Rip Channel Research Articles

Physical modeling of three‐dimensional intermediate beach morphodynamics

Experiments have been performed in a large wave tank in order to study the morphodynamics of rip current systems. Both accretive and erosive shore‐normal wave conditions were applied, the beach evolving through all the states within the intermediate beach classification, under the so‐called down‐state (accretive) and up‐state (erosive) morphological transitions. Results show that any prescribed change in the wave conditions drastically increases the rate at which the morphology changes. The surf zone morphology tends toward a steady state when running a given wave climate for a long duration. We quantitatively describe a full down‐state sequence characterized by the progressive evolution of an alongshore‐uniform bar successively into a crescentic plan shape, a bar and rip channel morphology, and a terrace. From the analysis of a large data set of dense Eulerian measurements and bathymetric surveys, we depict several feedback mechanisms associated with wave‐driven rip current circulation, wave nonlinearities and the seabed evolution. At first, a positive feedback mechanism drives a rapid increase in the rate of morphological change, beach three‐dimensionality, and rip intensity. By the time the sandbar evolves into a bar and rip morphology, a negative feedback mechanism, characterized by a decaying beach change rate and an increasing beach alongshore uniformity, overwhelms the former mechanism. An erosive sequence characterized by both an overall offshore bar migration and an increase in beach three‐dimensionality is also described.

Nearshore circulation over transverse bar and rip morphology with oblique wave forcing

ABSTRACTThere is a paucity of field data to describe the transition in nearshore circulation between alongshore, meandering and rip current systems. A combination of in‐situ current meters and surf zone drifters are used to characterize the nearshore circulation over a transverse bar and rip morphology at Pensacola Beach, Florida in the presence of relatively low energy oblique waves. Current speeds vary in response to the relative wave height ratio (Hs/h), which defines the degree and extent of breaking over the shoal. In the absence of wave breaking the nearshore circulation was dominated by an alongshore current driven by the oblique waves. As waves begin to break across the shoal (0.2<Hs/ h<0.5) the nearshore circulation is characterized by a meandering alongshore current. As conditions became more dissipative (Hs/h>0.5), the meandering current is replaced by an unsteady rip circulation that moves offshore between the shoals before turning alongshore in the direction of wave advance outside the surf zone. The increase in wave dissipation is associated with an increase in very low frequency (VLF) variations in the current speed across the shoal and in the rip channel that caused the circulation to oscillate between an offshore and an alongshore flow. The unsteady nature of the nearshore circulation is responsible for 55% of all surf zone exits under these more dissipative conditions. In contrast, only 29% of the drifters released from the shoal exited the surf zone and bypassed the adjacent shoal with the alongshore‐meandering current. While the currents had a low velocity (maximum of ~0.4 m s‐1) and would not pose a significant hazard to the average swimmer, the results of this study suggest that the transverse bar and rip morphology is sufficient to create an alongshore variation in wave dissipation that forces alongshore meandering and low‐energy rip circulation systems under oblique wave forcing. Copyright © 2013 John Wiley & Sons, Ltd.

Ability of beach users to identify rip currents at Pensacola Beach, Florida

Quasi-permanent rip current hot spots at Pensacola Beach, Florida, pose a significant hazard to beach users, largely because the hot spots are located at or close to the primary access points. While an increase in the number of lifeguards has led to a decrease in the number of drownings since 2004, the number of rescues and contacts has increased to over a 30,000 year. Despite warning signs at access points along the beach, it is not clear whether beach users are able to identify a rip channel or an active rip current. To assess beach users’ knowledge of rip currents and their ability to identify rip channels and currents, 97 surveys were conducted between June and September of 2010 at Pensacola Beach. Beach users were asked to identify rip channels in oblique photographs taken on green, yellow and red flag days when the potential for rip currents is low, medium and high, respectively. A majority of participants suggested that they could identify a rip channel or current (if present), but less than 20 % of beach users were able to identify the rip channels and currents. The majority of participants identified heavy surf areas as the location of the rips versus the relatively flat water of the current or the darker color water of the channel. Results further suggest that most beach users, and particularly local participants, are overconfident in their ability to identify rip channels and currents. The focus of beach users on heavy surf as an indication of the rip current potential and the overconfidence in identifying a rip channel or current affects the spatial distribution of beach users and to some degree the location of rescues and drownings. While it can be quite difficult for the average beach user to identify rip channels and active rip currents, the results of the study suggest a need for further education efforts to reduce the rip hazard, particularly in areas where lifeguards are not permanently stationed.

