THE RISING: Why Sea Level Is Increasing
As the Land Sinks
Around the Chesapeake Bay, sinking land is exacerbating the effects of sea level rise
Ghent neighborhood, Norfolk, Virginia. Credit: David Harp
Floodwater seeps onto streets during a high-water event in the Ghent neighborhood of Norfolk, Virginia. Scientists say that patterns of subsidence in coastal Virginia will make floods like this one more common. Photograph: David Harp
Around the Chesapeake Bay, sinking land is exacerbating the effects of sea level rise

AS TOURISM SLOGANS GO, "Hampton Roads: Where the Land Sinks" probably won't draw in crowds of visitors. But among scientists, that's the reputation that this region of Virginia is building.

Manmade climate change is driving up water levels all around the world. But communities on the Chesapeake Bay, including those in the densely populated Hampton Roads area near the estuary's mouth, have to contend with another threat that worsens the effect of rising water. Across Maryland and Virginia, the land is gradually sinking, a process that scientists call subsidence. These changes are caused by long-term geological shifts occurring across the Mid-Atlantic region. New research also suggests that human activities may be making the situation worse.

Subsidence adds to what is known as relative sea level rise, a term that describes the combined effects of rising oceans and the sinking of land surfaces. It's a bit like a ship going down in a gale: sinking towns dip closer down to the water, making them more vulnerable to storms and flooding. That's reason for concern in an area like Hampton Roads, where some towns already flood during run-of-the-mill high tide events.

"When the next big hurricane hits, it will be worse because of the land subsidence," says Jack Eggleston, a hydrologist with the U.S. Geological Survey (USGS) in Reston, Virginia.

This subsidence results from natural processes thousands of years old whose effects resemble a game of teeter-totter.

During the last ice age, a massive glacier called the Laurentide Ice Sheet stretched from Canada down into Pennsylvania and New Jersey. The ice was so heavy that it pushed down the land underneath it. At the same time, the land just outside of the glacier's edge — what we now call the Mid-Atlantic coast — reacted like the other end of a playground seesaw: it was forced up into what's called a glacial forebulge.

Then, around 18,000 years ago, all that ice started to melt, and the seesaw started to flip back. "Now that the ice has been removed, there's a rebound" in the land to the north of the Mid-Atlantic, says John Boon, an oceanographer and professor emeritus at the Virginia Institute of Marine Science in Gloucester. Similarly, "the forebulge that we have here is ... going back down."

In Maryland and Virginia, a network of GPS stations, operated by the National Geodetic Survey, has tracked this shifting in the land for several decades. Based on this and other data, researchers estimate that land surfaces around the estuary are falling by around 1.5 millimeters each year because of the ongoing rebound from the last ice age. But some towns on the estuary seem to be sinking a lot faster: the number for the Hampton Roads area is closer to 4 millimeters each year on average.

"This question kept coming up of what was causing this land subsidence," says Whitney Katchmark. She heads up the Water Resources Department in the Hampton Roads Planning District Commission, which advises towns in the region.

Source: Updating Maryland's Sea-Level Rise Projections
The rates of land sinking vary from place to place based on how much groundwater towns and businesses draw up, as this map shows. The red contour lines show the rates of subsidence, in millimeters per year, from 1940 to 1971. The fastest sinking sites are centered around the towns of West Point and Franklin. Both communities are home to large paper mills that pull up a lot of water from wells. Map: U.S. Geological Survey

The most obvious answer was groundwater withdrawal by municipal water utilities and other users.

To understand why, think of a jelly doughnut. Like the raspberry filling inside these morning treats, groundwater is stored in aquifers. That's the name for large formations of sand and clay that sit tens to hundreds of feet below the land surface. The Hampton Roads area gets its groundwater from the Potomac Aquifer, a formation that extends from North Carolina up into New Jersey. When you tap a well into one of these formations and draw out water continuously, it's a bit like removing the filling from your doughnut. Like with the pastry, if you remove too much water from an aquifer, the aquifer can collapse down on itself. When it does, the land above it will also begin to sag.

Researchers have observed this phenomenon in action across the country, most notably in the Houston-Galveston area of Texas. There, residents saw the land fall by as much as 10 feet during the course of the 20th century.

Coastal Virginia uses less groundwater than Houston and Galveston but still depends on wells to slake thirsts and support industries. In 2012 the Hampton Roads area, which is home to about 1.7 million people, got about 16 percent of its water from wells, Katchmark says. That added up to around 45 million gallons of groundwater drawn per day.

In 2013, Eggleston and a USGS colleague, Jason Pope, wrote a report issued by the agency on the causes of land subsidence in the southern Bay region — or from Gloucester County, Virginia, on the York River, to the North Carolina border. The report was commissioned by Katchmark's planning district. In it, the USGS scientists summarized a number of existing studies on the connection between groundwater withdrawal and the sinking of land surfaces in Virginia. While a few other factors can influence subsidence patterns to a small degree, a number of lines of evidence point the finger at water usage.

Eggleston and Pope reported, for instance, that areas where the land is falling the fastest tend to line up with where people have pumped out the most groundwater. In all, the scientists estimated that groundwater withdrawal caused about half of the subsidence seen around coastal Virginia during the last several decades. Groundwater withdrawal could also be worsening subsidence in Maryland, but scientists have not studied the problem there as thoroughly as they have in Virginia.

In that more southern state, some small solutions to this problem may be brewing, says Scott Kudlas, director of the Office of Water Supply in the Virginia Department of Environmental Quality (DEQ). In order to safeguard its water supply, the state set up a permitting program for groundwater withdrawal in 1992. Today, businesses and residential developments in coastal Virginia that use more than 300,000 gallons of well water in any one month out of the year are required to apply for a groundwater permit. To put that number into perspective, the average household uses around 9,000 gallons of water in a month.

As of 2013, Kudlas's department had issued close to 170 groundwater permits in the Hampton Roads region. A large number happen to be up for renewal in the next year or so. That could give the DEQ an opportunity to try to put new caps on how much water those big users draw up, Kudlas says: "A lot of decisions will be made in the next two years."

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