Offshore controls on nearshore rip currents

Abstract. We develop and implement a new method to take into account the impact of waves into the 3-D circulation model SYMPHONIE (Marsaleix et al., 2008, 2009a) following the simplified equations of Bennis et al. (2011) which use glm2z-RANS theory (Ardhuin et al., 2008c). These adiabatic equations are completed by additional parameterizations of wave breaking, bottom friction and wave-enhanced vertical mixing, making the forcing valid from the surf zone through to the open ocean. The wave forcing is performed by wave generation and propagation models WAVEWATCH III® (Tolman, 2008, 2009; Ardhuin et al., 2010) and SWAN (Booij et al., 1999). The model is tested and compared with other models for a plane beach test case, previously tested by Haas and Warner (2009)and Uchiyama et al. (2010). A comparison is also made with the laboratory measurements of Haller et al. (2002) of a barred beach with channels. Results fit with previous simulations performed by other models and with available observational data. Finally, a realistic case is simulated with energetic waves travelling over a coast of the Gulf of Lion (in the northwest of the Mediterranean Sea) for which currents are available at different depths as well as an accurate bathymetric database of the 0–10 m depth range. A grid nesting approach is used to account for the different forcings acting at different spatial scales. The simulation coupling the effects of waves and currents is successful to reproduce the powerful northward littoral drift in the 0–15 m depth zone. More precisely, two distinct cases are identified: When waves have a normal angle of incidence with the coast, they are responsible for complex circulation cells and rip currents in the surf zone, and when they travel obliquely, they generate a northward littoral drift. These features are more complicated than in the test cases, due to the complex bathymetry and the consideration of wind and non-stationary processes. Wave impacts in the inner shelf are less visible since wind and regional circulation seem to be the predominant forcings. Besides, a discrepancy between model and observations is noted at that scale, possibly linked to an underestimation of the wind stress. This three-dimensional method allows a good representation of vertical current profiles and permits the calculation of the shear stress associated with waves and currents. Future work will focus on the combination with a sediment transport model.

On approximately planar beaches, rip channel development is often preceded by a period in which jet‐like rip currents develop in apparently random locations, and dissipate after minutes to tens of minutes. The subsequent development of sharp‐edged, trough‐like channels extending across the surf zone suggests that rip currents on planar beaches cause local erosion. Conversely, channels are known to cause localized offshore‐directed flow, and once channels have formed on approximately planar beaches, rip currents no longer occur in apparently random locations, but are restricted to the locations of the channels. Apparently, the excavation of channels by rip currents on planar beaches triggers a positive feedback between the morphological development and the flow. However, theoretical analysis indicates that, when depth increases with distance from shore, and sediment transport is treated as a function only of local flow conditions, channel development in the vicinity of a rip current may not occur. In numerical simulations (using a “cellular” model of nonbathymetrically driven rip currents) in which sediment flux on a planar beach is driven by approximately realistic rip current velocity patterns, deposition occurs under parts of the rip currents (especially in the seaward half of the surf zone), and these “rip ridges” cause a negative feedback. In these simulations, as in most models treating surf zone sediment transport, sediment flux is assumed to be strictly a function of local hydrodynamic conditions. However, Observations of sediment‐laden rip‐current plumes extending well beyond the surf zone suggest that suspended sediment transport is not always in equilibrium with local conditions. Other simulations employ a treatment of suspended‐sediment transport that allows for a lag in the adjustment of fluxes to the changes in local hydrodynamic conditions that the sediment is advected through. With this nonlocal sediment‐transport, the flux of sediment out of the surf zone in a rip current is increased, ridges do not develop under any part of the rip currents, and the morphodynamic feedback becomes positive.

The setup/setdown of the mean water level induced by incident waves on a beach has been hypothesized to be unstable and, thereby, a source of horizontal circulation, rip currents, and rhythmic topography but a direct proof was lacking. This paper examines the dynamics of a small‐amplitude circulation coupled with the subsequent perturbation on the mean water level and the wave radiation stress for normal, regular wave incidence on a nonerodible and nonbarred beach. Two cases are considered. In the first one, the wave refraction by currents and depth variations is neglected, whereas in the second one, wave refraction is accounted for. In the first case, it is proved by analytical methods that there is no instability. It is shown that the coupling between flow, mean free surface, and wave radiation stress provides a restoring force that reinforces gravity in the surf zone, while in the shoaling zone, the coupling opposes gravity. This destabilizing effect in the shoaling zone is too small to overcome gravity force. Although this restoring/destabilizing force cannot produce instability, it causes a small correction on the classical long wave phase speed that becomes now anisotropic. In contrast, when wave refraction is included in the model, the motionless basic equilibrium becomes unstable and circulation cells tend to develop. It is argued that, even without wave refraction, if sediment transport and topographic evolution is accounted for, the situation can be reversed and the basic equilibrium can be unstable. Therefore rip currents in nonbarred beaches cannot be explained from an instability originated only by the setup/setdown unless wave refraction or topographic evolution is taken into account.

