Rip Neck Research Articles

Vertical Distribution of Rip Currents Generated by Intersecting Waves in a Sandbar–Groin Systems

To analyze the impacts of groins, sandbars, and channels on the three-dimensional features of rip currents, we conducted experimental investigations on the vertical distribution of rip currents under intersecting waves along barred beaches with channels. This study employed ADV flow velocity measurements at two distinct locations: within channel and on a sandbar. The results indicate that in nodal sections within channel and on a sandbar, the rip head region manifests surface flow characteristics, characterized by high velocities near water surface. In a rip neck location, the vertical distribution of rip currents on a sandbar exhibits greater variability, whereas within channel, the distribution is more homogeneous. The vertical distribution of rip currents in nodal sections within channel aligns with the logarithmic distribution law. The vertical distributions of the alongshore velocity of rip currents display a consistent pattern of higher at the top and lower at the bottom. However, this pattern varies slightly depending on the wave period. The presence of groins influences the fluctuation characteristics of the alongshore velocity of rip currents by regulating the nearshore circulation system. This results in the alongshore velocity of rip currents in channel pointing towards the groin. In contrast, the alongshore velocity of rip currents on sandbar, situated farther away from groin, exerts a weaker effect on the alongshore velocity of a rip current.

Field Measurements of a High-Energy Headland Deflection Rip Current: Tidal Modulation, Very Low Frequency Pulsation and Vertical Structure

Headland rips, sometimes referred to as boundary rips, are rip currents flowing against natural or artificial obstructions extending seaward from the beach, such as headland or groynes. They can be driven either by the deflection of the longshore current against the obstacle or by alongshore variation in breaking wave height due to wave shadowing in the lee of the obstacle. The driving mechanism therefore essentially depends on the angle of wave incidence with respect to the natural or artificial obstruction. We analyze 42 days of velocity profile measurements against a natural headland at the high-energy meso-macrotidal beach of Anglet, southwest France. Measurements were collected in 6.5–10.5-m depth as tide elevation varied, during the autumn–winter period with offshore significant wave height and period ranging 0.9–6 m and 8–16 s, respectively, and the angle of wave incidence ranging from −20 ∘ to 20 ∘ . Here we analyze deflection rip configurations, corresponding to approximately 24 days of measurements, for which the current meter was alternatively located in the rip neck, rip head or away from the rip as wave and tide conditions changed. Deflection rips were associated with large offshore-directed velocities (up to 0.6 m/s depth-averaged velocities) and tide modulation for low- to moderate-energy waves. The vertical profile of deflection rips was found to vary from depth-uniform in the rip neck to strongly depth-varying further offshore in the rip head with maximum velocities near the surface. Very low frequency motions of the rip were dramatic, ranging 10–60 min with a dominant peak period of approximately 40 min, i.e., with longer periods than commonly reported. The strong offshore-directed velocities measured well beyond the surf zone edge provide new insight into deflection rips as a dominant mechanism for water and sediment exchanges between embayed (or structurally-controlled) beaches and the inner-shelf and/or the adjacent embayments.

Rip current simulation on some beaches in coastal Quang Nam province

This paper presents modelling results of rip currents on the main beaches along coastal Quang Nam province including Ha My, Binh Minh, Tam Thanh and Rang beaches during two typical wind seasons: Northeast monsoon (Northeast wind direction, wind levels: 4, 5, 6) and Southwest monsoon (Southeast wind direction, wind levels: 4, 5) using Mike 21 model. Calculation results show that during the Northeast monsoon, the rip current formed in all beaches. In the scenario of level 4 of wind speed, average rip speed was about 40–50 cm/s. In particular, at Tam Thanh beach area, the rip was a typical one with the components such as feeder current, rip neck and rip head. With the level 5 of wind field, the formation of the rip was clearer, the speed of the rip was stronger, average value was about 50–60 cm/s. Meanwhile, with the level 6 of wind field, the typical rip structure was broken, creating local eddies or longshore currents at some positions, but strengthened at other positions. During the Southwest monsoon, the rip current did not form at the beaches and the longshore currents were dominant.

