Presentation by Christopher Daly (Institut Universitaire Europeén de la Mer, Brest) at the XBeach X (10th Year Anniversary) Conference, during Delft Software Days - Edition 2017. Wednesday, 1 November 2017, Delft.

1. Simulating accretion and cusp formation
at Nha Trang Beach, Vietnam
CHRISTOPHER DALY
IUEM, BREST, FRANCE
FRANCE FLOC ’H, RAFAEL ALMAR, PEDRO ALMEIDA

2. Context
• 7-day field experiment at Nha Trang, Vietnam, Nov/Dec 2015.
• Rapid berm and cusp formation observed following a small storm event.
• Can this be simulated in XBeach? (operational model)

3. Presentation Outline
• Introduction
• Field Experiment
• Modelling Approach
• 1D Simulations: Accretion
• Validation of model settings for Nha Trang
• 2D Simulations: Cusp Formation
• Scenarios varying wave climate and sediment composition
• Conclusion and Outlook

4. Introduction: Field Experiment
• ADCP at 15m depth + 4 Pressure sensors in inner surf and swash zone.
• Sediment samples to determine D50 (0.5 mm = coarse sand).
• Daily topographic surveys using RTK-GPS.

5. Introduction: Model Approach
• Swash zone processes have to be well-resolved to obtain accretion, which are
resolved differently in the surfbeat and non-hydrostatic modes of XBeach:
Relevant Physical Processes Surfbeat Mode Non-hydrostatic Mode
Wave skewness and asymmetry:
Net onshore transport over wave
period in shoaling and surf zone
Uses parameter (facua) to
modify orbital velocity
Resolved
Wave-wave interactions:
Swash-backwash interactions super-
imposed on infragravity waves –
sediment mobilization at swash base
Not resolved Resolved
Groundwater infiltration:
Allows sediment to settle at the
upper extent of the swash
Resolved using
groundwater module
Resolved using
groundwater module
Sheetflow transport conditions:
Modification of sediment transport
under high current velocities
Accounted for using
dilatancy implementation
Accounted for using
dilatancy implementation

6. 1D Accretion: Model Setup
• 6-day, non-hydrostatic, 1D simulations starting on 28 Nov. on 1 x 300m grid.
• Real-time wave timeseries with morphology enabled (morfac = 1).
• Default model settings used (Kingsday version) except for:
bdslpeffdir (talmon) ; facsl (0.15) ; gwflow (on) ; bedfriction (manning) ; dilatancy (on)
RMSE (m) including
all parameters
All params 0.114
RMSE (m) excluding
single parameters
dilatancy 0.106
bedfriction 0.149
gwflow 0.186
facsl 0.217
bdslpeffdir 0.485

7. 1D Accretion: Beach Profile Comparisons
• Maintains measured beach slopes, simulates onshore transport and forms the berms
observed above SWL, but slightly overpredicts accretion.

8. 1D Accretion: Wave Height Comparisons
• Hs trend well predicted (R = 0.94), but magnitude overpredicted (RMSE = 0.15m).
• Spectrum predicted well in gravity band, overestimated in infragravity band.

9. 2D Cusp Formation: Model Setup
• 3-day, non-hydrostatic, 2D simulations starting on 28 Nov. on 250 x 250m grid.
• Real-time & spectral wave input with morphology enabled (morfac = 1/10).
• Varied sediment size (D50) and wave conditions (Hs, Tp, σ, θ).

10. 2D Cusp Formation: Method of Analysis
• Contours of bathymetry extracted between -1.5 and 2 m elevation, with Fourier
analysis used to determine cusp length scales L (along-shore) and H (cross-shore).
• Longshore variability indicated by RMS bed level anomaly (Δ) over the entire domain
• H & L timeseries exctracted at mid-swash (tide level + 0.4m) + final value at 2.7 days.
H = 4.29 m
L = 17.9 m
Δ = 0.031 m

11. 2D Cusp Formation: Effect of Wave Height/Period
• Wave period has a strong influence on generation of the cusps compared to wave
height and steepness.
H = 0.04 m
L = 26.1 m
Δ = 0.001 m
H = 1.50 m
L = 15.3 m
Δ = 0.014 m
H = 4.51 m
L = 12.4 m
Δ = 0.039 m
H = 0.46 m
L = 14.9 m
Δ = 0.006 m
H = 5.33 m
L = 10.8 m
Δ = 0.040 m
H = 2.85 m
L = 6.38 m
Δ = 0.043 m
σ = 0°
θ = 0°
D50 = 0.5 mm

12. 2D Cusp Formation: Effect of Wave Dirctn./Sprdng.
• The cusps are mainly generated under narrow-banded, normally incident waves.
H = 3.65 m
L = 54.6 m
Δ = 0.035 m
H = 2.22 m
L = 46.0 m
Δ = 0.051 m
H = 6.84 m
L = 70.0 m
Δ = 0.096 m
H = 2.21 m
L = 24.3 m
Δ = 0.046 m
H = 2.08 m
L = 52.2 m
Δ = 0.079 m
H = 4.85 m
L = 32.3 m
Δ = 0.105 m
Hs = 1 m
Tp = 11 s
D50 = 0.5 mm

13. 2D Cusp Formation: Effect of Sediment Compositn.
H = 3.03 m
L = 18.6 m
Δ = 0.029 m
H = 1.40 m
L = 18.4 m
Δ = 0.015 m
H = 0.14 m
L = 20.8 m
Δ = 0.004 m
• Cusps form more easily on beaches with coarse sediment.
• For mixed bed (D50 = 0.5 & 0.2 mm) swash zone is armoured with coarse fraction.
• At end of simulation, finer sediment located on horns, but varies over tidal cycle.
H = 0.48 m
L = 17.0 m
Δ = 0.009 m
Hs = 1 m
Tp = 11 s
σ = 0°
θ = 0°

14. Conclusion
• Swash processes are well represented using the non-hydrostatic model and, coupled
with morphology, it is able to simulate berm and cusp formation.
• Cusps form under narrow-banded, near-normal, long-period wave conditions, and
also under both accretive and erosive conditions.
• Cuspate features have realistic spatial dimensions which change in line with different
wave forcing conditions and sediment characteristics.

15. Outlook
• Look more closely at wave height and bed level anomalies at the initiation of the
cuspate features and after their development.
• Modify sediment transport formulations to be more compatible with non-
hydrostatic mode (e.g. include acceleration skewness)?