Urban subsidence, the gradual or sudden sinking of the Earth's surface in urban areas, is a significant and often overlooked phenomenon. It poses substantial threats to infrastructure, public safety, and the overall resilience of cities worldwide. Understanding how urban subsidence is measured, the factors contributing to it, and the risks it imposes on infrastructure is crucial for urban planning and disaster mitigation.
Geodetic Measurement of Urban Subsidence
Geodetic techniques play a vital role in detecting, measuring, and monitoring urban subsidence. These methods provide precise data on ground elevation changes over time, allowing for the identification of subsiding areas and the quantification of subsidence rates. Some common geodetic techniques include:
- Interferometric Synthetic Aperture Radar (InSAR): This satellite-based remote sensing technique is widely used for mapping ground deformation over large areas with high accuracy (millimeter-level). InSAR compares radar images of the same area taken at different times to detect phase differences, which correspond to ground movement. Variations of this technique include:
Persistent Scatterer InSAR (PS-InSAR): Focuses on stable, reflective points (persistent scatterers) like buildings or rocks to provide highly accurate displacement measurements over time.
Small Baseline Subset (SBAS): Uses multiple interferograms with short spatial and temporal baselines to generate time-series deformation maps, improving accuracy in areas with lower coherence.
- Global Navigation Satellite System (GNSS): This includes systems like GPS. Continuously operating GNSS stations can provide precise 3D coordinates, including vertical displacement, allowing for the monitoring of subsidence at specific points. GNSS data is often used to validate InSAR results.
- Leveling Surveys: A traditional geodetic method involving precise measurements of height differences between points on the ground. While highly accurate, leveling is labor-intensive and provides point-based data rather than areal coverage.
- Light Detection and Ranging (LiDAR): An airborne or terrestrial remote sensing technique that uses laser pulses to create detailed 3D maps of the Earth's surface. Repeated LiDAR surveys can detect changes in elevation over time.
These techniques are often used in combination to provide a comprehensive understanding of subsidence patterns. For instance, InSAR can identify broad areas of subsidence, while GNSS and leveling can provide high-accuracy point measurements for calibration and validation. Recent studies highlight the increasing use of Sentinel-1 satellite data with open-source tools like SNAP and StaMPS for PS-InSAR analysis, enabling widerscale and more frequent monitoring.
Contributing Factors to Urban Subsidence
Urban subsidence is driven by a complex interplay of natural and anthropogenic (human-induced) factors:
Anthropogenic Factors:- Groundwater Extraction: This is a primary driver of subsidence in many urban areas globally. Excessive pumping of groundwater from aquifers for domestic, industrial, and agricultural use lowers the water table and reduces pore pressure. This causes the underlying sediments, particularly unconsolidated clays and silts, to compact under the weight of overlying materials and structures, leading to land sinking. This is projected to impact 19% of the global population by 2040.
- Urban Development and Construction: The sheer weight of buildings, infrastructure (roads, bridges), and other urban structures can compress underlying soft or loose soils, causing gradual settlement. This is especially pronounced in areas built on reclaimed land or young alluvial deposits. Rapid urbanization often involves construction without adequate assessment of the soil's carrying capacity.
- Mining and Resource Extraction: Underground mining for minerals, coal, oil, and gas creates voids beneath the surface. The collapse of these voids can lead to sudden and significant subsidence.
- Unregulated Waste Dumping: Large piles of municipal solid waste, especially on weak or marshy soils, can exert significant pressure, leading to land compression and subsidence.
- Drainage of Organic Soils: Draining wetlands or peatlands for urban development leads to the oxidation and decomposition of organic matter in the soil, causing it to shrink and the land surface to subside.
- Underground Tunneling: Construction of tunnels for transportation or utilities can alter subsurface stress and hydrology, potentially inducing localized subsidence.
- Natural Compaction of Sediments: Young, unconsolidated sediments, particularly in river deltas, coastal plains, and alluvial fans, naturally compact and consolidate over time under their own weight or the weight of overlying layers.
- Tectonic Activity: Earthquakes, faulting, and other tectonic movements can cause abrupt or gradual changes in land elevation, including subsidence. Glacial isostatic adjustment, where land previously covered by glaciers is still readjusting, can also contribute.
