Detailed quantification of delta subsidence, compaction and interaction with man-made structures: the case of the NCA airport, France

Abstract. River deltas are dynamic coastal systems and their evolutions are closely monitored as it often concentrates vital natural resources for the surrounding areas. Many deltas worldwide experience subsidence due to geological processes (sediment loading and compaction) or human activities (groundwater or hydrocarbon extraction, land reclamation). This causes shoreline erosion or wetland loss which represent serious issues for the population. In this study we investigate the dynamic of the Var delta (France) where reclaimed lands over sea have been built to host the Nice côte d'Azur airport (NCA). Actually, the stability of this infrastructure is a permanent concern since, in 1979, a newly built extension of the runway platform collapsed in the sea, causing important damages. The project of land extension stopped, but the present airport platform is still located on reclaimed land. Factors that can trigger such catastrophic landslide are thought to be linked to the delta activity and the artificial airport platform load. We used, therefore, Envisat InSAR data to measure accurately the ground deformation of the area that includes the Var delta and NCA airport. Combining data from ascending and descending orbits, we estimated the east–west and vertical components of the deformation and obtained very accurate displacement rate (with a 1σ error of 0.25 mm yr⁻¹). We found that nearly all the deformation is vertical and impacts the whole Var delta. The Var valley subsides at a very low rate (0.5–1 mm yr⁻¹) but downstream the subsidence rate increases and a clear jump is observed at the transition with the reclaimed lands (1–2 mm yr⁻¹). On average, the reclaimed lands subside at 3 mm yr⁻¹. Since the subsidence rate increases in correlation with the sediment thickness, we interpret it as the compaction of the delta quaternary sedimentary wedge. In addition, three areas subside faster (between 5 and 10 mm yr⁻¹), with one calling for more attention as it is the largest and overlooks the steep Var canyon. The progressive increase of subsidence rates toward the sea also suggests that the underwater parts of the margins could subside at rates well above 10 mm yr⁻¹.