Thresholds in Road Network Functioning on US Atlantic and Gulf Barrier Islands

Barrier islands predominate the Atlantic and Gulf coastlines of the USA, where population and infrastructure growth exceed national trends. Forward-looking models of barrier island dynamics often include feedbacks with real estate markets and management practices aimed at mitigating damage to buildings from natural hazards. However, such models thus far do not account for networks of infrastructure, such as roads, and how the functioning of infrastructure networks might influence management strategies. Understanding infrastructure networks on barrier islands is an essential step toward improved insight into the future dynamics of human-altered barriers. Here, we examine thresholds in the functioning of 72 US Atlantic and Gulf Coast barrier islands. We use digital elevation models to assign an elevation to each intersection in each road network. From each road network we sequentially remove intersections, starting from the lowest elevation. We use the maxima of the second giant connected component to identify a specific intersection-and corresponding elevation-at which functioning of the network fails, and we match the elevation of each critical intersection to local annual exceedance probabilities for extreme high-water levels. We find a range of failure thresholds for barrier island road network functioning, and also find that no single metric-absolute elevation, annual exceedance probability, or a quantitative metric of robustness-sufficiently ranks the susceptibility of barrier road networks to failure. Future work can incorporate thresholds for road network into forward-looking models of barrier island dynamics that include hazard-mitigation practices for protecting infrastructure.