The sinking situation of subsidence

Texans could be forgiven for having a sinking feeling in 2021. In many cases — particularly in Houston and along the Gulf Coast — Texas is literally sinking, and a new report suggests that the problem will get worse.

A recent report published in Science by a large collection of international researchers, including researchers from the U.S. Geologic Survey (USGS) examined the global threat of subsidence. Subsidence is the sinking of land due primarily to groundwater withdrawal such as for drinking water or irrigation.

The analysis estimated that the threat of subsidence will grow to affect 1.6 billion people worldwide by 2040. The analysis also found that subsidence occurs preferentially in very flat areas made up of alluvial sediments and coastal plains, particularly those in and near densely urban and irrigated areas with high water stress and high groundwater demand — that is, places like Houston and the Gulf Coast.

“Most of the geology in that area is sand, silt and clay,” said Jaime Flores, program coordinator at the Texas Water Resources Institute (TWRI).

Flores, whose background is in geology, added that in the Gulf Coast, there are a lot of soft layers of sediment that haven’t compacted yet and become rock. Eventually they will turn into sandstone and siltstone and clay stone. These are very young soils.”

The age and physical makeup of the soil matter for subsidence because “young” soil, and particularly silt and clay, can compact under the right conditions, such as groundwater removal.

Clay acts like plateletsonce you remove the water in it, the particles settle flat,” Flores said. With sand, those angular grains hold their shape against each other, and they don’t collapse as much as silt or clay.”

Flores explained that the history of human activity in the area, which included groundwater pumping and oil and gas extraction, and the unique nature of the soil under it has created a perfect storm for subsidence.

“The main driver of subsidence is groundwater pumping,” said Lucas Gregory, Ph.D., assistant director of TWRI. “If you’ve got the groundwater pumping, then you’ve got the potential for subsidence. It doesn’t necessarily mean it will happen, but it could.”

Groundwater pumping and similar extractive activities empty out the pore space in the soil, creating the opportunity for compression and subsidence. Both the Gulf Coast region and Houston specifically have a long history of all activities, but Gregory explained that the Houston area has tried to rein in its reliance on groundwater.

“Back in the day, Houston was basically all on groundwater, but over the last 5years, they’ve made a valiant effort to switch over to pretty much exclusively surface water for their primary drinking water supply,” he said.

Subsidence, flooding and urban growth

As the subsidence increases, so too does the risk of flooding, something with which Houston residents are quite familiar.

Both Gregory and Flores compared the subsidence and flooding dynamic in Houston to water trapped in a bowl.

“You would always have flooding in a place like Houston, just because it’s close to the coast and is the terminus of a lot of waterways, but subsidence just intensifies the risk,” said Gregory. “The challenge is, with subsidence, you basically create a deeper bowl that makes it harder for the water to flow out of an area.”

Flores similarly described the compacted clay layers that form underground in situations of subsidence like an impenetrable bowl keeping the floodwaters from being absorbed into the groundwater.

The whole coast basically has to deal with the same issues of the clays and the sands and the flooding. A lot of the flooding would be taken up in the past by the natural bogs and wetlands, but we have removed a lot of them to have industry out there in those areas, especially in Houston.”

While subsidence cannot be reversed, it can be slowed and mitigated through groundwater management and urban planning. The report in Science noted that the forecast of subsidence risk in Japan and Germany is expected to decline, “owing to effective groundwater management policies and population declines.

The Houston area’s two subsidence districts — the Harris-Galveston Subsidence District (HGSD) and the Fort Bend Subsidence District — have worked to monitor subsidence and mitigate it through a variety of water management efforts. For example, the HGSD has the goal of reducing the area to 20% reliance on groundwater by 2035, and managing water to promote aquifer recharge. In some places, the efforts have reduced subsidence in Houston to “negligible” levels.

The analysis in Science, the subsidence districts, and Rosario Sanchez, Ph.D. all pointed to another subsidence risk that places like Houston face: urbanization.

Sanchez, senior research scientist for TWRI, focuses on issues related to groundwater management along the border of the United States and Mexico. She pointed out that Mexico City is a dramatic example of subsidence around the world, having sunk over 32 feet in the last 60 years, according to the USGS.

Though the geology and history of land use of Mexico City are both very different from the Gulf Coast, the modern sinking of the city is directly related to the pumping of groundwater. It is also indirectly related to the population growth in the area, something Mexico City and Houston have in common. More people mean more demands for water.

“Houston is going the way of Mexico City because of the urbanization,” said Sanchez. “Even though they have the two subsidence districts already in place to prevent over-pumping, it’s not enough. The problem is really the exponential growth of the cities.”

Houston has not sunk quite as badly as Mexico City at only about 10-12 feet since the 1920s according to the USGSbut the population in the area is large and growing, suggesting subsidence will continue to be a pressing issue. According to the City of Houston and the U.S. Census Bureau, the Greater Houston Metro Area was home to over 7 million people in 2019. The Texas Demographics Center expects that the coming decade will add over 1.5 million to that number, with 8.7 million people living in the nine-county metro area by 2028.

According to the report in Science, the same dynamic that faces Houston is in play worldwide.

During the next decades, global population and economic growth will continue to increase groundwater demand and accompanying groundwater depletion and, when exacerbated by droughts, will probably increase land subsidence occurrence and related damages or impacts, the report read. 

Authors

As communications manager for TWRI, Kerry Halladay provides leadership for the institute's communications, including a magazine, newsletters, brochures, social media, media relations and special projects. 

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