Wants to Understand Role the Region’s Tectonics Play in Subsidence
The ground beneath our feet is not as solid as we think. Throughout Houston, the ground is shifting and settling. Evidence of this shifting can be seen across Houston, in everything from the settling of house foundations to the cracking of sidewalks.
Shuhab Khan, professor in the Department of Earth and Atmospheric Sciences in the College of Natural Sciences and Mathematics, studies Houston’s shifting landscape, specifically the sinking of land, a phenomenon known as subsidence. Khan believes that subsidence is contributing to the increased severity of flooding in certain areas.
Khan also argues that we need a more thorough understanding of what may be contributing to subsidence in the Houston area. “We know some of what is causing this subsidence, but we need to know more,” said Khan, who joined the University of Houston faculty in 2003.
Subsidence: The Sinking of Land
Similar to a sponge holding water, groundwater functions as structural support by filling the tiny pores in the clays and sediments that form our ground. When groundwater is removed, the ground starts to shrink and compact, shriveling up like a dried sponge.
In other words: the ground sinks.
Historically, most of our city’s water has been drawn from the aquifers lying deep underneath Houston. That fact, combined with the 1920s-era oil drilling, has led to some areas of Houston losing as much as 10-12 feet of elevation.
To combat subsidence, the Harris-Galveston Subsidence District was created in 1975 by the Texas Legislature. The district’s efforts, which included the shift from using groundwater to surface water, have been largely successful in slowing the rate of subsidence.
Increased Rates of Subsidence in North and Northwest Houston
However, in areas with high rates of development, such as the north and northwest sections of Houston, there is still a high reliance on groundwater, which is cheaper, as a main source of water. These areas are seeing increased rates of subsidence.
Areas with significant losses include:
- Spring Branch lost 4 feet since 1975
- Jersey Village lost 2 feet since 1996
- Greenspoint lost almost 2 feet in the last ten years
These areas also saw historic flooding during the 2015 Memorial Day Flood and the 2016 Tax Day Flood.
“When the ground starts sinking, it forms bowl-like depressions, which increase the amount of time that floodwaters linger in an area,” Khan said.
Other Factors May Contribute to Subsidence
Khan believes there may be other factors that affect Houston-area subsidence.
“People have made very simplistic explanations for the subsidence in Houston, without taking into consideration the tectonics of this region,” Khan said.
Subsidence is, at its most fundamental, a shifting of the ground. Although the primary cause of subsidence may be due to the removal of water, the manner in which the ground sinks can vary depending on the geology of the subsurface.
One of the factors Khan believes needs further investigation is the possible link between salt movement in the subsurface and subsidence.
Salt Movement in Subsurface May Contribute to Subsidence
Deep underneath Houston, there are extensive salt formations. In some areas, this salt has moved upwards, similar to a bubble rising to the surface, forming salt domes that lie a few hundred meters under the surface. In other areas, this salt forms a layer miles underneath the surface.
“At high pressures, salt behaves almost like a liquid, with the salt in the subsurface slowly moving toward the Gulf of Mexico,” Khan said.
If the removal of water causes the ground to sink, the movement of salt in the subsurface may be altering how the ground sinks.
Movement of salt in the subsurface can leave behind formations known as salt rollers and salt welds.
“Salt rollers are chunks of salt that have been left behind after the salt has moved. Subsurface salt is soft, almost jelly-like in its consistency,” Khan said. “These salt rollers act as lubrication for the ground to slide along on top of it.”
Salt welds are areas where the salt formation has become so thin that the ground above and below fuses together, creating weak spots that can generate new faults.
“The simple explanation is that the removal of groundwater causes subsidence, and if you stop groundwater extraction, then you stop subsidence. But is that the only contributing factor or is subsidence also happening because of natural processes, such as the movement of salt in the subsurface, that are triggered by the removal of water?” Khan said. “We need to take into account all the possible factors.”
- Rachel Fairbank, College of Natural Sciences and Mathematics