Concrete Blocks Could Calm Deadly Waves
Ripple structures on the seafloor could force waves underneath ships and buoys, protecting the ships from storms, according to one engineer's calculations.
CREDIT: Reza Alam, UC Berkeley
You might call it a sea-based safe harbor. Structures added to the seafloor could create a patch of calm water even in the midst of rough waves, according to one mechanical engineer's calculations.
The calm spots could help protect offshore wind farms and oil drilling platforms, or create safe areas for fishermen during storms, said Mohammad-Reza Alam, who performs research and teaches at the University of California, Berkeley.
For now, the safe-spot structures are still firmly in the world of ideas. Alam estimated it would take 10 or 15 years of research to know if the structures would be commercially viable, but he's beginning to take the first steps on that decadelong journey. In February, he published a paper about the idea in the journal Physical Review Letters. Now, he's devising a lab test setup using a large water tank, he told TechNewsDaily.
Alam's proposal is to place a series of rock or concrete blocks on the seafloor around the area where calmer waters are desired. As much as possible, the blocks should be placed so they create carefully defined rippled patterns called sinusoidal waves, he said. The idea also requires that the ocean in that area has layers of slightly different water, he added. For example, around Arctic regions, the ocean tends to have a layer of colder, saltier water at the bottom and warmer, less salty water nearer the surface.
As long as the water is right, any incoming waves that pass over the concrete blocks should get forced down underneath the surface of the ocean. The waves should then continue to move underwater, just below the bottom of the buoy or boat protected in the calm spot. Once the waves encounter the next set of blocks, however, the blocks should send the waves back to the surface.
The idea works in part because it is already possible for waves to move underwater, instead of on the surface of the sea. In parts of the ocean that have layered water, underwater waves ride along the boundaries of the layers, called thermoclines. The waves are called internal waves or interfacial waves, and they're "very similar to surface waves," Alam said.
The seafloor structures transform surface waves into internal waves, or vice versa, by transferring energy between the surface and the thermocline, Alam explained.
He calls the effect of the ocean-floor structures "cloaking," as in "invisibility cloak," because ships inside the calm spot wouldn't affect the water around them. During this effect, the surrounding waves would act as if the ships simply weren't there. Without the cloaking, any floating buoy or boat would alter the shape of the ocean waves around it, in the way that rocks in a river bend streaming water around them. "It's a cloak against ocean waves," Alam said.
The idea for making ocean calm spots came to Alam after reading about physicists who make invisibility devices for microwaves and other electromagnetic waves, which include light waves. Scientists have created invisibility devices for many phenomena that act as waves, including sound waves, he pointed out, so he wondered if could do the same for water.
There's plenty for Alam to study in the coming years. Besides the tank setup, he is looking to fine-tune the math he uses to represent the ocean. Right now, his equations assume just two layers of water — cold and warm — but the real ocean has far more layers that act more like a continuum than like a set of stripes. "So we're working basically to understand what's going to happen if you have that continuous stratification," he said.
He presented his work most recently on Nov. 18 at a conference hosted by the American Physical Society.