formation of e stuarine t urbidity m axima in partially mixed estuaries
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Formation of Estuarine Turbidity Maxima in partially mixed estuaries
H.M. Schuttelaars1,2, C.T. Friedrichs3 and H.E. de Swart1
1: Institute for Marine and Atmospheric research, Utrecht University, Utrecht, The Netherlands2: Faculty of Civil Engineering and Geosciences, TU Delft, The Netherlands3: Virginia Institute of Marine Science, Virginia, USA
An example of a plume of water, heavily laden with suspended sediments, entering an estuary.
Photo by: Chesapeake Bay Program
Introduction• In many estuaries Estuarine Turbidity Maxima are observed
Classical model for formation of ETM due to convergence of river flow and gravitational circulation
• During stratified conditions: 1 ETM where~ 1002 kg m-3
• During mixed conditions: 2 ETMs
• During stratified conditions ETM generally weaker
• first one at ~ 1002 kg m-3
• second, weaker ETM 30 km downstream of 1st one
Observations in the York river, Virginia, USA (Lin & Kuo, 1999)
Research questions:
•Can the convergence of sediment at two different locations be modelled?• Which conditions result in the formation of two ETMs?
Hypothesis: The density distribution in the estuary controls the position, strength and number of ETMs that will be observed.
Model Approach
Geometry:
• weakly convergent• flat bed
Forcing:
• sea side: M2 water elevation• river side: fresh water flux
Sediment: • uniform, fine sediment (ws = 0.001 m s-1)• non-cohesive
• Water Motion: 2 DV (width averaged) shallow water equations • Suspended load transport:
• Horizontal eddy viscosity and diffusivity neglected• Influence of stratification on vertical eddy viscosity and diffusivity through Richardson number:
• Density: diagnostic
Az = Az0 (1 + A Ri)-p
Kz = Kz0 (1 + K Ri)-q (Officer, 1976)
With Ri ~ g H / 0 UT2
• advection-diffusion equation• deposition• erosion ~ (x) |u|
• Morphodynamic equilibrium: no net sediment transport
This requirement results in the spatial structure of the erosion coefficient
Analytical solution method:
Net Sediment Transport, that still depends on the erosion coefficient (x)
Velocities u and w Concentration C
Width-Integrated residual concentration:
First Experiment: Estuary is vertically stratified ( = (x,z))
One ETM is observed around 80 km
• One ETM is found around 80 km.• 20 km upstream of 2ppt.
Width-Integrated residual concentration:
Second Experiment: Estuary is well mixed ( = (x))
Two ETMs are observed, 2nd one 20 km downstream of 1st
• 2 ETMs are observed• ‘Classical’ ETM around 80 km• 2nd ETM 20 km downstream of 1st one• 2nd ETM less pronounced
Conclusions
Further research: • Which physical mechanism results in the
second ETM (quite straightforward with analytical model)?
• Why is the ETM not pushed upstream with stronger stratification?• Parameter dependency of position of ETM
• Diagnostic model useful in gaining insight in formation of ETMs• During mixed conditions two ETMs will form• During stratified conditions only one ETM will form• Stratification weakens the ETM
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