Water-level controls on macro-tidal rip currents

Field measurements and numerical modelling have been used to investigate the water-level control of rip current dynamics on a macro-tidal beach. Field data collected over 32 complete tidal cycles, spanning a range of wave and tide conditions, demonstrate that rip current strength and behaviour is modulated at the semi-diurnal frequency by tide-induced changes in the water-level over bar/rip morphology. Peak flow speeds in the rip neck 〈uv〉 correspond to the time of maximum wave breaking 1.5h before and after low water.Alongshore-directed water surface gradients ∂η/∂y were measured along the feeder channel and around the ends of the inter-tidal bar, with head differences O(0.1m). The numerical model reproduced ∂η/∂y with a good level of skill and showed that ∂η/∂y and 〈uv〉 increase with the proportion of breaking waves Qb over the inter-tidal bar; but 〈uv〉 was maximised during peak Qb, maximum ∂η/∂y occurred when wave breaking moved offshore to the sub-tidal bar and Qb was reduced. Around low water, the forcing of the rip current by the alongshore pressure-driven feeder current was reduced by the decrease in Qb over the bar and feeder regions, but an offshore flow through the rip channel was maintained by a localised intensification of ∂η/∂y around the ends of the inter-tidal bar.

Video-imaging of transient rip currents on the Gold Coast open beaches

Murray, T., Cartwright, N., Tomlinson, R., 2013. Video-imaging of transient rip currents on the Gold coast open beaches.Transient rips vary in location and occurrence making them hard to predict, and unlike topographically controlled, fixed or semi-permanent rips, they are believed to be controlled by surf zone hydrodynamics rather than beach and surf zone morphology. This paper reports on the first video analysis of transient rip behaviour in the Southern Hemisphere. Transient rips were identified from a high camera located approximately 100 m above mean sea level at Narrowneck, Gold Coast, Australia. Transient rip occurrence, duration and flow behaviour were identified from the video and occurrence was correlated with environmental factors including beach state, wave, tide and wind characteristics. Transient rips were found to display a mushroom cloud rip head and a narrow trailing rip neck consistent with characteristics often displayed by topographically controlled rips. Transient rip events were characterised by short life-spans (30 – 236 s) and low temporal frequency, but high temporal variance, occurring for only 0.52% of the time in the video. Around 66% of transient rip events identified occurred when the beach was generally alongshore uniform in a shore-attached terrace state. Transient rip occurrence was also found to be significantly greater on the ebb tide as opposed to the flood tide and when wave breaking was absent on the outer bar. There was a lack of a clear correlation between averaged wave and wind conditions and transient rip formation. Current and future study is focussing on the effects of wave groupiness and sea state on transient rip occurrence, spacing, duration and intensity.

Laboratory experiment on rip current circulations over a moveable bed: Drifter measurements

This study describes a laboratory experiment on rip current circulations over a moveable bed. Rip current characteristics over eight contrasting nature‐like beach morphologies are investigated. The seabed varied from reasonably alongshore uniform to strongly alongshore nonuniform with crescentic patterns and bar‐rip morphologies, representative of a full morphological down‐state sequence. The same offshore shore‐normal waves were generated by the wavemaker for the eight situations with the same mean water level to study the sensitivity of rip current characteristics as a function of the beach morphology only. In each case, a 30 to 60 min video run was used to track a large number of drifters released within the surf zone. Results show the presence of classic rip current patterns with counterrotating cells and a relatively narrow offshore‐directed jet varying from shore‐normal to strongly skewed. Wave‐driven circulations were strongly unstable. Computed standard deviations of flow intensity and direction provide high‐resolution information on the spatial variability of rip current instabilities. Highly pulsating and weakly directionally variable offshore‐directed flow is observed in the rip neck for well‐developed bar‐rip morphologies that turns into a weakly pulsating and highly directional variable rip current flow with decreasing beach alongshore nonuniformity. Proposing a definition of rip current intensity based on the rip current circulation geometry, rip current intensity was found to linearly increase with increasing measure of beach alongshore nonuniformity within both the low‐energy and moderate‐energy rip current regimes. To date, our laboratory experiment provides the first extensive quantitative rip current information during a full down‐state sequence for a given wave condition.

