S'A' Talke , H'E' de Swart, H'M' Schuttelaars

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Feedback between residual circulations and
sediment distribution in highly turbid estuaries
an analytical model

• S.A. Talke , H.E. de Swart, H.M. Schuttelaars

Netherlands
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Overview

• Observation Some estuaries have extremely high
  suspended sediment concentrations and fluid mud
  layers at their Estuarine Turbidity Maximum
• We present a simple model to explain how
  turbidity currents affect the longitudinal
  distribution of suspended sediment

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Observationsof Ems Estuary
350 km
(Tidal Weir)
Longitudinal Survey
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Sediment Concentrations
Low Flow Conditions Q 20 m3/s
Variations in sediment concentration huge(!)
along longitudinal axis ?variation from lt 100
mg/L (marine) to gt 10 g/L (brackish) Thus,
longitudinal distribution of sediment
concentration affects density gradients
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Longitudinal Distribution of Turbidity and
Salinity
Downstream
Upstream
What controls shape, extent and position of ETM?
Q35 m3/s
Large salinity and turbidity gradients in
longitudinal direction ? Gradients oppose each
other downstream of ETM ? Gradients in same
direction upstream of ETM
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Development of a simple model
(1) Assumptions
ltTidally Averagedgt
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Development of a simple model
(2) Gravitational Circulation
Longitudinal Salinity Profile
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Development of a simple model
(3) Add sediment dynamics
(1)
(2)
(4)
(3)

• Vertically integrated flux of sediment vanishes
  during equilibrium conditions

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Model Sensitivity Study
Z
?Parameters
K 100 m2/s
X
Q 15 m3/s q 0.03 m2/s
H 7 m
Av 0.001 m2/s
C 2 g/L
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Morphodynamic Equilibrium
As reference concentration is increased,
?Longitudinal spread of ETM increases ?Location
of ETM stays the same! ? At ETM, dc/dx is
zero. Thus, location set by the balance
between freshwater flow and salinity
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Morphodynamic Equilibrium
Gravitational Circulation
Example for High sediment Concentration c 200
g/l s 12 psu
Turbidity Circulation
Combined Circulation
Velocity structure ?Reduced residual
circulation downstream Maximum turbidity
currents and salinity currents occur at the
same (downstream) location ? Turbidity currents
zero at maximum ?persistent near-bottom upstream
flow of gt 2 mm/s ? gt 200 m per day!
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Morphodynamic Equilibrium
Affect of varying freshwater flow, q
Three Basic Conditions predicted by model Low
Flow ?Sediment concentration largest at upstream
boundary Intermediate Flow?ETM forms with
asymmetric profile High Flow? Sediment is
flushed out of estuary
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Morphodynamic Equilibrium
Affect of varying eddy viscosity and diffusivity
As the eddy viscosity is increased, the ETM moves
downstream
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Morphodynamic Equilibrium
Affect of varying depth
As depth is increased, ?the ETM moves upstream
?asymmetry of ETM shape enhanced
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Dredging Scenario
H7 m (2005, Ems)
Conceptual Model for the effect of deepening a
river (low flow conditions)
H 5m (1980, Ems)
High Sediment concentration ? Less vertical
mixing (50) ?ETM moves upstream ? Higher
Concentration causes larger upstream
spread Increased depth ? ETM moves upstream
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Discussion

• Recall longitudinal transect of sediment
  concentration
• ?Consistent with model for low flow conditions!
• Sediment piles up at
• Tidal weir

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Discussion

• No feedback between salinity field and turbidity
  currents
• Tidally varying processes not included
• Literature and measurements show this to be
  important

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Sediment Concentration at a cross section
Strong variation in sediment concentration w.r.t.
tidal phase
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Cross Sectional Measurements
Conventional stratification
Salinity 3ppt less at bottom than middle of water
column!
Salinity over a tide
During flood, salty clean water is moved over
fresh, muddy water In upper layer, normal
salinity stratification (interaction in lateral
direction?) ? Complex, tidally varying mixing
and residual circulation
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Conclusions

• Longitudinal gradients of both sediment
  concentration and salinity drive tidally-averaged
  circulation
• Large Sediment concentrations result in an
  increased upstream extent of turbid zone
• The upstream migration of ETM in Ems is likely
  both due to increased water depth and reduced
  mixing due to high sediment concentrations

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