DSD-INT 2017 Breaking bar migration induced by infragravity waves - Michallet
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The document discusses the effects of infragravity waves on sediment transport and bar migration in beach morphodynamics, utilizing a physical model with various wave conditions. It presents findings on the differing sediment transport mechanisms associated with onshore and offshore bar migration, influenced by long and short wave interactions. The study incorporates video analysis to examine flow dynamics and sediment concentration, providing insights into the complexities of coastal sediment behavior.
DSD-INT 2017 Breaking bar migration induced by infragravity waves - Michallet
- 1. BREAKING BAR MIGRATION INDUCED BY INFRAGRAVITY WAVES H. Michallet M.V.L. Rocha and P.A. Silva With many thanks to: E. Barthélemy, Joël Sommeria, F. Bonnel XBeachXConference – Delft - 1.11.2017
- 2. • LEGI Wave flume 36 m / 0.55 m • Controlled wave maker irregular waves • Hydrodynamics Froude scaling length scale 1:10 • Loose bed Shields / Rouse scaling • Morphodynamics (Grasso et al. 2009, 2011) • Boundary layer (Berni et al. 2013, 2017) • Liquefaction (Scholtès et al. 2015) • Morpho / long waves (Rocha 2016) lightweight sediment d50 = 0.64 mm r = 1.18 Physical model of beach morphodynamics
- 3. 2 conditions of 2 groups of bichromatic waves Tp = 1.7 s Tg = 14 s differing in LF component C1 C2 beach profiles and changes surface elevation maximum mean minimum
- 4. wave skewness asymmetry Wave height short waves LF Net sediment transport C1 : onshore bar migration C2 : offshore
- 5. same Tp = 1.7 s Tg = 14 s less LF amplitude (less grouping) C1 C3 C1 : offhore bar migration C3 : onshore HSW HLF AS
- 6. wave propagation wave gage Video evidence 200 frames/s of phase lags in boundary layer flow and sediment transport (here 1/10 frames)
- 7. Video analysis of succesive frames to get velocity fields (CIV) and sediment concentration (pixel brightness) Need to identify : air entrainment sediment concentration gradients / grey scale
- 9. Normalized pixel brightness Horizontal velocity Bed response to breaking waves Plug flow jet impact increasing erosion, suspension …
- 10. Horizontal velocity Sediment concentration Maximum erosion and transport of a dense bed layer at the end of the group
- 11. Low-pass filtered horizontal velocity Sediment concentration Maximum erosion and transport of a dense bed layer at the end of the group / LF off-shore flow
- 12. Sediment flux / surface elevation vertically integrated net transport average concentration undertow Net transport primarily in the bed layer, with LF trend
- 13. net transport over 4 runs 4 runs of same condition C1 for 1 run from video
- 14. 10 successive runs of same condition C2 bar formation, off-shore migration
- 15. • Light-weight sediment small-scale physical model + CIV-UVMAT : further insight into sediment transport mechanisms (e.g. Roelvink & Stive, 1989, Sleath, 1999) • Evidence of leading flow acceleration induced bed motion (plug flow vs. sheet flow, see Berni et al., 2017, likely to occur in the field, see large scale experiments by Anderson et al. 2017) • Moving bed layer increases along the wave group, advected at LF • For similar short waves, increase / decrease in LF energy promotes off-shore / onshore bar migration Conclusions