Barrier islands as coupled human–landscape systems
Dylan McNamara, Eli Lazarus
There are nearly 300 barrier islands between Maine and Texas, and of these, at least 70 are intensively developed. Mean population density along the U.S. Atlantic and Gulf coasts are the highest in the country. Such concentrated development exists and continues despite the fact that barrier islands are transient landscapes, not only over geologic time scales of millennia but also within human and economic time scales of centuries to decades. Populated barrier islands are inherently vulnerable
... natural hazards such as sea-level rise, cumulative erosion, and storm events; this vulnerability drives humans to actively modify barrier geometry and environments. The most common manipulations are beach nourishment, to mitigate shoreline erosion, and increases to dune height or seawall construction to prevent flooding and damage from overwash during storm events. Over time scales of years to decades, hazard-mitigation actions impact natural, spatio-temporal barrier processes such as washover deposition and planform transgression, which in turn affect future efforts to manage, control, or prevent barrier change. Through their maintenance and persistence, interventions against coastal hazards represent a significant dynamical component of developed barrier-island system evolution, such that, within the past century, human actions and natural barrier-island processes have become dynamically coupled. This coupling leads to steady-state barrier island behaviors that are fundamentally new. The only way to understand how developed barrier islands will respond to climate change over decadal time scales is to treat these settings as strongly coupled human-natural systems. Over time scales longer than centuries, human interventions may be coupled only weakly to long-term barrier dynamics. Short of major technological advancements or sweeping decisions to transform these environments into comprehensively geoengineered terrains, high-density development on U.S. barrier islands will likely cease to exist in its current configuration. ***Please note that this is a preprint and not peer-reviewed. This work is provided by the authors as a means to ensure open access to scholarly work on a non-commercial basis.*** 3 INTRODUCTION For developed barrier islands around the world, hazard and risk are both increasing. Intensifying and often compounded coastal hazards include shoreline erosion, storm flooding, and sea-level rise. Likewise, risk is increasing as a function of population density and infrastructural development in coastal zones worldwide. As a result, coastal environments -and barrier islands, especially -are being transformed by management decisions, hazard-mitigation interventions, and deliberate manipulation of coastal morphology. Mainstream (as opposed to heterodox) economics defines risk as the likelihood of a hazard event -physical change of a given magnitude driven by a natural event or process -multiplied by the value of assets and infrastructure susceptible to damage from that event. (By this definition, in the absence of valued assets, an environment may be subject to hazard without risk.) A recent report by the U.S. National Research Council applies a somewhat broader scope, defining coastal risk "as the potential for coastal hazards, such as storm surge-induced flooding and wave attack, to cause adverse effects on human health and well-being; economic conditions; social, environmental, and cultural resources; infrastructure; and the services provided within a community" (NRC 2014, p. 1). Coastal management and engineering interventions to reduce risk and mitigate hazard impacts often have unintended consequences, including complex feedbacks between human activities and physical coastal change that researchers are just beginning to understand. Barrier islands comprise approximately 10% of the world's open-ocean coastline, and more than 400 barrier islands fringe the seaboards of the United States; collectively, they represent 24% of barrier-island shoreline length worldwide, a quantity more than twice the next ***Please note that this is a preprint and not peer-reviewed. This work is provided by the authors as a means to ensure open access to scholarly work on a non-commercial basis.*** 4 highest national total (Mexico; 11%) (Stutz and Pilkey 2011). There are nearly 300 barrier islands between Maine and Texas, accounting for over 16,800 km 2 (~6500 mi 2 ) of land area, or 1.5% of coastal shoreline county area along the Atlantic and Gulf Coasts. Of these, at least 70 are intensively developed. Between 1945Between -1975 for urban development increased by 153% (over ~500 km 2 or ~200 mi 2 ); in 1975, urban development comprised 14% of Atlantic and Gulf Coast barrier island land area, when the national average was only 3% (Dolan 2000). In many states, especially through the Mid-Atlantic, barrier islands host a disproportionate amount of high-value housing stock (Nordstrom 2000). Moreover, these same places have experienced marked increases in population. According to recent Census data, the mean percent historic population change for coastal shoreline counties in the U.S. (defined as "counties are directly adjacent to the open ocean, major estuaries, and the Great Lakes") increased by 39% between 1970-2010, but coastal shoreline counties along the U.S. Eastern Seaboard and Gulf Coast saw a 62% change. (The U.S. mean for all counties was 52%.) Mean population density in Atlantic and Gulf Coast shoreline counties (556 people mi -2 ) already exceeds the categorical (446 people mi -2 ) and national (105 people mi -2 ) densities. (Note that the U.S. Census calculates density in terms of square miles, by convention.) Of the total number of housing units in the U.S. in 2010, 39% were in coastal shoreline counties, along with 42% of all U.S. seasonal homes. Nearly 4 million new housing units appeared in coastal shoreline counties between 2000-2010, representing an 8% increase overall; seasonal units increased by 18%. At last count, 52% of U.S. households with an annual household income greater than $150,000 were in coastal shoreline counties (NOAA 2013). ***Please note that this is a preprint and not peer-reviewed. This work is provided by the authors as a means to ensure open access to scholarly work on a non-commercial basis.*** 5 The inherently transient nature of the barrier island landscape (FitzGerald et al. 2008 ) makes development on barrier islands inherently vulnerable to natural coastal hazard, and drives humans to actively modify barrier geometry and environments. The most common manipulations are beach nourishment, to mitigate shoreline erosion, and increases to dune height or seawall construction to prevent flooding and damage from overwash during storm events. As human interventions modify the natural environment to make it more accommodating for development, those modifications affect spatio-temporal changes resulting from natural physical processes, which in turn affect subsequent interventions. For example, beach nourishment alters distribution of barrier overwash (and washover) in space and time, barrier height and width, and spatial patterns of change in planform shoreline position. Adjustments to (or prevention of) these barrier processes and traits in turn affect management and engineering decisions regarding future nourishment projects. The ubiquity, maintenance, and persistence of beach nourishment, dune construction, cliff stabilization, and seawalls, jetties, and other structures mean that interventions and modifications now function as intrinsic morphodynamic components of the developed barrier system -and thus control how developed barrier islands evolve as landforms. Over intermediate time scales of years to decades, human actions and barrier island processes are therefore dynamically coupled. Werner and McNamara (2007, p. 399) describe the constituent parts of a human-landscape system in which dynamic coupling is particularly strong: ***Please note that this is a preprint and not peer-reviewed. This work is provided by the authors as a means to ensure open access to scholarly work on a non-commercial basis.*** 6 "...human-landscape coupling should be strongest where fluvial, oceanic or atmospheric processes render significant stretches of human-occupied land vulnerable to large changes and damage, and where market processes assign value to the land and drive measures to protect it from damage. These processes typically operate over the (human) medium scale of perhaps many years to decades over which landscapes become vulnerable to change and over which markets drive investment in structures, evaluate profits from those investments and respond to changes in conditions."