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RATIONALE / OBJECTIVES / RESEARCH APPROACH & METHODS / IMPACT PLAN / REFERENCES

Project overview

Rationale (top)

Rip currents are traditionally known as relatively narrow, seaward-flowing currents that originate in the surf zone and extend seaward of the breaking region. 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.

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'. 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). 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).

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).

 

PPT_argus_annotate

Figure 1. Rectified ARGUS video image showing 4 rip current systems from the MATRIX pilot experiment at Perranporth, Cornwall (Austin et al., 2009). 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. The rips are separated by transverse bars and retained within the surf zone by a longshore bar.

 

Rip_circ

Figure 2. Results from GPS drifters deployed during the MATRIX pilot experiment (Austin et al., 2009). Zoomed-in section of the ARGUS image representing Rip 4 superimposed with the Lagrangian flow pattern recorded using specialist GPS drifters. The flow pattern is dominated by a large anti-clockwise rotating eddy, but several offshore-directed 'squirts' are also apparent. The contour lines represent bathymetry.


Rip currents are driven by cross-shore and longshore gradients in the radiation stress and the mean water surface, resulting from wave energy dissipation patterns associated with bar/rip morphology and wave-current interaction (Sonu, 1972). The strength and character of the rip system is temporally variable because the forcing (incident waves) and controlling factors (nearshore morphology) change over time. In addition, the tidal modulation of the location of wave breaking processes introduces a further complicating element to rip dynamics.

Rip dynamics can be considered over the following dominant time scales:

Weeks to months: Variation in rip dynamics over longer timescales is mainly due to changes in the nearshore morphology forced by changes in the wave climate. Cell circulation systems are generally best developed on beaches with prominent transverse bars and rips (Brander, 1999), allowing for the formation of relatively steep water surface gradients and, therefore, strong rip currents. Along the north Cornish coast, nearshore bar and rip morphology becomes increasingly pronounced and three-dimensional during the relatively calm summer months (Poate et al., 2009). Accordingly, rip currents intensify and the risk posed by rips to surf zone water users increases in summer, coinciding with maximum visitor numbers (Scott et al., 2009).

Days: Nearshore morphology is relatively constant from day to day and changes in the rip dynamics over this time scale are primarily driven by variations in the incident wave conditions (e.g., wave height, period and angle). Nearshore current velocities do not automatically increase with the incident wave energy level, however, and the response is strongly dependent on the antecedent morphology. For example, MacMahan et al. (2010) found some evidence that rip velocities on a beach with transverse bar-rip morphology were strongest under intermediate wave conditions, and weakened under both increasing and decreasing wave energy conditions.

Hours: Changes in rip dynamics over a time scale of hours occur mainly due to the tidal modulation of the wave energy dissipation over the bar morphology (Figure 2). Rip current velocities tend to be maximum when wave breaking over the bar is most intense and minimum when waves do not break over the bar (bar either dry or seaward of the surf zone). Tidal modulation of the breaker pattern can cause rip systems to be 'turned on and off' and this process may be implicated in mass rescue events, because these seem to be linked to a critical tide level (Scott et al., 2009).

Minutes: Rip current velocities may also vary over a time scale of minutes and this phenomenon is referred to as rip pulsing, and may be related to 'squirts'. The three main explanations for rip pulsing are shear waves (Reniers et al., 2007), standing infragravity waves (MacMahan et al., 2004) and wave group related variations in mass flux over nearshore bars, causing ponding of water in the trough and periodic releases through the rip channels (Brander and Short, 2001). Distinguishing conclusively between the different mechanisms of rip pulsing requires an instrument array that is even more comprehensive than proposed here; however, we will be able to measure rip pulsing as a function of wave, tide and morphological conditions, and hence assess any additional risk posed to surf zone water users.

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. 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.

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). 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. 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. 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.

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. Traditionally, rips are considered to flow roughly normal to the shoreline and extend seaward of the surf zone and field experiments using dye and person floats conducted in Australia and New Zealand (Brander and Short, 2000, 2001) generally support such circulation. However, more recent field experiments on a number of US and European beaches using specialist Lagrangian GPS drifters have indicated that rip currents frequently form extensive rotational surf zone eddies, whereby the offshore flow within the rip channel returns landwards over the intertidal bar from the outer edge of the surf zone (MacMahan, 2008; Austin et al., 2009). The rotational eddy is primarily contained within the seaward limit of the surf zone with only occasional ‘squirts’ piercing the breaker line.

