Study of Lateral Flow in a Stratified Tidal Channel‐Shoal System: The Importance of Intratidal Salinity Variation

Abstract

Lateral flow significantly contributes to the near-bottom mass transport of salinity in a channel-shoal system. In this study, an integrated tripod system was deployed in the transition zone of a channel-shoal system of the Changjiang Estuary (CE), China, to observe the near-bottom physics with high temporal/spatial resolution, particularly focusing on the lateral-flow-induced mass transport. These in situ observations revealed a small-scale salinity fluctuation around low water slack during moderate and spring tidal conditions. A simultaneous strong lateral current was also observed, which was responsible for this small-scale fluctuation. A high-resolution unstructured-grid Finite-Volume Community Ocean Model has been applied for the CE to better understand the mechanism of this lateral flow and its impact on salinity transport. The model results indicate that a significant southward near-bed shoal-to-channel current is generated by the salinity-driven baroclinic pressure gradient. This lateral current affects the salinity transport pattern and the residual current in the cross-channel direction. Cross-channel residual current shows a two-layer structure in the vertical, especially in the intermediate tide when the lateral flow notably occurred. Both observation and model results indicate that near-bottom residual transport of water moved consistently southward (shoal to channel). Mechanisms for this intratidal salinity variation and its implications can be extended to other estuaries with similar channel-shoal features.Plain Language Summary In channel-shoal systems, lateral processes sometimes can be of great significance, for example, influencing salinity and sediment transport, even some biogeochemical matters that influence the water environment. Salinity and sediment transport will have a further effect on navigability, ecology, and so on, for example, sediment deposition issues, which significantly decrease the shipping capacity. Consequently, knowing the accurate information about the flow field and finding a criterion for pronounced lateral processes are important to better understand hydrodynamics and sediment behaviors in channel-shoal systems. Here in this study, we conducted an in situ observation and used numerical simulation to detect and predict a strong lateral flow in the North Passage of the Changjiang Estuary. This shoal-to-channel lateral flow occurred around the end of ebb and caused an intratidal salinity variation at the observation site, close to the deep channel. Therefore, a link between pronounced lateral flows and typical salinity variation can be established. This link can also be found in other estuaries like Hudson River estuary. Findings presented here will help to easily distinguish pronounced lateral flows and better understand the effect of lateral flows.

Study of Sediment Transport in a Tidal Channel‐Shoal System: Lateral Effects and Slack‐Water Dynamics

“…To demonstrate the influence of groynes, the nearest groyne to the observational site was removed in the model. Earlier work (Figure 14 in Zhou et al (2019)) showed that such removal strongly reduced lateral flow during LWS. By keeping or removing the nearest groyne, we therefore have two cases with (Case 0) or without (Case 1) lateral flow during LWS, which can be used to quantify the effect of this groyne-induced impacts on lateral flow and sediment transport.…”

“…The present study further supports the importance of lateral salinity gradient to sediment transport, and particularly focuses on lateral salinity gradient during high and low water slack. Zhou et al (2019) reported a salinity-induced density gradient and the lateral flow thereon due to retention of high-salinity water by groynes. This companion study further investigated the lateral sediment transport due to this lateral flow during LWS, and also expanded findings to lateral process during HWS, extending our understanding of slack-water dynamics.…”

“…Zhou et al. (2019) reported that near‐bed lateral flows were strengthened by retention of high‐salinity water during ebb tides. This lateral flow greatly influenced lateral transport processes, as evidenced by a pronounced intratidal salinity variation near the deep channel.…”

“…However, salinity-induced density gradients which are widespread in estuarine environments can greatly impact groyne-related three-dimensional effects, such as lateral circulation, stratification, and near-bed transport of high-concentration water mass. Zhou et al (2019) reported that near-bed lateral flows were strengthened by retention of high-salinity water during ebb tides. This lateral flow greatly influenced lateral transport processes, as evidenced by a pronounced intratidal salinity variation near the deep channel.…”

Lateral Circulation and Associated Sediment Transport in a Convergent Estuary

“…Lateral circulation and lateral sediment transport have received increasing attention because lateral processes appear to play important roles in exchange flow (Cheng et al, 2017; Cheng & Valle‐Levinson, 2009; Lerczak & Rockwell Geyer, 2004; M. Li et al, 2014; Scully et al, 2009), estuarine stratification (Aristizábal & Chant, 2014; Scully & Geyer, 2012), and sediment trapping (Fugate et al, 2007; Huijts et al, 2006; Kim & Voulgaris, 2008; Lacy et al, 2014; Ralston et al, 2012). Several mechanisms have been proposed that drive lateral flows, including differential advection (Huijts et al, 2009, 2011; Lerczak & Rockwell Geyer, 2004; Nunes & Simpson, 1985), Ekman forcing (Cheng et al, 2017; Huijts et al, 2006), diffusive boundary layers (Chen & Sanford, 2009; Garrett et al, 1993), cross‐stream density gradients (Chen & Sanford, 2009; Scully et al, 2009; Zhou et al, 2019), channel curvature (Chant, 2002; Kim & Voulgaris, 2008), and wind forcing (Chen et al, 2009; Y. Li & Li, 2011) and interaction between barotropic tides and bathymetry variations in cross section (Valle‐Levinson et al, 2000; Zhu et al, 2018). In estuaries with lateral variations of bathymetry, these mechanisms compete with or reinforce each other.…”