U.S. Geological Survey National Shoreline Change

In coastal areas, the dynamic interfaces between water and land are often locations of concentrated residential and commercial development as well as Federal, State, and local municipal landholdings managed for recreation and conservation. These areas are frequently subjected to a range of natural hazards, which include flooding, storm effects, and coastal erosion. In response, the U.S. Geological Survey (USGS) is compiling existing reliable historical shoreline data along open-ocean sandy shores of the conterminous United States and parts of Alaska and Hawaii as part of the Coastal Change Hazards priority area. One component of this research effort documents changes in shoreline position, which are used as a proxy for coastal change. Shoreline position is one of the most monitored indicators of environmental change (Morton, 1996), and it is an easily understood feature marking the location of a beach through time.

A principal focus of the shoreline change effort has been to develop a consistent methodology for calculating shoreline change rates and reporting results that may be periodically updated when additional data or improved techniques are available. Since 2004, the USGS has been publishing results of shoreline monitoring work, organized and presented by coastal region, including the U.S. Gulf of Mexico coast (Morton and others, 2004), the Southeast Atlantic coast (Morton and Miller, 2005), California sandy shorelines (Hapke and others, 2006) and California coastal cliffs (Hapke and Reid, 2007), the New England and Mid-Atlantic coasts (Hapke and others, 2011), parts of the Hawaii coast (Fletcher and others, 2012), the Pacific Northwest coast (Ruggiero and others, 2013), and parts of the Alaska coast (Gibbs and Richmond, 2015; Gibbs and others, 2019). Updates to the Southeast Atlantic, Gulf, and Alaska coasts have also been published (Kratzmann and others, 2017; Himmelstoss and others, 2017; Gibbs and Richmond, 2017).

During Hurricane Irma in September 2017, Florida and Georgia sustained significant effects on beaches, dunes, barrier islands, and coral reefs. Extensive erosion and coral losses result in increased immediate and long-term hazards to shorelines that include densely populated regions. These hazards put critical infrastructure at risk for future flooding and erosion and may cause economic losses. This storm event presented an important opportunity for the USGS Coastal and Marine Hazards and Resources Program (CMHRP) to update shoreline datasets that could be used to evaluate coastal erosion along the southeastern U.S. coastline and potential vulnerability to future storms and changes in sea level. Shoreline positions were compiled, prior to and following Hurricane Irma, along the open-ocean sandy coasts of Georgia and Florida (fig. 1). Shoreline positions from the mid-1800s through 2018 were used to update the shoreline change rates for these regions. The shoreline positions and updated change rates can provide actionable information to homeowners, coastal communities, and managers of public and private properties to improve resiliency to long-term hazards.

This report is an update to previous reports for Georgia and Florida (Kratzmann and others, 2017; Himmelstoss and others, 2017) and includes updated rate-of-change calculations based on additional light detection and ranging (lidar) shoreline position data extracted using the profile method (Farris and others, 2018), improved rate metrics, and application of a proxy-datum bias correction on a transect-by-transect basis that quantifies potential bias and errors associated with integrating shorelines referenced to different proxies (Ruggiero and List, 2009). The proxy-datum bias accounts for the unidirectional onshore bias of proxy-based high water line shorelines relative to datum-based mean high water (MHW) shorelines. If this offset is not accounted for, shoreline change rates show slower shoreline retreat, progradation rather than retreat, or faster progradation than in reality (Ruggiero and List, 2009). The proxy-datum bias concept is explained further in Ruggiero and List (2009).

Several new datum-based MHW shorelines extracted from lidar elevation data are included in the analysis. Lidar dates range from 2006 to 2018. The full range of shoreline data is 1851 to 2018. The Florida coast has been divided into three subregions: Florida east coast (from the border with Georgia to Key Biscayne, Florida), Florida west coast (Anclote Key to Barefoot Beach), and Florida panhandle (Bald Point State Park to the Alabama border). Georgia is contained in a single dataset. The shoreline areas between Key Biscayne and Barefoot Beach, and Florida west coast and Florida panhandle have not been included in this study because of a lack of data and are marked as such on figure 1 between subregion boundary lines.

