Thoughts From Engineers: Hidden, But Not Harmless: The Unsettling Impacts of Land Subsidence on Urban Centers

The core assumptions that have shaped for decades how we plan, build, develop our cities and even live from day-to-day are losing ground. For example, most engineers and scientists would agree we’re now in an era of climate non-stationarity even though we presumed the opposite—climate stationarity—for years. Another assumption relates to the use and extraction of natural resources such as groundwater. We now know there are limits to how much we can safely withdraw to meet a variety of needs, from sustaining agriculture operations to supplying drinking water. These issues, among others, are increasingly relevant in the context of land subsidence in the United States.

Recent research by Ohenhen et al., “Land subsidence risk to infrastructure in US metropolises” (bit.ly/MetroSubsidenceRisk), builds on earlier investigations and shows that subsidence isn’t limited to coastal regions but is in fact widespread across the United States, affecting inland cities as well. Major news platforms shared the results earlier this year, but public attention has predictably moved on. Arguably, the topic needs to linger in the airwaves a bit longer.

Like all well-designed research, science can be lifesaving and provide key insights into the systems that underpin our lives. Not only do shifts in land elevation place already compromised and aging infrastructure at risk, but other threats (e.g., flooding associated with storm events) may become even more hazardous.

What does this research tell us about next steps? Some cities are leveraging different tools to mitigate the damage; other urban areas may be unaware of risk—or uncertain how to approach it.

Surface and Subsurface Investigations

This analysis was performed within 28 of the most populated urban areas in the United States. Point-based observations using global navigation satellite systems were combined with interferometric synthetic aperture radar (InSAR) datasets collected between 2015 and 2022 to generate spatial maps showing fine-scale land variability through time. Further data processing and analysis showed the degree of vertical land movement (VLM) within each urban area, specifically whether land was subsiding or uplifting. The authors not only determined percentage of land area within each city that was subsiding, but also estimated rate of subsidence, high-hazard areas, percentage of population living in subsidence zones and more. A few findings are discussed below, but the paper itself should be consulted for additional details.

Analysis showed that in each of the 28 cities studied, 20 percent of the land area is subsiding. In 25 of the 28 cities, more than 65 percent of the land area is sinking. Houston has the most significant land subsidence, with 42 percent of land area subsiding at a rate of more than 5 millimeters per year. On 12 percent of land area within Houston, subsidence is occurring at rates upward of 10 millimeters per year. Interestingly, the authors observed that eight cities of the total 28 experience subsidence rates above 3 millimeters per year. These cities have experienced more than 90 flood events since 2000. For an interesting article on how land subsidence compounded flooding effects in Houston, see Miller and Shirzaei, “Land subsidence in Houston correlated with flooding from Hurricane Harvey” (bit.ly/HoustonSubsidence).

Drivers, Tools and Long-Term Implications

Drivers of land subsidence can be natural and/or originating from human activity (i.e., anthropogenic). Several cities such as New York; Washington, D.C.; Denver; Indianapolis; Detroit and Chicago can attribute most elevation loss to land settlement following glacial retreat—a process known as glacial isostatic adjustment—while elevation variability on the western coast, in cities such as Portland and Seattle, can be explained in part due to tectonic plate activity. Despite the role of natural influences, this study validates findings that 80 percent of land subsidence can be attributed to groundwater withdrawals. The analysis that led to this conclusion is complex and—in very general terms—draws on extensive statistical analysis examining the effects of groundwater level changes on VLM. Strong correlations between VLM and groundwater level change were particularly evident in confined aquifer regions.

A few cities in the United States have a long history of land subsidence and have experimented with several tools to address the problem. Houston has struggled to manage land subsidence since the 1970s when residential areas were abandoned due to drastic elevation shifts and chronic water incursion. The Harris-Galveston Subsidence District was created soon after and began regulating groundwater withdrawals as the city renewed efforts to develop the necessary infrastructure for surface-water supplies.

Managed aquifer recharge (MAR) is being used in several areas as well. Phoenix actively manages groundwater through special-purpose districts and employs MAR technology to recharge overextended aquifers. The Sustainable Water Initiative for Tomorrow research center operates a MAR plant in Suffolk, Va., that treats and injects secondary effluent into the Potomac Aquifer System, the region’s primary source of water supply, at a rate of 1 million gallons per day. The process, which uses deep-well recharge technology and advanced carbon-based water treatment to match MAR water to the geochemical quality of the aquifer’s native supply, is arguably state-of-the-art as it works to slow land subsidence while continuing to provide a regional water supply.

In terms of addressing the issue of land subsidence, the challenge of the undertaking is clear. According to the authors, “A crucial first step is identifying location-specific drivers, as anthropogenic causes can be mitigated, whereas subsidence caused by natural processes often necessitates adaptation.” The authors continue, “separating these drivers is challenging as multiple subsidence mechanisms are sometimes superimposed at a singular location.”

Clearly, some communities have been surprised by land subsidence. It’s a slow invisible creep—until it isn’t. But there’s an upside to this discussion. Without minimizing the seriousness of what’s at stake or the scale of the problem in certain areas, we know which tools can mitigate harm—retreat and zoning in vulnerable areas, restrictions on groundwater withdrawals, building restrictions or reinforcements, awareness and full-scale monitoring for at-risk zones, and more. Perhaps most importantly in terms of a broader perspective, the issue validates a “systems-driven” mindset and paradigm to urban planning and resource management that upends the narrow presumption of unimpeded growth and groundwater use that has guided us for years. 

 

Chris Maeder CivilGEO Thoughts From Engineers