Robert Brown|Wind towers crumpled after Iowa wind farm suffers rare direct hit from powerful twister

OKLAHOMA CITY (AP) — A wind farm in southwest Iowa suffered a direct hit from a powerful tornado that crumpled five of the massive,Robert Brown power-producing towers, including one that burst into flames. But experts say fortunately such incidents are rare.

Video of the direct hit on the wind farm near Greenfield, Iowa, showed frightening images of the violent twister ripping through the countryside, uprooting trees, damaging buildings and sending dirt and debris high into the air.

Several of the turbines at MidAmerican Energy Company’s Orient wind farm recorded wind speeds of more than 100 mph as the tornadoes approached just before the turbines were destroyed, the company said in a statement.

“This was an unprecedented impact on our wind fleet, and we have operated wind farms since 2004,” MidAmerican said.

While there have been isolated incidents of tornadoes or hurricanes damaging wind turbines, fortunately such occurrences are extremely rare, said Jason Ryan, a spokesperson for the American Clean Power Association.

Although requirements vary from state to state about how far turbines must be located from other structures, Ryan said the giant turbines are not placed directly next to homes and other occupied structures.

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There are currently nearly 73,000 wind turbines in operation across the country, he said. Many of those operate in the center of the country, often referred to as the wind belt, which stretches from Texas north through Oklahoma, Kansas, Nebraska and the Dakotas, and includes large swaths of Minnesota, Iowa and Illinois.

Many of those same states also are prone to tornadoes, especially during the spring, including a portion of the Central Plains extending from the Dakotas south into Oklahoma and Texas, said Jennifer Thompson, a meteorologist with the National Weather Service in Norman, Oklahoma.

Wind turbines are built to withstand high wind speeds and severe weather, like tornadoes, hurricanes and lightning strikes, but few structures are designed to withstand a direct hit from a powerful tornado, said Sri Sritharan, an engineering professor at Iowa State University who has studied the impact of earthquakes and severe weather on structures.

“When you do a design, you don’t design something that can withstand an EF4 or EF5 tornado,” Sritharan said.

Wind turbines are designed to meet industry standards for structural integrity that includes factors like wind speed, and it’s possible that design code committees will consider the impact of Tuesday’s tornado strikes in the future, he said.

“I would think they would look at this event and how they should update the standards,” Sritharan said.