Rip Current Mechanisms and Prediction

Andrew KennedyDepartment of Civil Engineering and

Geological SciencesUniversity of Notre Dame

With help from Yang Zhang, Enrique Gutierrez, Kevin Haas, Brian Sapp, Maurizio Brocchini,

Luciano Soldini

Rip Currents• Narrow, offshore-directed currents beginning

inside the surf zone and extending further offshore

• All rip currents are directly driven by breaking waves– Indirectly driven by wind (generating waves), so wind

speeds can sometimes be a proxy for wave heights• To generate rip currents, must have changes in

breaking strength along the shoreline– Structures (pier, jetty, breakwater)– Sandbars/troughs– Natural unsteadiness of wave breaking

Rip Current Behavior•Rip Currents often have features in common, although details can be very different

•Feeder Currents

•Rip Neck

•Rip Head

•Return Flow

•Rip Currents caused by wave breaking more strongly in some places than others Narrow Laboratory Rip Current

Feeder Currents

Rip neck

Rip head

Return flow

Offshore Wave Generator

Shoreline

Laboratory `Open Beach’ Rip Current

Mean Velocities

•Cellular circulation -Offshore in Rip Neck, Onshore over bar•Often Closed Cells•Sometimes water is ejected offshore, particularly after large wave groups

Laboratory `Jetty’ Rip Current

Offshore Wave Generator

Shoreline

Wall

Mean Velocities – Jetty Rip

•Currents sweep along the shoreline, then turn offshore as they reach the wall (jetty)•May or may not form a very large circulation cell

What Makes a Dangerous Rip Current?Open Beach•Well-developed three dimensional sandbar bathymetry

•Strong wave breaking on bar, weak or no breaking in deeper rip channel

•Waves large enough to force significant currents in waist-to-chest deep water

•1 ft breakers unlikely to cause problems•2 ft breakers may cause problems•3-4 ft breakers have caused many problems•Large waves (6ft) may force large currents but generally keep weak swimmers out of the water

•Waves with near-shore normal incidence•As wave angle increases, you get longshore currents – washes out rips

•Longer wave periods often considered more dangerous

•Rip Current Pulsations•Not such a problem on Great Lakes?

What Makes a Well-Developed Open Beach Rip Current Bathymetry?

Basic Theory of Beach State Changes1.A storm or large waves creates a linear sandbar in moderately deep water2.In the days to weeks after the storm, smaller shore-normal waves push the bar onshore and create a three-dimensional bar-trough bathymetry• Self-reinforcing behavior

3.Growth will continue (and presumably rips can become stronger) until interrupted 4.Rip Bathymetry can be destroyed by strong alongshore currents (from waves at a large angle) 5.Rip Bathymetry can be destroyed by large waves moving the bar offshore (point 1), or by strong longshore currents

Dune Acres, Indiana, October 11, 2007This topography could be very dangerous, given the right waves

Shallow bar with strong wave breaking

Deeper bar with little or no wave breaking

Even Deeper bar

Currents

First Person Accounts• “The waist-high water quickly

became shoulder height as the bigger swell moved through and the more powerful undertow swept them off their feet. And they were gone. All in 90 seconds.”

• “Two large waves hit suddenly and he was pulled under. He tried repeatedly to reach the surface, and when he finally did he had been swept out about 60 metres. The people on the beach had become mere specks and he panicked.”

• LESSON: Rips are very unsteady, and safety can change very quickly

Rip Currents in San DiegoCourtesy Rob Holman, OSU

What Makes a Dangerous Rip Current?Jetty or Pier Rips• Because these are permanent, bathymetry does

not change– Wave heights, directions control rip strength– These should always be clearly signposted

Rips From 3D Wave Directionality• May be more associated with swell waves• Do not have a preferred location• Highly transient• Not so well understood

•Can model currents by discretizing basic Navier-Stokes equations or approximate Navier-Stokes equations

•Require extremely detailed input

•Quite slow – can not run hundreds of times to examine sensitivity of results

•Good accuracy when given accurate inputs for waves, water levels and bathymetry

Problems for Rip Forecasting

•Requires a detailed bathymetry to compute

•These can change on a daily basis and are not available for forecast purposes

•Because of this, models are not used directly for forecasting

Modeling Rip Currents

Existing Forecast Methods•Existing methods rely on indexes (Lushine, Lascody, Schraeder) that use meteorological parameters to correlate with rip current data

•Advantages – straightforward, make use of available data

•Disadvantages – do not start from engineering principles, may be site specific, may be sensitive to uncertainties in predicted values

•Other possibilities – scaling parameters from engineering principles

•Knowledge of local conditions is a plus

•ECFL LURCS index from NWS Melbourne

•Can be used operationally by people who know nothing about rip currents

•Swell Height, Swell Period, Tidal Factor, Swell Persistence

•Rip Current risk: Low, Moderate, High

Great Lakes Rip Forecasting?Simplifications:

1. All waves are locally generated - no sea vs swell wave issues2. No tides – water levels are largely irrelevant

Important Factors for Open Beaches1. Wave heights (use wind strength as a proxy)2. Wave direction (use wind direction as a proxy)3. Wind duration (possibly less directly important)4. Pre-existing morphology (hugely important for open beaches)5. Bar Depth (controls strength of breaking)6. Not really possible to compute detailed currents as bathymetry is lacking

Important Factors for Jetty Rips1. Wave heights (use wind strength as a proxy)2. Wave direction (use wind direction as a proxy)3. May be possible to precompute detailed currents for given conditions,

look up most similar cases

High Surf also important, but different from Rip Currents

Involvement of Lifeguards•Lifeguards know local conditions better than anyone

•Good idea of existing conditions – developed morphology, etc•Can immediately judge conditions better than outsiders•Institutional memory of conditions during past dangerous events

•Lifeguards are on-the-ground observers that can validate/improve forecast techniques

•Compare lifeguard estimates of danger to predicted danger•Use lifeguards to evaluate rip current development for next day’s forecast

•Could set up web-based form that can be filled out from cellphone

Questions?