The Future of Developed Barrier Systems: 1. Pathways Toward Uninhabitability, Drowning, and Rebound

Many barrier islands and spits (collectively, "barriers") throughout the world are highly developed. As low-lying, sandy coastal landforms, barrier systems are naturally reshaped by processes associated with storms and sea-level rise (SLR). The resulting landscape changes threaten development, and in response, humans employ defensive measures that physically modify barrier geometry to reduce relatively short-term risk. These measures include the construction of large dunes, emplacement of beach nourishment, and removal of washover. Simulations conducted using a new coupled modeling framework show that, over decades to centuries, measures to protect roadways and communities alter the physical characteristics of barrier systems in ways that ultimately limit their habitability. We find that the pathway toward uninhabitability (via roadway drowning or community narrowing) and future system states (drowning or rebound) depends largely on dune management-because building dunes blocks overwash delivery to the barrier interior-and on initial conditions (barrier elevation and width). In the model, barriers can become lower and narrower with SLR to the point of drowning. The timing and occurrence of barrier drowning depends on randomness in the timing and intensity of storms and dune recovery processes. We find that under a constant rate of SLR, negative feedbacks involving storms can allow barriers that do not drown to rebound toward steady-state geometries within decades after management practices cease. Barrier islands and spits (collectively referred to as "barriers") can naturally keep up with sea-level rise primarily through a process called overwash. During overwash, sand from the beach is washed landward past the dunes by storm waves, leading to increases in barrier height (elevation) and width. Tall dunes, built to protect roadways and oceanfront properties, prevent overwash from elevating the existing barrier landscape. Here we use a new model to show that over many decades to centuries, an unintended consequence of rebuilding tall dunes in the aftermath of storms, which then block future overwash, is the narrowing and lowering of barriers relative to sea level. In some cases, this leads to complete drowning of the barrier interior. In other cases, once humans stop rebuilding dunes, the landscape recovers in as little as a few decades. Counterintuitively, with higher dune design heights and lower/wider initial barrier geometries, barriers become uninhabitable sooner Randomness in dune-storm interactions dictates whether or not a barrier drowns after management ceases Simulations suggest barrier systems can recover in height, width, and cross-shore position quickly (within decades) after management ceases