STEP-WISE COASTAL RETREAT RELATED TO FLOODED RIVER VALLEY TRIBUTARY JUNCTIONS

Understanding the factors influencing coastline retreat rates during transgression is important for coastal planners in the face of ongoing sea-level rise. Most models of coastline response to sea-level rise focus exclusively on interactions between the rate of rise and topography, ignoring dynamic bathymetry and sedimentation. In this paper, we present a box model that shows that the upper shoreline of wave-dominated estuaries (bayhead deltas), which commonly contain populous urban and industrial centers, become stabilized and their rate of retreat decreases as they transgress across a tributary junction. Our model is based on two important observations from river valleys and estuaries across the world. First, valley cross-sectional area is conserved across tributary junctions. Second, estuarine water depth is largely a linear function of fetch in estuaries with an excess supply of fine-grained materials due to the importance of waves reworking fine-grained sediments. The decrease in retreat rate at tributary junctions is caused by a decrease in total estuarine volume accompanied by a conservation of sediment within the estuarine system as a tributary junction is transgressed. Our modeling results highlight the importance of inherited topography created during falling sea levels on shaping the nature and rate of transgression during the ensuing sea-level rise. In particular, tributary junctions act as pinning points during transgression.