Complex retreat behavior in experimental barrier island response to base level rise

Abstract

Barrier islands act as natural barriers between the ocean and the mainland. These islands create protected, low energy environments behind them that are important ecological and economic hubs. Barrier islands are naturally low-laying features and are susceptible to drowning from sea level rise. However, previous work suggests that barrier islands can retreat upslope to keep up with sea level rise and maintain their subaerial extent. To explore this idea, we performed physical experiments where barrier islands were subjected to a constant relative sea level rise (RSL) and 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 “stick-slip” motion comprising stationary intervals alternating with backward 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 towards the mainland until the island becomes too wide for waves to carry sediment all the way 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.