Elsevier

Ocean Engineering

Volume 228, 15 May 2021, 108905
Ocean Engineering

Hydrodynamic characteristics of lateral withdrawal in a tidal river channel with saltwater intrusion

https://doi.org/10.1016/j.oceaneng.2021.108905Get rights and content

Highlights

  • A 3-D hydrodynamic and variable density model is built and validated.

  • Hydrodynamic fields of lateral withdrawal with saltwater intrusion are analyzed.

  • Non-uniform mainstream velocity reduces upper division width and enlarges circulation.

  • Influence of mainstream velocity is larger than that of salinity stratification.

  • The sediment entering the intake would increase and some measures should be used.

Abstract

Saltwater intrusions occur widely in tidal river channels, significantly changing the vertical distribution of mainstream velocity and density (salinity). They are vital to the hydrodynamic conditions of lateral withdrawal. In this study, a 3-D numerical model was constructed and validated through experimental tests, and the hydrodynamic characteristics of lateral withdrawal under saltwater intrusion were investigated. Additionally, the vertical distribution effects of mainstream velocity, withdrawal velocity, and density stratification were analyzed. Under non-uniform mainstream velocity conditions, with larger surface velocity than bottom velocity, the division width of the upper layer is smaller, while the local secondary circulation is stronger than that of the lower layer. Secondary circulation is top-shaped and weakens as the withdrawal velocity increases. When salinity stratification exists, the division width of the lower layer decreases, and the secondary circulation is stronger than that of the upper layer; meanwhile, the vertical water exchange increases. The influence of mainstream velocity is greater than that of salinity stratification. Under these two factors, the sediment entering the intake increases, and submerged vane, sill, and weir measures should be adopted. This research represents an advance in lateral withdrawal hydrodynamic research and provides support for engineering design in estuarine areas.

Introduction

Estuary areas are affected by sea and ocean tides. Saltwater intrusion often occurs in tidal river channels and is crucial for water utilization and management (Stancanelli et al., 2017; He et al., 2018). For a tidal river channel with saltwater intrusion, freshwater tends to be above the saltwater in the salinity gradient, while the salinity and density in the bottom layers are greater than those in the upper layers. The local hydrodynamic conditions are significantly different from those of non-tide freshwater river channels (Lian et al., 2015). In addition to salinity and density stratification, the mainstream velocity vertical distribution tends to be non-uniform, with greater longitudinal velocity in surface layers than in bottom layers (Wang et al., 2019). The vertical velocity distribution often becomes bi-directional owing to highly stratified flows, with a sharp difference in interfacial density and velocity (Krvavica et al., 2016). As the freshwater inflow, tidal variation, or average sea level decrease, the average estuary salinity and saltwater intrusion length increase. The vertical salinity stratification and velocity difference temporally differ with the freshwater inflow magnitude, tidal variation, and average sea level (He et al., 2018; Wang et al., 2019; Krvavica and Ružić, 2020).

Lateral intakes are widely built in river channels. Under the effects of lateral withdrawal and inertia, the flow of mainstream water gradually changes direction and enters the intake. A critical division width can be identified in the mainstream, defined as the width at which the local upstream inflow enters the intake, expanding under a greater withdrawal velocity and discharge ratio (Yousefi et al., 2011; El Kadi Abderrezzak et al., 2011; Montaseri et al., 2019). For a straight mainstream channel with lateral intake, the bottom division width tends to be smaller than the surface division width. The opposite occurs for a curved mainstream channel (Montaseri et al., 2019). Flow division zones, separation and reversal vortices, and singular points exist within the bed-shear stress vector field (Neary et al., 1999). Secondary circulation, closed recirculation, and helix-shaped recirculation that occur at the intake entrance have significant three-dimensional characteristics (Cao et al., 2003; Asnaashari et al., 2016; Momplot et al., 2017; Montaseri et al., 2019). They are closely related to the withdrawal velocity, position, and diversion angle of the lateral intake. The secondary circulation in the upstream part of the intake is larger than that in the downstream part, expanding as the withdrawal discharge increases (Cao et al., 2003). When centrifugal force decreases under a low withdrawal velocity, closed recirculation occurs and helix-shaped recirculation occurs as the centrifugal force increases under a high withdrawal velocity (Momplot et al., 2017). For a curved channel, bi-cellular circulations with clockwise center-region circulations and counterclockwise circulations exist near the inner bank and free surface, respectively (Montaseri et al., 2019).

