Water, water, everywhere—and it’s actually quite heavy.
Over 5 days in August 2017, Hurricane Harvey dumped feet of precipitation on southeastern Texas and neighboring states, including 51 inches (130 cm) of rain in one Houston neighborhood, a North American record. In addition to obscuring the ground beneath the deluge, the floodwaters bowed Earth’s crust under the land they inundated, a new study has found. The waters’ tremendous weight caused a measurable, albeit temporary, drop in the height of the region’s land.
By analyzing data from GPS stations located all over the region, researchers led by Adrian Borsa at Scripps Institution of Oceanography in La Jolla, Calif., showed that as the rain fell, the ground almost immediately began subsiding under the weight of the flood.
Borsa presented the findings Tuesday at the American Geophysical Union’s 2017 Fall Meeting in New Orleans, La. The researchers’ poster was part of a session on late-breaking research into the massive hurricanes that hit the United States over the past few months.
A Sinking Realization
Earth’s crust is “like a trampoline,” said Borsa, deforming under weight placed on top of it, then springing back when the load is gone. He estimated the relatively short burst of extreme rain during this storm to have weighed 127 gigatons.
A network of 477 GPS monitors with extremely fine scale resolution, down to 1 millimeter, detected a substantial vertical lowering of the land as the rain fell, Borsa reported. The average ground depression measured by monitors across Texas, Louisiana, Arkansas, and Mississippi was roughly 1 centimeter but reached as much as 1.8 centimeters at some individual stations.
Daily measurements of ground displacement are extremely useful but can be difficult to deal with, explained Christopher Milliner, a postdoctoral fellow at the Jet Propulsion Laboratory who also used GPS stations to measure vertical motions in the ground after this storm. The uncertainty of the measurement can be as large as the measurement itself, but in the case of this storm, the effect was clearly sizable and noticeable. Milliner and Borsa used slightly different methods to analyze their data but came to similar conclusions. In September, Milliner’s tweet about his preliminary findings, only 3 days after Hurricane Harvey, went viral as it captured the imaginations of Twitter users fascinated by hidden impacts of the massive storm.
— Chris Milliner (@Geo_GIF) September 4, 2017
After the hurricane was over but the flooding remained widespread, no one knew how long the water would take to go away. Fortunately, the GPS stations also proved useful for this part of the exploration. They showed a gradual elastic rebound of the land as the water dispersed. By approximately 3 weeks after the rains ended, the instruments confirmed that all the excess water had either drained away or evaporated.
This type of data analysis from the supersensitive GPS stations is vital, says Borsa, because “we would like to forecast how much water would be showing up downstream after a storm.” This kind of assessment could potentially also help managers of dams know how much water to release in advance of a flood to avoid later accumulations exceeding their dams’ safety limits and storage capacities.
Milliner agreed, saying the technology test prompted by Harvey has revealed a lot of unknowns about water storage on land after a storm. “In the future, we could hopefully apply this to other hurricanes to improve real-time estimates of flooding.”
—Shaena Montanari (email: [email protected]; @DrShaena), Freelance Science Journalist