Intermittent Retreat Behavior in Experimental Barrier Island Response to Constant Sea Level Rise and Wave Forcing

Barrier islands are naturally low-lying features and are susceptible to drowning from sea level rise. However, previous work suggests that barrier islands can retreat upslope to maintain their subaerial extent, though not necessarily at a constant rate even under steady forcing. To explore this idea, we performed physical experiments where barrier islands were subjected to a constant relative sea level rise (RSL) and a constant wave environment. We tracked the islands through time with overhead images and periodic laser elevation scans. Time series of island location show that the islands did not retreat at constant rates through the transgression. Rather, overall long-term retreat occurred through a punctuated retreat motion comprising steady retreat alternating with accelerated steps. The source of this complex behavior is a cycling between two morphologies: one where an island has a developed ridge and another where no ridge is present. Sea level rise allows waves to overtop the island and erode the ridge that once kept the waves at bay. That sediment is deposited behind the island, moving the barrier landward. Waves continue to push sediment back toward the mainland until the island becomes too wide for waves to carry sediment across the island. This begins a process of backfilling the overwash fan, eventually creating a new ridge. These simple experiments support previous theoretical suggestions that periodic overwash is a key part of a barrier island's behavior during a transgression, and that this can lead to punctuated retreat even when RSL rise is steady.