Presentation by Dano Roelvink, IHE Delft/Deltares, at the XBeach User Day 2018, during Delft Software Days - Edition 2018. Thursday, 15 November 2018, Delft.

1. 1
Dano Roelvink
IHE Delft & DELTARES, Netherlands
Adjusting bed slope effects
to improve
beach/dune behaviour
Dano Roelvink
IHE Delft Institute for Water Education & Deltares

2. 2
Contents
• Improving beach and berm behaviour
• Example applications
• Praia de Faro
• La Faute sur Mer
• Houtribdijk
• Coupling with aeolian model Duna
• Long-term application
• Conclusions

3. 3
XBeach modes
• Stationary
• Surfbeat
• Non-
hydrostatic

4. 4
XBeach
• Originally developed for modelling storm impacts on sandy
coasts
• Waves, currents, sediment transport and morphology change
in 1D or 2DH
• Now three modes of application:
• stationary wave model, fast for longer-term evolution;
includes term for longshore transport gradient
• surfbeat mode, resolving variation on scale of wave groups,
infragravity waves
• nonhydrostatic mode, short-wave resolving; also usable for
tsunami propagation

5. 5
Typical behaviour of Atlantic beaches in S &
SW Portugal
Successive observed
profiles at Praia de
Faro, Portugal, over a
two-year period,
collected as part of
the CROP Project
(Almeida et al., 2010)

6. 6
Typical behaviour of XBeach...
Simulated profile evolution
over 4 tidal periods.
sensitivity to Hm0 wave
height, for period Tp= 10 s..
sensitivity to wave period Tp,
for wave height Hm0 = 3 m.

7. 7
Possible solutions
• Detailed, intra-wave approach
• Resolve complex swash processes
• Many processes uncertain and computation very time-
consuming
• Still with many coefficients
• Heuristic approach
• Estimate equilibrium slope of swash zone from
observations: typically steeper for coarser grain size
• Introduce an up-slope term in the swash zone that nudges
the profile toward the desired bermslope

8. 8
Bermslope effect
• Only where H/h>1
b b
BS
bermslope
z z
q q
x x
a
¶ ¶æ ö
= -ç ÷¶ ¶è ø
Simulated ratio of wave
height/water depth ratio H/h
through four tidal cycles; the
darkest red indicates where
H/h>1, the área where the
bermslope effect is applied.

9. 9
No bermslope

10. 10
Bermslope = 0.1

11. 11
Bermslope = 0.2

12. 12
Effect of a longshore gradient
Accreting profile
Eroding profile

13. 13
• A simple approach is helpful in resolving the treatment of steep
beach face
• It can be tuned to existing situations or based on knowledge on
grain size dependence
• The method does not fixate the coastline as in earlier
approaches, but leaves the profile free to accrete or erode
• This makes it a useful model to couple with a dune growth
model
Conclusions

14. 14
New validation against large-scale data Oregon
Large Wave Tank
• Excellent agreement using
new ‘WTI’ settings
• But: slope of swash zone
much steeper in reality
1:7
1:15

15. 15
La Faute sur Mer – large-scale
C.H. Lashley, X. Bertin, D. Roelvink, Coastlab2018

16. 16
La Faute sur Mer – small-scale
C.H. Lashley, X. Bertin, D. Roelvink, Coastlab2018
AfterBefore

17. 17
Houtribdijk, NL
Robert McCall, Marlies van der Lugt

18. 18
New XBeach parameters
Keyword Meaning
bermslopetransport Turn on or off bermslope swash transport model
bermslopebed Turn on or off bermslope swash transport model for bed
load
bermslopesus Turn on or off bermslope swash transport model for
suspended load
bermslope Swash zone slope for (semi-) reflective beaches
bermslopefac Bed slope transport factor for bermslope model
bermslopegamma Wave height - water depth ratio to turn on bermslope
model in surf-beat
bermslopedepth Water depth to turn on on bermslope model in
stationary and nonh

19. 19
Coupling beach and dune
Profile
storm waves
moderate
waves
wind
tsunami H, # waves
vwind, dirwind ,
duration
Hm0, Tp, dirw,
zs, duration
Hm0, Tp, dirw,
zs, duration

20. 20
Coupled model approach
Processes
• Storm erosion and
moderate conditions
• Aeolian transport
and vegetation
growth
Models
• 1D XBeach instationary
or stationary
• 1D Duna model inspired
by Kroy, Herrmann,
Durán a.o.

21. 21
Duna model
Roelvink & Costas (subm.)
• Wind model Kroy et al.
(2002)
• Advection equation sand
with pickup according to
Bowen – implicit scheme
• Adapted critical shear stress
for moisture and slope
• Vegetation growth and
reduced shear stress
because of it
• Armouring
• Morphological updating
• Matlab implementation
Wind
model
With
vegetation
Without
vegetation

22. 22
• 2 yr simulation
• Wind
• Storms
• Moderate
• Random
timeseries per
season
• Realistic
seasonal
variation
• Large
fluctuations, little
net change
Application: Praia de Faro, 2009-2011
Roelvink and Costas (subm)

23. 23
• In 1D simulations, longshore transport gradient can be
included
• Assumption: transport gradient proportional to transport
• Coefficient lsgrad (~ 1 / length scale of transport change)
• Allows profiles to accrete or erode
Longshore transport gradient
( )1 0yb x
y
y
Sz S
p
t x y
S
lsgrad S
y
¶¶ ¶
- + + =
¶ ¶ ¶
¶
=
¶

24. 24
10-year simulation with repeated 2-year cycle
Dune height vs
progradation rate

25. 25
40-year simulation with strong accretion due to
longshore transport gradient

26. 26
• Simple heuristic adjustment in a few cells in swash zone
• Drastically improves behaviour of intertidal beach
• Allows for berm building and dune erosion
• Opens the way for long-term simulations with XBeach only
• Longshore transport gradient allows for structural erosion or
accretion
Conclusions

27. 27
• Re-calibrate testbed including bermslope effect
• Find dependence on grain size (and other factors?)
• Next challenge: reproduce longshore bars!
Next steps

28. 28
Long-term modelling strategies
• Hybrid model approach, e.g.
• Delft3D FM <-> ShorelineS <-> XBeach-Duna