Enhancing Coastal Infrastructure Resilience: three decades of InSAR Analysis of Nice Côte d’Azur Airport Subsidence

Coastal areas can be tremendously biodiverse and host a substantial part of the world’s population and critical infrastructure. However, there are often fragile environments that face various hazards such as flooding, coastal erosion, land salinization or pollution, earthquake-induced land motion, or anthropogenic processes. In this article, we investigate the stability of the Nice Côte d’Azur Airport, which has been built on reclaimed land in the Var River delta (French Riviera, France). This infrastructure, as well as the ongoing subsidence of the airport runways, has been a permanent concern since the partial collapse of the platform in 1979. Moreover, using InSAR data between 2003 and 2011, Cavalié et al. (2015) showed that parts of the airport platform were subsiding up to 10 mm/yr.

Understanding the mechanism and thus the evolution of sediment compaction is essential to evaluate the danger caused by the coastal subsidence. Therefore, in this study, we extended the observation period of InSAR measurements to better analyze the temporal evolution of the ground displacement on the Nice Côte d’Azur Airport platform in the hope of capturing the non-linear component of the deformation. Indeed, the relatively short period of observation (2003-2011) of the previous study (Cavalié et al., 2015) impeded the accurate detection of non-linearity in the surface displacement and thus to understand its dynamic. So, we used here the complete archive of SAR images acquired by ESA over a much longer period of time (28 years from 1992 to 2020).

Extending the observation window to study the long-term subsidence leads to substantial improvements in the understanding of the ongoing mechanisms along this coastal area. Indeed, the new analysis reveals a notable deceleration in the maximum downward motion rate, decreasing from 16 mm/yr in the 1990s to 8 mm/yr in the present day (for the fastest subsidence area).  We then used a simple analytical Burgers creep model to constrain the mechanisms and rheology at play. The data are properly explained by the phases of primary and secondary creep, highlighting a slow viscoelastic deformation at multiyear timescales.  Our study thus proves that the long-term InSAR data can improve our understanding of the surface processes and the subsurface material properties. Although the subsidence rate decelerates, at least for 28 years, our results show that the compaction of the sediment is still active and its future evolution is uncertain and still at stake. Indeed, if compaction bands are developing under the airport platform, creep processes could potentially lead accumulated material damage to failure.

This study underscores the critical role of remote monitoring in comprehending coastal land motion. We show here that employing advanced InSAR techniques offers a better understanding of actual hazards posed by the airport built on reclaimed lands. The findings advocate for ongoing monitoring initiatives to mitigate risks and enhance the resilience of coastal infrastructure.