Land Subsidence and coastal flooding hazrad in southeast Florida

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.

Figure 1. (left) “Sunny sky” flooding in Miami Beach. (right) Annual flooding occurrence in Miami Beach between 1998-2013 indicating a significant increase in tide flooding events (green) since 2006.

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.

Figure 2. (a) Location map of our study area in southeast Florida based on a satellite imagery. The map shows the location of documented coastal subsiding areas and planned GPS sites. Insert shows the location of the study area with respect to state of Florida. Map data: Google, Maxar Technologies. (b) Subsidence-induced structural damage to a building in Matheson Hammock Park. (c) Observed subsidence along seawall in Morningside Park. (d) Subsidence-induced structural damage at the entrance to the Kovens Conference Center at the Biscayne Bay Campus of Florida International University (FIU). (e) Observed subsidence along seawall in Haulover Park.