Estuaries and deltaic channels are dynamic environments at the transition from the river to the ocean (Figure 1). These coastal channels are of special importance for mankind in terms of transport (de Vriend et al., 2011) and ecological value (Bouma et al., 2005). The surrounding land is often densely populated (Edmonds et al., 2020), with 21 of the world's 30 largest cities being located next to estuaries (Ashworth et al., 2015).

Abstract. An extensive field campaign (EDoM) was executed in the Ems estuary, bordering the Netherlands and Germany, aiming at better understanding the mechanisms driving exchange of water and sediments between a relatively exposed outer estuary and a hyperturbid tidal river. Particularly the reasons for the large up-estuary sediment accumulation rates and the role of the tidal river on the turbidity in the outer estuary were insufficiently understood. The campaign was designed to unravel the hydrodynamic and sedimentary exchange mechanisms, comprising two hydrographic surveys during contrasting environmental conditions using 8 concurrently operating ships and 10 moorings measuring for least one spring-neap tidal cycle. All survey locations were equipped with sensors measuring flow velocity, salinity, and turbidity (and with stationary ship surveys taking water samples), while some of the survey ships also measured turbulence and sediment settling properties. These observations have provided important new insights into horizontal transport fluxes and density-driven exchange flows, both laterally and longitudinally. An integral analysis of these observations suggest that large-scale residual transport is surprisingly similar during periods of high and low discharge, with higher river discharge resulting in both higher seaward-directed fluxes near the surface and landward-directed fluxes near the bed. Sediment exchange seems to be strongly influenced by a previously undocumented lateral circulation cell driving residual transport. Vertical density-driven flows in the outerestuary are influenced by variations in river discharge, with a near-bed landward flow being most pronounced in the days following a period with elevated river discharge. The study site is more turbid during winter conditions, when the Estuarine Turbidity Maximum is pushed seaward by river flow, resulting in more pronounced impact of suspended sediments on hydrodynamics. All data collected during the EDoM campaign, but also standard monitoring data (waves, water levels, discharge, turbidity and salinity) collected by Dutch and German authorities is made publicly available at 4TU Centre for Research Data (https://doi.org/10.4121/c.6056564.v3; van Maren et al., 2022).

Abstract. An extensive field campaign, the Ems-Dollard Measurements (EDoM), was executed in the Ems Estuary, bordering the Netherlands and Germany, aimed at better understanding the mechanisms that drive the exchange of water and sediments between a relatively exposed outer estuary and a hyper-turbid tidal river. More specifically, the reasons for the large up-estuary sediment accumulation rates and the role of the tidal river on the turbidity in the outer estuary were insufficiently understood. The campaign was designed to unravel the hydrodynamic and sedimentary exchange mechanisms, comprising two hydrographic surveys during contrasting environmental conditions using eight concurrently operating ships and 10 moorings measuring for at least one spring–neap tidal cycle. All survey locations were equipped with sensors measuring flow velocity, salinity, and turbidity (and with stationary ship surveys taking water samples), while some of the survey ships also measured turbulence and sediment settling properties. These observations have provided important new insights into horizontal sediment fluxes and density-driven exchange flows, both laterally and longitudinally. An integral analysis of these observations suggests that large-scale residual transport is surprisingly similar during periods of high and low discharge, with higher river discharge resulting in both higher seaward-directed fluxes near the surface and landward-directed fluxes near the bed. Sediment exchange seems to be strongly influenced by a previously undocumented lateral circulation cell driving residual transport. Vertical density-driven flows in the outer estuary are influenced by variations in river discharge, with a near-bed landward flow being most pronounced in the days following a period with elevated river discharge. The study site is more turbid during winter conditions, when the estuarine turbidity maximum (ETM) is pushed seaward by river flow, resulting in a more pronounced impact of suspended sediments on hydrodynamics. All data collected during the EDoM campaign, but also standard monitoring data (waves, water levels, discharge, turbidity, and salinity) collected by Dutch and German authorities are made publicly available at 4TU Centre for Research Data (https://doi.org/10.4121/c.6056564.v3; van Maren et al., 2022).

<p>Channel beds in estuaries and deltas often exhibit a local depth maximum at a location close to the coast. There are two known causes of large-scale (i.e. >10 river widths along-channel) channel bed scours: width constriction and draw down during river discharge extremes, both creating a local flow acceleration. Here, we systematically investigate a potential third mechanism. We study the effect of tidal dominance on the equilibrium channel bed in estuaries with a 1D-morphodynamic model. In estuaries, a morphodynamic equibrium is reached when the net (seaward) transport matches the upstream supply along the entire reach. The residual (river) current and river-tide interactions create seaward transport. Herein, river-tide interactions represent the seaward advection of tide-induced suspended sediment by the river flow. Tidal asymmetry typically creates landward transport. The main reason for scour formation is the amplification of tidal flow through funnelling of tidal energy. Only for a scouring profile the drop in river induced current magnitude reduces the river-tide interaction term, so that the net sediment transport matches the upstream sediment transport. When tidal influence is relatively large, and when channel convergence is strong, a equilibrium is only obtained with a scouring profile. We propose a predictor dependent on the width convergence, quantified as S<sub>B</sub>, and on the ratio between the specific peak tidal discharge at the mouth and the specific river discharge at the landward boundary (q<sub>tide</sub>/q<sub>river</sub>). Scours develop if (q<sub>tide</sub>/q<sub>river</sub>)/S<sub>B</sub> exceeds 0.3. These results are independent of scale and allow the prediction of scour in estuaries under future changes.</p>