Publication: Assessing Current Coastal Subsidence at Continental Scale: Insights From Europe Using the European Ground Motion Service

Land subsidence increases the risk of flooding in low-lying coastal zones by amplifying relative sea-level rise (SLR). In this study, the current coastal land subsidence at the scale of Europe is assessed for the first time using the new Copernicus European Ground Motion Service (EGMS), released in 2022. The results suggest that nearly half of the low-lying coastal areas in Europe are currently subsiding at a rate faster than 1 mm/yr on average!

Coastal subsidence is higher on average in areas with more people, urban centers, and critical infrastructure. This raises concerns that coastal subsidence—and therefore relative SLR—tends to be underestimated in Europe and presumably in many other regions around the world. This study demonstrates the utility of emerging continental-scale land motion services like EGMS in better characterising the issue, and anticipating coastal risks and adaptation needs accordingly.

Key Findings

1. Challenges in VLM Calibration: The study compares the InSAR-based EGMS Ortho (Level 3) with nearby global navigation satellite systems (GNSS) vertical velocity estimates. It shows that the geodetic reference frame used to calibrate EGMS strongly influences coastal vertical land velocity estimates at the millimeter-per-year level. This calibration needs to be considered with caution.
2. Adjusted Assessments of Coastal VLM: After adjusting the EGMS vertical velocity estimates to a more updated and accurate International Terrestrial Reference Frame (ITRF2014), an assessment of VLM in European low-elevation coastal flood plains (CFPs) was performed. The findings indicate that nearly half of the European CFP area is subsiding at a rate faster than 1 mm/yr on average.
3. Urban and Harbor Vulnerabilities: The study reveals that urban areas and populations located in CFPs experience a near −1 mm/yr VLM on average (excluding the uplifting Fennoscandia region). For harbors, the average VLM is even larger, increasing to −1.5 mm/yr on average. This highlights the widespread importance of continental-scale assessments based on InSAR and GNSS to better identify areas at higher risk from relative SLR due to coastal subsidence.

Insights for Decision-Makers

1. Impact on Flood-Prone Coastal Areas: The research shows that European flood-prone coastal cities subside on average at 1 mm/yr (Fennoscandia excluded). This is critical information for urban planning and coastal management in light of increasing SLR.
2. Geocentric Reference Frame Considerations: The study underscores that the geocentric reference frame used to calibrate continental-scale land motion assessments significantly affects the results for geodetic (non-geophysical) reasons. This should be taken into account in future assessments.

Conclusion

Our study shows that there is a potential for a service for coastal adaptation practitioners, positioned downstream of EGMS or similar services in other regions worldwide. EGMS is a service of the “land” component of the European Earth Observation program Copernicus, but delivering actionable information to coastal users concerned with relative SLR requires additional analysis, as shown in this paper. Similar services can be developed in other regions of the world owing to the almost global coverage of the world’s coasts by the Sentinel 1 constellation of satellites. Given the challenges raised by SLR, it would be important to make sure that such operational services are further tailored to the needs of coastal practitioners concerned with understanding risks and planning for adaptation.

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