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Step-Wise Coastal Retreat of Bayhead Deltas Related to Flooded River Valley Tributary Junctions

Abstract

We present a numerical model that shows the transgressing upper shoreline of wave-dominated estuaries (bayhead deltas), which commonly contain populous urban and industrial centers, stabilize and their rate of retreat decreases at tributary junctions. The decreased rate of retreat across a tributary junction is caused by a decrease in total accommodation while sediment supply remains conserved. Our model predicts that bayhead deltas from smaller systems will be located closer to tributary confluences than their larger counterparts. An examination of modern bayhead deltas in Albemarle Sound, US Atlantic Coast reveals that bayhead deltas from smaller tributaries are located closer to tributary confluences than bayhead deltas associated with larger tributaries, supporting our model prediction. Our results highlight the importance of antecedent topography created during falling sea levels on shaping the nature of transgression during the ensuing sea-level rise. In particular, tributary junctions act as pinning points during transgression.