SEA walls with cracks and fissures are bound to be undermined by the waves,
and now researchers have figured out why some types of cracks do more damage
than others.
When waves hit a sea wall, they increase the pressure of air and water inside
cracks鈥攂ut this alone didn鈥檛 seem to be enough to explain the almost
explosive damage to many blocks of sea wall. Air seemed to be the culprit,
because cracks above the waterline tend to undermine sea walls, whereas
completely submerged cracks do little harm.
To work out what was going on, Gerald M眉ller and Guido Wolters of
Queen鈥檚 University Belfast built a mock sea wall in a 17-metre-long wave tank
and measured the pressure and velocity of the water and air at various points
inside artificial cracks.
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They found that each wave hitting the wall creates a pressure pulse that
travels through the air deep into the crack. In a half-submerged crack, the
pulse travels at roughly the speed of sound and rebounds when it reaches the end
of the crack, bursting back out of the sea wall and taking bits of wall with
it.
鈥淭he impact pressures travel into the structure and damage it from within,鈥�
Wolters told a meeting on waves organised by the American Society of Civil
Engineers in San Francisco last week. Gary Griggs, a marine geologist at the
University of California in Santa Cruz, agrees. 鈥淭he compression of the air in
the cracks acts as a sledgehammer that breaks up the rock.鈥�
In fissures that lie entirely under water, the pressure pulse travels at a
much slower rate. The wave weakens rapidly and cannot inflict the same amount of
damage as a wave travelling in air.