Gradual Inlet Expansion and Barrier Drowning Under Most Sea Level Rise Scenarios

The expected increase in rates of sea level rise during the 21st century and beyond may cause barrier islands to drown. Barrier drowning occurs due to a sediment imbalance induced by sea level rise, causing inlets to open and expand. It is still unclear how fast barrier islands can drown. To gain insight into the morphodynamics of barrier systems subject to sea level rise, we here present results obtained with a novel barrier island exploratory model, BarrieR Inlet Environment-Drowning, that considers inlet expansion beyond equilibrium size. We quantify how much of a barrier island chain is drowned by calculating the fraction of its length that is below mean sea level due to sea level rise. Results show that barrier drowning is mostly sensitive to the wave height and the rate of sea level rise. In the model, it takes 100s of years for barrier islands to start drowning in response to high rates of sea level rise (more than 5 mm/yr, for a typical coastal environment). This lag in barrier response is caused by a gradual decrease in the sand volume of the barrier. Higher rates of sea level rise cause earlier and more severe barrier drowning. Modeled barrier systems that face higher waves undergo more frequent inlet closures that lower the rate of drowning, but they also have a deeper shoreface that increases the rate of drowning. In model simulations, the latter process dominates over the former when sea level rise rates exceed 5 mm/yr. Model results fairly agree with available field data.