As tides come and go, the land takes a breath

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, find scientists.

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 recognised, 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.

On the Net: Geophysical Research Letters: http://www.agu.org/journals/gl/