- Dissolution of Subsurface Materials (Karst Topography): In areas underlain by soluble rocks like limestone, gypsum, or salt, groundwater can dissolve these rocks, creating underground cavities (karst features). The collapse of these cavities can lead to surface sinkholes and broader subsidence.
- Soil Composition and Properties: The type of soil and rock significantly influences susceptibility to subsidence. Soft clays and silts are more compressible than dense sands or bedrock.
- Erosion and Sedimentation: While erosion removes surface material, the deposition of new sediments, such as in river deltas, can add weight and contribute to the compaction of underlying layers.
Often, it is a combination of these factors that leads to significant urban subsidence. For example, groundwater extraction can exacerbate the natural compaction of alluvial sediments in a rapidly urbanizing coastal city. Climate change can also indirectly worsen subsidence by leading to increased groundwater demand during droughts or by causing sea-level rise, which makes coastal areas more vulnerable to the impacts of sinking land.
Infrastructure Risks from Urban Subsidence
Urban subsidence poses a wide range of substantial risks to infrastructure, leading to costly damage and safety hazards:
- Damage to Buildings and Foundations: Uneven sinking or differential settlement can cause cracks in walls, foundations, and structural elements of buildings. In severe cases, it can lead to structural failure and collapse. Even slight downward shifts can compromise structural integrity over time.
- Impacts on Transportation Networks:
Roads and Highways: Subsidence can cause cracking, undulations, and potholes in road surfaces, requiring frequent and expensive repairs and disrupting traffic flow. Differential settlement can create hazardous driving conditions.
Railways: Sinking land can misalign railway tracks, posing a derailment risk and necessitating costly re-leveling and maintenance.
Bridges and Overpasses: Differential settlement can stress bridge structures, abutments, and approach slabs, potentially leading to instability.
- Damage to Underground Utilities:
Pipelines (Water, Sewage, Gas): Ground movement can bend, crack, or break pipelines, leading to leaks, service disruptions, and potential contamination (e.g., sewage leaking into groundwater or gas leaks creating explosion risks). Malfunctioning drainage and sewage systems are common.
Underground Cables: Subsidence can damage underground power and communication cables, causing outages.
- Increased Flood Risk: As land subsides, especially in coastal or low-lying areas, its elevation relative to sea level or river levels decreases. This makes the area more susceptible to:
Coastal Flooding: During high tides and storm surges. Some coastal cities are sinking multiple times faster than the global sea-level rise, exacerbating this risk.
Inland/Riverine Flooding: Reduced ground elevation can impair natural drainage and increase the extent and duration of flooding from heavy rainfall or river overflows.
- Damage to Water Management Infrastructure:
Canals and Drainage Systems: Subsidence can alter the slope of canals and drains, impairing their efficiency and leading to waterlogging and flooding.
Dams and Levees: Differential settlement can compromise the integrity of dams and levees, increasing the risk of failure and catastrophic flooding.
- Economic and Social Impacts:
High Maintenance and Repair Costs: Constant repairs to damaged infrastructure place a significant financial burden on municipalities and property owners.
Property Devaluation: Properties in subsidence-prone areas may decrease in value.
Displacement of Populations: In severe cases, subsidence can render areas uninhabitable, forcing residents to relocate.
Water Insecurity: Contamination of freshwater sources by intruding seawater (in coastal areas) or pollutants from damaged pipes can worsen water scarcity.
Public Health Crises: Increased flooding can lead to sanitation problems and the spread of waterborne and vector-borne diseases. Exposure to chemical sludge or foul gases from damaged infrastructure can also impact health.
Recent research indicates that a significant percentage of urban areas in major cities worldwide, including those in the US, China, and Indonesia, are experiencing subsidence, putting millions of people and critical infrastructure at risk. For instance, studies have shown that some areas in cities like Jakarta are subsiding at rates of several centimeters per year, with some reports up to 10-25 cm/year in specific locations and periods. In the US, a recent study found that in 28 major cities analyzed, at least 20% of urban areas are sinking, with over 29,000 buildings in high-risk areas.
Addressing urban subsidence requires a multifaceted approach involving continuous monitoring using advanced geodetic techniques, sustainable management of groundwater resources, careful urban planning that considers geological conditions and soil-bearing capacity, and engineering solutions to mitigate the impact on existing and new infrastructure. Early warning systems and risk assessments are also crucial for protecting urban populations and assets.