Using a series of satellite methods to assess land subsidence in Alexandria
Résumé
Delta regions have been given considerable attention in recent years as these coastal areas are subject to subsidence processes that may exacerbate the exposure and risks to climate change-related sea level impacts. For instance, Hanson et al. (2011) considered an additional 0.5 m increase in relative sea level rise projections by 2070s for all large port cities established on delta regions. As a result of their analysis, Alexandria was ranked 11 in terms of exposure to coastal flooding. Within the Cecile project we have revisited the issue of land subsidence in Alexandria by: (1) examining the reprocessed data from the GPS station that is operational since 2001 near the historical tide gauge; and (2) investigating the results from the refined method of combining altimetry and tide gauge data. Both independent methods yield consistent rates of vertical land movements of 0.1+/-0.3 mm/yr at the GPS station and -0.4+/-0.2 mm/yr at the tide gauge, which are separated by 3 km. Details can be found in Santamaria-Gomez et al. (2012) and Wöppelmann and Marcos (2012), respectively. These observational results reveal that Alexandria is subject to a moderate rate of land subsidence for the past decades around 20 times lower than the values adopted by Hanson et al. (0.5 m between 2005 and 2070) to assess the exposure of populations and assets in delta regions. To further explore the areal extension of these point-wise results, we have used synthetic aperture radar (SAR) interferometry satellite techniques, namely the persistent scatterers interferometric analysis (PSI) to produce a mean velocity map over the period 2002-2010 corresponding to 49 SAR data acquired over Alexandria by the satellite Envisat. The results from the PSI analysis remarkably confirm the relative local stability between the tide gauge and the GPS stations. The two closest persistent scatterers to those stations give a consistent differential motion of 0.6 mm/yr, at the limit of the capability of the PSI technique in non-instrumented areas (about 0.5 to 1 mm/yr), however. Most interestingly, millimetre deformation rates are found in the city of Alexandria, which do not support the hypothesis of city-scale subsidence rates in excess of 5 mm/yr. This result is in agreement with previous geological field work in the eastern margin of the Nile Delta (e.g., Stanley and Warne, 1993). It reminds that coastal subsidence in large deltas is a site-specific process. While coastal exposure to climate extremes also depends on many other natural and human factors (e.g. coastal management practices), this work shows that the combined use of in-situ and satellite observations allows evaluating how the relative sea- and land level changes may aggravate or mitigate the existing coastal hazards.