Over the past decade, several coastal communities in
southeast Florida have experienced a significant increase in flooding
frequency, which has caused significant disturbance to property, commerce,
and overall quality of life. The increased flooding frequency reflects the
contribution of global, regional, and local processes that affect elevation
difference between coastal communities and rising sea level. In a recent
project, funded by the state of Florida, we monitor coastal subsidence in
southeast Florida using GPS and InSAR observations, in order to evaluate the
contribution of local subsidence to the increased coastal flooding hazard.
Preliminary results reveal that subsidence occurs in localized patches
(< 0.02 km
Several coastal communities in southeast Florida have been periodically subjected to flooding events, which have been induced by heavy rain, high tide, and storm surge. The frequency of the flooding events has increased over the past two decades causing property damage, transportation problems, an overall impact on daily life. Our recent study of flooding hazard in Miami Beach has shown that flooding frequency in the city doubled during the years 2006–2013 compared to the previous eight-year period of 1998–2005, mainly due to an increased number of high tide events (Fig. 1) (Wdowinski et al., 2016).
The increased flooding frequency reflects the contribution of global, regional, and local processes that affect elevation difference between coastal communities and rising sea level. Along the US Atlantic coast, the increasing coastal flooding has occurred mainly due to higher sea level, but has also been affected by land subsidence. In order to evaluate the contribution of land subsidence to coastal flooding hazard in southeast Florida, we began a new subsidence monitoring project, which is supported by the Florida Office of Insurance Regulation. The monitoring relies on two geodetic techniques, GPS and Interferometric Synthetic Aperture Radar (InSAR), as well as on visual field observations. The two geodetic techniques provide observations of surface changes over time with different spatial and temporal resolutions and, hence, complement one another. The project supports the construction of four continuous GPS (cGPS) stations, which are currently being constructed in four locations in southeast Florida. In addition, the project supports InSAR data processing of both archive and current data.
This study describes the geodetic monitoring project, which aims at quantifying the spatial extent and rates of coastal subsidence in southeast Florida. We first present field observations indicative of land subsidence in various locations along the southeast Florida coastline. Based on the observed subsiding areas, we selected locations for constructing four continuous GPS stations, which are described in the geodetic monitoring section on the study. We also describe the planned InSAR monitoring analysis as well as presenting preliminary InSAR results of archived ERS-1/2 data. Based on the preliminary InSAR results, we discuss the contribution of local land subsidence to coastal flooding hazard in southeast Florida.
Subsidence in Florida typically occurs at the local scale due to soil oxidation, sediments compaction, and sinkhole activity. Regional scale subsidence due to Glacial Isostatic Adjustment occurs in many sections of continental US, but is negligible in Florida (Sella et al., 2007; Kargar et al., 2016). Also, the tectonic stability of the Florida peninsula suggests negligible tectonic-induced subsidence in Florida.
In southeast Florida subsidence occurs mainly due to sediment compaction, as urban development took place, in part, on reclaimed marshland. Marshland subsidence is a natural process that is often compensated by sediment accretion during inundation events (Nicholls, 2004). However, in reclaimed marshlands, inundation prevention and lack of and sediment supply result in land subsidence. Differential subsiding urban areas often result in structural damage to building and structures, which can be used as proxies for land subsidence.
Along the southeast Florida coast, we observed structural damage to buildings in several coastal parks, including Matheson Hammock, Morningside, and Haulover, as well in the Kovens Conference Center at the Biscayne Bay Campus of Florida International University (Fig. 2). All four locations were constructed between 1930–1960 on reclaimed marshland. The observed structural damage in these locations suggest that local land subsidence is an active process reducing the elevation of some section of the southeast Florida coast.
In order to quantify the distribution and rate of coastal subsidence along the southeast Florida shoreline, we began a geodetic monitoring project, which is supported by the Florida Office of Insurance Regulation. The monitoring relies on two geodetic techniques, GPS and InSAR, which compliments one another. Continuous GPS measurements provide high temporal 3-D positioning observations at limited number of observation points with respect to an external reference frame. Whereas, InSAR provides high spatial resolutions observations of surface changes in line of sight (LOS) between the satellite and the surface, with respect to an arbitrary internal reference point. InSAR-based subsidence measurements are of higher quality in urban settings and arid environments than in vegetated areas, due to the scattering behaviour of each land cover. By using both measuring techniques, we will obtain detailed information of subsidence process in four selected locations and less detailed, but with very good spatial coverage of land subsidence along most of the urban sections of southeast Florida coastline.
