A survey of storm-induced seaward-transport features observed during the 2019 and 2020 hurricane seasons

Abstract

Hurricanes are known to play a critical role in reshaping coastlines, but often only impacts on the open ocean coast are considered, ignoring seaward-directed forces and responses. The identification of subaerial evidence for storm-induced seaward transport is a critical step towards understanding its impact on coastal resiliency. The visual features, found in the National Oceanic and Atmospheric Administration, National Geodetic Survey Emergency Response Imagery (ERI) collected after recent hurricanes on the U.S. East Atlantic and Gulf of Mexico coasts, include scours and channelized erosion, but also deposition on the shoreface or in the nearshore as deltas and fans of various sizes. We catalog all available ERI and describe recently formed features found on the North Core Banks, North Carolina, after Hurricane Dorian (2019); the Carolina coasts after Hurricane Isaias (2020); the Isles Dernieres, Louisiana, after Hurricane Zeta (2020); and the southwest coast of Louisiana, after Hurricanes Laura and Delta (2020). Hundreds of features were identified over nearly 200 km of coastline with the density of features exceeding 20 per km in some areas. Individual features range in size from 5 m to 500 m in the alongshore, with similar dimensions in the cross-shore direction, including the formation or reactivation of outlets. The extensive occurrence of these storm-induced return-flow and seawardflow morphologic features demonstrates that their role in coastal evolution and resilience may be more prominent than previously thought. Based on these observations we propose clarifying terms for return- and seaward-flow features to distinguish them from more frequently documented landward-flow features and advocate for their inclusion in coastal change hazards classification schemes and coastal evolution morphodynamic models.

Labeling Poststorm Coastal Imagery for Machine Learning: Measurement of Interrater Agreement

“…For example, the agreement scores for “washover visible in image” were consistently higher than the process “overwash.” So one could imagine asking if an image contained a recently active dune scarp instead of asking if collision had occurred. Alternate storm‐impact scales could also be used to refine questions (e.g., Leaman et al., 2020), as well as the inclusion of different impact features (e.g., Over et al., 2021).…”

Modeling the Morphodynamics of Coastal Responses to Extreme Events: What Shape Are We In?

“…One common process that Sallenger (2000) did not include is seaward-directed flow, or outwash (Over et al 2021; see figure 7 in Harter & Figlus 2017), which can occur when back-barrier water levels exceed those on the ocean side. Storm surge can inundate marshes or flood back-barrier lagoons.…”

“…Storm surge can inundate marshes or flood back-barrier lagoons. As forcing relaxes, this water returns seaward (Lennon 1991, Goff et al 2010, Harter & Figlus 2017, Goff et al 2019, Over et al 2021) and can scour new breaches or deepen existing channels. Alternatively, winds blowing across back-barrier sounds can generate surge along the backside of barrier islands.…”

“…The resulting small-amplitude (a few centimeters), very-low-frequency water-level fluctuations that can modulate infragravity waves and runup were observed on the Texas coast during Hurricane Harvey (2017) by Anarde et al (2020). Forerunner (Ekman) surge forced by alongshore winds and the Coriolis effect can elevate water levels well before storms arrive (e.g., Kennedy et al 2011), causing back-barrier lagoons to flood more easily, which leads to outwash (Goff et al 2010, Sherman et al 2013, Harter & Figlus 2017, Over et al 2021. Other contributors to unusual water levels include baroclinic gradients (Pringle et al 2019) and barotropic waves generated by moving fronts (e.g., Mercer et al 2002).…”

Modeling of Barrier Breaching During Hurricanes Sandy and Matthew

“…Water levels may become elevated in the back‐barrier with respect to the ocean as the result of a wide range of processes, including tide and surge phase lags, wave‐driven fluxes of water into the back‐barrier, runoff from upland precipitation, and varied response to atmospheric forcing between the back‐barrier and ocean, for example, wind‐driven setup along the back‐barrier and setdown in the ocean (Goff et al., 2010, 2019; Harter & Figlus, 2017; Sherwood et al., 2014). Extension of the Sallenger (2000) scale to include a regime dominated by seaward flow and sediment transport (“storm‐surge ebb,” Goff et al., 2010; or “outwash,” Over et al., 2021) allows for further resolution of processes that govern long‐term barrier island migration. Barriers with large back‐barrier bays susceptible to wind‐driven surge have been recognized previously as sites where these processes can be important.…”