Recent subsidence of the Venice Lagoon from continuous GPS and interferometric synthetic aperture radar

Coastal regions are increasingly affected by larger storms and rising sea level predicted by global warming models, aggravating the situation in the city of Venice where tidal-induced seasonal flooding coupled with natural and anthropogenic subsidence have been perennial problems. In light of accelerated efforts to protect Venice from the rise in sea level we assess land subsidence in the Venice Lagoon over the last decade. Through a combined analysis of GPS position time series from 2001.55 to 2011.00 for four stations installed by the Magistrato alle Acque di Venezia and thousands of observations of InSAR permanent scatterers using RADARSAT-1 images from 2003.3 to 2007.85, we determine that the northern lagoon subsides at a rate of 2-3 mm/yr, whereas the southern lagoon subsides at 3-4 mm/yr. The city of Venice continues to subside, at a rate of 1-2 mm/yr, in contrast to geodetic studies in the last decade of the 20th Century suggesting that subsidence has been stabilized. The GPS results indicate a general eastward tilt in subsidence and that the natural subsidence rate related to the retreat of the Adriatic plate subducting beneath the Apennines is at least 0.4-0.6 mm/yr. Our combined GPS and InSAR analysis demonstrates high spatial resolution in the vertical direction with a precision of 0.1-0.2 mm/yr with respect to a global reference frame. Continued efforts to secure the city of Venice from flooding must also take into account the significant local and regional subsidence rates as well as the expected rise in sea level.