Nearshore cell circulation is driven by incident wave energy dissipation through the generation of cross-shore and longshore gradients in the radiation stress and the mean nearshore water level. At a qualitative level this theoretical underpinning is reasonably well understood, but in practice it remains problematic to predict exactly when, and under what morphodynamic conditions, rip currents are at their strongest and pose the largest threat to surf zone water users. The leading hypothesis for this research project is that, regardless of the time scale under consideration (minutes to months), rip current flows are strongest when wave breaking is maximised over the bars and minimized over the rip channel. This depends critically on the hydrodynamic forcing, the tidal water level and the antecedent beach morphology - subtle changes in any of these factors may have significant repercussions for the rip circulation. Under conditions with strong rip current flows, the current is also most likely to extend beyond the surf zone, rather than developing a large eddy within the surf zone. The overall aim of this project is to test this hypothesis through a combination of beach monitoring, field experiments and sophisticated numerical modelling. A widely-applicable predictive scheme will be developed linking wave conditions, water level and beach morphology to rip speed and hazard, and the findings will be disseminated to the RNLI to help improve lifeguarding services to save lives.
More specifically, our objectives are to:
- Quantify the temporal variability in beach state and rip current characteristics over the daily to monthly time scale on two beaches using ARGUS video observations and GPS surveys.
- Collect two extensive field data sets to investigate and parameterise the relation between wave dissipation and rip dynamics over time scales ranging from minutes to weeks.
- Use the field data to improve, validate and calibrate a numerical model (XBeach) capable of simulating nearshore cell circulation and rip current dynamics.
- Produce generic rip current scenarios and hazard indicies based on field observations and numerical modelling to help the RNLI plan their lifeguarding activities and inform risk assessment and ublic awareness programs.
- Develop a decision-support system (DSS) to predict several days in advance, and for different stages of the tide, the risk presented by rip currents to surf zone water users.