Land Subsidence and coastal flooding hazrad in southeast Florida


Geodetic monitoring

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.