![]() ![]() In addition to rip current strength, the Lagrangian rip circulation pattern is also crucially important, because it controls the fate of any material or object caught in the rip. Hydrodynamic data collected on such a beach over a neap-spring tidal cycle will occupy a large Fr and H/h parameter space for addressing such scaling relationships. The ideal environment to further explore such relatively simple, but extremely useful ripscaling relationships is a beach with rip morphology that experiences a large tidal range and variable wave conditions. Previous field studies of rip dynamics seem supportive of these scaling relationships (MacMahan et al., 2006 Austin et al., 2009), but the amount of data is limited, particularly from beaches with a significant tidal range. Laboratory experiments (Dronen et al., 2002 Haller et al., 2002) have shown that rip current velocity, parameterised by the Froude number (Fr), is directly correlated with the wave breaking intensity on the bar crest, parameterised by the ratio between the local wave height to water depth (H/h). It would be useful if rip current velocity could be estimated for different wave and morphological conditions, without the need to conduct extensive field experiments and/or sophisticated numerical modelling. The rip current velocity is a critical parameter in controlling offshore sediment transport, material exchange between surf zone and inner shelf, and level of risk to surf zone water users. The rips are separated by transverse bars and retained within the surf zone by a longshore bar. The dashed and solid black lines represent the shoreline position and the seaward edge of the surf zone, respectively, and areas of wave breaking are represented by the lighest colours. Rectified ARGUS video image showing 4 rip current systems from the MATRIX pilot experiment at Perranporth, Cornwall (Austin et al., 2009). Our understanding of rip current dynamics has increased significantly over the last decade due to a number of comprehensive field and laboratory experiments (reviewed by MacMahan et al., 2006).įigure 1. Rip channels are a key component of rhythmic nearshore morphology, such as transverse/crescentic bars, and are typical of morphodynamically intermediate-type beaches (Figures 1 & 2). Flow velocities associated with cell circulation can be very significant with maximum flows in the rip neck (i.e., the narrowest section of the rip) of 1-2 m/s (MacMahan et al., 2006). Most rip currents are topographically constrained by bar morphology, rocky outcrops or coastal structures, but they also occur in the absence of a topographic expression when they are referred to as 'transient rips' or 'flash rips'. They are an integral part of the nearshore circulation system comprising of onshore mass transport over the bars, longshore currents in the feeder channels and offshore flows in the rip channel. Rip currents are traditionally known as relatively narrow, seaward-flowing currents that originate in the surf zone and extend seaward of the breaking region. 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. ![]() The overall aim of this project is to test this hypothesis through a combination of beach monitoring, field experiments and sophisticated numerical modelling. 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. 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. 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. 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. 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. ![]()
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