Subsidence Mapping, Characterization and Modeling: ESA-GMES Terrafirma Services
Raspini, Federico1; Moretti, Sandro1; Del Ventisette, Chiara1; Bianchini, Silvia1; Loupasakis, Constantinos2; Rozos, Dimitrios2; Banwell, Marie-Josée3; Cooksley, Geraint3
1Department of Earth Sciences - University of Florence, ITALY; 2National Technical University of Athens, GREECE; 3Altamira Information, SPAIN
Land subsidence, a generic term which usually refers to a gentle loss of ground elevation, is triggered by geological and/or anthropogenic factors. Among the natural causes of land downward motion, it is worth mentioning the long-term, natural compaction of recent unconsolidated fine-grained materials, and the drainage of peat-rich soils.
Some of the most common causes of land subsidence due to human activity are water pumping from underground reservoirs, backfill compaction under load imposition and reclamation of marshlands.
In most cases land subsidence is chiefly related to a combination of different causes and is often related to land- and water-use practices: slowly developed land subsidence phenomena is most likely to occur as a result of reservoir consolidation, since the natural compaction of unconsolidated fine-grained deposits is accentuated by ground-water pumping from aquitards with related water table decline. These phenomena can also manifest at normally consolidated fine-grained deposits where the ground-water drawdown can retrigger further compaction. Land subsidence induced by the overexploitation of aquifers is a very common natural hazard striking extensive areas worldwide. In many cases, subsidence affects urbanized areas and causes remarkable economic costs annually, as urban fabrics and infrastructures are directly involved in the land settling.
In several cases subsidence is a subtle phenomenon: gentle setting, extending over large areas and with a low deformation rate, develops slowly, almost imperceptibly. However, in cases where the ground deformation is uneven distributed in the affected area, the consequences of such phenomena could become severe.
According to the US National Research Council, 1991, the annual costs resulting from subsidence-related problem exceed 125 million of dollars in the USA. Although these costs are small in respect to those of many other geo-hazards (in the USA, total annual economic losses due to landslides have been estimated to range from 1 to 2 billion of dollars by Schuster & Fleming, 1986), the magnitude of the problems due to subsidence must not be underestimated.
Subsidence-related problems include economic, environmental and social aspects. Examples of successful efforts to mitigate specific subsidence problems exist and they include regulation of systems of groundwater pumping, alternative water supply and critical areas mapping. Adopted policies and solutions for land subsidence management can be various, but despite this variability, mitigation of subsidence requires action in three steps, as recognized by the US National Research Council, 1991: "First, basic earth-science data and information on the magnitude and distribution of subsidence are needed, to recognize and to assess future problems. Such data include geodetic, geologic, hydro-geological, hydrologic, soils, land land-use information." [...] "Second, research on subsidence processes and engineering methods for dealing with subsidence is needed for cost-effective damage prevention or control." [...] "Prediction of subsidence magnitudes, rates, and location is commonly hampered by incomplete understanding of specific details of the relevant processes and the inability to determine adequately subsurface conditions and physical properties of the deforming materials." [...] "And third, although many types of mitigation methods are in use in the United States, studies of their cost-effectiveness would facilitate choices by decision makers."
In synthesis: where mitigation methods have to be put into practice, detailed mapping, characterization and simulation of subsidence have to precede their design and implementation.
In the framework of the ESA GMES Terrafirma Extension project
(www.terrafirma.eu.com), the exploitation of Persistent Scatterer Interferometry within subsidence analysis has been promoted. Satellite interferometry, thanks to its wide spatial coverage and its millimeter accuracy, is ideally suited to measure the spatial extent and magnitude of surface deformation associated with subsidence phenomena. Since the first two-year Stage 1 (started in 2003), Terrafirma is aimed at supporting civil protection agencies and local authorities in charge of risk management, in order to transmit information and benefits for understanding patterns of geo-hazards-related ground motions.
Three validated case studies, where subsidence is caused by different factors, are presented: the urban area of Rome and the Gioia Tauro plain (Italy) and the Kalochori village (Greece). These case studies have been selected with the idea to cover the main activities that must be carried out when dealing with geohazard investigations.
- Mapping: one of challenging aspects when dealing with subsidence analysis and mapping is that extent and magnitude of the phenomenon are not easily detectable. In most cases, land subsidence phenomena present low deformation rates and take place for several decades, sometimes without being noticed at the beginning. Mapping is chiefly devoted to identify extent and rate of a land motion and to evaluate possible interference with urbanization, socio-economic activities and infrastructures. ERS1/2 and Envisat SAR imagery have been exploited to map ground deformation over the urbanized areas of the city of Rome (Central Italy).
-Characterization and monitoring: measuring through time the surface displacement fields induced by the event. This type of information is of great value especially in those urbanized areas endangered by movement and where the investigated phenomenon is going to threaten valuable elements at risk. PSI data are used for evaluating the spatial extent of subsidence, for reconstructing its temporal evolution and for identifying the main factors underpinning the phenomenon. Temporal and spatial evolution of the ground subsidence in the Gioia Tauro plain (Southern Italy) is investigated exploiting multi-temporal analysis of PSI data from historical ERS 1/2 (1992-2001) and recent Envisat (2002-2006) satellites, in ascending and descending geometry.
-Modeling: the ultimate purpose of this task is the integration of conventional in situ investigations, geotechnical modeling and measurement of past displacement derived from PSI analyses to support the identification of the casual factors of the occurring deformation. The subsidence phenomenon affecting the Kalochori village (Greece), due the plenty of available geological data has been selected, since it represents the perfect chance of applying a Finite Element simulation of water pumping-induced subsidence. Reliability of the obtained results has been validated through available PSI data.
National Research Council (1991). Mitigating losses from land subsidence in the United States. National Academy Press, Washington, DC.
Schuster, R.L. & Fleming, R.W. (1986). Economic losses and fatalities due to landslides. Bulletin of American Association of Engineering Geology. 23.