Suspended Sediment Transport in Rip Currents on a Macrotidal Beach

Thorpe, A., Miles, J., Masselink, G., Russell, P., Scott, T., Austin, M., 2013. Sediment Transport In Rip Currents on a Macrotidal BeachRip currents are offshore-directed flows in the surf zone that may be responsible for significant offshore transport of sediment. In this study high frequency surface elevation and current velocity data were collected alongside suspended sediment concentrations in a rip channel on a high energy, macro-tidal beach. At low tide (water depth over the rip channel (h) = 0.9 m), when the rip current was active (maximum offshore directed flow = ~ 0.4 m/s), net suspended sediment transport was directed offshore, and 88% of the flux was due to the mean flow component. The rip current was found to pulse at very low frequencies (VLF) and this contributed significantly to offshore sediment transport, accounting for 48% of total oscillatory flux (not considering direction). Infra-gravity (IG) frequency oscillations had a marginal contribution to offshore sediment transport within the rip (10% of total oscillatory flux) and incident wave transport was onshore (42% of total oscillatory flux). The relative influence of the wave component increased as tidal elevations increased and the rip current became inactive. For comparison, at mid-tide (h = 3.5 m) the total suspended sediment flux was in the offshore direction and four times less than during low tide. The mean flux accounted for 34% of the total transport at mid-tide. At high tide (h = 6.1 m) the total sediment transport was in the onshore direction due to the incident waves, with a 0% contribution from the mean flux.

Video observation of megacusp evolution along a high-energy engineered sandy beach: Anglet, SW France

Birrien, F., Castelle, B., Dailloux, D., Marieu, V., Rihouey, D. and Price, T.D., 2013. Video observation of megacusp evolution along a high-energy engineered sandy beach: Anglet, SW France.We present an 18-month period of video monitoring of both the nearshore sandbars and the megacusps at Anglet Beach, SW France. The study site covers a 2-km long stretch of beach that is constrained to the North by a groin that extends 400 m seaward. The beach is high-energy intermediate, mostly double-barred, with a steep beach face (~1/10) favouring the formation of cusps at a large range of lengthscales, from beach cusps (O(10 m)) to megacusps (O(100 m)). A megacusp is systematically observed against the groin as a result of the persistent presence of a topographic rip. Further away from the groin, about 4–5 megacusps are typically observed. The shoreline dynamics are strongly controlled by the geometry of the surfzone sandbar. In-phase shoreline-sandbar coupling was commonly observed, that is, megacusps in the alignment of the rip channels and seaward bulges in the shoreline facing the surfzone sandbar horns. Megacusps at Anglet Beach have a typical cross-shore amplitude of O(10 m). During a severe storm with significant wave heights over 7 m, a very erosive megacusp (hotspot) formed in the alignment of the rip channel. Prior to this storm, a seaward bulge in the shoreline was observed in this location. Accordingly, 40 m cross-shore variation change in shoreline position at this location was the signature of megacusp dynamics only. This hotspot was systematically observed at the same location during 7–8 months after the storm event, before the fair weather conditions in summer resulted in an overall accretion of the beach. Megacusps were smoothed out during rapid alongshore migration of the sandbar or for high-energy wave conditions. In addition, the evolution of the alongshore-averaged shoreline position shows the exact opposite trend as that of the alongshore-averaged sandbar position, yet with less pronounced cross-shore variations. Overall, our results highlight once again the strong links between shoreline and surfzone sandbar dynamics.

