Hong
Kong Coastline Gasps for Air
HKU Bulletin, October 30, 2004
Air pressure fluctuations in coastal areas can initiate a process
of underground ‘breathing’ that causes the soil under the
surface to absorb or repel air. Consequently, like every living organism
that inhabits them, coastal areas need to breathe. But in Hong Kong,
where coastlines are reclaimed and paved over, that can present a
problem.
The interaction between the sea and land is crucial to the breathing
process, according to University scientists who for the first time
described this interchange in an article published in Geophysical
Research Letters.
Air exists in the soil between groundwater levels and the surface.
When tides come in, the groundwater level rises, pushing the air in
the soil to the surface. When the tide recedes, air is drawn from
the surface into the soil.
“This breathing process is happening all the time,” said
Dr Jiu Jimmy Jiao, Associate Professor in the Department of Earth
Sciences who, with PhD student Li Hailong, conducted the research.
“But in Hong Kong, rapid urbanisation has created a lot of real
problems. A lot of areas are covered by paved surfaces, which have
very poor permeability,
and the air cannot get in or out very freely.”
Buckled concrete is one result of this. The air pressure under the
surface can be so great, particularly when heavy rains occur at the
time of quickly rising tides, that the pavement heaves up – a
problem seen in Hong Kong.
Another problem occurs when pavement prevents air from getting in
as the tide quickly recedes. This creates a vacuum, which sucks out
fine materials such as silts and fine sand. Dr Jiao has a photograph
of an engineer standing in an underground hole created by this effect
that was two metres square. The surface area can collapse into holes
like this.
Reclamation also contributes to the problem. Dr Jiao said some reclamation
sites were filled by extremely permeable rock boulders and some incorporated
buried old sea walls, unlike natural coastlines where sand, earth
and other materials were less permeable and could absorb water waves
from fluctuating tides. Reclaimed coastlines enabled tidal waters
to move inland up to a few hundred metres from the shore. And inevitably,
they were covered in paved surfaces, further aggravating the problem.
“The air gets trapped and because of that we have very high
pressure when the sea level rises up and very low pressure when the
sea level falls down,” Dr Jiao said.
“So far as we know only Hong Kong has this problem. Few places
in the world have experienced such intense urban growth as Hong Kong
has over the last half century. This has created some environmental
and engineering problems but at the same time also offered unprecedented
opportunities for novel research.”
That does not mean other areas are immune from the effects of pavement
and reclamation, though. Dr Jiao pointed out many coastal areas around
the world were highly developed, although at a pace less rapid than
Hong Kong.
“Coastal breathing is a natural process. Anywhere we modify
it, then we have problems,” he said.
However, these problems could be reduced by being aware of the problem,
he said. Land could be reclaimed by choosing the fill materials and
structures carefully and applying surfaces that allow an exchange
of air. Another solution is to install pressure release holes at specific
sites in paved areas.
Dr Jiao added that the findings might have application in biology,
in terms of studying the effects of coastal breathing on plants and
animals.
Hydrogeology
Research Group at HKU
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