We used the National Oceanic and Atmospheric Administration (NOAA) Office of Response and Restoration’s Environmental Sensitivity Index (ESI) geodatabases to calculate the linear kilometers of total shoreline and kilometers of hardened shoreline for each coastal county within the continental US (NOAA 2005).

To evaluate the relationship between potential drivers and the percentage of hardened shoreline in each US coastal county, we considered the following factors: 2010 housing density (units per square kilometer), 2010 gross domestic product (GDP, expressed in US dollars), coastal slope (%), accretion/erosion rates (meters per year [m yr–1]), geomorphology, mean tidal range (m), mean wave height (m), relative SLR (millimeters per year [mm yr–1]), storm frequency between the years 1970–2010, relative county shoreline position (north to south or west to east along the coast), and years since a ban on shoreline hardening was passed (sources included: the US Census Bureau, the National Ocean Economics Program, the US Geological Survey Coastal Vulnerability Index [USGS CVI] [for a description of the variables, see Hammer-Klose and Thiehler 2001], the Federal Emergency Management Agency’s US Presidential Major Disaster and/or Emergency Declarations [FEMA 2014], and federal and state legislation and permitting procedures).

We grouped all man-made structures (seawalls, bulkheads, riprap structures [revetments, breakwaters, groins/jetties], and hybrid seawall/bulkheads with riprap) to calculate the cumulative lengths of hardened shoreline. We divided each state’s ESI shoreline by coastal county and, for the Pacific and Atlantic coasts, by whether the shoreline was “open” (ie directly exposed to the ocean) or “sheltered” (ie located in a bay, sound, lagoon, or tidally influenced river). We did not divide the Gulf of Mexico coast into “open” or “sheltered” categories because much of the Gulf coastline (eg the Louisiana coastline, the Big Bend region of Florida) consists of reticulated wetlands that cannot be easily classified in this way. We then calculated the length (in kilometers) of tidal shoreline (total and armored), as well as the percentage of hardened shore for each county. Additionally, we calculated the length of tidal wetland shoreline (total and armored).