Video monitoring nearshore sandbar morphodynamics on a partially engineered embayed beach

De Santiago, I., Morichon, D., Abadie, S., Castelle, B., Liria, P. and Epelde, I., 2013. Video observation of the morphodynamics of nearshore sandbars on a partially engineered embayed beach.Embayed beaches are prevalent environments along the northern Spanish coastline. The present work analyses the temporal and spatial variability of nearshore sandbars on the pocket beach of Zarautz using daily video observations over 2 years. This partially urbanized beach can be divided into two well defined distinct areas: a natural section backed by a sand dune system to the East and an engineered section with rigid seawalls to the West. The objective of the present work is to evaluate if the engineered and natural sections of the beach present different morphological behaviors. The video image analysis was performed in two steps: First, sand bars were characterized visually and classified following standard classification schemes. Then, the outer sand bar migration was measured using automatic bar detection. Results show that the nearshore sandbars evolution covers a wide range of temporal and spatial variability. A noteworthy exception is the persistent presence of headland rips. The beach is mostly double-barred, with both bars able to go through all the states within the intermediate classification. The most common double bar morphological configuration is an inner low tide terrace coupled to well-developed megacusps, with an outer subtidal crescentic bar. Various preferred locations of rip channel formation are identified along the beach, suggesting that the effects of the headlands can propagate far away from the headlands towards the center of the embayment. Interestingly, the western engineered and more sheltered section of the beach sometimes exhibits a different beach state to that of the eastern section.

流れから波へのフィードバック機構がリップチャンネル形成に及ぼす影響

Current effect on waves (CEW) is examined for formation of rip channel systems with a barotropic model that consists of an Eulerian phase-averaged shallow water equation with a vortex-force formalism, WKB ray equations, and a bed evolution equation with the Soulsby-Van Rijn's total sediment flux formula. CEW acts on reducing the offshore extent of seaward rip currents through wave refraction on the currents, leading to modifying the budget of sediment flux and the associated surf-zone topography. Inclusion of CEW results in shoaling rip channels, deepening offshore mounds, and elongating alongshore spacing of rip channels with both the normal and oblique incidence of offshore waves.

SPH MODELING OF MEAN VELOCITY CIRCULATION IN A RIP CURRENT SYSTEM

A Lagrangian numerical model called Smoothed Particle Hydrodynamics is used to analyze rip current system generated by a single bar and a rip channel. The pattern of the wave-induced circulation cell over the bar, the oppositely-rotating circulation cell on-shore and a strong seaward-directed current in the rip channel is modeled numerically. The mean horizontal variations of rip current system as well as three-dimensional circulations are studied. The results in three-dimensional space reveal the wave-current interaction and flow patterns in different parts of rip channel, bar, and the trough located near shore. For comparison to experimental data, Eulerian nodes are introduced to the numerical model and SPH interpolation over neighboring Lagrangian particles is implemented to find fluid parameters at those specific nodes. This methodology leads to a better understanding of depth-integrated flows and a more accurate comparison of numerical results with experimental results. Model predictions are compared to laboratory measurements of Drønen et al. (2002) and show good agreement, including mean velocity profiles, mean surface elevation and three-dimensional velocity components.

FIELD MEASUREMENTS OF BEDFORMS IN A RIP CHANNEL ON A MACRO-TIDAL BEACH

A Sand Ripple Profiler (SRP) was deployed in a rip channel on a dissipative sandy beach to measure bedform height (∆), length (λ) and migration rate (Mr¬) throughout a macro-tidal cycle with an offshore significant wave height of 0.7 m and peak period of 10 s. At lower tidal elevations in the strong offshore flow of the rip current (maximum = 0.4 m/s) bedforms (∆ = 0.15 m, λ = 3 m) were found to migrate offshore (Mr = 0.21 m/hr). Outside of active rip current conditions (water depth (h) = >~2.5 m) bedforms were found to be of smaller scale (∆ = 0.09 - 0.12 m, λ = 1 - 1.2 m) migrating onshore at a rate of 0.35 m/hr at mid tide (h = 3.3 m) and 0.03 m/hr at high tide (h = 6.3 m). Onshore migration rates were found to increase with increased wave skewness and velocity variance.

LAGRANGIAN DRIFTER MODELLING OF AN EXPERIMENTAL RIP CURRENT

A non-uniform alongshore wave forcing on an experimental uneven mobile bathymetry create mean circulation on a rip channel. A 2D numerical hydrodynamic model that integrates the non-linear shallow-water equations in a shock-capturing finite-volume framework is used to validate the nearshore circulation, and drifters displacement.

