The Roles of Bathymetry and Waves in Rip‐Channel Dynamics

Abstract

The behavior and predictability of rip currents (strong, wave‐driven offshore‐directed surfzone currents) have been studied for decades. However, few studies have examined the effects of rip channel morphology on the rip generation or have compared morphodynamic models with observations. Here, simulations conducted with the numerical morphodynamic model MIKE21 reproduce observed trends in flows and bathymetric evolution for two channels dredged across a nearshore sandbar and terrace on an ocean beach near Duck, NC, USA. Channel dimensions, wave conditions, and flows differed between the two cases. In one case, a strong rip current was driven by moderate height, near‐normally incident waves over an approximately 1‐m deep channel with relatively little bathymetric evolution. In the other case, no rip was generated by the large, near‐normally incident waves over the shallower (∼0.5 m) channel, and the channel migrated in the direction of the mean flow and eventually filled in. The model simulated the flow directions, the generation (or not) of rip currents, and the morphological evolution of the channels reasonably well. Model simulations were then conducted for different combinations of the two channel geometries and two wave conditions to examine the relative importance of the waves and morphology to the rip current evolution. The different bathymetries were the dominant factor controlling the flow, whereas both the initial morphology and wave conditions were important for channel evolution. In addition, channel dimensions affected the spatial distribution of rip current forcings and the relative importance of terms.

Nearshore Flow Dynamics Over Shore‐Oblique Bathymetric Features During Storm Wave Conditions

“…The phase-resolving 3D numerical model Simulating WAves till SHore (SWASH) applies the nonlinear shallow water horizontal momentum equations and the nonhydrostatic vertical momentum equation using a finite difference scheme to simulate the water surface and velocity fluctuations (Zijlema, 2020;Zijlema et al, 2011). 3D models resolve the detailed physical processes required to best simulate wave transformations over complex bathymetry because depth-averaged (2D) models can underestimate nearshore flow velocities by neglecting nonlinear 3D dynamics (Christensen et al, 2024;Kumar et al, 2011;Marchesiello et al, 2015). Furthermore, a phase-resolving numerical model permits a direct comparison between XBR observations and modeling results (L. D. Szczyrba, Mulligan & Humberston et al, 2023).…”

“…These strong offshore-directed rip currents can scour channels in nearshore sandbars and alter patterns in wave dissipation, causing the local mean water level to slope downwards toward the rip channel (Haller et al, 2002). In a study of the beach at Duck, North Carolina, Christensen et al (2024) found that the morphology of the ripchannel exerts a stronger control on flow dynamics than incident wave conditions. Feedback between the channel and the flow regime can further reinforce offshore currents and erode the beach profile (Komar & McDougal, 1988).…”