Summary

The collapse of the Champlain South Condominium high-rise apartment block in a suburb of Miami, Florida, in 2021, which killed 98 people, drew attention to the potential dangers of massive urban development in vulnerable coastal regions. Now, a new study has revealed alarming signs of structural instability in high-rise buildings along the coast off Miami in South Florida. The research was carried out by researchers from the University of Miami, the University of Houston and the NASA Jet Propulsion Laboratory in the USA in collaboration with scientists led by Prof. Mahdi Motagh from the GFZ German Research Centre for Geosciences and Leibniz University Hannover.

Using satellite-based radar measurements, the research team documented subsidence of up to 8 centimetres in 35 of 197 coastal buildings in the period between 2016 and 2023, with newer buildings constructed since 2014 being the most severely affected, as was the northern section of the coast. Creep deformations of sand layers in the limestone subsoil of the region are suspected to be the main cause, which can be accelerated or triggered by the construction load. Increased water movement due to tidal forces or stormwater injection could also exacerbate such processes. The results show that such subsidence – contrary to previous assumptions – can occur years after construction and can in some cases also be triggered in existing buildings by neighbouring new construction. The study has been published in the journal Earth and Space Science.

Measuring subsidence of buildings with satellite radar

Satellite-based radar systems can be used to obtain information about the stability and motion of critical infrastructure such as buildings, dams, transport networks and bridges. In radar interferometry (InSAR – Interferometric Synthetic Aperture Radar), radar signals from objects such as balconies, rooftop air conditioning units and pavements are scattered back to the satellite flying approx. 700 kilometres above the Earth. Analysing of these echoes makes it possible to detect displacement of objects with millimetre to centimetre accuracy.

The research team used this technology to investigate the development of an approximately 15-kilometre-long coastal strip of the Barrier Islands off Miami, Florida, from Sunny Isles Beach in the north via Bal Harbor and Surfside to Miami Beach in the south.

The researchers from the USA, led by first author Dr Farzaneh Aziz Zanjani, PostDoc at the University of Miami, and Prof. Falk Amelung, also from the University of Miami and in 2019 a Helmholtz International Fellow at the GFZ, analysed data from the European Sentinel-1 satellite and created time series of surface displacement for the years 2016 to 2023 by combining 222 SAR images. The co-authors led by Mahdi Motagh, working group leader in GFZ Section 1.4 “Remote Sensing and Geoinformatics” and professor at Leibniz University Hannover, Germany, and Andreas Piter, doctoral student with Mahdi Motagh at Leibniz University Hannover, additionally analysed 121 data sets from the German TerraSAR-X satellite for the period 2017 to 2021.

Among other things, a new open-source software called “SARvey” was used to analyse the data, which was largely developed by Andreas Piter in collaboration between the GFZ and Leibniz University Hannover.

Results: Subsidence of coastal structures

The researchers observed significant subsidence of two to eight centimetres at 33 of 87 coastal structures in Sunny Isles Beach, Bal Harbor and Surfside; of the 110 coastal buildings in Miami Beach to the south, only two were affected. Overall, there were localised clusters along the coast. It was also found that newer buildings built after 2014 were particularly affected by the subsidence. According to the researchers, this may indicate a direct influence of the associated construction activities. In older affected buildings, the onset of subsidence usually coincides with nearby construction activities.

However, for the Champlain South Condominium, which collapsed in 2021, the team was unable to detect any precursory signs of motion, partly because the section which collapsed could not be imaged properly by the side-looking radar sensor used in this study.

Potential hazard: Sandy layers in the subsoil on Florida's coast

The researchers see the sandy subsurface geology of Miami and the surrounding area as the main – locally effective – cause of the subsidence and thus of the danger to the buildings on the coast. It consists mainly of limestone, which has been interspersed with sandy layers in the course of geological development, and poses unique challenges for urban development.

It is well known that new high-rise buildings can experience settlements of several tens of centimetres during and shortly after construction. However, the current study shows that subsidence can persist even years later.

The researchers hypothesise that the subsidence observed in Miami is primarily caused by the weight of the buildings and construction activity. This is accompanied by both vibrations and dynamic subsidence as a result of groundwater discharge following pumping. Both could lead to a gradual compaction of the sand grains within the sand layers embedded in the limestone. The daily tidal flow through the sand layers and the discharge of rainwater can also lead to subsidence by rearranging the sand grains or dissolving the limestone. Overall, subsidence is likely caused by a combination of these mechanisms, with their specific contributions varying depending on local geological conditions.

With the barrier islands off the coast of Miami already vulnerable to sea level rise and climate change, ongoing subsidence further increases the risk to the city's infrastructure.

Importance of monitoring subsidence for resilient urban development

The extent to which the developments observed here can also be transferred to other areas depends largely on the geology of the subsoil.

“Understanding and managing the risk of subsidence is crucial for sustainable urban development. The complexity of dealing with such risks requires a networked approach and interdisciplinary collaboration to promote the resilience of cities to withstand the challenges posed by natural and man-made factors,” emphasises Mahdi Motagh from GFZ.

“Advanced remote sensing technologies such as InSAR play a key role in monitoring urban development in vulnerable areas, as they enable early detection and evaluation of risks to urban elements exposed to ground deformation hazard. This data can serve as a basis for decisions on infrastructure development, land use and disaster preparedness.”

Funding:

• University of Miami Laboratory For Integrative Knowledge (U-Link) Climate Resilience programme
• German Federal Ministry for Digital and Transport
• University of Houston
• National Aeronautics and Space Administration. Grant Numbers: NNH21ZDA001N-DSI, 80NSSC 22K1096
• Helmholtz International Fellowship to Falk Amelung
• U.S. National Science Foundation. Grant Numbers: 2138259, 2138286, 2138307, 2137603, 2138296

Original publication

Aziz Zanjani, F., Amelung, F., Piter, A., Sobhan, K., Tavakkoliestahbanati, A.,
Eberli, G. P., Haghighi, M. H., Motagh, M., et al. (2024). Insar observations of construction‐induced coastal subsidence on Miami's barrier islands, Florida. Earth and Space Science, 11, e2024EA003852.
https://doi.org/10.1029/2024EA003852

Further Information

https://github.com/luhipi/sarvey