During the typhoon season, Hong Kong is susceptible to inclement weather such as persistent heavy rainstorms and typhoons. When typhoons strike, the associated strong winds and low atmospheric pressure may cause the sea level to rise (i.e. storm surges) and huge waves to surpass the seawall (i.e. overtopping waves), posing threats to the low-lying coastal or windy locations. This time, I have invited the colleagues from the Port Works Division of the Civil Engineering and Development Department (CEDD) to introduce how the department plans ahead before the onset of the typhoon season by restoring and strengthening the existing marine facilities to enhance the protective capacity, and also how it raises the efficiency of inspection of marine facilities with new technologies.
Building wave walls to reduce threat caused by overtopping waves
Currently, the CEDD is responsible for maintaining about 127 kilometres of seawalls and breakwaters, more than 320 piers and landing steps as well as other public marine facilities. Many of you may still have a vivid memory of the onslaught of super typhoon Mangkhut in Hong Kong two years ago, which caused a certain degree of damage to marine facilities. Among them, the Tseung Kwan O (TKO) Waterfront Park was hit by huge waves, with some railings and paving blocks damaged. Engineer of the Port Works Division of the CEDD, Miss TONG Pui-sze, says that, in order to reduce the threat of overtopping waves to coastal residents and facilities, the department has constructed a wave wall of about 600 metres in length and 1.1 metres in height at the TKO Waterfront Park to enhance the protection.
Design to withstand waves and provide leisure spaces
The wave wall was built with reinforced concrete with the adoption of a curved design to strengthen the protection to waterfront facilities. Similar designs have also been adopted at the seawalls on Peng Chau and in Stanley. One may ask whether a wave wall is the higher the better. According to Miss TONG Pui-sze, given that the waterfront park is a leisure open space for TKO residents, a balance has been struck in the design such that the wave wall will alleviate the impact of huge waves without obstructing the sea view for residents’ enjoyment. Similarly, the project has retained the original metal railings on one of the landings at the TKO Waterfront Park, and constructed a concrete wave wall at the entrance of the landing to prevent waves from damaging the nearby footpath and cycle track.
Using precast concrete units to strengthen breakwaters
Furthermore, during the onslaught of Mangkhut in Hong Kong, portions of the breakwater at the Aberdeen South Typhoon Shelter formed by rocks weighing five to six tonnes collapsed due to the impact of huge waves. After the incident, instead of using rocks heavier than the original ones to strengthen the damaged portions, the CEDD replaced them with specially designed precast concrete units (commonly called dolosse).
According to Engineer of the Port Works Division of the CEDD, Mr CHENG Cheuk-hin, the dolosse are shaped like the Chinese character “工” with a 90-degree twist (thus called twisted blocks in the trade), weighing about 3.4 tonnes per unit. Interlocked with each other, the units form a sturdy protective layer that is more effective in withstanding the impact of waves than natural stones. Besides, as dolosse are manufactured artificially, mass production is possible whenever necessary to enable maintenance work to complete against time.
Applying new technologies to inspect marine facilities
To ensure that marine facilities remain stable under prolonged wave attacks, it is imperative to have in place a comprehensive inspection plan and timely maintenance. Engineer Miss TONG Pui-sze says that the CEDD has introduced new survey technologies such as the Integrated Multibeam Echo Sounder and the Laser Scanner System in recent years. With the ability to capture the overall structural condition of the marine facilities in a short time, these new technologies serve to enhance the accuracy and efficiency of inspections.
According to her, the Multibeam Echo Sounder works by transmitting sound waves and receiving echoes underwater to measure the structural forms of underwater marine facilities such as seawalls and breakwaters through computer analysis, whereas the Laser Scanner System measures the conditions of marine facilities above water level. Compared to the past when diving teams had to be called upon to carry out such inspections subject to constraints of underwater visibility, tidal conditions and sea traffic, the new integrated system can effectively shorten the inspection time, quickly locate the damage locations, and boost repair work efficiency.
In recent years, as extreme weather conditions have become more and more frequent, all departments under the DEVB are well-prepared for the coming typhoon season. I would also like to remind the public that, during typhoons, they should stay away from dangerous places, avoid going to the seaside to look at the waves, stay home to take proper precautions, and stay tuned for the information released by the Government as there is no room for complacency.
7 June, 2020Back