Observations and conceptual modelling of morphological coupling in a double sandbar system

ABSTRACTNearshore sandbars, located in <10 m water depth, can contain remarkably periodic alongshore undulations in both cross‐shore position and depth. In a double sandbar system, the alongshore spacing of these morphological patterns in the inner sandbar may be identical to those in the outer sandbar. Although this morphological coupling has been observed previously, its frequency and predominance remain unclear. In this paper, we use a 9.3‐year dataset of daily low‐tide time exposure images from the double‐barred beach at Surfers Paradise (Gold Coast, Australia) to analyse the temporal and spatial characteristics of morphological coupling within a double sandbar system. We distinguish five types of morphological coupling between the inner and outer sandbars, of which four coincide with a downstate progression of the outer bar. Coupling is either in‐phase (with a landward perturbation of the inner bar facing an outer‐bar horn) or out‐of‐phase (with a seaward perturbation of the inner bar facing an outer‐bar horn), where the coupled inner‐bar features either consist of rip channels or, predominantly, perturbations of the low‐tide terrace. Cross‐correlation of the image‐derived inner‐ and outer‐bar patterns shows coupling to be a common phenomenon in the double sandbar system studied here, with coupling in 40% of the observations. In contrast to previous observations of sandbar–shoreline coupling at single‐barred beaches, in‐phase coupling (85% of all coupled bar patterns) predominates over out‐of‐phase coupling (15%). Based on our observations and bathymetries assimilated from the images for a restricted set of coupling events, we hypothesize that the angle of offshore wave incidence, wave height and depth variations along the outer sandbar determine the type of flow pattern (cell circulations versus meandering currents) above the inner bar and hence steer the type of coupling. Copyright © 2012 John Wiley & Sons, Ltd.

Role of morphological variability in the evolution of nearshore sandbars

Computations using a depth-averaged morphological process-based model of a double nearshore bar system have been used to test the hypothesis that bathymetries with small variability adapt more easily to new hydrodynamic conditions than bathymetries with distinctly imprinted crescentic patterns. The computations are used to investigate the assumption that nearshore bathymetries tend to evolve toward a rip-channelled pattern matching concurrent constant hydrodynamic forcing, if these conditions prevail for an extended period of time. In each computation an initially alongshore uniform double barred bathymetry, seeded with a small random bed-level perturbation, was forced by two sequential constant hydrodynamic conditions. For each set of conditions, four different computations show the effect of a later transition moment – and thus more distinctly evolved patterns – on the level of adaptation to the second condition. After the transition to the second condition, different hydrodynamic circulations occur due to differences in the bathymetry at the transition moments. Depending on how pronounced the existing features were at the moment of transition, these circulations either reinforce the existing bathymetric pattern or allow the bathymetry to evolve to a new rip-chanelled pattern with a spacing similar to the one that occurs if the second condition had been applied from the start. As hydrodynamic conditions generally change more rapidly than the adaptation time, which is at least in the order of days, it is highly unlikely that observed rip channel distances match length scales expected for concurrent hydrodynamic conditions, consistent with field observations (e.g. Holman et al., 2006). It is therefore concluded that nearshore patterns are formed by a combination of both the antecedent morphology – and thus antecedent hydrodynamics – and the current local hydrodynamic conditions, next to factors like sediment characteristics.

The morphodynamics of rip channels on embayed beaches

We use a nonlinear morphodynamic model to examine the formation and nonlinear evolution of surfzone rip channels on embayed beaches. Starting from a range of embayed beach bathymetries characterized by different length and curvature, and under different time-invariant and time-varying wave conditions, the numerical model can reproduce the flow circulation and morphological characteristics observed on natural embayed beaches: (1) normal beach circulation, characterized by rips similar to non-embayed beaches and the presence of headland rips, (2) cellular circulation, with either headland rips only occurring at one or both ends of the embayment or a single rip at the centre of the beach and (3) transitional circulation, where both topography and currents influence rip location and behaviour. Time-invariant simulations show that, under oblique-wave forcing, rip spacing is systematically larger updrift than downdrift. Headland rips are preferably observed for straight beaches, with no clear dependence on wave angle. Wave shadowing and resulting alongshore gradients in wave height against the headland are the primary driving mechanism for headland rips. The formation of a single central rip is observed for short, curved embayed beaches, with no clear dependence on the wave angle as well. We use a novel non-dimensional embayment scaling parameter to quantify the degree of headland impact on beach circulation. Our simulations with shore-normal waves and initially alongshore-uniform embayed beaches show the parameter is consistent with observations. Our simulations also suggest that for high wave obliquity or time-varying wave angle to the shore, the influence of the headlands can progressively propagate into the whole domain. A time-varying wave angle results in persistent migration of rips towards the downcurrent headland rip, the splitting of shoals, an increase in merging of rip channels and more alongshore-variable rip spacing. The longshore variability of rip channel wavelength along embayed beaches is consistent with the hypothesis that rips are self-organized patterns and is consistent with recent field observations.

