Barrier islands and spits tend to migrate landward in response to sea‐level rise through the storm‐driven process of overwash, but overwash flux depends on the height of the frontal dunes. Here, we explore this fundamental linkage between dune dynamics and barrier migration using the new model Barrier3D. Our experiments demonstrate that discontinuous barrier retreat is a prevalent behavior that can arise directly from the bistability of foredune height, occurring most likely when the storm return period and characteristic time scale of dune growth are of similar magnitudes. Under conditions of greater storm intensity, discontinuous retreat becomes the dominant behavior of barriers that were previously stable. Alternatively, higher rates of sea‐level rise decrease the overall likelihood of discontinuous retreat in favor of continuous transgression. We find that internal dune dynamics, while previously neglected in exploratory barrier modeling, are an essential component of barrier evolution on time scales relevant to coastal management.

Developed barrier systems (barrier islands and spits) are lowering and narrowing with sea-level rise (SLR) such that habitation will eventually become infeasible or prohibitively expensive in its current form. Before reaching this state, communities will make choices to modify the natural and built environment to reduce relatively short-term risk. Using a new coupled modeling framework, we simulate how, over decades to centuries, defensive measures to protect development (roadways and communities) alter the physical characteristics, and therefore habitability, of barrier systems. 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 – which influences overwash delivery to the barrier interior – but also on exogenous conditions (SLR and storminess), initial conditions (barrier elevation and width), and alongshore connectivity of management strategies. The timing and occurrence of barrier drowning depends on the rate of SLR and on stochasticity in the timing and intensity of storms and dune recovery processes. We find that negative feedbacks involving storms can allow barriers that do not drown to rebound toward steady-state geometries within decades after management practices cease. In the case of partial, early abandonment of roadway management (i.e., decades before the road is deemed untenable), we find that system-wide transitions to less vulnerable states are possible, even under accelerated SLR and increased storminess.

Tide gauge water levels are commonly used as a proxy for flood incidence on land. These proxies are useful for projecting how sea‐level rise (SLR) will increase the frequency of coastal flooding. However, tide gauges do not account for land‐based sources of coastal flooding and therefore flood thresholds and the proxies derived from them likely underestimate the current and future frequency of coastal flooding. Here we present a new sensor framework for measuring the incidence of coastal floods that captures both subterranean and land‐based contributions to flooding. The low‐cost, open‐source sensor framework consists of a storm drain water level sensor, roadway camera, and wireless gateway that transmit data in real‐time. During 5 months of deployment in the Town of Beaufort, North Carolina, 24 flood events were recorded. Twenty‐five percent of those events were driven by land‐based sources—rainfall, combined with moderate high tides and reduced capacity in storm drains. Consequently, we find that flood frequency is higher than that suggested by proxies that rely exclusively on tide gauge water levels for determining flood incidence. This finding likely extends to other locations where stormwater networks are at a reduced drainage capacity due to SLR. Our results highlight the benefits of instrumenting stormwater networks directly to capture multiple drivers of coastal flooding. More accurate estimates of the frequency and drivers of floods in low‐lying coastal communities can enable the development of more effective long‐term adaptation strategies.

Tide gauge records are commonly used as proxies to detect coastal floods and project future flood frequencies. While these proxies clearly show that sea-level rise will increase the frequency of coastal flooding, tide gauges do not account for land-based sources of coastal flooding and therefore likely underestimate the current and future frequency of coastal flooding. Here we present a new sensor framework for measuring the incidence of coastal floods that captures subterranean and land-based contributions to flooding. The low-cost, open-source sensor framework consists of a storm drain water level sensor, roadway camera, and wireless gateway that transmit data in real-time. During five months of deployment in the Town of Beaufort, North Carolina, 24 flood events were recorded. 25% of those events were driven by land-based sources – rainfall, combined with moderate high tides and reduced capacity in storm drains – and would not have been detected using tide gauge proxies. This finding suggests that tide-gauge proxies likely underestimate flood frequency in areas where the stormwater networks are at a reduced drainage capacity due to inundation by receiving waters. Our results highlight the benefits of capturing multiple drivers of coastal flooding by instrumenting stormwater networks directly. More accurate estimates of the frequency and drivers of floods in low-lying coastal communities can enable the development of more effective long-term adaptation strategies.

Predicting coastal infrastructure reliability during hurricane events is important for risk-based design and disaster planning, including delineating viable emergency response routes. Previous research has focused on either infrastructure vulnerability to sea-level rise and coastal flooding, or the impact of changing sea level and landforms on surge dynamics. This paper represents a multidisciplinary effort to provide an integrative model of the combined impacts of sea-level rise, landscape changes, and coastal flooding on the vulnerability of highway bridges-the only access points between barrier islands and mainland communities-during extreme storms. Coastal flooding is forward modeled for static projections of geomorphic change. First-order parameters that are adjusted include sea level and land surface elevation. These parameters are varied for each storm simulation to evaluate relative impact on the performance of bridges surrounding Freeport, Texas. Vulnerability is estimated by evaluating both the probability of structural failure given surge and wave loads as well as the time inundated. The probability of bridge failure is found to increase with storm intensity and sea level because bridge fragility increases with storm surge height. The impact of a shifting landscape on bridge accessibility is more complex; barrier island erosion and transgression can increase, decrease, or produce no change in inundation times for storms of different intensity due to changes in wind-setup and back-bay interactions. These results suggest that tying down bridge spans and elevating low-lying roadways approaching bridges may enhance efforts aimed at protecting critical infrastructure.