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