The modelling of rip channel in creation of rip currents

The modelling of rip channel in creation of rip currents

On the impact of an offshore bathymetric anomaly on surf zone rip channels

We use a nonlinear morphodynamic model to demonstrate that the presence of a single persistent offshore bathymetric anomaly strongly affects the formation, nonlinear evolution and saturation of surf zone rip channels. In the case of an offshore bump or trough and waves with oblique incidence, a rip channel shoreward of the anomaly is enforced by the more seaward alongshore variability in depth. The degree of rip channel enforcement is controlled by the strength of the rotational nature of surf zone rip current circulations, which is, in turn, driven by differential broken wave energy dissipation induced by wave refraction across the offshore bathymetric anomaly. The alongshore location of this forced rip channel is more stable with increasing offshore anomaly amplitude, decreasing offshore wave obliquity and decreasing bathymetric anomaly distance to the shore. Simulations show that rip channel behavior downdrift and updrift of the offshore perturbation are different. In our numerical experiments, downdrift rip channels have systematically larger alongshore scales, smaller alongshore migration rates and more erosive megacusps than those updrift. Rip channels therefore self‐organize into patterns of different alongshore scales and migration rates as a result of an alongshore perturbation in the wave forcing enforced by wave refraction across an offshore bathymetric anomaly. These simulations are qualitatively corroborated by video observations of sandbar behavior during a down‐state sequence at a site with a persistent offshore trough.

Implementation of the vortex force formalism in the coupled ocean-atmosphere-wave-sediment transport (COAWST) modeling system for inner shelf and surf zone applications

The coupled ocean-atmosphere-wave-sediment transport modeling system (COAWST) enables simulations that integrate oceanic, atmospheric, wave and morphological processes in the coastal ocean. Within the modeling system, the three-dimensional ocean circulation module (ROMS) is coupled with the wave generation and propagation model (SWAN) to allow full integration of the effect of waves on circulation and vice versa. The existing wave-current coupling component utilizes a depth dependent radiation stress approach. In here we present a new approach that uses the vortex force formalism. The formulation adopted and the various parameterizations used in the model as well as their numerical implementation are presented in detail. The performance of the new system is examined through the presentation of four test cases. These include obliquely incident waves on a synthetic planar beach and a natural barred beach (DUCK’ 94); normal incident waves on a nearshore barred morphology with rip channels; and wave-induced mean flows outside the surf zone at the Martha’s Vineyard Coastal Observatory (MVCO).Model results from the planar beach case show good agreement with depth-averaged analytical solutions and with theoretical flow structures. Simulation results for the DUCK’ 94 experiment agree closely with measured profiles of cross-shore and longshore velocity data from Garcez Faria et al. (1998, 2000). Diagnostic simulations showed that the nonlinear processes of wave roller generation and wave-induced mixing are important for the accurate simulation of surf zone flows. It is further recommended that a more realistic approach for determining the contribution of wave rollers and breaking induced turbulent mixing can be formulated using non-dimensional parameters which are functions of local wave parameters and the beach slope. Dominant terms in the cross-shore momentum balance are found to be the quasi-static pressure gradient and breaking acceleration. In the alongshore direction, bottom stress, breaking acceleration, horizontal advection and horizontal vortex forces dominate the momentum balance. The simulation results for the bar/rip channel morphology case clearly show the ability of the modeling system to reproduce horizontal and vertical circulation patterns similar to those found in laboratory studies and to numerical simulations using the radiation stress representation. The vortex force term is found to be more important at locations where strong flow vorticity interacts with the wave-induced Stokes flow field. Outside the surf zone, the three-dimensional model simulations of wave-induced flows for non-breaking waves closely agree with flow observations from MVCO, with the vertical structure of the simulated flow varying as a function of the vertical viscosity as demonstrated by Lentz et al. (2008).

