Formation of Estuarine Turbidity Maxima in partially mixed estuaries H.M. Schuttelaars 1,2, C.T. Friedrichs 3 and H.E. de Swart 1 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 An example of a plume of water, heavily laden with suspended sediments, entering an estuary. Photo by: Chesapeake Bay Program

Contents Introduction and Motivation Model Setup Model Results Conclusions and Discussion

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 1 st one Observations in the York river, Virginia, USA (Lin & Kuo, 1999)

During stratified conditions: 1 ETM During mixed conditions: 2 ETMs (Lin and 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: M 2 water elevation river side: fresh water flux

Sediment: uniform, fine sediment (w s = 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 A z = A z0 (1 +  A Ri) -p K z = K z0 (1 +  K Ri) -q (Officer, 1976) With Ri ~ g  H /  0 U T 2 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, 2 nd one 20 km downstream of 1 st

2 ETMs are observed ‘Classical’ ETM around 80 km 2 nd ETM 20 km downstream of 1 st one 2 nd 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