Sediment Suspension by Straining‐Induced Convection at the Head of Salinity Intrusion

Abstract

The tidal straining can generate convective motions and exert a periodic modification of turbulence and sediment transport in estuarine and coastal bottom boundary layers. However, the evidence and physics of convection and sediment suspension induced by tidal straining have not been straightforward. To examine these questions, mooring and transect surveys have been conducted in September 2015 in the region of the Yangtze River plume influence. Field observations and scaling analyses indicate an occurrence of convective motions at the head of saline wedge. Theoretical analyses of stratification evolution in the saline wedge show that unstable stratification and resultant convection are induced by tidal straining. Vertical turbulent velocity and eddy viscosity at the head of saline wedge are both larger than their neutral counterparts in the main body, largely enhancing sediment suspension at the head of saline wedge. Moreover, sediment suspension in both neutral and convection‐affected flows is supported by the variance of vertical turbulent velocity, rather than the shearing stress. Finally, the stability correction functions in the Monin‐Obukhov similarity theory can be simply derived from the local turbulent kinetic energy balance to successfully describe the effects of tidal straining on turbulent length scale, eddy viscosity, and sediment diffusivity in the convection‐affected flow. These recognitions may provide novel understanding of estuarine turbidity maxima, and the dynamical structure and processes for coastal hypoxia.

Role of Particle Resuspension in Maintaining Hypoxic Level in the Pearl River Estuary

“…For the CBS marked HC1 when the front created by shelf salinity intrusion passed by at 21:00 on 15th July 2020 (Figure 8b), the bottom shearing stresses were smaller than the threshold for incipient particle movement (Figure 8a). This indicates that this CBS event is not induced by bottom stresses, but rather by convection induced by tidal straining due to shelf salinity intrusion (see Zhang & Wu, 2018 for the details). Other three CBS events marked HC2, HC3, and HC4 all occurred during maximum flood tides, and they were generated by bottom shearing stresses induced by tidal stirring because their bottom stresses all exceeded the incipient threshold (Figure 8a).…”

“…Our recent studies presented the physical dynamic structures and oxygen processes for controlling the spatial distribution and maintenance of summertime hypoxia in the Pearl and Yangtze River estuaries, respectively (Cui et al., 2019; Zhang & Wu, 2018). The frontal zone presents a two‐layer gravitational circulation associated with winds, tides, and the buoyancy of river discharge in the Pearl River Estuary (Wong et al., 2003; Zu & Gan, 2015).…”

“…The surface flow of the gravitational circulation is seaward, and the bottom flow is toward the head of the estuary. The coastal hypoxia was found to occur near the front created by shelf salinity intrusion where DO vertical diffusion from river plume is highly restrained by a mid‐depth barrier layer with weak mixing (see Figure 13 in Cui et al., 2019), and concentrated bottom suspensions locally appear due to particle resuspension induced by tidal straining (Zhang & Wu, 2018). Nevertheless, the contribution of oxygen consumption caused by particle resuspension to the DO level of bottom hypoxia has not yet been clarified in this region.…”

Phytoplankton Blooms off a High Turbidity Estuary: A Case Study in the Changjiang River Estuary

“…At the same time, the Kuroshio subsurface water intrusion determines the bottom nutrient conditions in the shelf (Fan & Song, ), which supplies nutrients for phytoplankton blooming through diapycnal mixing or upwelling (Tseng et al, ; Yang et al, ). On the other hand, strong tidal‐induced mixing suspends the bottom sediments and forms a turbidity maximum zone (TMZ) in the inner estuarine area (Figure ; Li & Zhang, ; Wu et al, ), while strong stratification restricts sediment resuspension into the upper layers in the plume area (Wang et al, ; Zhang & Wu, ). The complicated hydrodynamics create a dramatic turbidity gradient, which results in variable euphotic depths across the shelf (Bian et al, ).…”

Biological–physical oceanographic coupling influencing particulate organic matter in the South Yellow Sea

“…Additionally, the combination of primary production and sediment oxygen consumption and accumulation rates can also provide a perspective of the carbon budget regarding POC production and burial (Song et al, 2016;Qiao et al, 2017). Other than interpretation of direct observations, both hydrodynamic and ecosystem dynamic models have been applied to study POC dynamics in terms of cross-shelf transport, sediment resuspension, and biological production in marginal seas (Schartau et al, 2007;Zhou C. et al, 2015;Zhang and Wu, 2018).…”