Effect Of Sediment Concentration Research Articles

Coupled two-dimensional model for heavily sediment-laden floods and channel deformation in a braided reach of the Lower Yellow River

It is difficult to predict the detailed morphodynamic processes induced by heavily sediment-laden floods in alluvial rives, due to the complexity in topography and the significant channel evolution rates, especially in a braided reach of the Lower Yellow River (LYR). A two-dimensional morphodynamic model using a coupled solution approach was firstly developed, with the effects of sediment concentration and bed deformation being directly considered in the hydrodynamic governing equations. The finite volume method with the HLL-MUSCL scheme was adopted in the numerical solution, which can obtain second-order accuracy in space and time. In addition, unstructured meshes were used to accommodate the irregular boundaries. The proposed model was validated using measurements in the braided reach of the LYR during the flood events of 2020 and 2004, with general agreement obtained between the calculated and measured hydrographs of flow and sediment concentration. The highest accuracy was achieved in the simulation of discharge, with a Nash-Sutcliffe efficiency coefficient (NSE) of 0.92. The simulation accuracy in sediment concentration was improved by the coupled approach, with the relative error of the calculated peak value decreasing from 17 % to 9 %. In addition, transport characteristics of graded suspended sediments were well simulated, with the fine fraction being transported downstream and the medium fraction depositing in the study reach. Influences of channel adjustments on the sediment transport processes were investigated during a flood event. The results demonstrated that the incised channel in 2020 was characterized by a larger water depth and smaller velocity, as compared with the shallow channel in 2004. Consequently, the peak discharge decreased and the flood propagation time was extended under the 2020 topography, associated with the lower sediment discharge at the outlet due to the reduced sediment transport capacity.

Sediment erosion characteristics of Pelton turbine runner: Effects of sediment concentration and diameter

The stable operation of Pelton turbine under sediment conditions is the key guarantee of the hydropower storage power generation system. This research focuses on sediment erosion characteristics of a full-size Pelton turbine runner under rated working conditions through numerical simulation. The primary objective is to study the influences of different sediment concentrations (0.6 %, 0.45 %, and 0.15 %) and diameters (1 mm, 0.1 mm, 0.01 mm, and 0.001 mm) on Pelton turbine runner. Primarily, Runner efficiency is most affected by the concentrations of sediment present, followed by the diameters. Additionally, the relationships between the different sediment concentrations and diameters and the sediment erosion characteristics of the runner are established by comparing the results of experimental and numerical simulations. The sediment concentration mainly affects the surface erosion amounts and raising the sediment concentration exacerbates the surface erosion amounts. The sediment diameters primarily affect the distribution of erosion regions on the runner. Sediment particles with large diameters (d ≥ 0.1 mm) tend to create concentrated erosion regions on both sides of the water outlets of the buckets and at the splitter edge. Conversely, sediment particles with small diameters (d < 0.1 mm) result in additional dispersed erosion regions on the surfaces of the buckets.

One-dimensional morphodynamic model for a tail reach of the Yellow River estuary

A one-dimensional morphodynamic model was improved to study the unsteady flow and sediment transport processes in a tail reach of the Yellow River estuary (YRE). In this model, the modified equations for flow hydrodynamics take the effects of sediment concentration and bed evolution into consideration. The improved model was validated using measurements in the tail reach of the YRE for the years 2003 and 2015, and the model showing an excellent data fit for discharge, water level, suspended sediment concentration and the cumulative volume of channel evolution. The effect of different cross-sectional spacings was also studied using the model results in the tail reach, with various numerical tests of the 2015 flood event conducted. It was found that an appropriate cross-sectional distance of less than 3.14 km should be adopted when simulating the process of bed deformation. Moreover, the interaction between runoff and tide in 2015 was studied on the basis of the simulated water levels and an empirical relation for the location of tidal limit was established. The results show that the tidal limit in 2015 was 15.84 km downstream of Xihekou.

Numerical Study on the Effect of Sediment Concentration on Jump Characteristics in Trapezoidal Channels

Hydraulic jump is a phenomenon observed in open channel flow, and studying it has been a challenge for scientific communities. Most of the previous studies on this topic have been devoted to the cases of clear water in rectangular geometry. However, in nature river flows carry sediment loads; meanwhile, trapezoidal stilling basins are proved to be more economical as well. In this paper, the RNG turbulence model has been used to simulate jump characteristics in a symmetrical trapezoidal channel of 45° side slope for clear water flow and three different sediment concentrations of 4%, 8% and 12% and a range of initial Froude number between 1.5 and 12. The results indicate that by increasing the sediment concentration the bed shear stress amounts and energy loss ratio increase while the length of jump ratio decreases. In comparison with the clear water flow, for the case of Fr1 = 7 and sediment concentration of 12%, the length of jump ratio decreases by 18.1%, the sequent depth ratio increases by 17.2% and the energy loss ratio increases by 3.2%. Furthermore, the bed shear stress at the beginning and at the end of the jump is innovatively investigated. The results show that the shear stress at the initial depth section is not dependent on the sediment content, but at the secondary depth of flow, it is highly dependent on the suspended sediment loads content.

