Subsidence, or the sinking of land due to movement beneath the Earth’s surface, has been recorded over several decades in the Cy-Fair area. SILENTLY SINKING Long-term subsidence eects • Damage to buildings and infrastructure • Increased ooding risk • Elevation changes • Compacting of aquifers
An aquifer is an underground rock structure containing water. Original land surface elevation 1 2
The USGS report stated some of the highest levels of subsidence in the Greater Houston area since the 1990s have been in the northwest portion of Harris County, although there is some uncertainty about the full picture due to limited historical data. Results show areas in Cy-Fair had an average of 0.87-2.17 centimeters of displacement per year between 2016-20. A GPS station located o FM 1960 near Windfern Road showed the highest rate of subsidence during that time period. It saw 60.25 cumulative centimeters of displacement since measuring there began in 1999. Northwest of Cy-Fair near the Waller area, however, the rate of subsidence was only 0.25 centimeters per year and had a cumulative rate of 11.34 centimeters since 1999. The UH report also found that northwestern Harris County is experiencing “substantial negative displacement” due to population growth and development as well as the use of groundwater from aquifers. Wells can be drilled into an aquifer—a large, underground water-bearing rock—and water can then be pumped out for residential and industrial use. According to the USGS report, which covers a new model created to study land displacement and the ow of groundwater from 1897-2018, groundwater withdrawals from the Gulf Coast Aquifer System over time have led to more than 300 feet of groundwater-level declines in the Chicot, Evangeline and Jasper aquifers. Groundwater use across the Greater Houston region caused as much as 9 feet of subsidence. However, once regional water authorities began to convert to surface water use instead of groundwater, Chicot and Evangeline aquifer groundwater levels began to recover. Development leads to more groundwater pumping to meet water needs, said Mike Turco, who is the general manager for the Harris- Galveston Subsidence District, which regulates groundwater and monitors subsidence in the Houston region. “As you add more people, you add more businesses, you’re going to have more needs for water resources, and diversifying those sources from groundwater to an alternative like surface water helps the subsidence issue and prevents subsidence for us,” Turco said. Data from HGSD shows subsidence has contributed to ooding across the Greater Houston area. However,
Silt & clay
Gravel & sand
After long-term groundwater withdrawals, clay and silt layers compact.
Prior to excessive groundwater use, clay and silt layers are loosely packed.
SOURCES: HARRISGALVESTON SUBSIDENCE DISTRICT, SHUHAB KHAN, ROBERT MACECOMMUNITY IMPACT
the Earth’s surface, according to a University of Houston geological study released in August. The resulting sinking is a problem experienced all over the Greater Houston area. Shuhab Khan, a UH geology professor and one of the August report’s authors, said he believes there is a balance to using groundwater sustainably. “Groundwater is the cleanest water all over the world,” Khan said. “It is a means for drinking, for agriculture, for industry, and when we start pumping more water than the amount of water that is replenishing [aquifers], that balance is gone.” The UH study states its ndings also imply subsidence may be responsible for fault movement in the Greater Houston area based on analysis of land displacement. Over time, subsidence can cause damage to property, pipes and roads, Khan said. To prevent subsidence from worsening, regional organizations are working to decrease groundwater use. Eorts include a $1.4 billion expansion of the North Harris County Regional Water Authority’s Northeast Water Purication Plant in Humble. This plant treats Lake Houston water and turns it into drinking water for parts of Harris County and surrounding areas, including Cy-Fair. Upon completion in 2024, ocials said the plant will increase treated water capacity from 80 million gallons per day to 400 million gallons per day. Subsidence in Cy-Fair Experts have tracked land deformation in Houston’s growing suburbs over the past couple of decades. According to the UH report, land within Houston’s city limits is not sinking substantially, but suburbs are impacted more.
Parts of Cy-Fair saw signicant rates of land depression—dubbed subsidence—from 2016-20, according to geological studies released in January and August.
CENTIMETERS OF DISPLACEMENT -2.45 to -1.75
-1.75 to -1.25 -0.20 to -0.10
-1.25 to -0.85 -0.10 to -0.00
-0.85 to -0.45
SOURCES: SHUHAB KHAN, UNIVERSITY OF HOUSTON, JOHN ELLIS, OKLAHOMATEXAS WATER SCIENCE CENTERCOMMUNITY IMPACT
phenomenon to document and study. … It can result in permanent loss of land, increased risk of ooding and damage to infrastructure,” Ellis said. Subsidence is chiey caused by pumping water from underground reserves, which compacts sublayers of clay and silt in aquifers beneath
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John Ellis—who authored the Jan. 12 study and serves as the center’s Gulf Coast studies chief—said subsidence worsens ooding in watersheds by “altering the base ood elevations.” “[Subsidence] is an important
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