Effects of bathymetry, wave environment and sea level on rip current dynamics in a wave-resolving model
Rip currents and nearshore circulation are important as they influence sediment transport and have the potential to be hazardous to beach users. Obtaining field observations of the flow characteristics is expensive thus the need for cheaper options to gain understanding into how they work. Nearshore wave models can be used to provide insight on rip development despite being unsuitable for providing exact forecasts of beach response. This study aimed to simulate rip current formation under varying bathymetric and wave conditions.A nearshore Boussinesq wave model (FUNWAVE) was used to simulate wave-driven currents in two bathymetric conditions: one domain based on bathymetric data collected from Ocean Beach, Dunedin, New Zealand, and the other using the same bathymetric data averaged to create an alongshore uniform domain with a single rip channel. Simulations were run that tested the effects of incrementally increasing the complexity of the wave field on both bathymetries. Variables included in the wave field were frequency distribution, directional spread and mean incidence. One set of wave conditions was also selected for use in simulations that represented high and low tide conditions at Ocean Beach.Simulations with large significant wave heights (1.5 m) were less likely to retain material in the surf zone with ~10-15% fewer tracers retained compared to simulations with 0.5 m waves. Simulations of the alongshore averaged bathymetry all formed rip currents near the channel while more complex wave conditions generated eddies and multiple exit pathways from the surf zone. Complex conditions in both the bathymetry and wave field lead to the formation of multiple exit pathways from the surf zone. With increasingly “real-life” conditions, rip current dynamics became less tied to bathymetric variability. Larger significant wave height was the most significant contributor to wave orbital velocity increase. Simulations with a large significant wave height featured a higher proportion of the domain exceeding the erosive threshold for wave orbital velocity.Rip currents at Ocean Beach vary depending on the wave conditions with significant wave height and directional spread being particularly important. In addition, the amount of sediment exiting the surf zone is controlled by different processes that depend on wave conditions. For less energetic conditions the number of exit pathways will be important, in more energetic conditions sediment will likely leave the surf zone regardless of the directional spread or mean incidence of the wave field.
Advisor: Suanda, Ata; Smith, Abby
Degree Name: Master of Science
Degree Discipline: Marine Science
Publisher: University of Otago
Keywords: rip currents; waves; model; wave-resolving; FUNWAVE; Ocean Beach Dunedin; sediment; retention zones
Research Type: Thesis