1 University of Utrecht Modelling lateral entrapment of sediments in well-mixed estuaries Photo: mudbanks in the Ems Huib de Swart Karin Huijts, Henk Schuttelaars, Arnoldo Valle-Levinson Institute for Marine and Atmospheric Research Utrecht

2 University of Utrecht Lateral entrapment of suspended sediment: James River estuary Introduction Washington Chesapeake Bay 100 km Sediment trapped at the left bank From ADCP data A. Valle-Levinson (view into estuary)

3 University of Utrecht Introduction Here: simple model to study physical processes that cause lateral trapping of sediment Specific focus : 1.Coriolis deflection of tidal currents and of along-estuary density-driven flow 2.Lateral density gradients Main question: why is sediment in James River estuary trapped at the left bank?

4 University of Utrecht Geometry Local (L~10 km) Along-estuary uniform Here: lateral bathymetry of James estuary 3D Shallow water equations Tidal estuary Density gradients (prescribed) Coriolis M 2 + M 0 Sediment mass balance equations Advection diffusion equation for suspended sediment No mean lateral sediment transport Non-cohesive sediment, uniform size M 2 + M 0 Domain and equations of motion Model sea river

5 University of Utrecht Analysis Model where primes~tides and bars~time mean Scaling and perturbation analyses → reduce eq's to essential physics Analytical solutions: and Tidal flow equations Residual flow equations

6 University of Utrecht Effects of tides and density gradients on the transport is investigated separately by substituting and : 1. Tides: 2. Along-estuary density gradient: 3. Lateral density gradient: Mean lateral sediment transport Mean lateral transport due to -D Model + By definition: T = -D

7 University of Utrecht Results Mean sediment concentration (mg l -1 ) Case 1: Tides and along-estuary density gradient -D 3 cm/s 0.5 cm/s Highest concentrations near the bed → Lateral near-bed flow crucial for lateral transport trapping at right bank Transport Mean lateral sediment transport induced by…

8 University of Utrecht Mean density-driven flow Results Case 1: Tides and along-estuary density gradient In deep channel: inflow Coriolis deflection => Sediment is transported to the right bank

9 University of Utrecht Trapping at left bank Case 2: Tides and both horizontal density gradients Results Mean flow component due to lateral density gradient (cm/s) ← max. 6 cm/s Mean sediment concentration Mean sediment transport due to… …along-estuary density gradient …tides …lateral density gradient -D 3 cm/s 0.5 cm/s 6 cm/s Transport salter fresher

10 University of Utrecht Tidal amplitude Comparison to observations Courtesy observations: A. Valle-Levinson

11 University of Utrecht Mean flow Comparison to observations Asymmetric bed profile! Symmetric }

12 University of Utrecht Mean concentration Comparison to observations Lateral density gradient mechanism dominates in James River!

13 University of Utrecht Conclusions Sediments are trapped at the left bank of the James-transect, as Lateral near-bed flow induced by lateral density gradient induces mean sediment transport towards that side Tides erode bed sediments The effects of tides and density gradients on lateral sediment trapping can be studied separately, providing insight in underlying mechanisms.