Study of Inner Harbour Waves and Their Reduction
Findings and Recommendations
ACE Paper (38/99)
In recent years, wave conditions in Victoria Harbour have become rougher. As a result, cargo handling, berthing and similar activities have become more difficult, and passage on small vessels has become less comfortable in certain parts of the harbour. Public concern over marine safety in the harbour has increased. In view of this, the Civil Engineering Department of the Hong Kong Government commissioned the Department of Mechanical Engineering of the University of Hong Kong in February 1996 to conduct the Study of Inner Harbour Waves and Their Reduction. The main purposes of the study were to investigate the wave conditions in the harbour, identify critical problem areas and develop a prototype design of a wave energy-absorbing seawall suitable for use in Hong Kong. The study included an extensive investigation comprising database development, field wave assessment which is supplemented by ship-wave assessment and photogrammetric/aerial-photographic survey on harbour waves, investigation of ship-generated waves, development of wave-absorbing seawall structures, and design outline of the seawall structure for engineering applications. An additional study on the Impact of Planned Reclamation on Waves was requested in October 1996 to investigate the cumulative effect of proposed harbour reclamation on waves. The entire project was successfully completed in February 1999.
2.The main study has indicated that under normal weather conditions, wind-generated waves and offshore swells are low and pose no significant problems to the operation of vessels and port activities in the harbour. The deteriorating wave condition is mainly the result of the increased marine traffic and waves reflected from vertical solid seawall around the harbour also contribute to the build-up of wave energy. The study shows that wave heights vary according to the pattern of marine activities. From mid-night to dawn when the marine traffic in the harbour is light, the sea is relatively calm. Thereafter, as more and more ships move in the harbour, the sea surface becomes more disturbed. The highest waves are found in the morning peak hours around 7:00 to 8:00. By that time, strong background waves have built up and are maintained at a fairly high level throughout the day. At around 17:00 to 18:00 during the afternoon, another peak period of wave disturbance occurs. The sea calms down gradually throughout the night as fewer ships are moving in the harbour.
3.Based on the extensive wave measurements at 18 wave monitoring stations around the harbour and the aerial-photographic survey, the existing harbour wave field can be classified into four characteristic wave regions according to the significant wave height (Hs) distribution. It reveals that waves in the western part of the harbour are generally higher than those in the eastern part and are particularly strong in the region from Western District Public Cargo Working Area to Macau Ferry Terminal along the northern coast of Hong Kong Island.
4.The functions of seawall structures in Victoria Harbour should include not only the shore protection and harbour activity support, but also the wave energy dissipation to minimise their impact on the harbour wave climate due to their reflectivity. Most sections of existing seawalls, however, were designed based on geotechnical and structural stability at specified load level and risk measurement. Wave energy dissipation at seawalls was not an important technical issue in previous planning and designs. As one of the key issues of this study, effective wave absorbing seawall models suitable for harbour utilisation in Hong Kong have been investigated comprehensively. Recommendation of the preferred seawall models has been made based upon literature review, analysis on wave dissipation mechanisms and efficiency, evaluation on engineering suitability and feasibility, and results of physical model tests under wave conditions similar to the actual wave field in the Study Area. The outline engineering design of the selected seawall model to be tested in situ has been produced.
5.For studying the cumulative effect of planned reclamation on harbour waves, four scenarios of reclamation as advised by the Secretary of Planning, Environment & Lands (SPEL) are used, namely the base year 1996, the 1996 committed reclamation, the mid-term reclamation (year 2006), and the long-term reclamation (year 2011). Since ship-generated waves are the primary source of harbour wave formation and development, it is thus important to study the effect of marine traffic growth on harbour waves. The growth in marine traffic based on the predictions of the MARAD Study by the Marine Department, i.e. the 1996, 2006 and 2011 marine traffic conditions have been used for this purpose.
6.In summary, the wave climate in the harbour due to ship disturbances, wave reflection at boundaries, ocean swells and normal winds are studied under specified scenarios of navigation and reclamation in the present and strategic planning years. The following conclusions and recommendations can be drawn from the study:
The deteriorating wave condition in Victoria Harbour is mainly due to the increased marine traffic, especially those high speed, high power vessels of the displacement type. The predicted significant wave height in the area near Sai Ying Pun would be greater than 1.0 m by 2011. In comparison with ship-generated waves, wind-generated waves and ocean swells have no significant contribution to the harbour wave system under normal weather conditions.
The progressive reclamation has only a passive impact on the wave climate. It reduces the available area for wave energy dissipation, causes wave energy re-distribution and alters the regional wave agitation pattern in terms of location and area. This indirect impact on waves is relatively gentle over a large region, but can be severe in a local area such as Sai Ying Pun.
The growth and diversion in marine traffic and wave energy redistribution upon reclamation off Green Island and Central* will significantly worsen the wave agitation problem in the western part of the harbour with the greater impact coming from marine traffic growth and vessel speed. The control of high speed, high power vessels of the displacement type travelling through the area is considered to be the most effective measure to mitigate the wave agitation problem. However, the realistic correlation between navigation characteristics of a multiple vessel system and features of ship-generated waves requires further investigations.
Wave energy absorbing seawalls should be used for all new reclamation works in Victoria Harbour. It reduces waves in the vicinity of the seawall and the background waves in the harbour due to low reflection. Some improvement can be achieved by locally modifying the existing vertical seawall at Sai Ying Pun. Since the existing land use along this critical length of seawall may not permit such works, the feasibility and cost-effectiveness of this option will need to be further investigated.
In the future design of harbour reclamation and marine traffic arrangement, their impact on waves should continuously be assessed by the computer simulation, long-term wave monitoring, and physical modelling. Mitigation measures should also be investigated and tested.
Professor Allen T. Chwang
Sir Robert Ho Tung Chair Professor
Head of Department
Department of Mechanical Engineering
The University of Hong Kong
The Study was based on Government's reclamation proposals as at 1996. According to the latest information provided by the Planning, Environment and Lands Bureau, the proposed Central Reclamation has been reduced by about 50%. On the other hand, the Administration is reviewing the scale and scope of reclamation off Green Island.