Sinkhole formation and subsidence along the Dead Sea coast, Israel
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<p>The dissolution of salt-bearing evaporites in the Ebro valley alluvial karst, subject to intense irrigation, produces sinkholes characterised by very high subsidence rates. Here, numerous large subsidence depressions and sinkholes were filled and used for the construction of human structures (e.g., roads, railways, industrial states) that are currently affected by severe damage. The ground deformation in many of these sinkholes is characterised by progressive sagging, eventually punctuated by the sudden occurrence of nested collapses. The latter events have the potential of causing traffic accidents involving fatalities (e.g. roads, railways). Two critical aspects for assessing the hazard associated with these active sinkholes and managing the risk are the precise delineation of the areas affected by ground deformation, and the assessment of the displacement rates and their spatial-temporal patterns.</p><p>This work is focused on the so-called Papiro active sinkhole, which affects the Logro&#241;o highway (N-232a), its service road, and some adjacent buildings. The interpretation of old aerial photographs reveals that this sinkhole is nested within a large subsidence depression, which has been progressively buried and is currently traversed by the Logro&#241;o highway. The recent occurrence of three collapse sinkholes (2006, 2013, 2021) in this section of the Logro&#241;o highway suggests that this is one of the highest risk sites of Zaragoza city area. Subsidence in the Papiro sinkhole has been monitored applying two techniques: (1) high-precision leveling (since 2015); and (2) terrestrial laser scanner (TLS, since 2020). A 108 m long leveling line with a spacing between benchmarks of 3 m was installed along the service road next to the highway. Deformation profiles constructed from measurements taken every three months indicate: (1) a stationary 57 m long deformation zone; (2) constant deformation rates over the monitoring period; (3) a maximum subsidence rate in the central sector of the sinkhole of 1.89 cm/yr; (4) a secondary high-subsidence spot close to the sinkhole edge, coinciding with the location of the 2013 collapse event. The surface displacement models produced by the comparison of point clouds obtained with TLS confidently capture ground deformation with a very high spatial resolution in areas where rates are higher than 4 cm/yr. Interpretation of the consistent displacement data and surface deformations indicates that the sinkhole is characterised by sagging deformation with some displacement accommodated by collapse faults at the margins. The two techniques provide complementary results. High-precision leveling, thanks to its utmost accuracy, allows defining the edges of the deformation zone and resolving small spatial-temporal variations, whereas TLS offers high spatial coverage and resolution. Ongoing monitoring might allow to capture deformation patterns preceding an expected future collapse, and to gain insight into potential precursory deformation applicable for early warning.</p>
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<p>Enclosed topographic depressions are characteristic of karst landscapes on Earth. The scale and morphological characteristics of such depressions are variable, but the most common depression type is a sinkhole (doline). Certain karst depressions that are much larger than sinkholes and that display gentler slopes and more complex three-dimensional shapes are known as uvalas. A single uvala typically contains numerous sinkholes within it. The developmental relationship between sinkholes and uvalas has been subject of debate, however, mainly because long developmental timescales impede direct observation in classical limestone karst, where such features are most commonly reported.</p><p>Here, we describe the development of five uvalas and numerous associated sinkholes in an evaporite karst setting on the eastern shore of the hypersaline Dead Sea. This karst landscape evolved rapidly over a 25-year period from 1992 to 2017 in response to the anthropogenically-driven decline in the Dead Sea level. Our remote sensing data and field observations show that both the sinkholes and the uvala-like depressions formed through subsidence in a very close spatio-temporal relationship. While many sinkholes developed initially in clusters, the uvalas developed around such clusters as larger-scale and gentler depressions that are structurally distinct both in space and time.</p><p>In agreement with inferences for examples in limestone karst settings, the uvalas in this evaporite karst setting do not form by a simple coalescence of sinkholes. Instead, these evaporite-karst uvalas form through subsidence (sagging), interpreted here as in response to distributed subsurface dissolution and physical erosion within a mechanically unstable subsurface volume (e.g. a groundwater conduit network). Sinkholes, on the other hand, are interpreted as discrete subsidence responses within that volume to smaller-scale zones of highly localised material removal (e.g. individual groundwater conduits). Our observations and interpretations are consistent with numerical modelling of subsidence produced by the development of multiple void spaces at progressively deepening levels. Morphometrically, our results also agree well in several respects with a recent re-evaluation of uvalas in some classical limestone karst areas. Consequently, this study helps to clarify the nature, occurrence and genesis of uvalas in karst systems generally.</p>
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<p>Sinkholes linked to cover evaporite karst in urban environments still represent a challenge in terms of clear identification and mapping considering the anthropic rehash and the presence of man-made structures.</p><p>We propose and tested a methodology to identify the subsiding features in an urban area within a cover evaporite karst environment, through an integrated and non-invasive multi-scale approach combining seismic reflection, DInSAR, leveling and full 3D GPR.</p><p>The analysis was conducted in a small village in the Tagliamento valley (Friuli Venezia Giulia region, NE Italy) named Quinis, where sinkholes are reported since a long time as well as the hazard linked to their presence: within the years, several houses have been demolished and at present many of them are damaged.</p><p>First we applied each methodology independently and after we compared, combined and integrated them to obtain more coherent and cross-validates results. Seismic reflection imagined the covered karst bedrock identifying three depocenters; DInSAR investigation allowed to identify an area with higher vertical velocities; leveling data presented a downward displacement comparable with DInSAR results; 3D GPR, applied here for the first time in the study and characterization of sinkholes, clearly defined shallow sinking features imaging also under a shallow dense pipe network. Combining all the obtained results with accurate field observations we identified and map the highest vulnerable zones.</p><p>The final result is the combining of the geophysical, DInSAR and leveling information, while also locating the damaged buildings, the local asphalt pavement breaks or renovation and the buildings which are nowadays demolished, by using vintage photographs and historical maps. The data are consistent, being the most relevant present damages and the demolished building within the zones with higher sinking velocity on the base of both leveling and DInSAR. Geophysically imaged depocenters lie within the most critical area and perfectly correlate with the local pavement damages.</p><p>In a complex geological and hydrological framework, as in the study area, a multidisciplinary and multi-scale approach is mandatory to identify and map the zone most affected by sinking phenomena. While punctual data such as borehole stratigraphy, local groundwater level variations with time, extensometers measurements and geotechnical parameters are useful to highlight local hazard due to occurring deformation, the proposed integrated methodology addresses a complete and quantitative assessment of the vulnerability of the area. It&#8217;s fundamental, especially in anthropized environments, using different integrated techniques, without forgetting the role of the fieldwork of the geologists who can detect the precursors or already occurred, even elusive, signs of the ongoing or incipient sinking.</p>
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