NEW DUNE EROSION MODEL FOR THE NETHERLANDS

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NEW DUNE EROSION MODEL FOR THE NETHERLANDS

• M.R.A. van Gent
• M. Boers
• J.H. de Vroeg
• J. van de Graaff
• H. Steetzel
• F. Diermanse
• D.J. Walstra
• Client

WL Delft Hydraulics WL Delft Hydraulics WL
Delft Hydraulics Delft University of
Technology Alkyon WL Delft Hydraulics WL
Delft Hydraulics Rijkswaterstaat
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Presentation overview

• Introduction
• Large scale dune erosion tests
• A new dune erosion equilibrium profile
• A new dune safety assessment tool
• Conclusions

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Part 1 Introduction

• Dutch policy on flood prevention and the
  assessment of dune safety

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Dutch Policy on Flood Prevention

• Every polder has a normative probability for
  failure (Holland P lt 10-4 per year)
• Every five years there is a general assessment of
  the flood defence system
• The Dutch government provides Hydraulic Boundary
  Conditions and Assessment Guidelines

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Normative Hydraulic Boundary Conditions
(HBC)Comparison between 2006 and 2001
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Assessment of the Dunes

• Assessment Guidelines developed in 1984
• However wave period Tp is fixed on 12 s
• Items of work
• What is the effect of a larger Tp on the dune
  safety?
• How can we implement this effect in guidelines?

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Part 2 Large scale dune erosion experiments

• Studying the effect of the wave period on
• dune erosion in the Delta flume of Delft
  Hydraulics

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Model set-up
depth scale factor nd 6 steepness factor
S0 2 grain size D50 200 µm
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Profile measurements
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The effect of the wave period on the dune
erosion profile

• An increasing wave period gives
• more dune retreat
• a milder slope
• equal vertical position of the dune foot

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The effect of the wave period on the dune
erosion volume

• For normative conditions, an increase of the wave
  period Tp from 12 s to 18 s (prototype) gives an
  increase of the erosion volume of 25 (after 2.5
  hour in prototype).

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Part 3 A new dune erosion equilibrium profile

• Extending the existing profile equation with the
  effect of the wave period

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Comparison of the erosion volume above MWL
T 18 s
T 15 s
T 12 s

• The comparison is reasonable for a large domain
  (including earlier small and large-scale tests)

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Part 4 A new dune safety assessment tool

• For the safety assessment to the normative
  probability, the following uncertainties are
  encountered
• water level
• wave height
• wave period
• sediment grain size
• storm duration
• accuracy of the deterministic dune erosion model
• These uncertainties are included in a
  probabilistic model

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Guideline model

• To provide the user a simplified model, a
  guideline model has been developped that
  approximates the probabilistic model

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Part 5 Conclusions

• The normative Hydraulic Boundary Conditions for
  the Netherlands have increased significantly
• Dune erosion experiments in the Delta flume have
  shown that an increase of the wave period leads
  to an increase of the amount of dune erosion
• The new profile equation for dune erosion gives
  an acceptable prediction of the dune retreat and
  dune erosion volumes. The model does not
  accurately predict the profiles below water.
• A new guideline model has been develloped that
  accurately approximates the probabilistic
  computations of the dune erosion volumes.

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