Rip currents or undertow in the surf zone?

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Rip currents or undertow in the surf zone?
Annual CoastView-meeting, Bologna, March 2004

• Niels Vinther and Troels Aagaard
• Institute of Geography, University of Copenhagen,
  Denmark

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Rip currents or undertow
Longshore bar
Rip head
Longshore bar
Feeder channels
Rip head
From the web.
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This project

• Based on data from four existing field
  experiments Skallingen 1995, 2000, 2002 and
  Egmond 2002
• Field experiments include measurements of current
  velocities/directions, sediment concentrations,
  wave heights/periods and morphological surveys.
• The final aim of this study is to set up a
  general model based on hydrodynamic and
  morphological parameters to distinct between 2D
  and 3D circulation patterns in the surf zone.

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Skallingen
TR1.5 m (1.3 and 1.7) Hmean0.5 m D50160-200 ß
0.007 (nearshore zone), 0.02-0.03 (intertidal
zone).
After Aagaard et al., 1995
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Egmond
TR1.65 (1.4 and 2.1) Hmean 1.3 m D50 250-300
µm ß 0.02-0.03 (nearshore zone) and 0.1-0.02
(intertidal zone)
After Kroon et al., 2003
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Bore-model to predict mass transport across
longshore bar

• C(gh)0.5
• B00.1
• A0.9H2
• Hrmswave height
• hdepth
• Twaveperiod

Svendsen, 1984
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Skallingen 1995
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Skallingen 2002
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Egmond 2002
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Mass transport versus trough area/dist to rip
Skallingen 1995
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Mass transport versus trough area/dist to rip
Skallingen 2002
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Mass transport versus trough area/dist to rip
Egmond 2002
?
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Mass transport versus trough area/dist to rip for
all data sets
Mistake? Or is it the damning point of using the
model globally?
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Zoom in on mass transport versus trough area/dist
to rip
Slope of separation line 1 (m/s)
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Some questions

• Is it a globally applicable model?
• How can we improve the model?
• How can we get the required data
• Can the model be integrated in the Argus
  routines?

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Some answers

• We need more data - preferably from other field
  sites.
• More 3D data (cell circulation).
• From Argus-images we can get trough area, level
  of bar crest and distances to rips
• From off-shore buoys we can get wave
  heights/period and water levels

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Conclusions

• Surf zone circulation is depended not only on
  hydrodynamics but also on surf zone bar
  morphology
• When currents in the trough exceed 1 m/s,
  undertow is initiated and vice versa
• Deviations at Egmond, may turn down the model as
  a global model
• The model may be integrated in the Argus-routines

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Thank you!
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Extra slides
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Skallingen field site
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Egmond field site
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Instrumentation
Current velocity/direction Wave heights Wave
periods Morphology
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Calculation of trough areas
SK1995
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Skallingen 2000
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Mass transport versus trough area/dist to rip
Skallingen 2000
No positive data