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

© L.M. Keyzer et al, . All rights reserved

The Rhine ROFI: a tidal river plume along the Dutch coast

2© L.M. Keyzer et al, . All rights reserved

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

3© L.M. Keyzer et al, . All rights reserved

Tidal plume fronts are advected bytidal straining

The Rhine ROFI: the near-field region

4

(De Boer, 2006)

© L.M. Keyzer et al, . All rights reserved

Why we need to understand impact of fronts

Zandmotor(a mega nourisment)

Port of Rotterdam

5© L.M. Keyzer et al, . All rights reserved

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

6© L.M. Keyzer et al, . All rights reserved

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

© L.M. Keyzer et al, . All rights reserved

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)

© L.M. Keyzer et al, . All rights reserved

• 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

9© L.M. Keyzer et al, . All rights reserved

© Deltares

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

© L.M. Keyzer et al, . All rights reserved

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)

11© L.M. Keyzer et al, . All rights reserved

HW +2hrs

Sa

linity

Wind direction

Sa

linity

STRAINS 18m site

12

• Stratified

• ‘Plunging head’ indicates front

• Front almost reached shore

© L.M. Keyzer et al, . All rights reserved

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

© L.M. Keyzer et al, . All rights reserved

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

© L.M. Keyzer et al, . All rights reserved

• 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

15© L.M. Keyzer et al, . All rights reserved

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

16© L.M. Keyzer et al, . All rights reserved

Onshore vs offshore

Sa

linity

Sa

linity

Ekman transport

Wind direction

‘No wind’ plume

Contour: 32 ppt

17© L.M. Keyzer et al, . All rights reserved

Upwelling vs downwelling

Salin

ity

Sa

linity

Ekman transport

Wind direction

‘No wind’ plume

Contour: 32 ppt

18© L.M. Keyzer et al, . All rights reserved

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

© L.M. Keyzer et al, . All rights reserved

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

© L.M. Keyzer et al, . All rights reserved

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

© L.M. Keyzer et al, . All rights reserved

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 l.m.keyzer@tudelft.nl 22

𝐡

ureturn

© L.M. Keyzer et al, . All rights reserved

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.