Harvey floodwater was so heavy it sank Houston by two centimeters ... temporarily

Chris Milliner at NASA's Jet Propulsion Laboratory at the California Institute of Technology says Houston actually sank two centimeters under Harvey's floodwaters. Chris Milliner at NASA's Jet Propulsion Laboratory at the California Institute of Technology says Houston actually sank two centimeters under Harvey's floodwaters. Photo: Brett Coomer, Staff Photo: Brett Coomer, Staff Image 1 of / 33 Caption Close Harvey floodwater was so heavy it sank Houston by two centimeters ... temporarily 1 / 33 Back to Gallery

Considering all the misery that Harvey inflicted, as a hurricane and tropical storm, it's doubtful that anyone noticed that Houston also sank a bit. At least that's the conclusion of a California geophysicist.

Chris Milliner, a postdoctoral fellow at NASA's Jet Propulsion Laboratory at the California Institute of Technology, used observations from the Nevada Geodetic Laboratory and the University Nevada-Reno statistics department to determine that the Houston metro area was two centimeters lower because of the load of the widespread floodwaters on the Earth's crust.

The simple explanation? Water is heavy. About a ton per cubic meter.

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"GPS data show that (the flood) was so large it flexed Earth's crust, pushing Houston down," Milliner said Monday via Twitter.

Elaborating on Tuesday, Milliner said there's no reason to worry. It should be a passing thing.

"This should be a temporary drop," he said in an e-mail. "Once floodwaters recede, we should expect a similar, but opposite elastic response of the crust, i.e., uplift. Similar to if you were to jump on-and-off your mattress."

Milliner referred to the phenomenon as local elastic subsidence. He said it is seen in most areas that experience significant seasonal changes in water or ice.

Those commenting on Milliner's tweet suggested the measurements could reflect compacting of the soil. He said that was unlikely, as a number of the measurements were taken where monitors rest on bedrock. There had to be more involved.

Subsidence has been a concern in the Houston area for years, mostly occurring because of groundwater use from shallow aquifers. The Harris-Galveston Subsidence District was created in 1975 to manage groundwater pumping. It has about 8,000 permit holders.

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The district's manager, Michael Turco, said he was unaware of Milliner's tweet. Subsidence should not be an issue after a flood, he said, because less groundwater is being used.

"I would not expect any short term thing like that to have any impact on overall subsidence" Turco said.

Aquifer levels actually go up after big rain events, leading some people to conclude that recharge has occurred. Turco said that's an illusion. It just means that irrigation and other pumping is not taking place as usual.

"It's just less water being used," he said.

The notion of subsidence taking place because of the weight on the crust intrigued him enough to reach out to Milliner to get the data for himself. Houston already has a problem with sinking, so it doesn't need any help.