Formation Of Turbidity Maximum Research Articles

Formation of Turbidity Maximum in the Modaomen Estuary of the Pearl River, China: The Roles of Mouth Bar

AbstractAn estuarine turbidity maximum (ETM) functions as a filter and plays a key role in transporting suspended sediment. The features of ETM and their formation mechanisms vary with different types of estuaries. Their responses to morphological factors require further study. In this study, the Modaomen Estuary of the Pearl River was selected to examine the formation mechanisms of the ETM and roles of the mouth bar. Hydrodynamics, suspended sediment concentration, and sediment size distribution were measured via cruise shipboard in the mouth bar zone during the dry season of 2020, which indicated that the ETM occurred in the central and western bars. Furthermore, numerical modeling of an idealized estuary based on Delft3D was performed to explore the effects of the existence of a mouth bar on the dynamic structures causing the ETM, including saltwater intrusion, residual longitudinal circulation, and bed sediment resuspension. The results indicated that the existence of a mouth bar contributed to the occurrence of double residual circulations and enhanced bed shear stress on the mouth bar, which favor the ETM formation. In general, two different dynamic regimes were responsible for the ETM formation, that is, sediment trapping by saltwater intrusion and sediment resuspension induced by tidal currents. The mouth bar played a key role in these two mechanisms, including altering the estuarine circulation and bed shear stress. The results contribute to a better elucidation of the ETM formation in estuaries and provide scientific guidelines for estuarine management and engineering.

Turbidity maximum formation and its seasonal variations in the Zhujiang (Pearl River) Estuary, southern China

Real-time observations in the field and numerical simulations (with Delft3D) were combined to study the formation, distribution and the relevant influencing factors of turbidity maximum (TM) in the Zhujiang (Pearl River) Estuary (ZE). The spatial distribution pattern of the TM varies with the longitudinal distributions of salinity and suspended sediment concentration (SSC). The SSC is enhanced and the TM is intensified during dry seasons, whereas the center of the TM moves upstream by a distance of 10 km during wet seasons. The formation of the TM is influenced by a complex combination of numerous factors, including tides, river discharges and topography, wherein sediment resuspension and vertical circulation dominate the formations and variability of the TM.

Turbidity maximum formation in a well-mixed macrotidal estuary: The role of tidal pumping

Traditionally, vertical circulation (induced by gravity circulation and tidal straining), tidal pumping, and resuspension are suggested as the major processes for the formation and maintenance of the estuarine turbidity maximum (ETM). Due to strong mixing, tidal pumping is considered as the dominating process in macrotidal estuaries. To analyze field observation data, the classical empirical decomposition method is commonly suggested, but the tidal pumping flux (TPF) based on this method may lead to erroneous conclusions about the mechanisms of ETM formation because the effects of advection induced by the horizontal SSC gradient and fine bed sediment supply are ignored. If these effects are included, the TPF clearly reproduces the convergence patterns and thus demonstrates its role in the formation of the ETM. By a simplified analytical solution, the TPF is the result of the competition between the downstream flux induced by the river current together with the lag in sediment response and the upstream flux induced by tidal asymmetry and the lag. Field observations in the well-mixed macrotidal Yalu River estuary (located between China and North Korea) were analyzed. Tidal pumping is identified as the dominant mechanism of its ETM formation, and the position of the ETM for different river discharges and sediment settling velocities can be predicted by the concept of tidal pumping by numerical and analytical procedures. The present study provides a typical example of how to evaluate the tidal pumping contributions on ETM formation using the combined information provided by field data, numerical modeling results, and analytical solutions.

A two-phase numerical model for suspended-sediment transport in estuaries

This paper presents a full 2-D X/Z numerical model for sediment transport in open channels and estuaries using a two-phase (fluid–solid particle) approach. The physical concept and the mathematical background of the model are given and test-cases have been carried out to validate the proposed model. In order to illustrate its feasibility for a real estuary, the model has been applied to simulate the suspended-sediment transport and the formation of turbidity maximum in the Seine estuary. The numerical results show that the main characteristics of estuarine hydro-sediment dynamics in the Seine estuary are in fact reproduced by the proposed model. A qualitative agreement between the numerical results and the actual observations has been obtained and is presented in this paper.

