Presentation by Gerasimos Kolokythas (Flanders Hydraulics) at the XBeach X (10th Year Anniversary) Conference, during Delft Software Days - Edition 2017. Wednesday, 1 November 2017, Delft.

1. 1 1
MORPHODYNAMIC ANALYSIS OF
INTERVENTION SCENARIOS
AT THE BELGIAN COAST
UNDER THE MASTERPLAN ‘FLEMISH BAYS’

2. 2
Outline
• Introduction
• Model set-up & performance
• Scenarios
• Results
– Velocity field
– Sedimentation-erosion patterns
– Sediment transport
– Evolution of beach profiles
• Discussion
• Conclusions

3. 3
Introduction
 Masterplan ‘Flemish Bays’
• long term coastal protection strategy (up to 2100)
• sheltered connection between Zeebrugge harbor
and the Scheldt estuary
 Proposed mitigating scenario (E3)
• an artificial barrier island, a detached breakwater
and beach nourishment at the east of Zeebrugge

4. 4
Model set-up
• curvilinear
• 55000 computational nodes
150x363 cells
• Cell sizes:
23 x 25m (surf zone Knokke)
240 x 210m (offshore)
 Computational grid
 WTI-calibration1 settings
Chezy roughness = 50 m½/s
 D50 = 300μm
1 Van Geer, P.; den Bieman, J.; Hoonhout, B.; Boers, M. (2015). XBeach 1D - Probabilistic model:
ADIS, settings, Model uncertainty and Graphical User Interface. 65 pp.

5. 5
Condition Hs (m) Tp (s) Θwave (oN) Vwind (m/s) Θwind (oN) Weight factor
SW125 1.25 5.4 225 9.6 225 0.4254
N175 1.75 6.0 0 9.0 0 0.1934
W175 1.75 6.0 270 11.5 270 0.0683
W275 2.75 7.0 270 16.7 270 0.0220
Model set-up
 Forcing
• representative tidal cycle
• reduced wave forcing - 4 wave conditions
 Simulation period
• hydrodynamic time: 31.2 days
• morphological time: 9 years (morfac = 105.9)

6. 6
Model performance
 Validation: based on sedimentation/erosion
volumes at beach and shoreface sections
1TAW: Belgian datum level (Tweede Algemene Waterpassing) – Average low water level at Ostend (1834-1853)
Baai van
Heist
Duinbergen
Albert
Strand
Knokke-
Zoute
Lekkerbek Zwin
Above low water (above +1.39 m TAW1) – (x 1000 m3/year)
Measured trend
1997-2010
1 9 4 -40 6 8
Model -1 11.8 -0.6 -46.8 -45 30.6
Shoreface (between +1,39 and -4,11 m TAW) – (x 1000 m3/year)
Measured trend
1997-2010
52 88 17 -4 -8 13
Model -5.3 25.8 1.3 -12 -20.4 42.5
Offshore (below -4,11 m TAW) – (x 1000 m3/year)
Measured trend
1997-2010
0 0 24 -10 -4 21
Scen011 0 0.2 0.2 -18.3 26.9 6.2
Total profile (from -4,11 m TAW to the dike) – (x 1000 m3/year)
Measured trend
1997-2010
52.5 97 45 -54 -6 43
Model -6.3 37.8 0.9 -77.1 -38.6 79.3
• Bluish cells: accretion
• Redish cells: erosion

7. 7
Scenarios
Reference E3
 Scenario E3: an artificial barrier island, a detached breakwater and beach nourishment

8. 8
Results  Velocity field (flood - ebb)
flood ebb
• increased at:
o the passages island-detached
breakwater - Zebrugge
• reduced at:
o the east tip of the island and
at Zwin

9. 9
Results  Sedimentation – erosion patterns (9 years)
• Reference scenario
o Strong beach erosion
• Scenario E3
o Strong erosion at passages/west
tip. Deposition mainly in the
sheltered area.
o Eroded sand at weather side of
island moves to offshore & east
o Eroded sand at east tip moves
towards the beach.
o Erosion at the beach substantially
reduced.
E3
Reference

10. 10
Results  Sediment transport rates (9 year-averaged)
• increased at:
o the passage island-detached
breakwater
o the passage detached
breakwater- Zeebrugge
o along the weather side of the
island
• reduced longshore transport
along the protected beach
E3
Reference

11. 11
Results  Longshore sediment transport (yearly averaged-
integrated at the coastal zone)
1500 m
erosion accretion
• Reference scenario
o Strong eastward transport
- peak at west Zwin (x’≈ 20km)
• Scenario E3
o Reduced eastward transport (>50%)
- peak moved to the west (x’≈ 15km)

12. 12
Results  Evolution of beach profiles
1
2
3

13. 13
Discussion  Effect of avalanching on island erosion
• wetslp
- critical bed slope for the initiation
of avalanche
- lower values -> more erosion
• wetslp = 0.10
- enhanced sed/ero patterns
mainly at the tips of the island
wetslp=0.10
wetslp=0.26
physical meaning?
very low angle of repose
of submerged sand (~6o)

14. 14
Discussion  Effect of avalanching on island erosion
wetslp=0.10
wetslp=0.26
year 0year 3
wetslp=0.10
wetslp=0.26
year 6
wetslp=0.10
wetslp=0.26
year 9
wetslp=0.10
wetslp=0.26

15. 15
• The proposed mitigating Scenario E3 achieves:
– Protection of the coast (reduction of the longshore
transport)
but…
– Erosion at the west tip
– Weather side of the island sensitive to wave attack /
also eastward transport
– Erosion between island breakwater and Zeebrugge port
– Deposition of sediment in the sheltered area
Conclusions