PREDICTION OF DUNE EROSION

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PREDICTION OF DUNE EROSION

• L. C. van Rijn
• Deltares/Delft Hydraulics

NCK March 2008
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CONTENTS(15 sheets)

• 1) Analysis of scaling laws and scale tests
• 2) Mathematical modelling of dune erosion
• 3) General dune erosion prediction rule
• Research done within EU-projects
• SANDS
  (Improvement of laboratory facilities)
• CONSCIENCE
  (Management of coastal erosion)

NCK March 2008
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ANALYSIS OF SCALING LAWS/TESTS

• Scale modelling has been done by Vellinga (1986)
• 
  by Delft Hydraulics (2004, 2006)
• using a typical coastal profile (REFERENCE CASE)
• - super storm conditions (prob 1 to 10000)
• - storm surge level of 5 m NAP
• - offshore significant wave height Hs,o7.6 m and
  Tp12 s
• - sediment diameter of 0.225 m

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• Analysis of scaling laws and scale tests

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ANALYSIS OF SCALING LAWS AND TESTS

• Overall result of Vellinga (1986) based on all
  scale tests
• - Dune erosion (above storm surge level) 300
  m3/m after 5 hours of storm
• (coastal recession of approx 30 m for a dune
  height of 10 m above SSL)
• Re-analysis focussing on
• 1) tests in Schelde flume and Deltaflume (larger
  depth scales)
• 
  model (used) distortion
• 2) tests with Ratio of R _______________________
  _______ ? 1
• 
  scaling law distortion

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ANALYSIS OF SCALING LAWS AND TESTS
Rel. Large flumes dune erosion for reference
case250 m3/m (15 smaller)

• Overall result of Vellinga (1986) based on all
  scale tests

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MATHEMATICAL MODELLING OF DUNE EROSION

• Process-based Process model CROSMOR
• - profile model starting at deep water
• - wave by wave approach (probabilistic model)
• - shoaling, refraction, bottom friction and wave
  breaking (rollers)
• - velocity asymmetry based on Isobe and Horikawa
  (UongtUoff)
• - bed load and suspended load for each single
  wave
• New elements
• - inclusion of long wave velocity related to
  short waves
• - inclusion of increased turbulence due to wave
  collision (enhancement factor in swash zone)
• - sliding of dune front and formation of small
  bar at toe of dune,if local slopegt input value
  (50o)

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MATHEMATICAL MODELLING OF DUNE EROSION
Most important is modelling of erosion at dune
front

• Model calibration based on 1 test from
  Delta-flume 2006

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• Model calibration based on 1 test of
• REFERENCE CASE in Delta-flume 2006
• Hs,o1.5 m Tp4.9 s ho4.5 m d500.2 mm

Surge level
measured after 6 hours
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• Effect of wave period
• REFERENCE CASE in Delta-flume 2006
• Hs,o1.5 m Tp4.9 to 7.4 s s ho4.5 m d500.2
  mm

T7.4 s
Surge level
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• Effect of wave spectrum
• REFERENCE CASE in Delta-flume 2006
• Hs,o1.5 m ho4.5 m d500.2 mm

T7.4 s
Surge level
Single peak
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• To verify the model FIELD DATA 1953 storm
• - surge level 1.5 to 3.9 m above MSL (over 30
  hours)
• - Hs,o4.9 to 6.3 m and Tp,o8.8 to 10 s
• - Mean erosion 90 m3/m along Delfland section
  (recession 10 m)

measured
T7.4 s
Surge level
Model overpredicts
Single peak
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• Dune erosion prediction for REFERENCE CASE
• (on full prototype scale)
• - surge level 5 m above MSL (over 5 hours)
• - Hs,o7.6 m and Tp,o12 s

Crosmor
T7.4 s
Surge level
Vellinga (present policy)
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• Summarizing Dune erosion for REFERENCE CASE
• Vellinga-Rijkswaterstaat rule 300 m3/m after 5
  hours
• Scale tests S- and D-flume 250 m3/m after 5
  hours
• CROSMOR 170 m3/m
  after 5 hours (40 smaller)
• 
  (overprediction for 1953 storm)
• DUROSTA (Steetzel 1993) 160 m3/m after 5
  hours
• IF math models are correct
• weak spots along coast may be much stronger
  than we think???????

Crosmor
T7.4 s
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Dune Erosion Rule
Crosmor
T7.4 s
Surge level
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Dune Erosion RuleDune erosion as
function of storm surge level and d50
d500.15 mm
Crosmor
T7.4 s
Surge level
d501 mm