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 (Figure 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.
Coastal subsidence 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 (Figure 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.
Continuous GPS
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. We constructed a network of four continuously operating GPS (cGPS) stations that monitor land subsidence with accuracies of millimeters per year (Figure 1). Each station is equipped with a precise GPS antenna, a monument containing the antenna, solar panels, cellular communication, and an explanation plaque (Figure 2). The spaceborne radar observations enable the detection of building movements over wide areas, also with an accuracy of millimeters per year. Preliminary results of radar observation acquired during 1994-1999 reveal that localized subsidence occurs in Miami Beach with a magnitude of up to 3 mm/yr, in urban areas built on reclaimed marshland.
Figure 1. Satellite image (google map) of the Miami-Dade County showing the location of four cGPS units for monitoring coastal subsidence.
Figure 2. Pictures of cGPS stations in Haulover Park (left) and FIU BBC campus (right).
InSAR analysis
InSAR data analysis of archived ERS-1/2 data yielded significant results with the desired accuracy of 1-2 mm/yr (Fiaschi and Wdowinski, 2020). The ERS-1/2 data were acquired during 1993-1999 and cover a total period of 7 years. We processed the data using the Small Baseline Subset (SBAS) algorithm (Berardino et al. 2002) with 24 acquisitions and 95 interferometric pairs. Data analysis includes multi-looking (1x5), topography phase removal (1 arc-second SRTM), ESA’s Precise Orbits, Goldstein adaptive filter (Goldstein and Werner 1998), co-registration using the Delaunay Minimum Cost Flow (MCF) algorithm (Costantini 1998), and Atmospheric Phase Screen removal by applying spatial (low-pass) and temporal (high-pass) filters. More details of data analysis are provided in Fiaschi and Wdowinski (2020).
Our ERS-1/2 InSAR analysis focused on subsidence within the city of Miami Beach, which have been subjected to periodic flooding. Miami Beach is a densely populated barrier island, roughly 10 km long and 2.5 km wide at its widest point (Figure 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 (~97%) was stable during the 1993-1999 observation period (green in Fig. 3a). Several localized subsiding areas (< 0.02 km2) were detected mostly in the western and eastern parts of the city (yellow in Fig. 3a). The detected subsidence rate is generally in the 1-3 mm/yr range, with uncertainty level of 0.6-0.8 mm/yr. Uncertainties are calculated for each pixel, based on their multi-temporal coherence values.
Figure 3. Vertical velocity map over Miami Beach from 1993–1999 and displacement time series (from Fiaschi and Wdowinski, 2019).
(a) vertical velocity map obtained with the SBAS technique. The black circles mark the location of the extracted displacement time series.
Red lines mark the location of roads affected by flooding during 1998–2012 (data from Wdowinski et al., 2016). Base image source: Esri,
DigitalGlobe, GeoEye, Earth Geographics, CNES/Airbus DS, USDA, USGS, AeroGRID, IGN, and the GIS User Community. (b) Vertical
displacement time series of the four selected points.
Shimon Wdowinski's research pages on:
Proceedings to the Tenth International Symposium on Land Subsidence (TISOLS) meetings