Using ADV for cohesive sediment settling velocity measurements

Using an Acoustic Doppler Velocimeter (ADV) to simultaneously measure the turbulent diffusivities and Suspended Sediment Concentrations (SSC) at one point in the water column provides an in-situ approach to measure the settling velocity of cohesive sediments. This approach does not alter the ambient turbulence, SSC, salinity and biological constituents that are the major factors affecting floc density and size, and thus, the settling velocity. The settling velocity measured using this technique is much higher than that obtained from the traditional Owen Tube method, which eliminates the effects of ambient turbulence. Using the settling velocities measured from this new approach and the erosion rates from in-situ measurements in a numerical study of the dynamics of turbidity maximum in the York River, the simulated results were quite reasonable. This is encouraging because it was the first time that a modeling study used in-situ measured settling velocities and erosion rates together and correctly predicted the formation of turbidity maximum. Although this is a promising approach, there are improvements that should be included to establish the procedures of using an ADV for measuring the settling velocity of cohesive sediments.

Generation of overtides and compound tides in Amazon estuary

Tidal propagation in estuaries is affected by friction and fresh water discharge, besides changes in the depth and morphology of the channel. Main distortions imply variations in the mean water level and asymmetry. Tidal asymmetry can be important as a mechanism for sediment accumulation and turbidity maximum formation in estuaries, while mean water level changes can affect navigation depths. Data from several gauges stations from the Amazon estuary and the adjacent coast were analyzed and a 2DH hydrodynamic model was configured in a domain covering the continental shelf up to the last section of the river where the tidal signature is observed. Based on data, theoretical and numerical results, the various influences in the generation of estuarine harmonics are presented, including that of fresh water discharge. It is shown that the main overtide, M4, derived from the most important astronomic component in the Amazon estuary, M2, is responsible for the tidal wave asymmetry. This harmonic has its maximum amplitude at the mouth, where minimum depths are found, and then decreases while tide propagates inside the estuary. Also, the numerical results show that the discharge does not affect water level asymmetry; however, the Amazon river discharge plays an important role in the behavior of the horizontal tide. The main compound tide in Amazon estuary, Msf, generated from the combination of the M2 and S2, can be strong enough to provoke neap low waters lower than spring ones. The results show this component increasing while going upstream in the estuary, reaching a maximum and then slightly decaying.

The formation mechanisms of turbidity maximum in the Pearl River estuary, China

The formation and dynamics of turbidity maxima (TM) in the Pearl River estuary (PRE) are not well understood but the existence of TM in the estuary have great potential engineering and environmental impacts. Based on the measurements of two large-scale field surveys conducted in the wet season of 1978 (July of 1978) and the dry season of 1979 (March of 1979), the existence and seasonal variations of TM in the PRE focusing on the two major natural navigation channels have been studied. The sediment transport fluxes over two consecutive tidal cycles have been analyzed in detail. The analysis results reveal that the formation mechanisms of TM in the estuary are rather complex. In general, gravitational circulation, tidal trapping, and sediment resuspension and deposition processes are the primary TM formation mechanisms in the PRE. The clockwise back flow pattern around Lingding Island also leads to the formation of TM in the West channel of the PRE. The occurrence of TM far upstream of the salt water wedge is the result of the complex hydrodynamic and sediment transport processes generated by the runoff of the major rivers.

Modelling the Turbidity Maximum in the Seine Estuary (France): Identification of Formation Processes

A two-dimensional depth-averaged model and a three-dimensional model for tide propagation and fine sediment transport are used to determine the respective role of several processes on the turbidity maximum behaviour in the Seine estuary. Consolidation processes have been taken into account in the 2DH model. Despite simplified sedimentological formulations and even without any gravitational effect, a turbidity maximum, close to the observations, is reproduced during spring tides. This result confirms the large contribution of tidal pumping on the turbidity maximum formation. The sediment behaviour (settling, erodability) acts in modifying the concentration and location of the tidal-induced turbidity maximum. The bathymetry influences the tidal propagation, but also the location of deposited sediment patterns and consequently, modifies the location of the turbidity maximum. The three-dimensional model allows accounting for all the forcing while the 2DH model does not include the salinity gradients and thus the density circulation, nor the vertical profile of velocity and concentration, which change the deposited sediment distribution. Density gradients act in maintaining the fine sediment in the estuary and in stretching the dynamical accumulation towards the upper limit of the salinity gradients. The turbidity maximum results from the superposition of both processes: (a) the dynamical accumulation of suspended sediment, related to tidal pumping and river flow, modelled by bathymetry and dependent on sediment behaviour, (b) salinity gradients, only related to river flow and bathymetry, which modify its structure.