Can Barrier Islands Survive Sea‐Level Rise? Quantifying the Relative Role of Tidal Inlets and Overwash Deposition

Abstract

Barrier island response to sea-level rise depends on their ability to transgress and move sediment onto and behind the barrier, either through flood-tidal delta deposition or via overwash. Our understanding of these processes over decadal or longer timescales, however, is limited. Here we use a recently developed barrier island model (BRIE) to better understand the interplay between tidal dynamics, overwash fluxes, and sea-level rise on barrier coasts and barrier island stratigraphy. Model results suggest that in microtidal environments with large alongshore sediment transport fluxes, tidal inlets are ephemeral and migrate rapidly. These conditions lead to effective deposition of flood-tidal deltas and allow inlets to constitute most of the landward sediment flux. Whether barrier islands can survive sea-level rise depends on the combined landward sediment flux from overwash and flood-tidal delta deposition, likely making barrier islands with artificially stabilized inlets (via jetty construction or maintenance dredging) more vulnerable to sea-level rise. Key Points:• Flood-tidal deltas built by ephemeral and rapidly migrating tidal inlets contribute significantly to barrier landward movement in response to sea-level rise • Additional landward sediment flux from tidal inlets can help barrier islands keep pace with sea-level rise • Barrier stratigraphy is not always a good indicator of formative landward sediment fluxes

Impacts of sediment removal from and placement in coastal barrier island systems

“…In the back-barrier environment, dredging of active or relict flood-tidal delta sediments may increase a barrier's long-term vulnerability to SLR (Nienhuis and Lorenzo-Trueba, 2019). Numerical modeling and field observations demonstrate that flood-tidal delta deposition is an important control on long-term barrier migration, in some cases stabilizing the barrier structure by providing a shallow platform for landward-migrating islands to build upon (Simms and others, 2006;Mallinson and others, 2010;Shawler and others, 2019).…”

“…Numerical modeling and field observations demonstrate that flood-tidal delta deposition is an important control on long-term barrier migration, in some cases stabilizing the barrier structure by providing a shallow platform for landward-migrating islands to build upon (Simms and others, 2006;Mallinson and others, 2010;Shawler and others, 2019). Additionally, flood deltas deposited by migrating inlets store sediment that can be reincorporated into the active beach and shoreface during long-term barrier retreat, potentially contributing to decadal-and centennial-scale resiliency (Moslow and Heron, 1978;Mallinson and others, 2010;Nienhuis and Lorenzo-Trueba, 2019). For landward-migrating barriers responding to SLR, a lack/disruption of flood-tidal delta sedimentation caused by sediment removal such as dredging may cause island narrowing, as well as potential disintegration of the barrier structure (Smith and others, 2008;Nienhuis and Lorenzo-Trueba, 2019).…”

“…Additionally, flood deltas deposited by migrating inlets store sediment that can be reincorporated into the active beach and shoreface during long-term barrier retreat, potentially contributing to decadal-and centennial-scale resiliency (Moslow and Heron, 1978;Mallinson and others, 2010;Nienhuis and Lorenzo-Trueba, 2019). For landward-migrating barriers responding to SLR, a lack/disruption of flood-tidal delta sedimentation caused by sediment removal such as dredging may cause island narrowing, as well as potential disintegration of the barrier structure (Smith and others, 2008;Nienhuis and Lorenzo-Trueba, 2019). Moreover, Walters and others (2014) and Lorenzo-Trueba and Mariotti (2017) noted that modeled long-term barrier shoreline retreat rate is tied to the extent of fringing back-barrier marshes, which similarly serve to provide a platform for island migration and are usually formed directly atop flood-tidal delta deposits (Barnhardt and others, 2002;Shawler and others, 2019).…”

“…Flood-tidal deltas can also act as long-term sinks of sediment and, in some cases, act in combination with ebb-tidal deltas to starve downdrift barrier islands of alongshore sediment delivery over decadal timescales (Nienhuis and Ashton, 2016), a process that can be exacerbated by inlet dredging and stabilization (for example, Chincoteague Inlet, Hardaway and others, 2015;and Ocean City Inlet, Leatherman, 1979). However, over multidecade to centennial timescales, sediments stored in flood-tidal deltas can be released to the shoreface during landward barrier migration, helping maintain resiliency in the face of long-term drivers of change such as sea-level rise (SLR) (Nienhuis and Lorenzo-Trueba, 2019).…”

Barrier breaching versus overwash deposition: parameterizing the morphologic impact of storms on coastal barriers

“…1) (Sopkin et al, 2014). Breaching is likely to become more common as a result of sea-level rise and barrier island flooding (Nienhuis & Lorenzo-Trueba, 2019a;Passeri et al, 2020). At the same time, washover deposition is a critical landward-directed sediment flux that can support barrier aggradation and prevent barrier drowning.…”