Geomorphic Response of a Coastal Berm to Storm Surge and the Importance of Sheet Flow Dynamics

Abstract

During a storm, as the beach profile is impacted by increased wave forcing and rapidly changing water levels, sand berms may help mitigate erosion of the backshore. Yet, the mechanics of berm morphodynamics have not been fully described. In this study, twenty‐six trials were conducted in a large wave flume to explore the response of a near‐prototype berm to scaled storm conditions. Sensors were used to quantify hydrodynamics, sheet flow dynamics, and berm evolution. Results indicate that berm overtopping, and offshore sediment transport were key processes causing berm erosion. During the morphological evolution of the beach profile, two sand bars were formed offshore that attenuated subsequent wave energy. The landward extent of that energy was confined to the seaward foreshore inhibiting inundation of the backshore. Net offshore‐directed transport was dominant when infragravity motions increased in the swash zone. Conversely, the influence of incident‐band motions on sediment transport was relatively greater in the inner‐surf zone. Near‐bed flow velocities and sheet flow layer thicknesses were larger in the swash zone than in the inner‐surf zone. This paper also provides a valuable analysis between morphology‐estimated total sediment transport rates and rates derived from in situ measurements. Sheet flow dynamics dominated foreshore cross‐shore sediment processes constituting the largest portion of the total sediment transport load throughout the berm erosion.

Beach and Dune Subsurface Hydrodynamics and Their Influence on the Formation of Dune Scarps

“…The experimental beach profile consisted of a berm seaward of a dune. The berm eroded over the first 11 of 47 total trials (Pontiki et al., 2023) (Figure 3). After the berm eroded and the berm sediments were transported offshore, the remaining beach profile was concave.…”

“…The experimental wave conditions were scaled from observed conditions during Hurricane Sandy in 2012 (Pontiki et al., 2023). The water levels were based on data observed at the Atlantic City NOAA Tides and Currents Station (NOAA, 2023), and the significant wave heights and wave periods throughout the duration of the storm were based on the random phase spectral model WAVEWATCH III offshore of Mantoloking (WW3DG, 2019).…”

Simulating Erosive and Accretive Conditions in the Swash: Applications of a Nonlinear Wave and Morphology Evolution Model

“…[32] present a thorough review on the dynamics of sand transport in the swash and on the use of practical models to simulate it. Reference is also made to works that delve into the intricacies of swash zone dynamics, like the experimental work of [33,34], the modelling works of [35][36][37] and the combined experimental-numerical works of [38,39]. Modelling attempts in relevant literature are typically based on the reproduction of sets of seminal experiments, like the ones collected and presented by [40].…”