Exponential growth of private coastal infrastructure influenced by geography and race in South Carolina, USA

Homeowners in coastal environments often augment their access to estuarine ecosystems by building private docks on their personal property. Despite the commonality of docks, particularly in the Southeastern United States, few works have investigated their historical development, their distribution across the landscape, or the environmental justice dimensions of this distribution. In this study, we used historic aerial photography to track the abundance and size of docks across six South Carolina counties from the 1950s to 2016. Across our roughly 60-year study period, dock abundance grew by two orders of magnitude, the mean length of newly constructed docks doubled, and the cumulative length of docks ballooned from 34 to 560 km. Additionally, we drew on census data interpolated into consistent 2010 tract boundaries to analyze the racial and economic distribution of docks in 1994, 1999, 2011, and 2016. Racial composition, measured as the percentage of a tract’s population that was White, positively correlated with dock abundance in each year. Median household income and dock abundance were only correlated in 2011. Taken together, these metrics indicate the growing desire for direct estuary access, however, that access does not appear to be equally spread across racial groups. Because docks enhance estuarine access and demarcate private property, our study provides longitudinal insights into environmental justice concerns related to disparate private property ownership. We found a persistent correlation between the racial characteristics of an area and dock abundance, strongly indicating that White South Carolinians have had disproportionately greater private water access for the past two decades.

Methods
Dock data was collected via historic aerial imagery of the South Carolina coast. Pre-1990 imagery was obtained from the University of South Carolina library, 1994 and 1999 imagery was obtained from the South Carolina Department of Natural Resources, and 2011 imagery was obtained from the US Department of Agriculture National Agriculture Imagery Program’s Geospatial Data Gateway (https://nrcs.app.box.com/v/gateway/folder/19350726983). Census data was obtained from the NHGIS and Historical Housing Unit and Urbanization Database at the tract level using their crosswalk files to interpolate 1990 and 2000 data to 2010 tract geographies.

Docks were tracked across decades in ArcGIS Pro and statistical models were run using R. Greater methodological detail is provided in the "Historic Infrastructure Methodology" file in the "Historic_Dock_Supplemental" folder on Zenodo. All pre-1990 images therein are reproduced with permission of the University of South Carolina library.