Runaway Barrier Island Transgression Concept: Global Case Studies

The regime of accelerating sea-level rise forecasted by the IPCC (2013) suggests that many platform marshes and tidal flats may soon cross a threshold and deteriorate/drown as back-barrier basins transform to intertidal and subtidal areas. This chapter explores how marshes may succumb to rising sea level and how the loss of wetlands will increase the extent and the overall depth of open water in the back-barrier, causing greater tidal exchange. Here, we present a conceptual model that depicts how increasing tidal prism enlarges the size of tidal inlets and sequesters an increasingly larger volume of sand in ebb-tidal delta shoals. The conceptual model is based on empirical relationships between tidal prism and inlet parameters, as well as field and theoretical hydraulic studies of tidal inlets showing that long-term basinal deepening intensifies the flood dominance of existing inlet channels and transforms some ebb-dominated channels to flood-dominated channels. This condition leads to sand movement into the back-barrier, which builds and enlarges flood-tidal deltas, filling the newly created accommodation space. The model hypothesizes that sand contributed to the growth of the ebb and flood tidal delta shoals will be at the expense of barrier reservoirs. This will result in diminished sand supplies along the coast, eventually leading to fragmentation of barrier island chains and the transition from stable to transgressive coastal systems. Several historical studies of barrier island systems throughout the world demonstrate barrier response to changing tidal prism and illustrate different stages of this conceptual model.