Fine sediment transport into the hyper-turbid lower Ems River: the role of channel deepening and sediment-induced drag reduction

Abstract

Deepening of estuarine tidal channels often leads to tidal amplification and increasing fine sediment import. Increasing fine sediment import, in turn, may lower the hydraulic drag (due to a smoother muddy bed and/or sedimentinduced damping of turbulence), and therefore, further strengthen tidal amplification, setting in motion a process in which the sediment concentration progressively increases until the river becomes hyper-turbid (Winterwerp and Wang, Ocean Dyn 63(11-12):1279-1292, 2013). To advance our understanding of the relative role of bed roughness and bed topography on sediment import mechanisms and sediment concentration, a Delft3D numerical model has been setup for an estuary which has been deepened and as a consequence experienced a strong increase in suspended sediment concentration: the lower Ems River. This model is calibrated against present-day hydrodynamic and sedimentary observations, and reproduces the basic sediment transport dynamics despite simplified sedimentological formulations. Historic model scenarios are semi-quantitatively calibrated against historic high and low water observations, revealing that changes in hydraulic roughness and deepening are probably equally important for the observed tidal amplification. This model is subsequently used to better understand historic changes in the hydrodynamic and sediment transport processes in the lower Ems River. Import of fine sediment has increased because of larger tidal transport, even though the degree of tidal asymmetry may not have significantly changed. The resulting rise in suspended sediment concentration reduced hydraulic drag, amplifying the tidal range. Export of fine sediment became less because the river-induced residual flow velocity decreased with deepening of the channel.

SPM response to tide and river flow in the hyper-turbid Ems River

“…The model was calibrated against measured data, showing proper simulation of the tidal propagation in the Ems River (Van Maren et al 2015). The model domain covers the Ems River from Knock in the northwest to the weir at Herbrum in the south.…”

“…Between 1960 and 1993, about 1.8 Mton of sediment (mainly mud) was removed from the Port of Emden and brought on land. This removal stopped in 1994, and Van Maren et al (2015) suggest that this triggered the increase in SPM in the river. 2.…”

“…Generally, the Ems's river discharge varies between around 20 and 400 m 3 /s, with an average of around 100 m 3 /s and exceptional peak values as high as 1200 m 3 /s (Van Leussen 1994). This river flow is measured at Versen, about 40 km upstream op Herbrum.…”

The hyperturbid state of the water column in estuaries and rivers: the importance of hindered settling

“…The first is of the Amazon shelf, with concentrations of more than 100 g/l (Kineke and Sternberg 1995), while the average bed shear stresses are estimated by model studies to be of the order of 1 Pa (Gabioux et al 2005). The second example is the Ems River, where concentrations of over 30 g/l have been observed (Talke et al 2009), while bed shear stresses are up to 2 Pa (Van Maren et al 2015). We compute the order of magnitude of the tide-averaged dimensionless erosion parameter for these systems, using its definition (11).…”

“…Several studies have made this connection by using the combined knowledge of measurements and model results (Chernetsky et al 2010;De Jonge et al 2014;Van Maren et al 2015). A possible feedback mechanism that could result in such a transition was proposed by .…”