10 Mar 2014

Great walls 'could stop tornadoes'

1:18 pm on 10 March 2014

Building three "Great Walls" across Tornado Alley in the US could eliminate the disasters, a physicist says.

The barriers - 300m high and up to 260km long - would act like hill ranges, softening winds before twisters can form, the BBC reports.

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Photo: AFP

They would cost $US16 billion to build but save billions of dollars of damage each year, said Prof Rongjia Tao, of Temple University, Philadelphia.

He unveiled his idea at an American Physical Society meeting in Denver.

Every year hundreds of twisters tear through communities in the great north-south corridor between the Rocky and Appalachian mountain ranges.

The proposed walls would not shelter towns - they would not be strong enough to block a tornado in motion.

Instead, they would soften the clashing streams of hot southern and cold northern air, which form twisters in the first place, Prof Tao said.

"If we build three east-west great walls, one in North Dakota, one along the border between Kansas and Oklahoma, and the third in the south in Texas and Louisiana, we will diminish the threats in Tornado Alley forever," he said.

As evidence, he points to China - where only three tornadoes were recorded last year, compared to 803 in the US.

China too has flat plain valleys running north-south, but the difference is they are broken up by east-west hill ranges.

Although only a few hundred metres high, they are enough to take the sting out of air currents before they clash, Prof Tao believes.

Back in the US, he notes that the flat farmlands of Illinois experience wildly varying risks of twisters.

"Washington County is a tornado hotspot. But just 100km away is Gallatin County, where there is almost no risk," he told the BBC . "Why? Just look at the map - at Gallatin you have the Shawnee Hills."

These act like a barrier 200-250m high, protecting Gallatin, he says.

"We may not have east-west mountain ranges - like the Alps in Europe - we can build walls. We've already been doing computer simulations and next we aim to build physical models for testing [in wind tunnels]."

Rather than create an eyesore, the walls could be "attractively" designed, says Prof Tao.

He cites the Comcast skyscraper in Philadelphia - also about 300m high, and built with a reinforced glass exterior - and says the tornado wall could be built of glass too.