Simulating barrier island response to sea level rise with the barrier island and inlet environment (BRIE) model v1.0

Barrier islands are low-lying coastal landforms vulnerable to inundation and erosion by sea level rise. Despite their socioeconomic and ecological importance, their future morphodynamic response to sea level rise or other hazards is poorly understood. To tackle this knowledge gap, we outline and describe the BarrieR Inlet Environment (BRIE) model that can simulate long-term barrier morphodynamics. In addition to existing overwash and shoreface formulations, BRIE accounts for alongshore sediment transport, inlet dynamics, and flood-tidal delta deposition along barrier islands. Inlets within BRIE can open, close, migrate, merge with other inlets, and build flood-tidal delta deposits. Long-term simulations reveal complex emergent behavior of tidal inlets resulting from interactions with sea level rise and overwash. BRIE also includes a stratigraphic module, which demonstrates that barrier dynamics under constant sea level rise rates can result in stratigraphic profiles composed of inlet fill, flood-tidal delta, and overwash deposits. In general, the BRIE model represents a process-based exploratory view of barrier island morphodynamics that can be used to investigate long-term risks of flooding and erosion in barrier environments. For example, BRIE can simulate barrier island drowning in cases in which the imposed sea level rise rate is faster than the morphodynamic response of the barrier island. that provide a range of ecosystem services (Barbier et al., 2011; McLachlan, 1983). Despite their importance, there is a critical gap in our ability to predict how barriers will respond to coastal change generally and sea level rise (SLR) specifically. A necessary condition for barrier islands to migrate landwards and keep up with SLR is sufficient sediment transport from the barrier front to the top and back via overwash fan deposition and flood-tidal delta formation (Armon and McCann, 1979; Inman and Dolan, 1989; Kraft, 1971; Lorenzo-Trueba and Ashton, 2014; Mallinson et al., 2010; Moore et al., 2010). There are few constraints, however, on the potential magnitudes of these landward sediment fluxes and how these fluxes vary as a function of the coastal setting, wave climate, or SLR. Recent models (e.g., Lorenzo-Trueba and Ashton, 2014) have suggested formulations for overwash fluxes, but the potential role of tidal fluxes, their feedbacks with overwash deposition, and the resulting ability of barriers to keep pace with SLR remain unclear. Here we present the BarrieR Inlet Environment (BRIE) model to address this fundamental knowledge gap. Transgression in the model is driven by two main processes: overwash sedimentation and flood-tidal delta deposition (Leatherman, 1979; Pierce, 1969, 1970). To date, models that have aimed to assess barrier island change over geological timescales typically account for only storm overwash, which is more suitable for a cross-sectional framework. Tidal inlets, however, have been suggested to contribute a large fraction of the transgressive sediment movement in a number of field studies (Pierce, 1969, 1970). The BRIE model extends the formulations of Lorenzo-Trueba and Ashton (2014) (LTA14) in the alongshore direction and incorporates tidal inlet morphodynamics through Delft3D-derived parameterizations (Nienhuis and Ashton, 2016, NA16). The purpose of the model is twofold: (i) to better understand long-term barrier island morphodynamics, including the effects of, for example, sea level rise, human development (jetties, beach nourishment), or storm pattern changes, and (ii) to improve paleoenvironment reconstructions. Section 2 of this paper provides a background on barrier island environments and recent model developments. In Sect. 3, we discuss model formulations, including overwash fluxes, alongshore sediment transport, and tidal inlet morphodynamics. Section 4 includes a model run that demonstrates the capabilities of the BRIE framework, including inlet dynamics alongshore and the generation of alongshore stratigraphic profiles. Section 5 explores model sensitivity to grid and time resolution, as well as a comparison to other barrier island models. We conclude with a few exploratory results and a discussion of potential model applications.