Modelling of hyperconcentrated flood routing and channel evolution in the lower Weihe River

Hyperconcentrated floods with sediment concentrations higher than 300 kg/m3 often occur in the lower Weihe River. These floods result in extremely high water levels and severe channel evolution and may pose a threat to people and property in certain reaches. Therefore, it is necessary to accurately simulate hyperconcentrated flood routing processes and the associated channel evolution in the lower Weihe River. A one-dimensional (1D) coupled morphodynamic model was developed to simulate the 1978 hyperconcentrated flood in the lower reach between Lintong and Huayin, and the effects of sediment concentration and bed evolution on flood routing were considered in this model. The calculated hydrographs of the discharge and sediment concentration agreed well with the data observed at hydrometric sections, with corresponding correlation coefficients higher than 0.93. The calculated channel evolution amount in the lower Weihe River was 0.14 × 108 t based on the 1D numerical model, which was lower than the observed value (0.22 × 108 t). In addition, we investigated the effects of the thickness of the active layer and the size distribution of the incoming suspended load on the model results. The maximum sediment concentrations at hydrometric sections and the cumulative channel evolution volume of the study reach increased with an increasing active layer thickness, and with a decreasing mean median diameter of the suspended load. Two metres is a reasonable active layer thickness for simulations of hyperconcentrated flood events in the lower Weihe River.

Experimental study on the effect of sediment concentration on trapezoidal hydraulic jump

The effect of suspended sediment concentration on the characteristics of a hydraulic jump in a symmetrical trapezoidal channel was studied experimentally and compared with a classic jump. The first test series of the classical jump was performed in a rectangular flume, while the second series was conducted with two different trapezoidal side slopes. In both cases, the tests were conducted under clear water and with different concentrations (4, 8 and 12%) of fine sand of 0.22 mm as suspended sediment. The results of the tests showed that, with an increase in sediment concentration, the jump length ratio and the sequent depth ratio decreased and the energy dissipation increased. The present study considered three different sections (two sections of trapezoidal shape with side slopes of 60° and 75° and one rectangular section). A comparison of the results in flow with Fr1 = 10 for all three sections for a sediment concentration of 12% with clear water showed that the jump length ratio decreased by 14%, 22% and 18%, the sequent depth ratio decreased by 12%, 21% and 31% and the energy dissipation ratio increased by 9%, 12% and 11%, respectively. Some dimensionless relations for the prediction of jump characteristics were also developed.

Experiments on measuring and verifying sediment trapping capacity of grass strips

Vegetation-restored hillslopes not only reduce erosion but also trap sediment from uphill silt-laden inflow. To investigate the sediment trapping mechanism of grass strips, a series of crossed experiments with various factors, including slope (5–20°), sediment concentration (40–160 g L−1), and unit flow rate (7.5–45.0 L min−1 m−1) were conducted using a 10-m-long and slope adjustable soil plot. The duration of each experiment was longer than required time to reach the stable state of sediment trapping, which ensures that the whole process of sediment trapping can be monitored and the maximum sediment trapping yield (sediment trapping capacity, Rm) can be obtained. The results showed that gentler slopes generated higher instantaneous sediment trapping efficiency (ISTE) and greater Rm. Higher sediment concentration or larger unit flow rate led to lower ISTE but greater Rm. The impact of slope on Rm amplified when the sediment concentration increased, implying that the interaction among these factors affects sediment trapping process. The sediment trapping effect of the first 2-m width grass strips performed better than the subsequent strips. 90% of sediment deposition occurred within first half of the time needed to reach the stable state. Slope had similar effect intensity on sediment trapping in each section of grass strips, so did flow rate, whereas the effect of sediment concentration was concentrated primarily in the first 5-m width. The standard regression coefficients of the multiple curve regression equation showed that the intensities of the impacting factors on Rm were as follows: slope > grass strips width > unit flow rate > sediment concentration. It indicated that slope plays a key role in controlling Rm. These results are helpful in assessing the effects of vegetation restoration on sediment transport at a hillslope scale.

