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Rip Current Mechanisms and Prediction

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Rip Current Mechanisms and Prediction Andrew Kennedy Department of Civil Engineering and Geological Sciences University of Notre Dame With help from Yang Zhang ... – PowerPoint PPT presentation

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Title: 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 days forecast
  • Could set up web-based form that can be filled
    out from cellphone

18
Questions?
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