Intratidal and Fortnightly Variability of Vertical Mixing in a Macrotidal Estuary: The Gironde

Abstract

Intratidal and fortnightly variability of turbulence at the mouth of a macrotidal estuary is explored in this study. Profiles of turbulent kinetic energy dissipation were estimated from a Vertical Microstructure Profiler, and velocity shear measurements and current velocities were collected with a vessel‐mounted Acoustic Doppler Current Profiler. Gradient Richardson numbers were quantified using density measurements from a Conductivity, Temperature, Depth profiler and squared vertical shear quantified from current velocities. All measurements were collected over one complete semidiurnal tidal cycle in both neap and spring tide conditions. Vertical eddy viscosity (Az) was quantified from the available data and was used as a proxy for vertical mixing. Results showed intratidal asymmetries in bottom‐generated turbulence during neap with Az larger during ebb tide (∼10−2 m2/s) than flood tide (∼10−3 m2/s) with the opposite occurring during spring tide with larger Az during flood (10−1 m2/s) than ebb (10−2 m2/s). Lateral processes produced elevated near‐surface mixing decoupled from bottom‐generated turbulence at the end of flood and ebb tide during neap and spring. The secondary flows were driven by Coriolis, and these flows either enhanced (neap tide, end of flood) or maintained (spring tide, end of ebb) total shear through slack tides producing near‐surface to midwater column turbulence. These results are the first of their kind to show midwater column mixing from lateral circulation driven by Coriolis in a well‐mixed system, which vary from the lateral processes previously shown to destabilize water columns in estuaries more influenced by stratification.

Intermittency in Estuarine Turbulence: A Framework toward Limiting Bias in Microstructure Measurements

“…Our results also identified small-scale intermittency in near-surface turbulence driven by vertical shear that is reinforced by lateral flows associated with a channel shoal interface (Collignon and Stacey 2013) and convective instabilities from the lateral straining of density gradients (Huguenard et al 2015). Recently, Ross et al (2019) found mid-water column turbulence generated by lateral flows during slack tides, which was influenced by Coriolis acceleration in a relatively wide system. Therefore, if significant channel-shoal morphology is present or the system is dynamically wide, then the number of casts averaged in a burst should increase to reduce the effects of undersampling.…”

Positive Feedback Between the Tidal Flat Variations and Sediment Dynamics: An Example Study in the Macro‐Tidal Turbid Hangzhou Bay

“…So research concerning their hydrodynamics, sediment dynamics, vegetation, fluid mud, bed evolution, and ecosystem in tidal flats is still on-going. In previous studies, much attention has been paid to the characteristics of hydro-sediment dynamics in macro-tidal turbid estuaries, such as turbulence, sediment dynamics and transport (e.g., Arndt et al, 2007;King et al, 2019;Ralston et al, 2012;Ross et al, 2019;B. Wang et al, 2011).…”

“…Tidal pumping dominates suspended particle matter transport in well-mixed estuarine systems (Becherer et al, 2016). However, much attention has been paid on hydro-sediment dynamics in the macro-tidal estuaries (e.g., Jalón-Rojas et al, 2021;Ross et al, 2019;Tu et al, 2019), as well as the effects of tidal flat on hydro-sediment dynamics (e.g., Gao et al, 2018;Nidzieko & Ralston, 2012;Schaefer et al, 2022b). Large-scale morphodynamic behavior, such as that found in HZB, has been insufficiently examined.…”

Observations of Near-Surface Mixing Behind a Headland

“…In estuaries, near-surface mixing can arise from lateral processes that is decoupled from bottom stress. The lateral straining of velocity shears, lateral density gradients and Coriolis acceleration have all promoted vertical mixing to form near the surface or subsurface during mid to late ebb [17][18][19]. These previous studies have highlighted the importance of channel-shoal morphology in the development of mixing driven by lateral processes.…”