: The rip current field resulting from the transformation of surface gravity waves over offshore submarine canyons has been studied. Employing a wave transformation model and a wave-induced circulation model over observed bathymetry we find that wave height variations associated with undulations in the canyon contours cause rip current circulation cells with alongshore spacing of O(100m) even though the nearshore bathymetry displays no variations at these length scales. Further, the predicted rips correspond to observed rip currents during the Nearshore Canyon Experiment (NCEX). Motivated by these results we study the relationship between O(100m) scale variations in offshore bathymetric contours and the resulting rip current field in the nearshore. We have then extended our work to the study of the rip current field during almost a month of the NCEX experiment ranging from Oct 9, 2003 through November 1, 2003. The location, strength and persistence of the rip current during this time and the predictability of the rip system as a function of the characteristics of the incoming wave field has been has been analyzed in detail.

Cross‐shore transport of larvae, pollutants, and sediment between the surf zone and the inner shelf is important for coastal water quality and ecosystems. Rip currents are known to be a dominant pathway for exchange, but the effects of horizontal temperature and salinity gradients are not well understood. Airborne visible and infrared imaging performed on the California coast shows warm and cool plumes driven by rip currents in the surf zone and extending onto the shelf, with temperature differences of approximately 1°C. The airborne imagery and modeled temperatures and tracers indicate that warm plumes exhibit more lateral spreading and transport material in a buoyant near‐surface layer, whereas cool plumes move offshore in a subsurface layer. The average cross‐shore extent of warm plumes at the surface is approximately one surfzone width larger than for cool plumes. Future work may explore the sensitivity of nearshore plumes to density patterns, wave forcing, and bathymetry.

This paper presents new laboratory data on long-wave (surf-beat) forcing by the random breaking of shorter gravity water waves on a plane beach. The data include incident and outgoing wave amplitudes, together with shoreline oscillation amplitudes at long-wave frequencies, from which the correlation between forced long waves and short-wave groups is examined. A detailed analysis of the cross-shore structure of the long-wave motion is presented, and the observations are critically compared with existing theories for two-dimensional surf-beat generation. The surf beat shows a strong dependency on normalized surf-zone width, consistent with long-wave forcing by a time-varying breakpoint, with little evidence of the release and reflection of incident bound long waves for the random-wave simulations considered. The seaward-propagating long waves show a positive correlation with incident short-wave groups and are linearly dependent on short-wave amplitude. The phase relationship between the incident bound long waves and radiated free long waves is also consistent with breakpoint forcing. In combination with previous work, the present data suggest that the breakpoint variability may be the dominant forcing mechanism during conditions with steep incident short waves.

Rip currents in the southern Baltic Sea multi-bar nearshore zone

Rip currents are the most dangerous hazard at surf beaches. Rip currents in South Florida have previously not been studied. Beach profiles for three Florida beaches (Miami Beach, Lido Beach, Sarasota, and Pensacola Beach) and one Georgia beach (South Cumberland Island) were chosen for surveying because of their variable sand bar heights. Rip current hazard at each beach was assessed by lifeguard rip rescue and drowning statistics. A relationship was found between sand bar height, beach slope and rip current hazard. Rip current measurements in South Florida, which involved utilizing GPS drifters, laser rangefinder and drone-imaged fluorescent tracer dye, showed that the speed ranged from 0.1-0.5 m/s, which is fairly slow compared to such measurements undertaken in California and Australia. The effect of rip currents on swimmers was analyzed based on the drag force acting on swimmers and the power they generate to overcome the currents when swimming against them. The drag force and power increase quadratically and cubically, respectively, with the increase of rip current and swimming speeds. Hence, even rip currents of low velocity can be dangerous and swimming against the current should be avoided if possible. Strong rips in California have been shown to exhibit a circulatory pattern, which could bring a floater back to the safety of a shallow sand bar. Field measurements of rip currents in South Florida clearly defined the flow characteristics of a nearly straight-line current, sometimes deflected to the east-southeast. Therefore, the traditional approach of swimming left or right, parallel to the shore is the best escape strategy, but not against the longshore current if present. A logistic regression analysis was conducted to predict the occurrence of rip currents based on beach conditions. The logistic model showed that wave height, wave period and wind speed were statistically significant factors in rip generation. Rips were found to be most commonly generated by relatively small, non-threatening waves (e.g., 0.6 to 0.9m in height). These physical factors, along with social and safety considerations, pose a significant problem for coastal management.

Rip currents are narrow, seaward directed flows in the surf zone that can pose a serious threat to swimmers. This issue has received attention particularly on swell dominated coasts (such as the US, Australia, France and UK) where numerous field experiments have been undertaken. However, the threat of rip currents is less recognised on wind-sea dominated coasts such as the North Sea, even though a consistent number of swimmers drift offshore (in rip currents) and require rescue by surf lifeguards each year (for example at Egmond aan Zee, The Netherlands). In August 2011, a five day field experiment SEAREX (Swimmer Safety at Egmond – A Rip current Experiment) was conducted. Lagrangian velocity measurements were taken with drifter instruments and human drifters that were tracked via GPS. Three flow patterns were observed in the experiment: (1) a locally governed circulation cell, (2) a pattern in which the drifter initially floats offshore and then is advected by a strong tidal alongshore-directed current and (3) a meandering longshore current between the shoreline and the bar. A variety of rip current velocities were measured with the strongest being approximately 0.6 m/s. The field data was hindcasted with the numerical model XBeach. Based on this model the sensitivity of rip currents towards wave height, water level and rip channel depth was investigated. Both field and model data show that offshore velocities in a rip increase with increasing wave height and decreasing water level, but that the rip channel depth imposes an upper limit on the rip current velocity.

