Reply on RC2

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).