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- Title
The Roles of Bathymetry and Waves in Rip‐Channel Dynamics.
- Authors
Christensen, D. F.; Raubenheimer, B.; Elgar, S.
- 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. Plain Language Summary: Rip currents are strong offshore‐directed currents that form on beaches worldwide where they pose a major risk to beachgoers. Despite rip currents being studied for decades, the influence of the seafloor shape on rip current generation and strength remains uncertain. Here we use a numerical model to simulate flows and evolution of the seafloor for two rip currents observed on a sandy beach near Duck, NC, USA. The model successfully reproduced the observed trends and was used to investigate the rip current forcing mechanisms for different seafloor shapes and wave conditions. The seafloor configuration was found to be the dominant factor controlling rip current generation. A 1‐m deep channel created a stronger and more persistent rip current compared with a 0.5‐m deep channel during similar wave conditions. The rip currents were primarily driven by alongshore differences in water level with a local minimum within the channel. The water levels forced two converging shore‐parallel "feeder" currents transporting water into the offshore‐directed rip current. Moreover, waves changed direction over the deep channel, resulting in higher wave energy on the channel sides and lower energy in the channel center. Alongshore variations in wave heights thereby modulated the strength of the feeders. Key Points: Channel dimensions are the dominant factor controlling the occurrence and strength of the rip currentThe channel evolution is affected by both the initial channel dimensions and the wave conditionsThe spatial distribution of rip current forcings and the relative importance of terms depend on the channel dimensions
- Subjects
RIP currents; WATER currents; CONTROLLED low-strength materials (Cement); WATER levels; BATHYMETRY; MEANDERING rivers; OCEAN energy resources
- Publication
Journal of Geophysical Research. Earth Surface, 2024, Vol 129, Issue 1, p1
- ISSN
2169-9003
- Publication type
Article
- DOI
10.1029/2023JF007389