"You had to be underground," The Weather Channel's Mike Bettes put it, "in order to survive this tornado."
He was not exaggerating. The massive storm that hit Moore, Oklahoma yesterday -- featuring winds up to 200mph -- didn't simply put those aboveground at risk of being swept up in its funnel. The tornado also took the smallest objects of everyday life -- down to pebbles and even dust -- and effectively converted them into bullets and shrapnel. Much of the carnage that so often results from tornadoes is the result of a terrible phenomenon known by an appropriately terrible euphemism: "debris impacts."
Larry Tanner is the manager of the Debris Impact Test Facility for the Wind Science and Engineering Research Center at Texas Tech University. In that capacity, he focuses on the (relatively) optimistic side of devastating storms: sheltering people from them. One of Tanner's jobs is to test shelters and their components in his lab, creating several-hundred-mph winds and debris storms -- which can't be mathematically simulated -- to analyze those shelters' ability to withstand a natural storm. Another of his jobs is to work with FEMA to assess the performance of storm shelters after tragedies like the Oklahoma tornadoes of 1999. Or the Missouri tornado of 2011.
Or, now the Oklahoma tornado of 2013.
There are thousands of shelters in Oklahoma -- to the extent, Tanner told me, that the state "has been the most active shelter state, probably, within the United States." Yesterday, however, that wasn't enough. The storm was deadly in part because there were so few places -- underground places -- for people to escape to. As Weather Nation's Paul Douglas noted last night, fewer than one in 10 Oklahomans have access to the basements that stand the best chance of keeping them safe when a "monster" -- another appropriately awful euphemism -- strikes.
So why weren't there more underground shelters for people to escape to yesterday? Why aren't houses in the vast stretch of prairie known as Tornado Alley routinely equipped with storm cellars?
There are, unsurprisingly, several reasons. But what they come down to is this: Yesterday's tornado was deadly, ultimately, not just because of the vagaries of the sky. It was deadly, too, because of the fickleness of the ground.
Ground That's Made of Clay
The relative dearth of storm cellars in Oklahoma may be partially attributed, as things so often can, to environmental factors. The soil in the state is composed largely of clay -- and that's particularly true in central Oklahoma, where Moore is located. ("Soils in the Central Rolling Red Prairies," geologists at Oklahoma State put it (pdf), "are dark and loamy with clayey to loamy subsoils developed on Permian shales, mudstones, sandstones and/or alluvial deposits under tall grasses.")
The ground in central Oklahoma tends to be soft and moist -- right down to the bedrock that sits, generally, some 20 to 100 feet below the surface.
Here's the problem with that when it comes to building basements and underground shelters: Clay is particularly fickle as a foundation for construction. When loamy soils absorb rainwater, they expand. And when the weather's dry, they contract. This inevitable and yet largely unpredictable variability makes basement-building a particular challenge, since it makes it nearly impossible to establish firm foundations for underground construction.
And while above-ground homes can be built on these somewhat shaky foundations, adding the element of open space in the form of a basement is a nearly impossible feat of engineering. There is a chance your house, its basement surrounded by glorified mud, will eventually simply topple into itself.
To mitigate this, contractors have been experimenting with steel reinforcements for basements, bolstering underground walls with steel beams that are drilled directly into the bedrock below. The problem here, though, is that much of Oklahoma's bedrock is composed of limestone (pdf), which, just like the soil above it, absorbs water. And which, when it's sapped of moisture, becomes chalky.
So, you can try to enforce your basement with steel; ultimately, though, the steel will be anchored to rock that is "rock" only in the broadest sense of the word.
'Bill Gates's Wealth'
There's an obvious corollary to these challenges. If you're going to attempt to build a steel-reinforced shelter -- the only kind that, despite its drawbacks, would stand a good chance of keeping you safe during a storm of yesterday's magnitude -- it's going to be expensive. Underground shelters cost thousands of dollars, at minimum, and many of them much more than that. Which are not sums that many families, especially given the current economic conditions, have at their disposal. As Paul Douglas summed it up: For most people, "it's cost-prohibitive to dig basements."