انواع جریان های برگشتی ساحلی، جریان ها و مخاطرات Rip current types, circulation and hazard

۱٫ Introduction Many global beaches are characterized by the presence of narrow and concentrated seaward flowing rip currents that extend from close to the shoreline, through the surf zone, and varying distances beyond. Rip currents are fundamentally driven by the action of breaking waves (Bowen, 1969) and are therefore found on a range of beach types (Wright and Short, 1984; Lippmann and Holman, 1989; Masselink and Short, 1993; Scott et al., 2011a; Loureiro et al., 2013) along oceanic, sea and lacustrine coasts exposed to different wave climates. It is well established that rip currents are important for the transport and crossshore mixing of heat, pollutants, nutrients and biological species (Talbot and Bate, 1987; Shanks et al., 2012; Sinnett and Feddersen, 2014). However, rip currents have long been of both scientific and societal interest mostly due to the coastal hazard they represent. Their flow is often sustained over sufficient temporal periods (hours–days) and mean velocities (often N0.5 m/s) enable transport of large volumes of sediment offshore, particularly during storm events (e.g. Cook, 1970; Thornton et al., 2007; Loureiro et al., 2012b, Castelle et al., 2015). This can accentuate localized shoreline and dune erosion making them a threat to shoreline infrastructure and coastal communities. However, arguably the greatest impact that rip currents present to society is through the hazard they represent to beach users who find themselves caught in one (Fig. 1). Indeed, the earliest studies on rip currents (e.g. Davis, 1925; Shepard, 1936) brought attention to the hazard by coining the term ‘rip current’ in an attempt to differentiate them from the conceptually misleading terms ‘undertow’ and ‘riptide’, which were gaining popularity in the public vernacular at the time and are unfortunately still often incorrectly used by the public and media to describe rip currents. Rip current flows can quickly carry unsuspecting bathers of all swimming abilities (Drozdzewski et al., 2012, 2015) into deeper water (Fig. 1), often against their will, where a combination of exhaustion and panic too often results in a drowning death (Brander et al., 2011). Each year hundreds of people drown and tens of thousands more are rescued from rip currents globally (e.g. Klein et al., 2003; Hartmann, 2006; Gensini and Ashley, 2009; Brewster, 2010; Brighton et al., 2013; Scott et al., 2011b; Arun Kumar and Prasad, 2014; Arozarena et al., 2015; Barlas and Beji, 2015) and it is now well established that they are the primary physical hazard facing recreational bathers on surf beaches worldwide (Brander, 2015; Brander and Scott, 2016). While the severity of the rip hazard to bathers has been shown to be influenced by various demographic, social, behavioural, knowledgebased and emotional factors (Sherker et al., 2010; Hatfield et al., 2012; Williamson et al., 2012; Caldwell et al., 2013; Woodward et al., 2013, 2015; Brannstrom et al., 2014), in terms of physical factors it is primarily dictated by a combination of rip current flow speed and circulation patterns (Scott et al., 2014). As outlined by Brander and MacMahan (2011), our understanding of rip current flow behaviour has had a strong influence on existing global rip current hazard safety messaging promoted to the public, particularly in terms of self-escape strategies. From this perspective it is therefore useful to consider the historical progression of scientific research and knowledge in relation to both rip current flow and the rip current hazard (Fig. 2). Fig. 2 presents temporal patterns of rip current related publications in internationally refereed journals based on a detailed search of the literature. A total of 236 publications from 1925 to April 2016 were sourced and the publication list and criterion for their inclusion are provided in Supplementary Material. Publications were coded into five dominant subject themes, some with sub-themes. Some papers (if applicable) were coded with multiple themes. As evident from Fig. 2a, scientific interest in rip currents experienced a relatively slow and modest progression until a marked and rapid increase post-2000 that continues today. In terms of themes, early studies on rip currents were clearly qualitative in nature either providing a descriptive review of existing rip current knowledge or describing different types of rip currents and there is some evidence of increased interest in both areas in recent years (Fig. 2b). There has been a noticeable increase in numerical process-based modelling studies, particularly since 2000, while the number of physical laboratory studies and conceptual empirical based models (e.g. Wright and Short, 1984) have remained relatively low over time (Fig. 2d). The increase in modelling studies is clearly a reflection of improved computing power and theoretical framework just as the rapid increase in rip current field measurements since 2000 (Fig. 2c) is largely due to technological advances and reduced costs of data gathering equipment (e.g. MacMahan et al., 2005, 2009; Schmidt et al., 2003). This is evident from the gradual temporal increase in Eulerian measurements, which record rip current flow past a fixed point and typically involve deployment of one or more flowmeters in a rip current system. While Eulerian approaches offer limited spatial coverage, they have been particularly useful in examining and understanding the temporal variability of rip current flow, such as the tidal modulation of rip current velocity and flow pulsing at infragravity frequencies (e.g. Sonu, 1972; Aagaard et al., 1997; Brander and Short, 2001; MacMahan et al., 2004, 2006). Also apparent from Fig. 2d is the increase in Lagrangian field measurements in the last decade. Lagrangian methods involve observing, or measuring, trajectories of specific fluid parcels through the rip current system and are useful for providing a two-dimensional representation of the spatial variability of rip current circulation and surface velocity patterns over time (e.g. Schmidt et al., 2003; Spydell et al., 2007; Austin et al., 2010; MacMahan et al., 2010a; Houser et al., 2013; McCarroll et al., 2014b; Winter et al., 2014; Scott et al., 2016). The move towards Lagrangian measurements is reminiscent of the earliest field measurements of rip current flows conducted near the Scripps Institute of Oceanography at La Jolla, California (Shepard et al., 1941; Shepard and Inman, 1950) using floating objects and drogues. In terms of studies related to the rip current hazard, while the earliest rip current publications (e.g. Davis, 1925; Shepard, 1936; Shepard et al., 1941) acknowledged the drowning hazard represented by rip currents, a dearth of dedicated hazard research existed until the data-driven models/forecasts studies by Lushine (1991) and Lascody (1998). However, since 2010 there has been a rapid proliferation of interest in the rip current hazard, particularly from social science studies (Fig. 2e) relating to human understanding, perception of, and behaviour in relation to the rip hazard (e.g. Drozdzewski et al., 2012; Brannstrom et al., 2014; Woodward et al., 2015). In this time, there has also been increased interest in statistical data reporting (e.g. Brighton et al., 2013; Arozarena et al., 2015; Barlas and Beji, 2015) and physical studies involving measuring or modelling swimmer behaviour in rip currents (e.g. McCarroll et al., 2014a; McCarroll et al., 2015; Castelle et al., 2016; Van Leeuwen et al., 2016). Overall, it is evident from Fig. 2 and Supplementary Material that scientific interest in rip currents is high, with continued rapid growth largely being driven by hazard related studies, followed by field measurements and modelling works.