In fact, MacMahan et al. (2010) synthesised the results of three drifter deployments and found that, on average, only 15-20% of the drifters released in the feeder current exited the surf zone per hour. Figure 2 shows an example of such a rip circulation pattern obtained with a small number of GPS drifters deployed during the MATRIX pilot experiment on Perranporth (Austin et al., 2009). The size and intensity of the rotational eddy was dependent on tidal stage, being strongest around low water when wave breaking occurred on the bar. It was further observed that as energy levels increased above a certain threshold, the eddy diminished and longshore currents became dominant

It is unclear under what conditions rotational surf zone eddies develop and when distinct rip currents form that extend beyond the surf zone. The implications for nearshore sediment transport, surf zone flushing and beach safety are obvious, though, and comprehensive measurements of the Eulerian and Lagrangian flow field under a range of morphodynamic conditions are expected to provide useful insights into the occurrence of these two rip circulation types.

Beach Safety

Rip currents are an important coastal phenomenon. Strong rip current flow velocities are the main hazard to surf zone water users. According to lifeguard records, over 68% of incidents ('rescues') on UK beaches can be attributed to rip currents (Scott et al., 2007). A similar percentage is reported from Australia and the USA. Worldwide hundreds of people drown each year due to rip currents.

Beach safety is a significant issue in regions such as the southwest of England, where the vast majority of the 10 million visitors per year frequent the beaches in the region. Beach leisure activities have expanded due to the increased popularity of water sports and the extension of the traditional summer beach usage season into the spring and autumn, placing increased pressure on lifeguarding activities.

Of particular concern are mass rescue events, when, within a few hours, upwards of 150 people per beach have required simultaneous rescue at multiple locations (> 10 beaches) due to rip currents (Scott et al., 2009). Records show that these events occur each summer when 'optimum' conditions of wave, tide and beach configuration coincide with a large beach population. Perranporth beach, one of the two study sites, has the highest occurrence of mass rescue events within the region (Figures 1 & 2).

The importance of rip currents for worldwide beach safety is well recognised by coastal scientists and lifeguards, but their dynamics remain poorly understood, especially in large tidal environments where they display a pronounced variability in current intensity depending on the tide level (Austin et al., 2009). This project, Dynamics of Rip currents and Implications for Beach Safety (DRIBS), aims to increase our quantitative understanding of rip current dynamics and associated beach hazards on beaches with a large tidal range through analysis of field data and numerical modelling.

Crucially, the research will be carried out in collaboration with the RNLI to ensure relevant information and tools are disseminated appropriately to the end users (lifeguard managers, lifeguards and the general public). This project is relevant to the RNLI, but also matches a key objective of the Department of the Environment and Rural Affairs (DEFRA) in their recently published 'High Level Marine Objectives', which emphasises the need to create a safe marine environment by eliminating and/or educating about coastal hazards (link).

Objectives (top)

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. 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. 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. 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. 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. The overall aim of this project is to test this hypothesis through a combination of beach monitoring, field experiments and sophisticated numerical modelling. 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.

More specifically, our objectives are to:

  • Quantify the temporal variability in beach state and rip current characteristics over the daily
    to monthly time scale on two beaches using ARGUS video observations and GPS surveys.
  • Collect two extensive field data sets to investigate and parameterise the relation between
    wave dissipation and rip dynamics over time scales ranging from minutes to weeks.
  • Use the field data to improve, validate and calibrate a numerical model (XBeach) capable of
    simulating nearshore cell circulation and rip current dynamics.
  • Produce generic rip current scenarios and hazard indicies based on field observations and numerical modelling to help the RNLI plan their lifeguarding activities and inform risk assessment and ublic awareness programs.
  • Develop a decision-support system (DSS) to predict several days in advance, and for
    different stages of the tide, the risk presented by rip currents to surf zone water users.

Research approach and methodology (top)

The research approach comprises three main elements, namely, beach monitoring, field experiments and numerical modelling. Considerable emphasis will be placed on assessing the implications of the research for beach safety and tools will be developed to assist the RNLI with their lifeguarding duties.

