spreading to outer Mirs Bay and Southern Waters. Despite significant upgrades of the water pollution control infrastructure over the past two decades, red tides and HABs still occur frequently in Hong Kong and present formidable challenges to fisheries management.
The team is working on setting up an early warning system for HABs. As the core of the forecast and management system, the tidal circulation and transport of pollutants are determined by hydrodynamics. A 3-D hydrodynamic model using structured grids, i.e. Delft3D, has been applied and validated for Hong Kong waters before (Lee and Qu, 2004). To provide a better representation of the irregular coastlines including that of islands, an unstructured-grid model (Delft3D-FM) is attempted to be adopted. In this paper, the performance of Delft3D-FM in hydrodynamic simulation of coastal waters was examined by a series of benchmark tests. The computational results were compared with analytical solutions or laboratory findings. At the end, the simulated tidal flushing processes in Hong Kong harbours were compared with previous studies using Delft3D.