Land subsidence

What Is Land Subsidence?

Land subsidence is the gradual settling or sudden sinking of the Earth's surface due to subsurface movement of earth materials. This geological phenomenon falls under the broader category of environmental finance due to its significant economic impact and the financial risks it poses to infrastructure and communities. It often results from human activities, such as the excessive withdrawal of groundwater or petroleum, or from natural processes like the compaction of sediments or dissolution of soluble rocks. Understanding land subsidence is crucial for urban planning, risk management, and assessing long-term property value in vulnerable areas.

History and Origin

While natural land subsidence has occurred throughout geological history, human-induced subsidence became a significant concern with the advent of extensive groundwater pumping and resource extraction. More than 80% of identified land subsidence in the United States is a consequence of human impact on subsurface water resources.¹⁹ The U.S. Geological Survey (USGS) has highlighted that sustainable development of land and water resources depends on an improved scientific understanding and detection of subsidence.¹⁸ Historical cases, such as in California's Central Valley, demonstrate how extensive groundwater pumping for agriculture has caused significant ground lowering, with estimates showing subsidence of up to 28 feet in some areas.¹⁷ The increasing development of land and water resources threatens to exacerbate existing subsidence problems and initiate new ones.¹⁶

Key Takeaways

• Land subsidence is the sinking or settling of the Earth's surface.
• It is largely caused by human activities, predominantly groundwater extraction.
• Subsidence poses significant financial risks to real estate, infrastructure investment, and public safety.
• Monitoring and mitigation efforts are critical for areas susceptible to this geological hazard.
• The economic consequences can be substantial, leading to higher maintenance costs and reduced asset valuation.

Interpreting Land Subsidence

Interpreting land subsidence involves understanding its causes, rates, and potential impacts. The rate of subsidence, often measured in centimeters or inches per year, indicates the severity of the issue. For instance, northern Jakarta, Indonesia, is sinking at an alarming rate of up to 4.9 cm (almost 2 inches) each year.¹⁵ This metric helps urban planners and policymakers gauge the urgency of implementing mitigation strategies. High rates of subsidence can lead to increased frequency and severity of flooding, damage to buildings, roads, and utilities, and saltwater intrusion into freshwater aquifers.¹⁴,¹³ Understanding these rates is essential for assessing the direct costs related to subsidence, including failure of constructions, infrastructure, and water defense structures, which incur high maintenance expenses and can result in substantial financial damages.¹² It also informs considerations of environmental risk in investment decisions.

Hypothetical Example

Consider a coastal city, "Portville," that relies heavily on groundwater for its municipal water supply. Over decades of rapid urbanization and population growth, the city's water utility has continuously increased its extraction of water from underground aquifers. Initially, the effects are subtle, perhaps only noticeable through minor cracking in older buildings. However, as the water table drops significantly, the fine-grained sediments in the aquifer system begin to compact. This compaction leads to widespread land subsidence.

In a hypothetical scenario, Portville's downtown area, built on historically marshy land, starts to sink by an average of 5 cm per year. This sustained subsidence causes severe structural damage to multi-story buildings, necessitating costly foundation repairs or even condemnation. Roads and underground pipes develop significant cracks, leading to frequent water main breaks and sewer system failures, requiring substantial capital expenditure for repairs. The city's port facilities, designed for a specific sea level, now find themselves progressively lower relative to the sea, making them more vulnerable to tidal flooding and storm surges, disrupting shipping operations. This gradual sinking affects local businesses and residents, impacting the city's overall financial stability.

Practical Applications

Land subsidence has numerous practical applications in areas like engineering, financial planning, and public policy. In civil engineering, understanding subsidence rates is crucial for designing foundations that can withstand ground settlement, especially for large structures or critical infrastructure. For the insurance industry, subsidence data is vital for assessing risk and pricing policies, particularly in flood-prone or geologically unstable regions.