Very low frequency motions of a rip current system: observations and modeling

Biscarrosse beach, located on the French Aquitanian Coast, is a high-energy meso-macrotidal double-barred beach. The outer and the inner bars exhibit most of the time crescentic patterns and transverse bar and rip morphology, respectively. Breaking waves over these three-dimensional features induce strong rip currents that are responsible of several drowning accidents each year. To improve our knowledge on this kind of complex environment, an intensive 5-day field experiment was carried out in June 2007 at Biscarrosse Beach. The present study is focused on Very Low Frequency motions (VLF) of a rip current system over a well-developed bar-rip morphology. Using both a drifter experiment and virtual drifter modeling, the study aims at analyzing the rip current pulsations and the drifter retention in the surf zone. The main results show the oscillating behavior of the rip currents, in particular within the rip neck where the VLF pulsations are intense (reaching 1m/s on time scales of 10 to 30 minutes). In addition, most of the drifters are retained within the surf zone (about 80%), with the other 20% exiting the surf zone. These results are reproduced by our numerical model, which shows that shear instabilities of the rip current can be the cause of such retention/expulsion proportions. In addition, here we present the spatial variability of the VLF motion over the entire rip current system.

Rip current system over strong alongshore non-uniformities: on the use of HADCP for model validation

Modeling and understanding topographically-controlled rip currents remains a challenging task. One of the reasons is the lack of intensive, high-spatial resolution, flow field measurements in the rip channel vicinity. During the ECORS (DGA-SHOM) intensive field measurements, an intertidal inner-bar rip channel was instrumented with fixed eulerian current meters. In addition, for the first time in such a system, a Horizontal ADCP (HADCP) was implemented in the vicinity of the rip current, on the sandbar edge, for horizontally profiling wave induced-currents. Results show that the HADCP provides unique information on the shear in the vicinity of the rip neck, which is particularly useful for model calibration. The HADCP data was compared with local flow measurements for various tide and wave conditions, showing a very good agreement at a 5 m range. Restrictions and recommendations for HADCP implementation in the field are pointed out. The use of HADCP for horizontally profiling rip current circulations would benefit from being deployed outside of the breakers to measure the cross section of the rip head where sediment plumes and bubbles are essentially surface dominated. In this rip current system area, which would suffer from acoustic opacity only during high energy conditions, the rip current jet is strongly unstable owing to the current shear. HADCP would provide unique information on the rip current instabilities and vortex shedding in this poorly understood area of the rip current system.

Rip Currents with Varying Gap Widths

The behavior of rip currents with changing bar and gap lengths was tested in a series of laboratory experiments using video-tracked floating drifters. Qualitatively, currents showed strong differences in overall character with changing topography. As narrow gaps became wider, the circulation tended to first travel parallel to the shoreline instead of immediately offshore, although strong offshore-directed flows were found for all topographies. Quantitatively, the width of the rip neck was found to increase slowly when compared to the increasing width of the rip channel. Maximum offshore and longshore velocities showed little dependence on bar or gap lengths, and were found to scale well with the predicted rate of generation of circulation. Finally, volumetric offshore flow rates through the rip channel showed no proportionality with the bar length as predicted by the mass transport hypothesis of rip current strength.

Transient rip currents and nearshore circulation on a swell‐dominated beach

Eulerian and Lagrangian measurements of nearshore circulation on a longshore uniform beach with perpendicularly incident swell waves are described. Both the Eulerian and Lagrangian data show a variable rotational part of the current field that is distinct from, but coexists with, that due to infragravity waves. A well‐defined feature of the flow field includes transient rip currents, which occur in varying locations and are not topographically controlled. Drifter trajectories show that the rips behave like a shallow plane jet, consisting of a constrained neck region that flows into a spreading head region. There is continuous modulation of the flow within the rip neck. Estimates of the length scale across the rip neck using two‐particle statistics are 20–30 m. Well‐defined discrete vortex features are observed at the initiation point of one of the necks and in one of the rip heads. Estimated dispersion values suggest that mixing is greatly enhanced outside the surf zone by the intermittent presence of the rip heads. The dispersion calculated from both inside and outside the surf zone appears to show scale dependence with a power law close to 4/3 in a similar way to oceanic turbulence at much larger scales.