These hydrodynamic phenomena affect withdrawal efficacy and sediment movement. A larger division width increases sediment settlement at the intake entrance, while stronger turbulence, vortices, and secondary circulation reduce the withdrawal efficacy and increase sediment settlement (Tavakoli et al., 2019; Serajian et al., 2020). Extensive research has been conducted on the hydrodynamic characteristics of lateral withdrawal, bathymetry effects (Yousefi et al., 2011; SEYEDIAN, 2014), mainstream velocity (Herrero et al., 2015), withdrawal velocity (Lucas et al., 2015) and angle (Meselhe et al., 2012; Montaseri et al., 2019), intake width (Hashid et al., 2015), and hydraulic structures such as vanes, plates, and weirs (Moghadam and Keshavarzi, 2010; Rosier et al., 2011; Serajian et al., 2020). However, existing research mainly focuses on freshwater in non-tidal channels, and research on tidal channels with density stratification and vertical differences in mainstream velocity is scarce. With increases in water demand in estuary and offshore areas, local water intake increases, and the hydrodynamic characteristics of lateral withdrawal in tidal river channels requires urgent study.

Using the lateral intake in a tidal river channel of east Guangdong Province, China, as a study case, this work aims to build and experimentally validate a 3-D numerical model and to investigate the hydrodynamic characteristics of lateral withdrawal under the effects of saltwater intrusion. This research is divided into five sections. Section 1 presents the introduction; Section 2 presents the research methods, including the numerical simulation, experimental test, and model scenarios; and the results are described in Section 3. The lateral withdrawal flow field under different mainstream velocity vertical distributions, withdrawal velocity, and density stratifications are analyzed. The results are discussed in Section 4, and the main conclusions from this research are drawn in Section 5.

Section snippets

Governing equations

The 3-D hydrodynamic numerical model was based on the incompressible continuity and momentum equations, shown in Equations (1), (2), respectively.ρt+(ρu)=0(ρu)t+(ρuu)=-p+μρ2u+ρgwhere t is the time, u is the velocity component, p is the pressure component, μ is the dynamic viscosity coefficient, g is the gravity, and ρ is the fluid density.

The VOF method was adopted to simulate the freshwater, seawater, and their mixture. The equation for the volume fraction is shown in Equation

Model validation

The numerical model was validated (Fig. 3) through experimental tests. The validation model mesh was set according to the engineering arrangement. Its differences with latter simulation model scenarios are as follows: the bank slope is inclined (~30°, as shown in Fig. 2 b), the bottom intake elevation is −3.5 m, and the bottom boundary is set as a wall condition.

Different mainstream velocities (Vm) and withdrawal velocities (Vw) were adopted in the model validation; results are shown in Fig. 3.

Discussion

Saltwater intrusion induces vertical differences in mainstream velocity and water density (salinity). The effects of mainstream velocity distribution on the hydrodynamic characteristics of lateral withdrawal are more significant than those of density stratification. The division width of the bottom layer is significantly larger than that of the surface layer, and the bottom secondary circulation is reduced. This differs from the uniform mainstream velocity scenario, where the flow field,

Conclusions

The hydrodynamic characteristics of lateral withdrawal are crucial to the pump intaking efficiency and long-term operation. By combining numerical simulations and experimental tests, this study revealed the hydrodynamic characteristics of lateral withdrawal in a tidal river channel with saltwater intrusion, and the effects of non-uniform mainstream velocity, withdrawal velocity, and density (salinity) stratification were analyzed. The main conclusions are as follows:

  • (1)

    Under a non-uniform

CRediT authorship contribution statement

Wei He: Conceptualization, Methodology, Data curation, Formal analysis, Writing – original draft, Funding acquisition. Aili Jiang: Conceptualization, Formal analysis, Resources, Writing – review & editing. Jian Zhang: Resources, Supervision, Writing – review & editing. Hui Xu: Methodology, Formal analysis, Supervision. Yang Xiao: Formal analysis, Resources. Sheng Chen: Methodology, Validation. Xiaodong Yu: Methodology.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This research was supported by the National Natural Science Foundation of China, China (51909070), the Natural Science Foundation of Jiangsu Province, China (BK20190488), the Fundamental Research Funds for the Central Universities, China (B200201024), and the State Key Laboratory of Hydraulic Engineering Simulation and Safety, China (HESS-2019).

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