Direct observations of estuarine dispersion results of a recent dye experiment
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Direct observations of estuarine dispersion: results of a recent dye experiment. Bob Chant Rutgers University. River. Pritchard’s Paradigm. Seaward at the surface. Pressure gradient and Velocity. But estuaries are often highly 3-dimensional with appreciable

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

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Direct observations of estuarine dispersion:

results of a recent dye experiment

Bob Chant

Rutgers University


River

Pritchard’s Paradigm

Seaward at

the surface

Pressure gradient and Velocity


But estuaries are often highly 3-dimensional with appreciable

cross channel gradients of salinity and velocity.

Do these processes impact stream-wise dispersion?

Not a new question-- But observations are lacking.

Current understanding is based on models (Zimmerman, Okubo, Fischer, Smith)

or indirect methods.


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

NSF PI’s Chant, Geyer and Houghton.

Salinity section May 21, 2001

2-days prior to injection


Pilot Dye Study May 23-24, 2001. 4 more experiments planned for May, 2002

Pilot study coincided with microstructure measurements by Hartmut Peters.

2002 injections

will occur at

spring and neap tide conditions


Ship Track and Dye Trace

Currents and salinity

Injection May 23, 2001

9:15-9:30 AM

Salinity and velocity cross

sections at time of release

Figure 2 Study Site


Dye’s salinity distribution at

release

Currents and salinity

Upper layer

Lower layer

39 kg of fluorecene dye


Patch 2 (12:20 - 14:00)

Depth integrated dye

1.8 kg

26 kg


40.94

40.9

Cross Channel Sections during Flood

2 hours after release


Patch 2 (12:20 - 14:00)

Integrated dye above 14 psu

Depth integrated dye

26 kg

1.8 kg


25 kg

26 kg

Patch 3

Patch 2


Patch 3 (14:00 - 15:40)

Depth integrated dye

Integrated dye above 14 psu

2.3 kg

25 kg


44 kg

Patch 3 (14:00 - 15:40)

Patch 4 (15:00 - 17:10)

Depth integrated dye

Depth integrated dye


A series of cross-channel section during late ebb

Currents from ADCP


1

5

8

6

2

1

7

3

8

4


Patch 4 (15:00 - 17:10)

Integrated dye above 14 psu

Depth integrated dye

16 kg

44 kg


1

0.5

0


Vertical mixing estimates


H > 10 m

H < 10m

<S>=15.8

<S>=15.5

we=0.09 mm/s

Kv=2.3* 10-3 m2/s

we=0.05 mm/s

Kv= 1.0-3 m2/s

<S>=15.4

<S>=15.2

we=0.05 mm/s

Kv=1.7* 10-3 m2/s

we=0.28 mm/s

Kv=1.0* 10-2 m2/s

<S>=15.2

<S>=14.4

Max Ebb

we=0.3 mm/s

Kv=1.3* 10-2 m2/s

we=0.31 mm/s

Kv=1.1* 10-2 m2/s

<S>=14.2

<S>=13.3

Max Flood


1

0.5

0

?

Slack before flood

Day 1

Slack before ebb

Day 2


+.5

12

16

m/s

-1.0

Current

May 24

30 hours after release

End of Ebb


FLOOD

Along 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)

Integrated dye above 14 psu

Depth integrated dye

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. Diapycnal

transport occurs primarily on flank

4) Perfect dispersive process-- coupling between secondary

flow spatial variations in mixing and strong ebb shears. Suggests

artificial decomposition of diffusive processes is ...

5) Dye distribution after one tidal cycle is not consistent with simple

tidal straining ideas.

6) While spread of dye is consistent with advection by estuarine flows (but mixing

may 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.


Fresh

Salty


2

1

1

3

5

4

5

40 kg

16 kg

3

2

4

26 kg

25 kg

44 kg

Max 35

36 kg


Simulation of Dye Release in Hudson

Ebb

Flood

*Simulation by Rocky Geyer


Patch 1 (11:00 - 12:15)

Integrated dye above 14 psu

Depth integrated dye

16 kg

0.6 kg


* H > 10m

* H < 10m

Dye weighted salinity of each cast


.. Or is it driven by secondary

circulation and mixing on

the margins?

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

NSF PI’s Chant, Geyer (WHOI) and Houghton (LDEO)

Is diapycnal transport primarily associated

with vertical mixing?

Note: May dominate diapycnal

mixing process in global ocean


End of Ebb

End of Flood

End of Flood


Two crude estimates of effective horizontal dispersion….

…both suggest Aeff=1000 m2/s

30 hours after release


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