Governments and municipalities use subsidence monitoring to inform urban planning and development, guiding where new construction can safely occur and where mitigation measures, such as regulated groundwater pumping or managed aquifer recharge, are necessary. The economic consequences of subsidence, including direct damages to property and public services, often necessitate significant public investment. A paper from the Federal Reserve Board highlights how climate change-related natural disasters can affect corporate loan costs, illustrating how such environmental risks translate into financial sector concerns.¹¹

Limitations and Criticisms

While land subsidence is a well-understood geological process, predicting its precise extent and rate can be challenging due to the complex interplay of geological factors, human activities, and environmental variables. One limitation is the difficulty in accurately modeling subsurface conditions and groundwater flow, which are crucial for forecasting future subsidence. Monitoring methods, while advanced, may not always capture localized rapid sinking events, such as sinkholes, which can occur suddenly and without extensive warning.¹⁰

From an economic perspective, quantifying the total financial impact of land subsidence can be complex. Direct costs, such as property damage and infrastructure repair, are often measurable, but indirect costs—like diminished economic productivity, reduced tourism, or increased public health risks from compromised water systems—are harder to ascertain. Critics of certain mitigation strategies, such as the construction of massive seawalls, point to potential environmental drawbacks like the disruption of marine ecosystems and the risk of trapping polluted water, turning enclosed bays into stagnant, toxic lagoons. The⁹se interventions also raise concerns about their substantial cost and potential to exacerbate economic inequality by disproportionately affecting coastal communities.

##⁸ Land Subsidence vs. Sea Level Rise

Land subsidence and sea level rise are distinct but often interconnected phenomena, particularly in coastal areas. Land subsidence refers to the sinking of the land itself, often driven by geological processes or, more commonly, by human activities like the over-extraction of groundwater. It ⁷is a local or regional phenomenon, varying significantly from one location to another based on geology and water management practices.

In contrast, sea level rise is the increase in the average global sea surface height, primarily caused by thermal expansion of ocean water and melting glaciers and ice sheets due to climate change. It is a global phenomenon, though its local effects can be influenced by ocean currents and gravitational changes.

The confusion between the two often arises because both phenomena contribute to an increase in relative sea level, meaning the effective height of the water relative to the land. When land subsides, it effectively lowers the ground, making coastal areas more vulnerable to inundation even if the absolute sea level remains constant. When coupled with global sea level rise, land subsidence significantly amplifies the risk of coastal flooding, erosion, and saltwater intrusion, as seen in cities like Jakarta, where both factors contribute to the city's precarious situation., Ad⁶d⁵ressing coastal vulnerability often requires considering both land subsidence and sea level rise in tandem to develop comprehensive adaptation strategies.

FAQs

What causes land subsidence?

The primary cause of land subsidence, particularly human-induced subsidence, is the excessive withdrawal of groundwater from underground aquifers. When water is pumped out, the subsurface sediments can compact, causing the ground above to sink. Other causes include the drainage of organic soils, underground mining, and natural processes like the dissolution of soluble rocks (leading to sinkholes).

##⁴# How does land subsidence affect infrastructure?
Land subsidence can severely damage infrastructure. Buildings may experience cracked foundations, roads can buckle, and underground pipes for water and sewage can break, leading to significant repair costs. It also increases the risk of flooding, impacting transportation networks and critical facilities. Thi³s necessitates considerable municipal bonds and public funds for repairs and mitigation.

Is land subsidence reversible?

In most cases, land subsidence caused by aquifer system compaction is largely irreversible. While reducing groundwater pumping can slow or halt further subsidence, the compaction of the soil layers that have already occurred is usually permanent. Efforts like managed aquifer recharge, where water is intentionally added to aquifers, can help stabilize the ground and replenish groundwater levels, but they cannot fully reverse the physical lowering of the land.

Which areas are most vulnerable to land subsidence?

Coastal lowland areas and regions with extensive groundwater extraction for agriculture or urban use are particularly vulnerable to land subsidence. Cities built on soft, alluvial soils, especially those experiencing rapid population growth and high water demand, are also at high risk. Examples include parts of California's Central Valley, coastal cities in Texas, Florida, and globally, cities like Jakarta.,

#²#¹# What are the financial implications of land subsidence for property owners?
For property owners, land subsidence can lead to a decrease in property value due to structural damage, increased insurance premiums, and higher costs for maintenance and repairs. In severe cases, properties may become uninsurable or uninhabitable, potentially leading to significant financial losses. The cumulative effect can impact the broader investment portfolio for real estate holdings in affected regions.