the influence of wind on the evolution of freshwater fronts in the … · 2020. 5. 6. · in the...
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The influence of wind on the evolution of freshwater fronts in the Rhine ROFI
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
Tidal plume front
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The Rhine ROFI: a tidal river plume along the Dutch coast
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In the near-field region, thedynamics are dominated by tidalplume fronts.
Every tidal cycle, a new tidal plume front is formed.
The tidal plume fronts from previoustidal cyles can still be present whilea new one is formed.
The Rhine ROFI: the near-field region
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Tidal plume fronts are advected bytidal straining
The Rhine ROFI: the near-field region
4
(De Boer, 2006)
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Why we need to understand impact of fronts
Zandmotor(a mega nourisment)
Port of Rotterdam
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Fronts can impact sediment transport
Shelf processes Near shore processes
front
ureturn
(Horner-Devine et
al., 2017)
1. Front propagates onshore
2. Return flow moves suspended sediment offshore
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STRAINS measurement campaigns
10 km
PI’s:
J.D. Pietrzak,
A.R. Horner-Devine,
A.J. Souza
7
ADCP
ADCP
Mooringincl. 4 CTDs
Mooringincl. 4 CTDs
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Role of the wind
How does the wind direction affect:
o Location of the river plume?
o Cross-shore velocities?
o Tidal plume fronts?
8
𝐡
ureturn
STRAINS campaign: strongest fronts are found under downwelling winds
(Rijnsburger, 2018)
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• 3D: 20 sigma layers
• Open boundary conditionso Tide
o Temperature and salinity (WOA2013)
o Steric height correction
• Forcing:o Wind and atmospheric pressure
(ERA5)
o Heat-flux model
o River discharges
• Unstructured grid
Dutch Continental Shelf Model
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© Deltares
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Model validation
onshore
offshore
10
ebb
flood
Cro
sssho
re
ve
locity
Sa
linity
Alo
ng
sh
ore
ve
locity
Measurements (CTD and ADCP) Model output
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Tidal cycle
HW HW +2 hrs HW +4 hrsHW -2 hrsHW -4hrs
Sa
linity
gra
die
nt[p
pt/km
]S
alin
ity
(su
rfa
ce)
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HW +2hrs
Sa
linity
Wind direction
Sa
linity
STRAINS 18m site
12
• Stratified
• ‘Plunging head’ indicates front
• Front almost reached shore
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HW +2hrs
Sa
linity
Wind direction
Sa
linity
STRAINS 18m site
13
Cro
ssshore
velo
city
onshore
offshore
• Strong onshore velocities in upper layer
• Offshore directed return flow in bottom layer
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HW +2hrs
Sa
linity
Wind direction
Sa
linity
STRAINS 18m site
14
Alo
ngshore
velo
city
ebb
flood
• Also vertical structure in alongshore velocities,
although uniformly directed
• Important for differential advection of tidal plume
fronts
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• Neap tide, eastern wind (5 m/s)
• What is the role of the wind?
Modify wind direction
What is the role of the wind?
Sa
linity
Wind direction
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Reference case: no wind
Compare 4 different wind directions
o Onshore (NW)
o Offshore (SE)
o Upwelling (NE)
o Downwelling (SW)
What is the role of the wind?
Sa
linity
No wind Contour: 32 ppt
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Onshore vs offshore
Sa
linity
Sa
linity
Ekman transport
Wind direction
‘No wind’ plume
Contour: 32 ppt
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Upwelling vs downwelling
Salin
ity
Sa
linity
Ekman transport
Wind direction
‘No wind’ plume
Contour: 32 ppt
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Upwelling vs downwelling: crossshore velocity
Upwelling wind Downwelling wind
Sa
linity
Cro
sssho
re
velo
city
onshore
offshore
19
Stronger stratification and larger crossshore velocities found under downwelling winds
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Upwelling vs downwelling: alongshore velocity
Upwelling wind Downwelling wind
Sa
linity
Alo
ng
sh
ore
velo
city
ebb
flood
20
Differential advection is stronger under downwelling winds
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Upwelling vs downwelling: fronts
Upwelling wind Downwelling wind
21
Sa
linity
gra
die
nt
[pp
t/km
]
Tidal plume front propagated/advected faster under downwelling winds
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Conclusions
• 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
𝐡
ureturn
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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.