Beach monitoring

Preliminary results from beach surveys conducted as part of WHISSP (link) suggest that the beach morphology is highly variable along the north Cornish coast with well developed bar/rip systems present in summer, but a largely featureless beach or very subdued bar/rip morphology in winter (Poate et al., 2009; weeks-to-months time scale). This work will be extended in DRIBS through monthly topographic and bathymetric 3D surveys of two key beaches on the north Cornish coast (Porthtowan, Perranporth), complemented by analysis of ARGUS video data already collected at the same locations (Figure 3). The beach surveys will provide the best spatial resolution of the changing intertidal morphology, allowing determination of the bar prominence, rip dimensions and location, and beach sediment volumes (Figure 4). The ARGUS data will provide the best temporal resolution by providing daily representations of the intertidal topography and wave breaking patterns (refer to Figure 1).

 

argus_fig

Figure 3. ARGUS video camera system at Perranporth. The camera system will be modified for the DRIBS project to include a third zoom camera providing greater resolution across the expected study region.

 

 

Surveying_fig

Figure 4. Monthly topographic beach surveys will be conducted by the DRIBS project team at the University of Plymouth using an ATV mounted RTK-GPS system.


Field experiments

We will conduct a 6-week field campaign on two high-wave, large-tidal beaches along the north Cornish coast where mass rescue events of upwards of 150 people per beach have required simultaneous rescue due to rip currents. During each of these campaigns, we will install several instruments in the surf zone that will measure waves, tides and rip currents at fixed locations.

In addition, we will use a large number of specialist drifters that measure the complete rip current pattern. The drifters will be released in the surf zone and will move according to the nearshore current pattern. Their location will be continuously monitored (using GPS) and the data from the drifters will provide useful information not only on the strength of the rip current, but also on the type of flow pattern. The drifters are designed to behave like human beings and their movement therefore mimics that of passive bathers.

Rip current experiments will include accurate measurements of the morphology and bathymetry, comprehensive water level and velocity measurements within the rip channel and neighbouring bars, and offshore directional wave measurements. The key quantities to be measured in the proposed field experiments include:

  • Regular inter- and sub-tidal bathymetry collected by RTK-GPS and echo sounder surveys.
  • Spatial patterns of wave energy dissipation, and radiation stress and water surface elevation gradients across the sub- and intertidal bar morphology.
  • Single point measurements and vertical profiles of the Eulerian flow velocity both within
    the rip and feeder channels and over the surrounding bars.
  • Lagrangian sub-surface drift across the cell circulation system using GPS-tracked drifters (Figure 6).
  • Seaward boundary conditions to force surf zone circulation models.

 

Drifter_fig

Figure 6. GPS drifters used to measure Lagrangian flow field of the rip current investigated during the MATRIX pilot experiment at Perranorth beach.


This is the first time such comprehensive measurements will be made in a macro-tidal setting. Experiment timings (early and late summer) have been selected to make sure that the rip morphology during the experiments will be significantly different (weeks-to-months time scale), whilst the duration of the field deployments will ensure sufficient variability in wave conditions (daily time scale). The key aspect of the field experiments is that due to the large tidal range (> 6 m), the forcing of the cell circulation system will vary considerably over a single tidal cycle, with rip currents being ‘turned on and off’ depending on the tidal stage (hourly time scale; refer to Figure 7).

 

tide_mod

Figure 7. Tidal modulation of rip current velocity during MATRIX pilot study. The strongest rip flows (> 0.6 m/s) prevail around low tide when wave breaking occurs over the nearshore bars.

 

This will enable us to test our leading hypothesis by pinpointing exactly under what conditions the rip current system is at its strongest. The combination of Eulerian and Lagrangian flow measurements will also help investigate offshore-directed squirts, which may be related to rip pulsing (minute time scale). The exposure of most of the bar/rip morphology at spring low tide will enable installation, maintenance and surveying-in of the instrumentation with relative ease and will further ensure accurate morphology data.

Numerical modelling

The work in DRIBS will be underpinned by numerical modelling throughout all phases of the work and will be supported by the Visiting Researchers (Prof Dano Roelvink and Dr Ap van Dongeren). The key numerical tool to be used is XBeach, a recently developed, process-based public-domain model that is capable of resolving the hydrodynamic processes most relevant to rip current dynamics (e.g., wave shoaling, refraction and energy dissipation, wave-current interaction; longshore and rip current velocities, infragravity wave motion and tidal translation; Roelvink et al., in press). XBeach will be used to plan and optimise the instrument deployment during the field experiments by running the model prior to the field experiments using actual bathymetry and expected wave/tide conditions. After the field experiments, XBeach will be validated and calibrated using the field data and the model will be implemented to help identify the most important processes in driving the rip circulation through comparison between field data and model results obtained using different model parameters and settings. XBeach will also be used to run scenario type simulations over individual tidal cycles using idealised beach morphology (e.g., subdued bar, pronounced bar, low-­tide bar, mid-­tide bar, etc) and varying wave/tide conditions to identify when and under what conditions rip currents are particularly strong and also what the character of the rip circulation is. The model output will be condensed to provide a simple diagnostic look-up-type tool (e.g., set of charts/tables) for daily use by beach lifeguards based on prevailing conditions.

