Rip Current Mechanisms and Prediction

1. Rip Current Mechanisms and Prediction Andrew Kennedy Department of Civil Engineering and Geological Sciences University of Notre Dame With help from Yang Zhang, Enrique Gutierrez, Kevin Haas, Brian Sapp, Maurizio Brocchini, Luciano Soldini

2. 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

3. 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 Rip head Return flow Rip neck Feeder Currents Narrow Laboratory Rip Current

4. Laboratory `Open Beach’ Rip Current Offshore Wave Generator Shoreline

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

6. Laboratory `Jetty’ Rip Current Offshore Wave Generator Wall Shoreline

7. 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

8. 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?

9. What Makes a Well-Developed Open Beach Rip Current Bathymetry? • Basic Theory of Beach State Changes • A storm or large waves creates a linear sandbar in moderately deep water • 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 • Growth will continue (and presumably rips can become stronger) until interrupted • Rip Bathymetry can be destroyed by strong alongshore currents (from waves at a large angle) • Rip Bathymetry can be destroyed by large waves moving the bar offshore (point 1), or by strong longshore currents

10. Dune Acres, Indiana, October 11, 2007 This topography could be very dangerous, given the right waves Even Deeper bar Deeper bar with little or no wave breaking Currents Shallow bar with strong wave breaking

11. 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

12. 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

13. Modeling Rip Currents • 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

14. 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

15. 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

16. Great Lakes Rip Forecasting? • Simplifications: • All waves are locally generated - no sea vs swell wave issues • No tides – water levels are largely irrelevant • Important Factors for Open Beaches • Wave heights (use wind strength as a proxy) • Wave direction (use wind direction as a proxy) • Wind duration (possibly less directly important) • Pre-existing morphology (hugely important for open beaches) • Bar Depth (controls strength of breaking) • Not really possible to compute detailed currents as bathymetry is lacking • Important Factors for Jetty Rips • Wave heights (use wind strength as a proxy) • Wave direction (use wind direction as a proxy) • May be possible to precompute detailed currents for given conditions, look up most similar cases • High Surf also important, but different from Rip Currents

17. 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

18. Questions?