Geology of a Storm

Before, during and after Hurricane Harvey

As hurricanes Harvey and Irma laid waste to huge swaths of Texas, Florida and other states in the southeastern United States, geologists were swept up in a whirlwind of research before, during and after the storms.

U.S. Geological Survey hurricane response crews worked as the storms approached to install storm-tide sensors at key locations.

Housed in vented steel pipes a few inches wide and about a foot long, the sensors are installed on bridges, piers and other structures that have a good chance of surviving a storm surge during a major tropical storm. The information they collect helps define the depth and duration of a storm-surge, as well as the time of its arrival and retreat. That information helps public officials assess storm damage, discern between wind and flood damage, and improve computer models used to forecast future floods, according to the USGS.

In the case of Harvey, five crews from the USGS Texas Water Science Center were deployed to install scientific instruments in advance of the storm, according to Lynne Fahlquist, a hydrologist with the center.

Some of the equipment will measure the height and intensity of the storm surge while other instruments, such as streamgages, will monitor real-time water levels of inland rivers and streams, Fahlquist said. The field crews gather scientific data from the instruments immediately after the storm has passed, which will provide critical information needed for accurate flood forecasting.

Image Caption

USGS scientist Charles Hartmann installs a storm-tide sensor in preparation for Hurricane Harvey in Carancahua Bay, Texas. Photo by George Umphres, USGS.

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As hurricanes Harvey and Irma laid waste to huge swaths of Texas, Florida and other states in the southeastern United States, geologists were swept up in a whirlwind of research before, during and after the storms.

U.S. Geological Survey hurricane response crews worked as the storms approached to install storm-tide sensors at key locations.

Housed in vented steel pipes a few inches wide and about a foot long, the sensors are installed on bridges, piers and other structures that have a good chance of surviving a storm surge during a major tropical storm. The information they collect helps define the depth and duration of a storm-surge, as well as the time of its arrival and retreat. That information helps public officials assess storm damage, discern between wind and flood damage, and improve computer models used to forecast future floods, according to the USGS.

In the case of Harvey, five crews from the USGS Texas Water Science Center were deployed to install scientific instruments in advance of the storm, according to Lynne Fahlquist, a hydrologist with the center.

Some of the equipment will measure the height and intensity of the storm surge while other instruments, such as streamgages, will monitor real-time water levels of inland rivers and streams, Fahlquist said. The field crews gather scientific data from the instruments immediately after the storm has passed, which will provide critical information needed for accurate flood forecasting.

Erosion Effects

After the storm, researchers at the University of Texas Institute for Geophysics, prepared to resurvey Lydia Ann Channel and Aransas Pass with marine geophysical instrumentation and collect sediment samples.

“Researchers believe that these locations have been subjected to substantial and measurable erosion and sediment transport associated with the storm surge and its ebb, and that investigating these locations will provide valuable insights into the impact of storm surges on barrier and estuarine systems,” said Institute spokesman Anton Caputo.

This work was scheduled for September. University of Texas researchers previously surveyed these areas in 2009, Caputo said.

“As geologists, we are highly interested in understanding how a storm impacts the sediments, and thus foundation, of the coastal zone, said Jeff Goff, senior research scientist at the institute.

“Most particularly, we are concerned with how storms either remove or add sand to the barrier island system, and thus affect such island’s long-term viability for supporting communities and protecting the estuarine ecosystem. Sand can potentially be eroded from the shoreface and then transported into the bays by storm surge, or sent out to sea on the ebb of the surge. In either case, substantial volumes of sand may be removed from the barrier islands by storms, negatively impacting the coastal zone unless sand is replenished by either natural or artificial means,” Goff said.

“From post-storm imagery, we can see in some areas where the storm moved sand from the shoreface, over the barrier island and toward the bay. Farther south, however, particularly around Port Aransas, San Jose Island and Matagorda Island, we see more evidence that water flowed seaward, eroding the beach foredunes in the process. We hypothesize that, in these areas, either that the ebb of the surge, or wind-driving of bay waters out to sea on the back side of the storm, caused significant seaward transport of coastal sediments. Our upcoming survey around Port Aransas is intended to search for submarine evidence of these processes,” he said.

“We found one important time line for geologic impact in the Port Aransas tide station. It indicated a slow buildup, over about 24 hours, of water height to about five feet above normal, but then a very rapid loss of nearly that entire amount over a span of just three hours. This asymmetric record shows that the seaward-directed flow was far stronger than the landward-directed flow, and thus a basis for predicting a net loss of sediment out to sea,” he said.

“While geography probably plays a larger role than geology in determining where the worst flooding is likely, information like sediment grain size distribution and identification of areas with dunes capable of protecting a stretch of coastline could potentially be coupled with surge models to predict sediment transport in a storm,” Goff added.

“Areas with significant dune systems whether manmade or natural can provide natural barriers to storm surge and thus mitigate flooding,” according to Sean Gulick, a research professor with the institute.

The regional geology in the Houston area comprises thick, rich surface soil inter-bedded with clay and fine-grained silt. As a low-lying city with minimal topography, the geology of Houston is not conducive to immediate recharge into the aquifers underlying the region. Therefore, in a storm in which 35 inches of rain fell in just over 48 hours, most of the rain could not be absorbed, explained Fahlquist.

‘Heavy’ Storm

The enormity of the Texas flooding was evidenced in a Labor Day tweet by Chris Milliner of the Jet Propulsion Laboratory. It was a map visualizing data from the Nevada Geodetic Laboratory. It showed that GPS data from special stations around Houston detected that the whole area had been pushed down roughly two centimeters by the weight of the water that fell during Hurricane Harvey, which — by estimates of as much as 33 trillion gallons of rainfall at 8.34 pounds each — was more than 137 billion tons, which deformed the Earth’s crust.

However, Mother Earth will bounce back.

“Any additional load on a section of crust requires an accommodation by that crust. However, if the load is only temporary, as is the case in a flood, then the accommodation (in this case downward) is also only temporary,” Gulick said.

USGS scientists responding to Hurricane Harvey include hydrologists, geologists, hydrologic technicians, computer scientists, geographers, physical scientists and more, Fahliquist said.

Geoscientists from the Institute for Geophysics taking part in the storm assessment included those in the areas of sedimentology, geomorphology, coastal processes, ocean modelers and more.

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