Direct observations of estuarine dispersion:results of a recent dye experiment

Bob Chant

Rutgers University

Pritchard’s Paradigm

Seaward atthe surface

Pressure gradient and Velocity

River

SUA

x

SASU

H

H

x

sK

z

sKwsusFlux Salt HV

xs

Kzs

KwsusFlux Salt HV

sux

sAeff

sss

uuu

Not a new question-- But observations are lacking.Current understanding is based on models (Zimmerman, Okubo, Fischer,

Smith) or indirect methods.

But estuaries are often highly 3-dimensional with appreciablecross channel gradients of salinity and velocity.

Do these processes impact stream-wise dispersion?

Salinity section May 21, 2001 2-days prior to injection

“Lagrangain studies of secondary circulation and mixing in a stratified channel”

NSF PI’s Chant, Geyer and Houghton.

Pilot Dye Study May 23-24, 2001. 4 more experiments planned for May, 2002Pilot study coincided with microstructure measurements by Hartmut Peters.

2002 injectionswill occur at spring and neap tide conditions

Figure 2 Study SiteInjection May 23, 2001

9:15-9:30 AMSalinity and velocity crosssections at time of release

Currents and salinityShip Track and Dye Trace

Upper layer Lower layer

Dye’s salinity distribution atrelease Currents and salinity

39 kg of fluorecene dye

Patch 2 (12:20 - 14:00)

Depth integrated dye

26 kg 1.8 kg

40.9

40.94

Cross Channel Sections during Flood2 hours after release

Patch 2 (12:20 - 14:00)

Depth integrated dye Integrated dye above 14 psu

26 kg 1.8 kg

25 kg

26 kg

Patch 3Patch 2

Patch 3 (14:00 - 15:40)

Depth integrated dye Integrated dye above 14 psu

25 kg

2.3 kg

Patch 4 (15:00 - 17:10)

Depth integrated dye

44 kg

Patch 3 (14:00 - 15:40)

Depth integrated dye

A series of cross-channel section during late ebb

Currents from ADCP

1

2

3

4

5

6

7

8

1

8

Patch 4 (15:00 - 17:10)

Depth integrated dye Integrated dye above 14 psu

44 kg16 kg

1

0.5

0

2zst

ssK

zst

sw

z

sKsw

v

e

ve

Vertical mixing estimates

we=0.09 mm/sKv=2.3* 10-3 m2/s

<S>=15.8

<S>=15.4

H > 10 m

<S>=15.5

<S>=15.2

we=0.05 mm/sKv= 1.0-3 m2/s

H < 10mMax Flood

we=0.05 mm/sKv=1.7* 10-3 m2/s

we=0.3 mm/sKv=1.3* 10-2 m2/s

<S>=15.2

<S>=14.2

Max Ebbwe=0.31 mm/sKv=1.1* 10-2 m2/s

we=0.28 mm/sKv=1.0* 10-2 m2/s

<S>=14.4

<S>=13.3

1

0.5

0

Slack before floodDay 1

Slack before ebbDay 2 ?

30 hours after release End of Ebb

m/s

+.5

-1.0

Current

12 16

May 24

FLOODAlong Channel sections during ebb

14:00 - 14:30

14:54 - 15:21

15:41 - 16:30

Dye Contour Intervals 0.2, 0.5, 1.0 2.0

Patch 5 (15:45 - 18:45)

Depth integrated dye Integrated dye above 14 psu

40 kg 20 kg

Conclusions

1) Lateral iso-pycnal advection of dye on flood

2) Mixing rates on flood are 10-3 and confined to bbm, suggestive of weak entrainment rates and thickening of BBL is due to flow convergence.

3) Mixing increases an order of magnitude on ebb. Diapycnaltransport occurs primarily on flank

4) Perfect dispersive process-- coupling between secondaryflow spatial variations in mixing and strong ebb shears. Suggestsartificial decomposition of diffusive processes is ...

5) Dye distribution after one tidal cycle is not consistent with simpletidal straining ideas.

6) While spread of dye is consistent with advection by estuarine flows (but mixingmay not be correct) observations clearly show that dispersion of dye is due to a 3-D tidal straining effect. Are estuarine flows then influenced by this mixing process?

7) Closure over-entrains on flood.

Salty

Fresh

Max 35

36 kg

1

16 kg

1

2

26 kg

2

3

25 kg

3

4

44 kg

4

5

40 kg

5

Simulation of Dye Release in Hudson

*Simulation by Rocky Geyer

Ebb Flood

Patch 1 (11:00 - 12:15)

Depth integrated dye Integrated dye above 14 psu

16 kg 0.6 kg

* H > 10m* H < 10m

Dye weighted salinity of each cast

.. Or is it driven by secondarycirculation and mixing on

the margins?

Is diapycnal transport primarily associatedwith vertical mixing?

“Lagrangain studies of secondary circulation and mixing in a

stratified channel”NSF PI’s Chant, Geyer (WHOI)

and Houghton (LDEO)

Note: May dominate diapycnalmixing process in global ocean

End of Flood

End of Ebb

End of Flood

30 hours after release

xssu

A

TLA

eff

eff

/2

Two crude estimates of effective horizontal dispersion….

…both suggest Aeff=1000 m2/s