IAHR 2015 - Modeling of wave attenuation by vegetation with XBeach, Van Rooijen, Deltares, 30062015
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Transcript of IAHR 2015 - Modeling of wave attenuation by vegetation with XBeach, Van Rooijen, Deltares, 30062015
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
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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