Precise GPS measurements require the installation of cGPS stations in coastal subsiding areas and monitoring the subsidence over a period of at least 3–4 years. This project supports the installation of four cGPS stations along the southeast Florida shoreline. Because during the writing of this proceedings the four cGPS stations are still in the planning stage, we describe here the site selection criteria and related activities, as well as the planned monitoring activities.
The first year of the project was devoted to site selection and obtaining permits to install the four cGPS stations. Apparently, this task was found to be much more difficult than anticipated, because most of the urban sections of southeast Florida coastline is privately owned. All of our attempts to install cGPS stations in backyards of private homes along the shorelines failed. Thus, we needed to select cGPS site locations on public land, which is a long process requiring permits by the local authorities. During the first year of the project, we identified fifteen possible sites on public land, as well as on private schools. Some locations were found to be unsuitable in terms of physical conditions (small plots, too many trees, or building plans for the area), whereas in some locations we were not able to obtain the required permit. By the end of the process, we identified four sites for cGPS station installation.
The selected locations include two sites in the Deering Estate, one in Morningside Park, and one in Haulover Park (Fig. 2a). The two sites in the Deering Estate are located on two different geological settings in two sides of the county park. The eastern site, located near the shoreline on peat soil, will be used to measure coastal subsidence. The western site, located on a limestone ridge, will be used as a control point to measure the stability of houses and infrastructure built on limestone. The two sites in Morningside and Haulover parks are located on unconsolidated sediments and will be used for monitoring coastal subsidence (Fig. 2c and e).
As soon as permits will be issued for the four sites, the cGPS stations will
be conducted and start operating. The station construction and data
downloading, archiving and processing will be conducted by UNAVCO
(
This geodetic monitoring project also support InSAR data processing of both
current and archived data. The analysis of current, Sentinel-1 data, did not
reveal yet significant results, because the observation time span
(2015–2019) is too short for obtaining velocity measurement with 1–2 mm yr
InSAR data analysis of archived ERS-1/2 data yielded significant results
with the desired accuracy of 1–2 mm yr
Vertical velocity map over Miami Beach from 1993–1999
and displacement time series (from Fiaschi and Wdowinski, 2019).
Our ERS-1/2 InSAR analysis focused on subsidence within the city of Miami
Beach, which have been subjected to periodic flooding (Fig. 1). Miami
Beach is a densely populated barrier island, roughly 10 km long and 2.5 km
wide at its widest point (Fig. 3). Our InSAR time series results reveal a
patch-like pattern of coherent velocity observations located mainly over the
built environment. The results indicate that most of the city
(
Increasing flooding frequency in several coastal communities along the
southeast Florida coastline resulted in damage to property, commerce, and
overall quality of life. The increased flooding frequency reflects
increasing rate of relative sea level rise, mostly due to ocean dynamic
contributions, but possibly also due to coastal subsidence. In order to
evaluate the contribution of land subsidence to the coastal flooding hazard,
we began a geodetic monitoring project that uses both cGPS and InSAR
observations. The project supports the construction of four cGPS stations
that will be constructed on subsiding public land along the southeast
Florida coastline. The project also supports InSAR monitoring of the urban
environment in southeast Florida. Preliminary results reveal that subsidence
occurs in localized patches with magnitude of 1–3 mm yr
GPS data will be archived by UNAVCO (
SW initiated the study and wrote the manuscript, SW and TOC carried the field survey including GPS site survey, SF conducted the InSAR analysis, SF and SW interpreted the InSAR results.
The authors declare that they have no conflict of interest.
This article is part of the special issue “TISOLS: the Tenth International Symposium On Land Subsidence – living with subsidence”. It is a result of the Tenth International Symposium on Land Subsidence, Delft, the Netherlands, 17–21 May 2021.
The authors thank UNAVCO and particularly John Galetzka for helping selecting and constructing the CGPS stations. This is contribution number 952 from the Southeast Environmental Research Center in the Institute of Environment at Florida International University.
This research has been supported by the Florida Office of Insurance Regulation.