Modeling formation and subsequent nonlinear evolution of rip channels: Time-varying versus time-invariant wave forcing

[1] We use a nonlinear morphodynamic model to demonstrate that time-varying forcing, in particular the time-varying angle of wave incidence, is crucial to the development of rip channels in terms of rip channel morphology, nonlinear behavior, longshore migration, and mean rip spacing. The time-varying angle of incidence leads to different mean rip spacings than the time-integrated time-invariant forcing and to systematically less developed bar and rip morphologies at more alongshore variable scales. This supports the common field observation of irregular and random alongshore rip spacings, and contrasts with the regular spacing predicted by existing time-invariant template, and instability models. Time-varying wave incidence also generally results in the onset of splitting of shoals and an increase in merging of rip channels. In addition, a time-varying angle of incidence with zero mean can drive a significant net alongshore migration of the rip channels. Abrupt changes in wave conditions are responsible for this net longshore migration through cumulative effects of the mismatch between wave conditions and bar and rip morphology orientation.

Numerical simulation of wave-induced currents combined with parabolic mild-slope equation in curvilinear coordinates

Researches on breaking-induced currents by waves are summarized firstly in this paper. Then, a combined numerical model in orthogonal curvilinear coordinates is presented to simulate wave-induced current in areas with curved boundary or irregular coastline. The proposed wave-induced current model includes a nearshore current module established through orthogonal curvilinear transformation form of shallow water equations and a wave module based on the curvilinear parabolic approximation wave equation. The wave module actually serves as the driving force to provide the current module with required radiation stresses. The Crank-Nicolson finite difference scheme and the alternating directions implicit method are used to solve the wave and current module, respectively. The established surf zone currents model is validated by two numerical experiments about longshore currents and rip currents in basins with rip channel and breakwater. The numerical results are compared with the measured data and published numerical results.

Field investigation of rip currents along the southern coast of the Caspian sea

This article presents, for the first time, an investigation into the presence of rip currents along the southern coast of the Caspian Sea. The study site is selected at the Anzali port where 3 people were drowned in one day of summer 2007. Bathymetric surveys were obtained twice during the summers of 2008 and 2009 and rip currents were measured for two days in the summer of 2008 and the summer of 2009 with different wave conditions, using five GPS drifters. An RCM 9 current meter was installed in the rip channel for credibility of the drifter velocity. The cross-shore location of their velocity maxima and the distance they extend offshore are just two important unknowns. These questions were investigated using drifter trajectories. An oblique and a classic rip current were observed in two experiments. A field validation of the drifters shows that correspondence between the drifters and RCM 9 measurements is good.

Implementation and modification of a three-dimensional radiation stress formulation for surf zone and rip-current applications

Regional Ocean Modeling System (ROMS v 3.0), a three-dimensional numerical ocean model, was previously enhanced for shallow water applications by including wave-induced radiation stress forcing provided through coupling to wave propagation models (SWAN, REF/DIF). This enhancement made it suitable for surf zone applications as demonstrated using examples of obliquely incident waves on a planar beach and rip current formation in longshore bar trough morphology (Haas and Warner, 2009). In this contribution, we present an update to the coupled model which implements a wave roller model and also a modified method of the radiation stress term based on Mellor (2008, 2011a,b,in press) that includes a vertical distribution which better simulates non-conservative (i.e., wave breaking) processes and appears to be more appropriate for sigma coordinates in very shallow waters where wave breaking conditions dominate. The improvements of the modified model are shown through simulations of several cases that include: (a) obliquely incident spectral waves on a planar beach; (b) obliquely incident spectral waves on a natural barred beach (DUCK'94 experiment); (c) alongshore variable offshore wave forcing on a planar beach; (d) alongshore varying bathymetry with constant offshore wave forcing; and (e) nearshore barred morphology with rip-channels. Quantitative and qualitative comparisons to previous analytical, numerical, laboratory studies and field measurements show that the modified model replicates surf zone recirculation patterns (onshore drift at the surface and undertow at the bottom) more accurately than previous formulations based on radiation stress (Haas and Warner, 2009). The results of the model and test cases are further explored for identifying the forces operating in rip current development and the potential implication for sediment transport and rip channel development. Also, model analysis showed that rip current strength is higher when waves approach at angles of 5° to 10° in comparison to normally incident waves.