FIU geophysics professor Shimon Wdowinski discusses a 2020 study where he found land subsidence in Miami Beach and in Norfolk, Va. Below he addresses key questions about subsidence and his research in the wake of the tragic building collapse in Surfside, Fla.
An expert in space geodesy, natural hazards and sea level rise Wdowinski is part of the FIU Institute of Environment and the Department of Earth and Environment. He holds a master’s degree in engineering sciences and a Ph.D. in geophysics from Harvard University. He also holds a master's degree in geology from Hebrew University.
1. What was the focus of your study?
We conducted this study because we were concerned about increased flooding in South Florida, especially in Miami Beach and in Surfside. We also studied Norfolk, Va., which has similar flooding occurrences. Rising sea levels introduce a major threat to these communities. Because rising sea levels and or land subsidence can increase coastal flooding, we wanted to understand how much of the threat is coming from the rising sea levels and how much from subsiding land.
2. What were your findings?
Our 2020 study of land subsidence and coastal flooding hazards in Miami Beach used Interferometric Synthetic Aperture Radar, or InSAR, data from the 1990s. The study was designed to provide local advisory organizations, such as the Southeast Florida Regional Climate Change Compact, with important information for improving flood hazard assessments. The study was to also assist with the development and implementation of more efficient flood emergency response and recovery plans. We found that most of the city of Miami Beach is stable except for several pockets that show subsidence at a rate of 1-3 millimeters per year. Most of the subsidence occurred in the western side of the city. However, we found one localized area of subsidence in the area of the Champlain tower. That area subsided at a rate of 2 millimeters per year between 1993 and 1999. Subsidence is a common, very slow movement of the ground that cannot be seen by human eyes but is detectable from space by InSAR.
3. What did the radar technology show you?
InSAR is widely used to detect and monitor surface changes of land, glaciers and water levels in wetlands. The technology has been widely used to monitor changes induced by processes that may lead to natural disasters including earthquakes, volcanoes, landslides, sinkholes and land subsidence. By comparing two or more radar observations from roughly the same location in space, we can detect small changes of the Earth’s surface with centimeter-level accuracy. Over time, changes can be observed with millimeter-per-year accuracy. In the case of the area where the Champlain Condo tower is located, the rate of subsidence detected from 1993 to 1999 was 2 millimeters per year, which is relatively low, but still above the detection level.
4. Is subsidence to be expected?
Subsidence is a common, very slow movement of the ground that cannot be seen by human eyes but is detectable from space. We didn't expect to see much settling on the east side, especially in an environment that is supposed to be pretty stable.
5. What generally causes subsidence on a barrier island like the one Miami Beach is built on?
Subsidence in urban and suburban areas are typically caused by soil consolidation. The western part of Miami Beach was built on reclaimed wetlands and has been subjected to more subsidence. The eastern side of the island is built on higher ground overlying limestone and is less likely to experience subsidence.