Land subsidence: An unassessed risk around the globe
September 27, 2016
Subsidence risk is not easily observable, making better understanding and assessment essential.
Land subsidence and sinkholes are caused by subsurface movement of earth. Both may cause significant damage to building foundations, underground utilities and pipelines, dams, bridges, construction projects and other critical infrastructure. These risks typically remain unassessed and hidden because of the lack of comprehensive loss data and challenges associated with estimating their impact.
Sinkholes are primarily a natural hazard associated the formation of cavities or voids from the dissolution of soluble underground rock or other subterranean material. Although the dissolution process is gradual, loss of subsurface support over the cavity can cause sudden collapse and sinkhole formation.
Land subsidence is the differential compaction of subterranean layers due to earth movement or groundwater depletion. While land subsidence is gradual and usually not observable, it is cumulative, generally irreversible and potentially destructive. Although there are several natural causes for land subsidence, according to the U.S Geological Survey (USGS), man-made activities, such as excessive pumping of groundwater for urban or agricultural use, cause most identified land subsidence in the U.S.
Land subsidence is a global problem. In the U.S., at least 45 states report varying levels of land subsidence. Likewise, large sections of France and England and parts of Germany, Spain, Italy and Eastern Europe are vulnerable to increased risk of subsidence. Megacities across the globe also face complex issues due to subsidence stemming from excessive groundwater pumping for their thirsty, growing populations. For example, Mexico City faces an acute risk of subsidence due to its vulnerable underground geology, excessive groundwater pumping, low rainfall and the limited replenishment rate of its aquifers. In fact, in some areas around Mexico City, the land has sunk about 30 feet in the last century.
Though subsidence in agricultural areas may go unnoticed, in urban areas, the risk of damage to building foundations, infrastructure and buried utilities can be significant. In addition to property damage risk, soil compaction can change the surface topography, increasing the risk of flooding, as seen in the Houston area recently. Although comprehensive information on repair costs is elusive, anecdotal information confirms that the damage caused by and remediation costs arising from land subsidence can be significant. For example, Stanford Woods Institute for the Environment estimates the remediation cost of land subsidence in California’s Santa Clara valley to be more than $756 million. Based on available data, a Swiss Re report estimates average annual damage cost for France to be EUR 340 million.
While subsidence risk is not easily observable, better understanding and assessment of this risk is needed. Until a solution presents itself, the stakeholders may consider formulating a water conservation program and a sustainable underground water management strategy that includes partnership with other businesses, the community and regulators. Businesses located in areas particularly susceptible to subsidence risk and dependent on groundwater must understand the finite nature of groundwater resources and plan accordingly. Businesses involved in large construction and infrastructure projects must consider land subsidence risk in their planning processes. In addition to new construction, all businesses in susceptible regions should consider implementing a periodic inspection program that can include, among other things, building settlement, foundation cracks and door/window alignment.
With droughts, water shortages and continued increases in demand for water, excessive pumping of groundwater will continue, potentially accelerating the global risk of land subsidence in many parts of world. All public and private sector stakeholders in susceptible regions need to work together to develop innovative solutions and risk management strategies to help reduce the risk for everyone in the community.