11th Warnemünde Turbulence Days on Linking Ocean Mixing and Circulation at Various Scales What: Forty-five participants from 10 countries met to discuss contemporary issues of marine turbulence with a focus on the linkage between mixing and overturning circulation on all scales (https://www.io-warnemuende.de/wtd-2023.html). When: 17–20 September 2023 Where: Rostock, Germany.

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Structure and intensity of estuarine exchange flow depend significantly on the eddy viscosity A_v profile which is dynamically linked to various forces (e.g., gravitational, tidal, wind-driven). The impact of winds on the exchange flow is complex due to its direct (local and remote changes in shear and density stratification) and indirect (modifications to A_v profiles) contributions. This study aims (i) to include wind entrainment effects in the tidally averaged A_v parameterization; (ii) to develop an analytical one-dimensional model for the wind driven exchange flow by using this novel parameterization and assess the tidally averaged dynamics over a relevant physical parameter-space, subdomains of which have not yet been explored numerically. This one-dimensional model is based on a balance between frictional forces and pressure gradient, calibrated with a tidally-resolving one-dimensional water-column model with second-moment closure. Structure and intensity of the resulting exchange flow profiles are analyzed with respect to three dimensionless parameters (the unsteadiness of boundary layer mixing U_n, scaled-directional wind stress W, and horizontal stratification S_i). While down-estuarine winds enhance the gravitational circulation, up-estuarine winds result in either a two-layer inverted circulation opposing the gravitational circulation, or a three-layer flow (favored by relatively strong S_i, weak W, and moderate U_n) that is up-estuarine at the surface with classical two-layer circulation underneath. Relative thickness of surface and bottom boundary layers affect both the intensity and the inflection depth of the exchange flow layers. Up-estuarine winds with W≳0.5 yield unstable stratification and reduce the exchange flow intensity with increasing W.

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A new parameterization for the estuarine turbulent eddy viscosity coefficient is developed considering the influence of wind forcing and feedback between stratification and shear. The emerging tidally averaged eddy viscosity profile A_v^T is parameterized as parabolic under well-mixed conditions, and is composed of a skewed-Gaussian-like form for the upper layer, and a parabolic form for the bottom layer under stratified conditions. The precise shape of the profiles depends parametrically on the bottom boundary layer thickness, the bulk Richardson number, and the Wedderburn number. The parameterized A_v^T profiles show excellent agreement with profiles obtained from numerical models. To explore the importance of vertically varying A_v^T with regard to exchange processes, an analytical model is designed. This one-dimensional model is based on a balance between frictional forces and pressure gradient. The resulting exchange flow is analyzed over the relevant parameter space that is associated with horizontal and vertical stratification through the bulk Richardson number, and the bi-directional wind stress via the Wedderburn number. Down-estuary wind enhances the up-estuary flow near the bottom and down-estuary flow near the surface driving an exchange flow pattern typically associated with gravitational circulation. Up-estuary wind results in either a two-layer inverted circulation opposing the gravitational circulation, or a three-layer flow that is up-estuarine at the surface with classical two-layer circulation underneath. Three-layer flow emerges with a weak wind. With increasing runoff velocity, three-layer flow transitions to a single layer flow under weak stratification conditions.

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Results from measurements are presented that were collected during a full tidal cycle in the Ems estuary, involving two landers and an anchored research vessel. The conditions were characterized by very weak winds, no wave effects, and low river run-off, so that the state was close to tide-only. We find that the lateral (i.e., cross-slope) transport of water and suspended particulate matter (SPM) much of the time shows a vertically layered structure, which is however subject to sudden transitions. Moreover, even on the small spatial scale of these measurements (i.e., within a distance of 200 m), a strong lateral variability is observed in the circulation patterns. We analyze its dynamics by means of dimensionless parameters. In addition, near-bottom peaks in SPM concentration are observed, notably during early flood. However, these episodic events have little effect on the overall transport of SPM, which involves the whole water column: the measurements show a high vertically integrated SPM signal during late ebb, resulting in an ebb-dominance in the transport at this position in the tidal channel.

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This paper presents thickness-weighted averaging (TWA) in generalized vertical coordinates as a unified framework for a variety of existing tidal-averaging concepts in seas and estuaries. Vertical profiles of resulting residual quantities depend on the specific vertical coordinate, which is held fixed during the averaging process. This dependence is demonstrated through the application to one-dimensional analytical tidal flow with sediment transport, to field observations from a tidal channel, and to model results from a two-dimensional estuary. The use of different coordinate systems provides complementary views on the residual dynamics and stresses the importance of a correct interpretation of residual quantities obtained by tidal averaging.

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Estuarine turbidity maxima (ETMs) are generated by a large suite of hydrodynamic and sediment dynamic processes, leading to longitudinal convergence of cross-sectionally integrated and tidally averaged transport of cohesive and noncohesive suspended particulate matter (SPM). The relative importance of these processes for SPM trapping varies substantially among estuaries depending on topography, fluvial and tidal forcing, and SPM composition. The high-frequency dynamics of ETMs are constrained by interactions with the low-frequency dynamics of the bottom pool of easily erodible sediments. Here, we use a transport decomposition to present processes that lead to convergent SPM transport, and review trapping mechanisms that lead to ETMs at the landward limit of the salt intrusion, in the freshwater zone, at topographic transitions, and by lateral processes within the cross section. We use model simulations of example estuaries to demonstrate the complex concurrence of ETM formation mechanisms. We also discuss how changes in SPM trapping mechanisms, often caused by direct human interference, can lead to the generation of hyperturbid estuaries.

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We present a systematic analysis of generation mechanisms for exchange flows in partially stratified estuaries using water column (1DV) and width-averaged (2DV) numerical models. We focus on exchange flows generated by eddy viscosity-shear covariance (ESCO). We identify two distinctly different physical mechanisms. The first, tidal ESCO circulation, results from interactions between the barotropic tide and temporal variations of the eddy viscosity. While this flow is mostly generated by direct interactions between the tide and eddy viscosity variations at the main tidal frequency, a similarly important contribution can be attributed to indirect interactions. These are more complex interactions involving eddy viscosity variations at other frequencies than the main tidal frequency (e.g., M4). The second mechanism is called gravitational ESCO circulation. This results from an amplification of the gravitational circulation through indirect interactions between the gravitational circulation and temporal variations of the eddy viscosity at any time scale. Tidal and gravitational ESCO circulations are generated by different mechanisms and have a different dependency on the phase and frequency of eddy viscosity variations and the density gradient. The relative contributions of gravitational circulation and tidal and gravitational ESCO circulation to the exchange flow are typically 1/3 each in tidally energetic well-mixed or partially stratified estuaries. The results are generalized using an idealized width-averaged model of the Scheldt River estuary. This model confirms the results of the water column model and additionally shows that temporal variations of turbulence not captured in the water column model have a significant effect on the exchange flow.@en