Long-Term Shoreline Change

Averaged rates of long-term shoreline change and the associated average values of rate uncertainty for the coasts of Georgia and Florida are presented in table 1. In each subregion, the averaged long-term rates are considered statistically significant when the average rate is larger than the average reduced n CI, or in other words, when the range of the rate plus or minus (±) the uncertainty is entirely accretional (positive) or entirely erosional (negative).

The coast of Georgia exhibited the highest average rate of erosion in the long-term, −1.8±0.09 meter per year (m/yr). In Georgia, erosion occurred at 39 percent of the shoreline change transect locations (table 1). The maximum long-term erosion rate was observed at Wolf Island, Georgia, −10.6±1.8 m/yr (table 1, fig. 1). The maximum long-term accretion rate was 18.6±8.7 m/yr, at Little St. Simons Island, Georgia (table 1, fig. 1). Accretion occurred at 61 percent of the Georgia shoreline change transect locations, making this subregion the most accretional overall in the long-term (table 1).

The average erosion rate was −0.4±0.02 m/yr on Florida’s east coast, where erosion occurred at 42 percent of transect locations in the long-term (table 1). Here, the maximum long-term erosion rate was −8.1±4.5 m/yr at south Little Talbot Island, Florida (table 1, fig. 1). The maximum long-term accretion rate in Florida’s east coast was 9.8±3.0 m/yr at Anastasia State Park (table 1, fig. 1). Accretion occurred at 57 percent of transect locations in this subregion (table 1).

The average erosional rate was −0.7±0.03 m/yr on Florida’s west coast, where erosion occurred at 45 percent of transect locations in the long-term (table 1). The maximum long-term erosion rate on this stretch of coast was −5.1±0.8 m/yr at Egmont Key (table 1, fig. 1). The greatest long-term maximum accretion rate on the Florida west coast is 8.6±5.9 m/yr at Honeymoon Island (table 1, fig. 1). Accretion occurred at 55 percent of transect locations in this subregion (table 1).

The average erosional rate was −0.9±0.03 m/yr on the Florida panhandle, where erosion occurred at 61 percent of transect locations making this subregion the most erosional overall in the long-term (table 1). The greatest long-term erosion rate here was −7.3±4.0 m/yr at the beach at Tyndall Air Force Base (table 1, fig. 1). The maximum long-term accretion rate on the Florida panhandle was 10.3±10.9 m/yr at Cape San Blas (table 1, fig. 1). Accretion occurred at 39 percent of shoreline change transect locations in this subregion (table 1).

Short-Term Shoreline Change

Averaged rates of short-term shoreline change and the associated average values of rate uncertainty for the coasts of Georgia and Florida are presented in table 2. In each subregion, the averaged short-term rates are considered statistically significant when the average rate is larger than the average reduced n CI.

The greatest short-term average erosion rate was in Georgia, −2.7±0.13 m/yr, where erosion occurred at 44 percent of the shoreline change transect locations (table 2). The greatest maximum short-term erosion rate was observed on the Florida panhandle at Cape San Blas, −42.4±54.4 m/yr (table 2, fig. 1). While this erosion rate is large, it is not statistically significant. The greatest short-term average accretion rate was in Georgia, 3.4±0.14 m/yr (table 2). The greatest maximum short-term accretion rate was measured on the Florida panhandle at Dog Island, 29.3±44.6 m/yr (table 2, fig. 1). Several of the short-term maximum accretion rates are not statistically significant (table 2).

The short-term average erosion rate for Florida’s east coast was −0.5±0.02 m/yr, and the average accretion rate was 0.8±0.02 m/yr (table 2). The maximum short-term erosion rate was −10.4±3.2 m/yr, observed at south Little Talbot Island, Florida (table 2, fig. 1). Greater average rates of erosion and accretion were measured on Florida’s west coast. Here, the short-term average erosion rate was −1.4 ±0.07 m/yr, and the average accretion rate was 1.6±0.06 m/yr (table 2). The maximum short-term erosion rate on Florida’s west coast was −13.5±5.4 m/yr at Three Rooker Bar (table 2, fig. 1). The short-term average erosion rate on the Florida panhandle was −1.5±0.06 m/yr, and the average accretion rate was 1.6±0.11 m/yr (table 2).