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 - PowerPoint PPT PresentationTRANSCRIPT
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Direct observations of estuarine dispersion:results of a recent dye experiment
Bob Chant
Rutgers University
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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
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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?
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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.
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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
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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
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Upper layer Lower layer
Dye’s salinity distribution atrelease Currents and salinity
39 kg of fluorecene dye
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Patch 2 (12:20 - 14:00)
Depth integrated dye
26 kg 1.8 kg
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40.9
40.94
Cross Channel Sections during Flood2 hours after release
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Patch 2 (12:20 - 14:00)
Depth integrated dye Integrated dye above 14 psu
26 kg 1.8 kg
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25 kg
26 kg
Patch 3Patch 2
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Patch 3 (14:00 - 15:40)
Depth integrated dye Integrated dye above 14 psu
25 kg
2.3 kg
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Patch 4 (15:00 - 17:10)
Depth integrated dye
44 kg
Patch 3 (14:00 - 15:40)
Depth integrated dye
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A series of cross-channel section during late ebb
Currents from ADCP
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1
2
3
4
5
6
7
8
1
8
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Patch 4 (15:00 - 17:10)
Depth integrated dye Integrated dye above 14 psu
44 kg16 kg
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1
0.5
0
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2zst
ssK
zst
sw
z
sKsw
v
e
ve
Vertical mixing estimates
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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
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1
0.5
0
Slack before floodDay 1
Slack before ebbDay 2 ?
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30 hours after release End of Ebb
m/s
+.5
-1.0
Current
12 16
May 24
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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
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Patch 5 (15:45 - 18:45)
Depth integrated dye Integrated dye above 14 psu
40 kg 20 kg
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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.
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Salty
Fresh
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Max 35
36 kg
1
16 kg
1
2
26 kg
2
3
25 kg
3
4
44 kg
4
5
40 kg
5
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Simulation of Dye Release in Hudson
*Simulation by Rocky Geyer
Ebb Flood
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Patch 1 (11:00 - 12:15)
Depth integrated dye Integrated dye above 14 psu
16 kg 0.6 kg
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* H > 10m* H < 10m
Dye weighted salinity of each cast
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.. 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
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End of Flood
End of Ebb
End of Flood
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30 hours after release
xssu
A
TLA
eff
eff
/2
Two crude estimates of effective horizontal dispersion….
…both suggest Aeff=1000 m2/s