Sediment Erosion Characteristics for the Potomac River and Is there a critical bed shear stress for deposition?

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Sediment Erosion Characteristics for the Potomac
River andIs there a critical bed shear stress
for deposition?

• Jerome P.-Y. Maa
• Virginia Institute of Marine Science
• College of William and Mary

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Experimental sites for erosion characteristics
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VIMS sea Carousel
4
-3
10
Douglas Pt and Maryland Pt (05)
-4
10
Gunston Cove (MD side, Aug. 04)
-5
Gunston Cove (MD side, Sept. 05)
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EROSION RATE (g/cm2/s)
-6
10
Gunston Cove (VA side, Sept. 05)
-7
10
-2
-1
0
10
10
10
APPRXIMABED EXCESS BED SHEAR STRESS (Pa)
-1
10
-2
? (s-1)
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Erosion is always near equilibrium during tidal
acceleration phases and ceased for other times
-3
10
0
0.2
0.4
0.6
0.8
1
BED SHEAR STRESS (Pa)
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A sediment sample from Gunston Cove site on
Virginia side
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Q Is there a critical bed shear stress for
deposition ?
Conclusions from previous studies were all based
on the interpretation of the Suspended Sediment
Concentration (SSC) time series, either in
laboratories or fields, with conflicting
results. Laboratory experiments from Mehta and
others suggested that there is a ?cd and when the
bed shear stress, ?b gt ?cd , there is no
deposition. Early field experiments (e.g.
Sanford and Halka, 1993), However, cannot
confirm the above statement, and that is the
problem. Is it possible to observe deposition
directly? We did in lab and working on
alternatives at field.
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Tidal level
other phases
Acceleration phase
other phases
Isovel (cm/s)
TSS (mg/L)
30 cm
An example of field measurements given by Sanford
and Halka (1993)
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Sanford and Halkas (1993) interpretation
Sanford and Halkas (1993) interpretation
?b
?b
?cr
?cr
?cd
?cd
Time
Time
E
D
C
C
Expected if E and D are mutully exclusive
Expected if E and D are mutully exclusive
Time
Time
E
Time
D
Time
Observed
The concerns are (1) the interpretation is based
on SSC at 30 cm above bed (2) D is downward flux,
not necessary deposition, (3) New finding E 0
after the tidal acceleration phases
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New interpretation
?b
?b
?cr
?cr
?cd
?cd
Time
Time
E
D
SSC
Upward flux
Downward flux
Time
Time
Because the downward flux, not necessary
deposition, is always there, but upward flux only
available when E gt 0.
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• Mehta (1991) suggested the possible existence of
  a stirred layer right above the bed, but so far,
  there is no proof.
• If we cannot observe it, for example, we cannot
  measure the stirred layer even in laboratory,
  then lets provide an experiment environment that
  a stirred layer cannot be developed. A relatively
  strong Secondary circulation in the VIMS lab
  carousel is a good candidate.
• Direct observation on when deposition occur

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Cross section of the VIMS Lab Carousel
0.4
Average ?b 0.37 Pa
?b (dyne/cm2)
?b distribution
0.2
ring speed8 rpm
0
1
1.05
1.1
1.15
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A general Pattern of the bed shear stress
distribution for the VIMS Carousel
?b distribution for different ring speeds
?b
?cd
Deposition length
0
1
1.05
1.1
1.15
Outer wall
R (m)
Inner wall
If ?cd does not exist, then we should observe
deposition at anyplace on the bottom. However,
we only see deposition at the corners where ?b
is small, especially at the inner corner.
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At a high bed shear stress (?0.9 Pa)
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Conclusions 1. Clear different erosion rates
were observed in the Potomac River a
spatial gradient with high erosion rate near the
turbidity maximum and low rate at other
areas. More experiments (in-situ and lab)
are necessary to address the temporal
gradient and cover the entire river. 2. Direct
observation of deposition shows that ?cd exists
clearly in a laboratory experiment. Field
observation is difficult because of the small
deposition depth. Observation of the
stirred layer is also not easy, but we are
working on this subject. 3. Downward flux in the
water column ? deposition on bed 4. If ?cd exist,
then erosion and deposition are mutually
exclusive, for cohesive sediment.