© L.M. Keyzer et al, . All rights reserved
The influence of wind on the evolution of freshwater fronts in the Rhine ROFI
Tidal plume front
L.M. Keyzer, S. Rijnsburger, J.D. Pietrzak, F. Zijl, M. Verlaan, M. Snellen, C.A. Katsman, D.C. Slobbe, R.P. Flores, A.J. Souza, A.R. Horner-Devine The Rhine ROFI: a tidal river plume along the Dutch coast
© L.M. Keyzer et al, . All rights reserved 2 The Rhine ROFI: the near-field region
In the near-field region, the dynamics are dominated by tidal plume fronts.
Every tidal cycle, a new tidal plume front is formed.
The tidal plume fronts from previous tidal cyles can still be present while a new one is formed.
© L.M. Keyzer et al, . All rights reserved 3 The Rhine ROFI: the near-field region
Tidal plume fronts are advected by tidal straining
(De Boer, 2006)
© L.M. Keyzer et al, . All rights reserved 4 Why we need to understand impact of fronts
Zandmotor (a mega nourisment)
Port of Rotterdam
© L.M. Keyzer et al, . All rights reserved 5 Fronts can impact sediment transport
Shelf processes Near shore processes front
(Horner-Devine et ureturn al., 2017)
1. Front propagates onshore 2. Return flow moves suspended sediment offshore
© L.M. Keyzer et al, . All rights reserved 6 STRAINS measurement campaigns
PI’s: J.D. Pietrzak, 10 km A.R. Horner-Devine, A.J. Souza
Mooring Mooring incl. 4 CTDs incl. 4 CTDs ADCP ADCP
© L.M. Keyzer et al, . All rights reserved 7 Role of the wind
STRAINS campaign: strongest fronts are found under downwelling winds (Rijnsburger, 2018)
How does the wind direction affect:
o Location of the river plume? 퐡
o Cross-shore velocities? ureturn o Tidal plume fronts?
© L.M. Keyzer et al, . All rights reserved 8 Dutch Continental Shelf Model
• 3D: 20 sigma layers
• Open boundary conditions o Tide o Temperature and salinity (WOA2013) o Steric height correction
• Forcing: o Wind and atmospheric pressure (ERA5) o Heat-flux model © Deltares o River discharges
• Unstructured grid
© L.M. Keyzer et al, . All rights reserved 9 Model validation
Measurements (CTD and ADCP) Model output Salinity
flood velocity
Alongshore ebb
offshore
velocity onshore Crossshore
© L.M. Keyzer et al, . All rights reserved 10 Tidal cycle
HW -4hrs HW -2 hrs HW HW +2 hrs HW +4 hrs
)
surface
(
Salinity
/km]
ppt
[ gradient
© L.M. Keyzer et al, . All rights reserved 11 Salinity
HW +2hrs
Salinity Salinity
• Stratified • ‘Plunging head’ indicates front • Front almost reached shore
Wind direction STRAINS 18m site
© L.M. Keyzer et al, . All rights reserved 12 HW +2hrs
offshore
Salinity
velocity Crossshore
onshore Salinity
• Strong onshore velocities in upper layer • Offshore directed return flow in bottom layer
Wind direction STRAINS 18m site
© L.M. Keyzer et al, . All rights reserved 13 HW +2hrs
flood
Salinity
velocity Alongshore
ebb Salinity • Also vertical structure in alongshore velocities, although uniformly directed • Important for differential advection of tidal plume fronts
Wind direction STRAINS 18m site
© L.M. Keyzer et al, . All rights reserved 14 What is the role of the wind?
• Neap tide, eastern wind (5 m/s)
• What is the role of the wind? Salinity Modify wind direction
Wind direction
© L.M. Keyzer et al, . All rights reserved 15 What is the role of the wind?
Reference case: no wind
Compare 4 different wind directions
o Onshore (NW)
Salinity o Offshore (SE) o Upwelling (NE) o Downwelling (SW)
No wind Contour: 32 ppt
© L.M. Keyzer et al, . All rights reserved 16
Onshore vs offshore
Salinity Salinity
Wind direction Contour: 32 ppt Ekman transport ‘No wind’ plume
© L.M. Keyzer et al, . All rights reserved 17
Upwelling vs downwelling
Salinity Salinity
Wind direction Contour: 32 ppt Ekman transport ‘No wind’ plume
© L.M. Keyzer et al, . All rights reserved 18 Upwelling vs downwelling: crossshore velocity
Upwelling wind Downwelling wind Salinity
offshore
velocity Crossshore onshore
Stronger stratification and larger crossshore velocities found under downwelling winds
© L.M. Keyzer et al, . All rights reserved 19 Upwelling vs downwelling: alongshore velocity
Upwelling wind Downwelling wind Salinity
flood
velocity Alongshore ebb
Differential advection is stronger under downwelling winds
© L.M. Keyzer et al, . All rights reserved 20 Upwelling vs downwelling: fronts
Upwelling wind Downwelling wind
gradient
/km]
ppt
[ Salinity
Tidal plume front propagated/advected faster under downwelling winds
© L.M. Keyzer et al, . All rights reserved 21 Conclusions © L.M. Keyzer et al, . All rights reserved
퐡
ureturn
• Location of the tidal river plume is strongly influenced by wind direction
• Under downwelling winds we find
o Thicker stratification
o Larger crossshore velocities
o Faster propagating tidal plume fronts
Lennart Keyzer [email protected] 22 References
de Boer, G. J., Pietrzak, J. D., & Winterwerp, J. C. (2009). SST observations of upwelling induced by tidal straining in the Rhine ROFI. Continental Shelf Research, 29(1), 263-277.
Horner‐Devine, A. R., Pietrzak, J. D., Souza, A. J., McKeon, M. A., Meirelles, S., Henriquez, M., ... & Rijnsburger, S. (2017). Cross‐shore transport of nearshore sediment by river plume frontal pumping. Geophysical Research Letters, 44(12), 6343-6351.
Rijnsburger, S., Flores, R. P., Pietrzak, J. D., Horner‐Devine, A. R., & Souza, A. J. (2018). The influence of tide and wind on the propagation of fronts in a shallow river plume. Journal of Geophysical Research: Oceans, 123(8), 5426-5442.