Post on 14-Jan-2016
S.A. Talke , H.E. de Swart, H.M. Schuttelaars
Feedback between residual circulations and sediment distribution in highly turbid
estuaries: an analytical model
Netherlands
•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
Overview
Observationsof Ems Estuary
Ems Estuary
Amsterdam
Utrecht
~350 km
Germany
Netherlands
0 km 45 km
100 km(Tidal Weir)
Longitudinal Survey
Variations in sediment concentration huge(!) along longitudinal axisvariation from < 100 mg/L (marine) to > 10 g/L (brackish)
Thus, longitudinal distribution of sediment concentration affects density gradients
Sediment Concentrations
Low Flow Conditions
Q ~ 20 m3/s
Distance from N. Sea (km)
Large salinity and turbidity gradients in longitudinal direction
Gradients oppose each other downstream of ETM
Gradients in same direction upstream of ETM
UpstreamDownstream
Q=35 m3/s
What controls shape,extent and position of ETM?
Longitudinal Distribution of Turbidity and Salinity
Development of a simple model(1) Assumptions
Non-cohesive, fine grained sediments
Constant settling velocity
Constant Eddy viscosity and diffusivity
Salinity well mixed
Constant Width
Constant Depth
<Tidally Averaged>
Development of a simple model(2) Gravitational Circulation
Z
X
Sea River
Essentially, gravitational circulation reproduced with a convergence zone
z
uA
zx
pz0 g
z
p
H
QubdzCso (1) (2) (4)(3)
Longitudinal Salinity Profile
Development of a simple model(3) Add sediment dynamics
z
uA
zx
pz0 g
z
p
H
Qubdz
Z
X
Sea River
(1) (2) (4)(3)
•Vertically integrated flux of sediment vanishes during equilibrium conditions
H
dzx
CKuC 0(6)
MorphodynamicEquilibrium
(5) 0)(
z
CKCw
z zs
Solution now closed to arbitrary constant, c*
c* is total integrated
bottom sediment
Cso
Data Points
CTD Profile
Exponential Profile
Model Sensitivity StudyParameters
Z
X
Model shows that distribution of sediment sensitive to these parameters
Ws = 1 mm/s
Av = 0.001 m2/s
Q= 15 m3/sq= 0.03 m2/sH = 7 m
C* = 2 g/L
K = 100 m2/s
As reference concentration is increased, Longitudinal spread of ETM increasesLocation of ETM stays the same!
At ETM, dc/dx is zero. Thus, location set bythe balance between freshwater flow and salinity
Morphodynamic Equilibrium
Morphodynamic Equilibrium
Example forHigh sedimentConcentration
c*= 200 g/ls* = 12 psu
Gravitational Circulation
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 maximumpersistent near-bottom upstream flow of > 2 mm/s
> 200 m per day!
Three Basic Conditions predicted by model:Low Flow Sediment concentration largest at upstream boundaryIntermediate FlowETM forms with asymmetric profileHigh Flow Sediment is flushed out of estuary
Affect of varying freshwater flow, q
Morphodynamic Equilibrium
Morphodynamic Equilibrium
As the eddy viscosity is increased, the ETM moves downstream
Affect of varyingeddy viscosity and diffusivity
Morphodynamic Equilibrium
As depth is increased, the ETM moves upstream asymmetry of ETM shape enhanced
Affect of varying depth
High Sediment concentration Less vertical mixing (50%)
ETM moves upstream Higher Concentration causes larger upstream spread
Increased depth ETM moves upstream
Conceptual Model for theeffect of deepening a river(low flow conditions) H= 5m
(1980, Ems)
H=7 m(2005, Ems)
Dredging Scenario
Discussion
• Recall longitudinal transect of sediment concentration
Consistent with model for low flow conditions!– Sediment piles up at
Tidal weir
Discussion
• No feedback between salinity field and turbidity currents
• Tidally varying processes not included– Literature and measurements show this to be
important
Sediment Concentration at a cross section
Strong variation in sediment concentration w.r.t. tidal phase
Cross Sectional Measurements
Salinity over a tide
Salinity 3ppt less atbottom thanmiddle of water column!
During flood, salty clean water is moved over fresh, muddy waterIn upper layer, ‘normal’ salinity stratification (interaction in lateral direction?)
Complex, tidally varying mixing and residual circulation
Conventionalstratification
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