The project also aim to develop innovative data-model assimilation tools to develop a decision-support system (DSS) with which rip current hazards and associated risk can be predicted several days in advance. Rip risk predictions developed in DRIBS will be based on a fully validated model outputs and bathymetry, and will cover several rip systems (Figure 1). Such information contributes significantly to planning and operational aspects of RNLI lifeguarding activities.

Impact Summary (top)

DRIBS is the first-ever comprehensive investigation of rip current dynamics on large tidal beaches and will significantly contribute to our quantitative understanding and modelling capabilities of rip current systems. The academic beneficiaries of this research include researchers and modellers involved with nearshore morphodynamics, especially those concerned with surf zone processes, beach morphology and coastal change. The greater insight into rip dynamics obtained from this study will also be beneficial to those involved with managing beaches and their hazards, specifically the life saving community (hence involvement of partner RNLI).

The main beneficiary of knowledge arising from this research is the Partner, the Royal National Lifeboat Institution (RNLI), who will use the outputs of this research to inform beach risk assessments, resource management, operational lifeguarding aspects, lifeguard training and national public awareness campaigns.

This research will benefit both the strategic and operational aspects of the RNLI lifeguarding activities, and will help them provide a better service, which ultimately saves lives. Other potential beneficiaries include all other lifeguarding organisations in the UK (e.g., Surf Life Saving GB, Royal Life Saving Society UK, Irish Water Safety, coastal councils), as well as overseas (e.g., Surf Life Saving Australia, Surf Life Saving New Zealand. Last, but not least, the coastal research and engineering community will benefit from the new knowledge and understanding provided by this research, especially those researchers interested in beach processes and nearshore currents.

Summary of benefits of the project for the RNLI:

Public education

  • The project will provide scientific data and research to support and develop the RNLI’s key safety messages. This information can then be circulated to the public using national and regional media campaigns and safety awareness programmes such as Beachwise, the RNLI’s annual beach safety campaign.
  • The project will provide information and data to help support and deliver the RNLI’s Surfers Safety Clinic’s. These clinics are currently being developed to provide a one-day workshop for competent surfers, covering basic rescue skills to assist water users who find themselves in trouble when lifeguard services are unavailable (i.e., out of season, non patrolled beach etc…)
  • The project will raise the public’s awareness of rip currents and their associated dangers during experiments through pro-active media support and providing lifeguards with the knowledge to deliver key information to the beach going public.

Lifeguard training

  • A series of regional lectures/workshops will follow the project, providing vital Rip related information/data to lifeguards, lifesaving clubs, beach managers and other interested bodies/persons. UoP and RNLI personnel will jointly deliver the lectures/workshops.
  • The full time lifeguards conference will also provide a good platform to deliver the key findings of the project to a wide audience. This will allow full time managers and supervisors to gain a good overview of the project whilst providing enough key information to cascade down to their lifeguards during local lifeguard inductions.
  • Information will be used to update Rip information within lifeguard training manuals and rescue techniques. Currently there is very little information specific to rip related hazards within lifeguard training manuals and practices (rips account for the largest environmental cause of incident).

Risk assessment & management planning

  • The project will provide robust and supporting evidence to develop rip hazard scenarios under different beach templates (e.g., with and without structures) to integrate within the RNLI’s current risk assessment systems. This will help to identify suitable control measures to reduce the risks of injury and deaths associated with rip currents.
  • Rip classification and hazard identification & rip hazard/risk prediction tools for resource management will help the RNLI better resource its rescue assets and personnel. With greater knowledge of rip related hazards and their identification, the RNLI will be better placed to manage beaches safely, keeping the public as safe as possible, whilst also ensuring its lifeguards are suitably trained.

References (top)

Austin, M.J., Scott, T.M., Brown, J.W., Brown, J.A. and MacMahan, J.H., 2009. Macrotidal rip current experiment: circulation and dynamics. Journal of Coastal Research, SI 56, 24-28.

