Land subsidence is the sinking of the Earth’s surface due to subsurface material movement. This geological process is a major concern for the Greater Houston region, which has experienced some of the fastest rates of sinking in the United States. The land has historically subsided by as much as three meters in some coastal areas, a phenomenon driven almost entirely by human activity.
Houston’s Susceptible Geology
The land beneath Houston sits on the Gulf Coast Aquifer System, which includes the Chicot and Evangeline aquifers. This system is composed of alternating layers of sand, gravel, silt, and highly compressible clays and shales. While sand and gravel store water well, the fine-grained clay layers are particularly vulnerable to compaction. When saturated, the clay holds significant amounts of water within its pore spaces, acting like a sponge and providing structural support to the overlying land.
The Primary Driver: Groundwater Extraction
The main cause of Houston’s sinking is the removal of groundwater from these vulnerable aquifers. Deep wells were drilled to meet the escalating demand for municipal and industrial water supplies. Extracting this water lowers the potentiometric surface, which is the water pressure within the aquifer system. This drop in pressure removes the buoyant support the water once provided to the clay layers.
The highly compressible clay layers then begin to compact and consolidate under the weight of the overlying earth. This process is known as aquifer-system compaction, and it causes the ground surface to permanently lose elevation. This compaction is mostly inelastic, meaning the clay cannot fully rebound and regain its original volume even if water levels are later restored. Subsidence was first documented near the Goose Creek Oil Field as early as 1918, but the problem became widespread as groundwater withdrawal increased dramatically after the 1940s.
Consequences for the Urban Landscape
The reduced land elevation resulting from compaction intensifies the region’s vulnerability to flooding. As the land sinks, it brings the urban landscape closer to sea level, making it susceptible to inundation from storm surges and heavy rainfall events. Coastal areas have suffered the most severe effects, leading to the abandonment of communities like the Brownwood subdivision near Galveston Bay.
Uneven sinking across the region alters natural drainage patterns for creeks and bayous. This irregular subsidence leads to the formation of new pooling areas where water collects, causing flooding in places that historically remained dry. The sinking also places stress on the city’s infrastructure network. Evidence of this strain is visible in damaged water pipelines, cracked roads and bridges, and foundation problems. Furthermore, the land movement can reactivate existing fault lines, causing surficial fissures and localized damage to property.
Strategies to Slow Land Subsidence
The primary strategy to combat land subsidence has been the mandated reduction of groundwater reliance in favor of surface water sources. The Texas Legislature created the Harris-Galveston Subsidence District (HGSD) in 1975 to regulate groundwater usage and prevent further sinking. The HGSD implements a regulatory plan that divides the region into areas with specific reduction targets. These regulations require water providers to transition to using surface water from lakes and rivers, such as Lake Livingston, which has significantly stabilized subsidence rates in coastal areas.
The HGSD enforces these mandates through a system of permitting, monitoring, and financial disincentives for non-compliance. Scientists monitor the effectiveness of these efforts using advanced technologies like GPS stations and satellite data to track land surface changes. The district also explores alternative water supplies, including brackish groundwater development and Aquifer Storage and Recovery (ASR).