A circulation description of a rip current neck

This paper considers wave-driven rip currents from a circulation-vorticity point of view. A highly simplified description of rip currents using continuous generation of point vortices at fixed locations is found to have a wide range of applicability around the rip neck. Rip scaling becomes straightforward, and is seen to collapse to a single form. Numerical experiments with this simplified system have the ability to predict well startup transients including previously unexplained peaks, and frequency-dependent effects of wave groups on rip currents. Quantitative mean currents in the rip neck are represented moderately well by this theory.

Mean currents and sediment transport in a rip channel

Measurements of hydrodynamics and suspended sediment transport were carried out along a cross-shore transect in a rip channel. A tidally induced threshold for rip flow initiation was found; this threshold depended upon the degree of wave energy dissipation and could be identified through a critical value of the ratio of significant wave height to water depth (γ s) in the rip neck. With normally incident waves, rip current velocities were well correlated with velocities predicted from a simple model involving the onshore mass transport in asymmetric breaking waves crossing the bar. Hydrodynamics and sediment transport were tidally modulated with strong offshore directed rip currents and associated offshore sediment transport due to the mean flow at low tide. At high tide when the rip was inactive, the tendency was for weak onshore directed mean flows and/or oscillatory incident waves transporting smaller amounts of sediment landward. The net result was a gradual lowering of the topographic relief along the rip channel with net deposition seaward of the rip. Outside the surf zone, transport was mainly landward due to oscillatory incident waves.

Flow characteristics along the rip current system under low-energy conditions

This paper aims to determine the flow signature of the rip current under low-energy conditions based on current meters at Herzliyya Beach, Israel, on the southeastern Mediterranean shore. Field experiments, conducted under relatively calm wave conditions ( T sig = 3−6 s, H sig = 0.3−1.1 m), included current and wave monitoring with morphological mapping. Current data were analysed for shore-normal and resultant components and the time series of instantaneous and smoothed velocities provided current and orbital wave velocities. Statistical and spectral processing yielded energy distributions, specific current discharge, and directional data. Velocity and velocity-time asymmetry increased from the feeders to the neck and subsequently slowed down towards the rip head. The directional range of the flow was narrow in channeled feeders and even better concentrated at narrow necks with higher velocities. The flows at the rip head were typically divergent. Systematic spatial change of energy spectra was observed along the rip system: the subharmonic frequency band dominated the feeders and the inner surf embayments and decreased gradually toward the rip neck with a simultaneous gradual increase in incident wave frequency. The infragravity band frequency was minor everywhere. Velocity spectra at oblique bars with sheltered areas differed from spectral changes at transverse bars with an exposed inshore. The velocity at the rip neck was above the threshold for grain movement whereas at the head, it was usually below the threshold. Textural parameters of the beach sand could not be systematically related to the current velocity, i.e., no environmental groupiness was evident along the rip system.

Flow characteristics at the rip current neck under low energy conditions

This paper presents the flow signature of the rip current neck as determined by fourteen near-bottom velocity records, with a comparison made to 63 recordings in the surf zone. The field experiments, including wave monitoring and morphological mapping, were conducted under relatively calm, dissipative wave conditions ( T sig = 3–6 s, H sig = 0.3-1.1 m) at Herzliya beach, Israel, on the Mediterranean shore. The current data were analysed for shore-normal and resultant components. The processing of the time series in the form of instantaneous and smoothed velocities, provided spectral energy, orbital and current velocity, specific current discharge and directional distribution data. The unique characteristic of the neck environment was found to be its continuous unidirectional seaward flow. Due to the channeling effect, this steady offshore drift constituted the fastest flow in the surf zone with a typically narrow (35°–94°) directional range. Through spectral analysis of the unidirectional return flow the morphodynamic process signature at the rip neck was defined by the relative contribution from the incident waves (78%), the subharmonic edge waves (14%) and the infragravity oscillations (8%). The pulsatory, jet-like rip model with concentration of energy at low frequencies is not corroborated by this study. The studied environment of the rip and bar pattern represented an inherited beach morphology, at an initial accretional, although not yet reflective, stage.