7 juli 2015

Modeling wave attenuation by

vegetation with XBeach

Arnold van Rooijen, Jaap van Thiel de Vries, Ap van

Dongeren, Robert McCall, Dano Roelvink, Ad Reniers

7 juli 2015

Content

• Introduction

• Model implementation

• Results

• Wave setup effect

• Discussion

• Conclusions and future perspectives

Introduction

7 juli 2015

• XBeach: eXtreme BEACH behavior model

• Storm impact model

• Short waves (energy balance)

• Infragravity waves (NLSWE)

• Tide / surge / flow (NLSWE)

• Sediment transport / morphology

• Dune erosion

• Breaching

• Overwash

www.xbeach.org

Roelvink et al. (2009)

Introduction

Why include vegetation effects in XBeach?

Vegetation can have a great effect on the local waves and currents

Waves can be attenuated significantly before arriving at the shore

It can therefore have a great effect on coastal safety

Effect on coastal safety

By reducing the local current and wave action, vegetation can provide a

sheltered area for sediment

Areas with relatively dense vegetation have great potential for increased

sedimentation and/or reduced coastal erosion

Effect on morphology

7 juli 2015

Implementation

Physical aspects

• Effect of vegetation on incident (short) waves

• Effect of vegetation on infragravity waves

• Effect of vegetation on wave setup

Schematization

• Vertically non-uniform vegetation (e.g. mangroves)

• Horizontally non-uniform vegetation (different types per location)

7 juli 2015

Implementation

• Infragravity waves and flow (Dalrymple et al. 1984):

• Incident (short) waves (Mendez & Losada, 2004; Suzuki et al., 2011):

7 juli 2015

3 33

3

, with

1 sinh 3sinh

2 3 cosh2

g

v

v D rms

Ec

x

gk ks ksC A H

k kh

1

2D DF C Au u

Results for Kansy (1999) experiments

7 juli 2015

• 1:10 scale experiments (Kansy, 1999)

• SINTEF wave flume (L = 40 m)

• In total 20 cases: 4 wave heights x 5

wave period

• Flexible submerged model vegetation

(kelp)

Vegetation characteristics:

Nv = 1200 m-2

bv = 2.5 cm

hv = 9 cm

CD = 0.4

Results for Kansy (1999) experiments

7 juli 2015

• Overall good agreement with measurements with limited

calibration (αbreak, CD)

• Shoaling of small-amplitude waves underestimated

• Dissipation due to wave breaking underestimated for

longer period waves

Wave setup effect

• Mendez & Losada (2004) assume linear wave theory

• sinusoidal waves

• dissipation due to work done, but net drag force = 0

• Wave skewness effect on mean water level (Dean & Bender, 2006)

• skewed velocity signal

• net force acts on vegetation

• As result vegetation force acts on

water column in offshore direction

• Lowering of mean water level

7 juli 2015

Wave setup effect

Implementation in XBeach:

• Wave shape model similar to Van Thiel de Vries (2009)

• Rienecker & Fenton (1981) method

• Weight function (w) computed as function of the wave phase (φ):

• Phase is estimated using empirical formulation by Ruessink et al. (2012):

7 juli 2015

8

1

cos 1 sini

w i i

i

u wA i t w A i t

1w

0.672

0.8151 tanh

2r

U

3

3

8r

HkU

kh

Wave setup effect

7 juli 2015

• 7 cases with irregular waves on a sloping bed (variation in wave

height vs. wave period)

• All cases run with and without vegetation

• Focus on wave propagation and wave setup Wu et al. (2011)

Wave setup effect

7 juli 2015

Discussion

• Using the wave shape model the mean water level is better estimated but still

overestimated

• Effect of wave setup has been studied:

• Analytically (Dean & Bender, 2006)

• Using lab experiments (Wu et al., 2011)

• Using a phase resolving model (e.g. Ma et al., 2013)

• Using a phase averaged model (Guannel et al., 2015)

• Several processes that may have an effect are not included here:

• Diversion of undertow

• Wave-induced in-canopy flow (e.g. Luhar et al., 2010)

• Drag coefficient

7 juli 2015

Conclusions & future perspectives

Wave attenuation by vegetation

• Wave attenuation by vegetation added to the XBeach source code:

• Short wave dissipation

• Infragravity wave (and flow) dissipation

• Wave setup effect

• Vertical schematization of plants and horizontal distribution of species

• Comparison with lab data (Kansy, 1999; Wu et al., 2011) shows good

agreement in predicted wave height using limited calibration

Wave setup effect

• Vegetation does not only dissipate waves, but also reduces wave setup!

• Improvement in mean water level calculation using wave shape model

7 juli 2015

Conclusions & future perspectives

XB-Veg: Storm impact model for vegetated coasts

• The current model is tested for (short) wave attenuation and the effect of

wave skewness on the mean water level

• Other effects are included, but need validation:

• Attenuation of infragravity waves

• Effects on morphology

• Vertically non-uniform vegetation

• Effect of wave-induced mean currents

• Drag coefficient

7 juli 2015

Thank you!

7 juli 2015

7 juli 2015

Introduction