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You Visit Tour. Webb Lion Fountain. June 1 2017. Photo David B. Hollingsworth

ODU/NASA Collaboration Challenges Conventional Wisdom on Subsidence

A groundbreaking new study by researchers at Old Dominion University and the NASA Jet Propulsion Laboratory (JPL) challenges decades of conventional wisdom about the sinking of land in southeastern Virginia.

That land in Hampton Roads is sinking is not in question. But David Bekaert, radar scientist at JPL, and Ben Hamlington, assistant professor of ocean, earth and atmospheric sciences at ODU, have gathered data suggesting that land subsidence is occurring at substantially different levels in different parts of the region.

"These results motivate and demonstrate the need to improve the understanding of vertical land motion (the earth rising and falling in small increments) here in Hampton Roads," Hamlington said.

This new information, computed by examining images taken from space, could ultimately have far-reaching effects in areas ranging from land use to flood insurance in low-lying Hampton Roads.

The researchers analyzed publicly available synthetic aperture radar (SAR) images acquired from the Japanese ALOS-1 satellite between 2007 and 2011. By processing this data with state-of-the-art techniques developed at JPL, the researchers produced the first high-resolution estimates of vertical land motion in Hampton Roads.

This initial examination of land motion shows definite trends. However, Bekeart said new data being collected by the European Space Agency's Sentinel-1 satellite will allow the research team to produce even more detailed findings.

"We have this initial finding, but the data that is currently being collected will allow us to improve a lot of the uncertainty in our calculations. This can help with the creation of detailed maps of vertical land motion," Bekaert said.

Land subsidence is one of three factors that make this region second only to New Orleans in susceptibility to flooding on the East Coast. The others are rising sea levels and a slowing Gulf Stream, which is piling up additional water on the mid-Atlantic region.

Data compiled over decades by the U.S. Geological Survey have been the traditional authority on land subsidence in southeastern Virginia. The agency has established that land in the area is sinking by 2 to 3 millimeters per year, because of the effect of glacial isostatic adjustment (the earth still rebounding from melting glaciers); ongoing shifts associated with the Chesapeake Bay meteor impact crater, and more localized groundwater pumping.

"The general thought on subsidence was that it occurs with relatively even and smooth contours," Hamlington said. But the new research suggests there are places where vertical land motion is as much as seven to 10 times the regional average. "If further study can validate these findings, this is information that should be incorporated into land use decisions and insurance flood maps."

Parts of Hampton Roads that are sinking far faster than average include Craney Island and the Portsmouth Naval Shipyard. Some parts of Hampton Roads are subsiding at significantly lower rates, and others have even experienced some uplift from 2007 to 2011.

An aerospace engineer, Hamlington has studied sea level rise for close to a decade by examining images taken from space, coupled with tide gauges. This research has become possible and feasible in the past two decades.

The current study is an effort to utilize similar technology to measure vertical land motion, something Hamlington says is significantly under-studied in the context of coastal sea level rise.

The limitations of the ALOS-1 data have prompted the research team to seek new sources of satellite data.

Through a $120,000 grant from NASA and the Commonwealth Center for Recurrent Flooding Resiliency -- which is jointly run by Old Dominion and the College of William & Mary -- the researchers will begin analyzing data from the Sentinel-1 satellite, which began transmitting material to Earth in 2015. The new satellite provides more frequent data from Hampton Roads, allowing for more precise estimates of subsidence in the region, even down to the neighborhood level.

"The Sentinel-1 data should provide the decision-making quality vertical land motion maps that are needed for Hampton Roads," Hamlington said.

Read the full report in Scientific Reports.

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