No need to remove trees from HK's maintained slopes to stop them falling

PUBLISHED : Monday, 23 May, 2011, 12:00am
UPDATED : Monday, 23 May, 2011, 12:00am

Whenever there is heavy rainfall we can expect reports of collapsed trees, such as the one that fell on a passing car in Tai Hang Road (and was featured on your front page photo on May 14).

This tree was located on a government-maintained slope covered with granite blocks. These create an impervious layer prohibiting the penetration of moisture to the underlying soil, and I assume this has contributed in no small way to the collapse. Without adequate water, trees become brittle and in the event of heavy rain cannot resist the massively increased loading. I believe that most of the trees that fall in urban areas do so because of similar water starvation.

The solution is not to remove all trees from maintained slopes, but to create horizontal cracks in the surface cover to allow some rainfall to permeate the soil. Such cracking will not affect the performance of the cover in preventing surface soil erosion and water inundation which could destabilise the slope.

There has been a recent debate about the lack of vegetation on a slope in Stubbs Road which was first sprayed with concrete and then coated with a product called Elegant Stone to give a rock wall effect. There is no doubt that the gradient is too steep for just vegetation to fix this slope, but the appearance could have been improved by the addition of climbers and creepers set into the concrete face of this wall. The Highways Department has successfully deployed this method on a near vertical slope in Kennedy Road.

Wall trees are an endearing characteristic of Hong Kong's retaining walls, and there is no reason why these indigenous species cannot be planted to enhance the look of bland walls.

Frank Lee, Mid-Levels


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