Plants can soften the appearance of noise barriers and improve the streetscape, Highways Department Chief Engineer Chow Chun-wah says. Special panels will be put on trial on two road sections in Kwun Tong and Tsuen Wan where noise-barrier retrofitting will begin in 2007.

Noise barriers and enclosures are often installed to protect residents from traffic noise. While they mitigate noise, they also have a visual impact. In this regard, a study was launched early this year to explore the greening and aesthetic design of noise barriers.

Bio barriers: Highways Department Chief Engineer Chow Chun-wah (left) says vegetated panels (centre) will adopted as part of the noise barrier on Tseung Kwan O Road. To alert our feathered friends, falcon stickers (right) have been put on the transparent noise barrier along Kam Tin Bypass to make the panel visible to birds.

Design practices

In an interview with news.gov.hk, Mr Chow said the study looked into overseas noise-barrier greening and aesthetic design practices. Many countries require noise barriers that are only two to three metres high, built of wood, stone or bricks. But needs are different in Hong Kong - where there are many high-rise buildings with residential premises close to highways.

The study also found due to site constraints and people's preference, Hong Kong usually uses transparent panels.

"In view of the study findings, we will mainly use transparent noise barriers in future projects. If site conditions allow, vegetated panels can be used as the lower part of noise barriers," Mr Chow said.

These panels are made of material in which seeds can be planted, to grow into plants that form a natural, green noise barrier. 

"However, maintenance is a great concern. If planting fails, the barriers will lose their visual appeal and function. Therefore, vegetated barriers require more frequent and careful maintenance."

Stone wool as barriers

The materials for vegetated barriers must be sound absorbing - and provide a medium in which plants can grow. Stone wool - a material made of molten rock which is spun like cotton candy and then solidifies - has high water-retention, is lightweight and sound absorbing, making it a good option for vegetated panels.

The chosen plants should come in different forms, colours and textures. They must be evergreen and fast growing, and able to tolerate pollution and drought with little maintenance. Some of the best choices are creeping fig, ivy or diverse-leaf creepers.

Mr Chow stressed the acoustic function of green barriers is the same as conventional ones, though the cost for building green panels is 10% to 20% higher.

To see if vegetated panels work well in Hong Kong, they will be adopted as part of the noise barriers to be retrofitted on Tseung Kwan O Road near Tsui Ping (South) Estate and Tsing Tsuen Bridge Tsuen Wan Approach. Work will start in 2007. The first is expected to be completed in 2009, the second in 2010.

Effectiveness monitored

"We will monitor and review the effectiveness of green noise barriers. If they prove to be successful, more vegetated panels will be used in our projects," Mr Chow  added.

On other greening measures, such as concrete panels with climbers, the engineering expert said the public prefer transparent panels since concrete ones may block views and light.

Earth mounds, which use plants in conjunction with soil-filled supporting structures made with concrete or recycled plastic, may take up more space but blend naturally with the environment, he said. Suitable sites in new highway projects are being identified for introducing such mounds. The department will also explore the feasibility of using enclosures with green rooftops.

Apart from the acoustic, aesthetic and greening aspects, engineers must also take into account the safety of our feathered friends, especially when designing noise barriers in areas with high bird density or in rural areas.

Barriers made visible

"With the advice of the Agriculture, Fisheries & Conservation Department, we have put falcon stickers on a section of transparent noise barriers along Kam Tin Bypass since June, to make the barriers visible to birds, following reports that bird carcasses were found," Mr Chow said.

Local and overseas experiences have shown that falcon stickers can cut the number of birds who fly blindly into the transparent barriers. The AFCD's initial assessment found the trial on Kam Tin Bypass worked well, with only one dead bird found in the two-month observation period.

In the same period, 20 bird carcasses were found along the section where no falcon stickers were posted.

To confirm the stickers' effectiveness, falcon stickers will be put on another section of transparent noise barrier along Kam Tin Bypass in early 2007.

Mr Chow noted so far there are no dead-bird reports for other roads fitted with transparent noise barriers.