Formation of E stuarine T urbidity M axima in partially mixed estuaries

1. Formation of Estuarine Turbidity Maxima in partially mixed estuaries 1: Institute for Marine and Atmospheric research, Utrecht University, Utrecht, The Netherlands 2: Faculty of Civil Engineering and Geosciences, TU Delft, The Netherlands 3: Virginia Institute of Marine Science, Virginia, USA H.M. Schuttelaars1,2, C.T. Friedrichs3 and H.E. de Swart1 An example of a plume of water, heavily laden with suspended sediments, entering an estuary. Photo by: Chesapeake Bay Program

2. Introduction • In many estuaries Estuarine Turbidity Maxima are observed Classical model for formation of ETM due to convergence of river flow and gravitational circulation

3. Observations in the York river, Virginia, USA (Lin & Kuo, 1999) • During stratified conditions: 1 ETM where r ~ 1002 kg m-3 • During mixed conditions: 2 ETMs • During stratified conditions ETM generally weaker • first one at r ~ 1002 kg m-3 • second, weaker ETM 30 km downstream of 1st one

4. 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.

5. Model Approach Forcing: Geometry: • weakly convergent • flat bed • sea side: M2 water elevation • river side: fresh water flux

6. Water Motion: 2 DV (width averaged) shallow water equations • Suspended load transport: • advection-diffusion equation • deposition • erosion ~ a(x) |u| Sediment: • uniform, fine sediment (ws = 0.001 m s-1) • non-cohesive • Horizontal eddy viscosity and diffusivity neglected • Influence of stratification on vertical eddy viscosity and diffusivity through Richardson number: • Density: diagnostic Az = Az0 (1 + gA Ri)-p Kz = Kz0 (1 + gK Ri)-q (Officer, 1976) With Ri ~ g Dr H / r0 UT2

7. Analytical solution method: Velocities u and w Concentration C Net Sediment Transport, that still depends on the erosion coefficient a(x) • Morphodynamic equilibrium: no net sediment transport This requirement results in the spatial structure of the erosion coefficient

8. First Experiment:Estuary is vertically stratified (r = r(x,z)) Width-Integrated residual concentration: One ETM is observed around 80 km

9. One ETM is found around 80 km. • 20 km upstream of 2ppt.

10. Second Experiment:Estuary is well mixed (r = r(x)) Width-Integrated residual concentration: Two ETMs are observed, 2nd one 20 km downstream of 1st

11. 2 ETMs are observed • ‘Classical’ ETM around 80 km • 2nd ETM 20 km downstream of 1st one • 2nd ETM less pronounced

12. Conclusions • 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 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