As tides come and go, the land takes a breath
Tuesday, July 20, 2004 Posted: 1:38 PM EDT (1738 GMT)
WASHINGTON
(AP) -- The millions of people around the world enjoying beach
vacations this summer may not be aware of it, but the land beneath
their feet is breathing.
As tides come and go, the water
causes changes in underground air pressure, forcing air and moisture in
and out of the ground along the shore, Jui J. Jiao of the University of
Hong Kong found in studying coastal areas near his school.
While
tidal changes in underground water levels along the shore were known,
changes in air pressure beneath the surface were not as well
recognized, Jiao reports in Geophysical Research Letters.
The
inhaling and exhaling isn't something likely to be noticed by
frolicking beachgoers, though Jiao believes organisms living in the
soil should be able to detect it.
"People can feel the process
only under very special conditions. In Hong Kong, people once saw the
water bubbles coming out of the fractures or joints of an asphalt
pavement and heard the noise of the air flow through fractures," he
said.
But that was a special case with a rapidly changing tide
and air and water moving through a limited number of underground
pathways.
"Under normal conditions, people usually cannot
directly detect this phenomenon. This is probably a reason why this
phenomenon has not been studied until recently," he said.
Jiao
said the breathing causes a constant exchange of air, water moisture
and any pollution that may be present between the soil and the air.
In
addition, the constant changes in pressure beneath the surface may have
an effect on buildings, airport and seaport structures and coastal
highways, which are usually designed to support downward acting loads.
Since
asphalt and building structures can block this air flow, the coastal
breathing may produce repeated cyclical -- up and down -- pressures on
these structures.
"It is well known that the strength of
materials under cyclic conditions is significantly lower than for
(single direction) loading," Jiao said via e-mail. "The performance of
concrete or asphalt surface under cyclic loading by tidal-induced air
pressure below the paved ground surface needs to be evaluated."
Douglas
L. Inman of the Scripps Institution of Oceanography in La Jolla,
California, agreed that the finding "will be very important to coastal
engineers working in areas of reclaimed land where it is important to
avoid having roads and structures pop-up."
Inman, who was not
part of Jiao's research team, said that the breathing may be more
common in areas of artificial fill, like Hong Kong.
"There should
be much less 'breathing' along natural coastal areas except where the
... rock consists of porous limestone or cobbles," he said.
Jiao
and his co-author, Hailong Li of the China University of Geosciences at
Wuhai, found that regular changes in below-ground air pressure which
were most affected by the rising and lowering of the water table with
the tides.
The process, which is lessened when it is raining, is relevant to coastal areas worldwide, Jiao said.
The
research was funded by the Hong Kong Research Grants Council, the
University of Hong Kong and the National Science Foundation of China.
Copyright 2004 The
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