9 juli 2015

Physical and numerical modelling of different nourishment designs

Alessio Giardino, Yorick Broekema, Jebbe van der Werf, Arnold van Rooijen, Michalis Vousdoukas

9 juli 2015

Why studying nourishments?

Coastal erosion is becoming a common threat to most of the countries (e.g. anthropogenic action, sea level rise)

Advances in dredging techniques Sand deposits easier accessible More attention towards flexible and environmental-

friendly type of solutions to coastal erosion problems

Nourishment designs (1)

Beach nourishments - Compensate local erosion - Mainly used in areas with narrow and low dunes - Lifetime: 1-5 years (relatively low) - Expensive solution (require pumping line to the beach)

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Nourishment designs (2)

Shoreface nourishments - Compensate erosion at larger scale - Mainly used in regions with relatively wide and high dunes - Lifetime: up to ≈ 10 years - Cheap solution - Large nourishment volumes required (efficiency ≈ 0.3 - 0.5)

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Objective of this research

Nourishments designs around the world often carried out based on experience and practical guidelines.

Objectives: - Better understanding of the physical processes (i.e.

hydrodynamics, waves and sediment transport) related to different nourishment designs, based on physical and numerical modelling.

- Results to be used in practical recommendations for the design of

nourishments.

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Physical modelling (1)

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Beach nourishment: Large Wave Channel Hannover (GWK)

L = 307 m, W = 5 m, d = 7 m D50 = 300 µm Vn = 1.3 m3/m

Tests: 2 h accretive wave conditions Hs = 0.51 m; Tp = 7 s Ω = Hs / (Tp x wss) = 3.6 3 h erosive wave conditions Hs = 0.9 m; Tp = 5.17 s Ω = 8.7

Shoreface nourishment (2 designs): Deltares Scheldt flume L = 56 m, W = 1 m, d = 1.2 m D50 = 134 µm Vn = 0.37 m3/m

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Physical modelling (2) (Walstra et al. 2011)

Tests: 24 h accretive wave conditions Hs = 0.10 m; Tp = 3 s Ω = Hs / (Tp x wss) = 2.3 16 h erosive wave conditions Hs = 0.17 m; Tp = 2.3 s Ω = 5.1

Numerical modelling (1)

Delft3D model (Lesser et al., 2004; Reniers et al., 2004)

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(1) Propagation of short wave energy(2) Wave energy dissipation due to wave breaking(3) Roller energy propagation(4) Roller energy dissipation

Shallow water equations(1) Continuity equation(2) Momentum equations

Waves (Instationary Roller model) HydrodynamicsWave, roller

forces

Water levels

Sediment transport computed with TRANSPOR2004 formula (Van Rijn 2007a,b,c) (bedload + suspended load)

Sediment transport

Morphological bed update

Exner equation

θ θ∂ ∂∂+ + = −

∂ ∂ ∂

cos( ) sin( )w g w gww

E c E cE Dt x y

α

γ

= − −

2

22 1 exp

dn

ww p w

ref

ED f EE

θ θ∂ ∂ ∂+ + = − +

∂ ∂ ∂2 cos( ) 2 sin( )r r r

r wE E c E c D Dt x y

β=

2 sin( ) rr

g EDc

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Beach nourishment

Beach nourishment: hydrodynamics

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Accretive conditions: Hs = 0.51 m and Tp = 7 s Erosive conditions: Hs = 0.9 m and Tp = 5.17 s

Beach nourishment: concentrations

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Accretive conditions: Hs = 0.51 m and Tp = 7 s Erosive conditions: Hs = 0.9 m and Tp = 5.17 s

Sediment sorting on a cross-shore profile (reference profile)

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Broekema et al. (in preparation) - 8 sediment fractions - 12 layers in the bed

Beach nourishment: effect of grain sorting

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Shoreface nourishment

Bathymetry evolution measurements (Walstra et al. 2011)

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Low nourishment design

High nourishment design

Beach erosion measurements

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Shoreface nourishments: waves

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Ref

eren

ce

Low

nou

r. H

igh

nour

.

Erosive tests: Hs = 0.17 m and Tp = 2.3 s

Conclusions

Measurements and model results for different nourishment scenarios have been compared. To conclude: - Higher velocities and concentration can be seen in proximity of the beach

nourishment - The model is capable of reproducing the sorting processes along the

profile (Broekema et al., in preparation) - Using larger grain size for the beach nourishment contribute to the

formation of a armour layer which can lead to a reduction in erosion - Different shoreface nourishment designs respond differently to different

wave conditions (i.e. erosive vs. accretive) - Shoreface nourishments higher in the profile are more effective in

reducing beach erosion due to larger reduction in wave height - More work is being carried out to: improve sediment transport formulations

for small-scale laboratory tests, assess alongshore vs. cross-shore effects, validation with field data

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9 juli 2015

Alessio.giardino@deltares.nl