Brander, R.W., 1999. Field observations on the morphodynamic evolution of a low-energy rip current system. Marine Geology, 157, 199-217.

Brander, R.W. and Short, A.D., 2000. Morphodynamics of a large-scale rip current system at Muriwai Beach, New Zealand. Marine Geology, 165, 27-39.

Brander, R.W. and Short, A.D., 2001. Flow kinematics of low-energy rip current systems. Journal of Coastal Research, 17, 468-481.

Dronen, N., Karunarathna, H., Fredsoe, J., Sumer, B.M. and Deigaard, R., 2002. An experimental study of rip channel flow. Coastal Engineering, 45, 223-238.

Haas, K.A., Svendsen, I.A., Brander, R.W. and Nielsen, P., 2002. Modeling of a rip current system on Moreton Island, Australia. Proceedings 28th International Conference on Coastal Engineering, ASCE, 784-796.

Haller, M.C., Dalrymple, R.A. and Svendsen, I.A., 2002. Experimental study of nearshore dynamics on a barred beach with rip channels. Journal of Geophysical Research, 107, 1-21.

MacMahan, J.H., Reniers, A.J.H.M., Thornton, E.B. and Stanton, T.P., 2004. Infragravity rip current pulsations. Journal of Geophysical Research, 109, doi:10.1029/2003JC002068.

MacMahan, J.H., Thornton, E.B. and Reniers, A.J.H.M., 2006. Rip current review. Coastal Engineering, 53, 191-208.

MacMahan, J.H., Brown, J.W. and Thornton, E.B., 2009. Low-coast handheld Global Positioning Systems for measuring surf zone currents. Journal of Coastal Research, doi: 10.2112/08-1000.1

MacMahan, J., Brown, J., Brown, J., Thornton, E., Reniers, A., Stanton, T., Henriquez, M., Gallagher, E., Morrison, J., Austin, M.J., Scott, T.M. and Senechal, N., 2010. Mean Lagrangian flow behavior on an open coast rip-channeled beach: A new perspective. Marine Geology, 268, 1-15.

Reniers, A., MacMahan, J., Thornton, E.B. and Stanton, T.P., 2007. Modeling of very low frequency motions during RIPEX. Journal of Geophysical Research, doi: 10.1029/2005JC003122.

Poate, T. Kingston, K.K., Masselink, G. and Russell. P.E., 2009. Response of high-energy, macrotidal beaches to
seasonal changes in wave conditions: examples from North Cornwall. Journal of Coastal Research, SI 56, 747-751.

Roelvink, D., Reniers, A.J.H.M., van Dongeren, A., van Thiel de Vries, J.S.M., Lescinski, J. and McCall, R., in press.
Modeling hurricane impacts on beaches, dunes and barrier islands. Coastal Engineering.

Scott, T.M., Russell, P.R.,Masselink, G., Wooler, A. and Short, A., 2007. Beach rescue statistics and their relation to
nearshore morphology and hazards: a case study for south-west England. Journal of Coastal Research, SI 50, 1-6.

Scott, T.M., Russell, P.E., Masselink, G. and Wooler, A., 2009. Rip current variability and hazard along a macro-tidal
coast. Journal of Coastal Research, SI 56, 895-899.

Schmidt, W.E., Woodward, B.T., Millikan, K.S., Guza, R.T., Raubenheimer, B. and Elgar, S., 2003. A GPS-tracked
surf zone drifter. Journal of Atmospheric and Oceanic Technology, 20, 1069-1075. Sonu, C.J., 1972. Field observation of nearshore circulation and meandering currents. Journal of Geophysical Research, 77, 3232–3247.

Talbot, M.M. and Bate, G.C., 1987. Rip current characteristics and their role in the exchange of water and surf diatoms
between surf zone and nearshore. Estuarine Coastal and Shelf Science, 25, 707-720.

Thornton, E.B., MacMahan, J. and Sallenger, A.H., 2007. Rip currents, megacusps, and eroding dunes. Marine Geology, 240, 151-167.

van Dongeren, A., Plant, N., Cohen, A., Roelvink, D., Haller, M. and Catalán, P., 2008. Beach Wizard: Nearshore bathymetry estimation through assimilation of model computations and remote observations. Coastal Engineering, 55, 1016-1027.

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Link to regional wave model data