Kim, I.; Lee, J.L.; Hwang, J.S.; Lee, S., and Lee, J., 2014. Vertical structure of rip current observed at Haeundae Beach. On 4 and 7 August, 2012, more than 200–300 people were swept away by the fast moving seaward current at Haeundae Beach, which is located on the southeastern corner of the Korean Peninsula, and they then had to be rescued Beach patrons were carried away 50 to 100 meters away from the shore by rip currents, to an area where swimming is restricted due to safety reasons. Rip currents are narrow currents that begin close to the shoreline and strongly flow seaward through the surf zone and beyond. Although rip currents can cause tragic accidents and are of considerable interest to scientists and to the general public, our current understanding of the spatial and temporal behavior of rip currents is limited. As a result, the Korean Meteorological Administration installed monitoring equipment that can assess weather conditions and issue rip current warnings on Haeundae beach starting...

Morphodynamic variability of surf zones and beaches: A synthesis

Rip current charactersitics and their role in the exchange of water and surf diatoms between the surf zone and nearshore

Arcuate duneline embayments (ADEs) are uneven rhythmic features cut into the frontal dune and beach, ranging between 800m to 2500m long in the longshore direction. Investigation of the causative processes of ADEs included: the perusal of historical aerial photograph records; rip current observations; identification of infragravity wave motions; and numerical simulation of wave refraction and focusing. ADEs were found to be erosional features that shift in location, but do not migrate in a consistent longshore direction. Infragravity wave signals identified in the swash run-up distance time-series and in nearshore orbital velocities were not maintained for sufficient duration to form rhythmic features of a similar length scale to the ADEs. Rip current erosion over the study period was observed to be localised and on much smaller spatial scale to the ADEs and several individual rip currents would fit into a single ADE. The focusing of waves at the beach due to refraction over offshore topographic irregularities was investigated by numerical simulation and provides a likely mechanism for the formation of ADEs at Waihi Beach with substantial agreement between peaks in longshore wave energy at the beach and the historical location of ADEs. We discern between the mechanism of rip-current-embayment formation (KOMAR, 1971, 1983a, b; KOMAR and HOLMAN, 1986) and that of direct wave attack on the back beach and dune due to wave refraction and focusing, and suggest that the application of the latter mechanism is responsible for their formation.

We present a hybrid machine learning and flow analysis feature detection method, RipViz, to extract rip currents from stationary videos. Rip currents are dangerous strong currents that can drag beachgoers out to sea. Most people are either unaware of them or do not know what they look like. In some instances, even trained personnel such as lifeguards have difficulty identifying them. RipViz produces a simple, easy to understand visualization of rip location overlaid on the source video. With RipViz, we first obtain an unsteady 2D vector field from the stationary video using optical flow. Movement at each pixel is analyzed over time. At each seed point, sequences of short pathlines, rather a single long pathline, are traced across the frames of the video to better capture the quasi-periodic flow behavior of wave activity. Because of the motion on the beach, the surf zone, and the surrounding areas, these pathlines may still appear very cluttered and incomprehensible. Furthermore, lay audiences are not familiar with pathlines and may not know how to interpret them. To address this, we treat rip currents as a flow anomaly in an otherwise normal flow. To learn about the normal flow behavior, we train an LSTM autoencoder with pathline sequences from normal ocean, foreground, and background movements. During test time, we use the trained LSTM autoencoder to detect anomalous pathlines (i.e., those in the rip zone). The origination points of such anomalous pathlines, over the course of the video, are then presented as points within the rip zone. RipViz is fully automated and does not require user input. Feedback from domain expert suggests that RipViz has the potential for wider use.

Beaches are one of the most popular recreational settings in most parts of the world. Indeed, marine tourism has been growing consistently in the last two decades. However, beaches are hazardous environments, and thousands of people die or sustain severe injuries while bathing or engaged in searelated recreational activities. Unfortunately, both academics and practitioners have largely overlooked the issue of beach safety and management. This paper presents the results of a research project developed in Southern Brazil -Beach Management and Safety Project, between the periods of 1995 -2001. The results suggest that two contributing factors had a significant part to play in accidents occurrences: 1) there is a clear correlation between the knowledge level of beach users about the natural environment (sea) and accidents and, 2) rip currents are unequivocally the main natural hazard for bathers. Apart from those, the research also identified that the number of people on the beach, and their behavior, also influence beach accidents. As the number of people increases, bathers tend to move to the backshore and surf zone, creating the conditions to for accidents to occur. As a practical result, the most significant one has been the reduction of fatal accidents by 80% since the introduction of the project in Santana Catalina's north beaches (100 Km).