Effect of Sediment Concentration on Hydraulic Characteristics of Energy Dissipation in a Falling Turbulent Jet

The effect of sediment on the hydraulics of jet energy dissipation is an urgent issue for high dams built on sediment-laden rivers. Accordingly, flume experiments were conducted using a ski-jump type energy dissipator in flows of four sediment concentrations (0 kg/m3, 50 kg/m3, 150 kg/m3, and 250 kg/m3) to determine the effects on discharge, flow regime, and hydrodynamic pressure in a plunge pool. The results demonstrate that the effect of sediment on discharge is constant, regardless of sediment concentration, when compared to fresh water. The width of the nappe decreased with increasing concentrations of sediment. The length of the jet trajectory increased with upstream water head. The time-averaged pressure and fluctuation pressure both exhibited peaks, describing the impact of the jet on the bottom of the plunge pool. The maximum time-averaged pressure and maximum fluctuating pressure both noticeably increased with upstream water head and slightly increased with sediment concentration for a given flow condition. The results also demonstrated that the dominant frequency of fluctuation trends to lower values, and that both the fluctuating energy and vortex scale increase with increasing sediment concentrations due to increased viscosity. These findings can be used to improve energy dissipation in dams on sediment-laden rivers.

Case study: Effects of sediment concentration on the wear of fluvial water pump impellers on Brazil's Acre River

The most important factor causing surface wear in hydraulic systems is the impact produced by abrasive particles suspended in the water flow. The impact conditions, such as mean particle velocity; particle mass; abrasive particle concentration in the liquid flow (number of particles per unit of liquid volume, size distribution of the particles); angle of attack at which the particles collide with the surface; and duration of the effect produced by the particles on the surface contribute to the development of the abrasion process. The wear resistance of the centrifugal pump impellers depends on the characteristics of the materials used in their manufacture and the abrasiveness of the suspended sediments in the pumped water. Despite special care in choosing the material used in its manufacture, abrasive wear is practically impossible to totally avoid. This work analyses the wear of ductile cast iron impeller blades of a centrifugal pump, used at the Fluvial Water Elevation Station (FWES) of the Acre River sedimentary basin in Brazil, due to the sediment concentration variation and the river water level. To determine the wear pattern and the relationship between the material and specific wear coefficient (k) due to increased sediment concentration, a ball-cratering microscale abrasive wear test was performed on samples of ductile cast iron, withdrawn from a used impeller, and abrasive suspensions at concentrations of 1, 2, 3, 5 and 10 g L−1 of river sediment in distilled water. The wear volume to the impeller blades due to the relative velocity of the mixture (water + sediment) and the wear accumulated in an annual hydrological cycle were estimated mathematically. The results showed that: i) the k remains constant regardless of the normal force (FD) applied in the tests in the abrasometer; ii) the wear tests showed the abrasive capacity of the sediments at different concentrations; and iii) the rotational control of the pump speed based on the sediment concentration and the river water level can contribute to a reduction in wear of up to 30% considering the hydrological cycle.

Improved criterion for plunge of reservoir turbidity currents

It is important to investigate the condition of turbidity current formation in reservoir sediment management. Previous criteria for turbidity current plunge are not applicable to hyperconcentrated turbidity currents. The governing equations for the motion of a turbidity current are refined, and an improved criterion for predicting the plunge of turbidity currents is derived based on the refined equations, with the effect of sediment concentration being included explicitly. The proposed criterion is calibrated using laboratory experimental data, which indicates that this improved criterion could be applicable to hyperconcentrated turbidity currents with sediment concentrations less than 400 kg/m3 as a kind of Newtonian fluid, and the plunging depth would be in direct proportion to the unit discharge to the power of 2/3 and would be in inverse proportion to the volumetric sediment concentration to the power of 1/4. The improved criterion is then verified using another three datasets, including measurements obtained from the prototype and model of the Xiaolangdi reservoir, and these verified results indicate that the predicted plunging depths agree well with the observed values, with a comprehensive correlation coefficient of 0·94. Therefore, the improved criterion can be used to predict the plunge of turbidity currents in the Xiaolangdi reservoir.

Numerical Modeling of Sediment Transport and Its Effect on Algal Biomass Distribution in Lake Pontchartrain Due to Flood Release from Bonnet Carr&amp;#233; Spillway

In order to protect the city of New Orleans from the Mississippi River flooding, the Bonnet Carre Spillway (BCS) was constructed from 1929 to 1936 to divert flood water from the river into Lake Pontchartrain and then into the Gulf of Mexico. During the BCS opening for flood release, large amounts of freshwater, nutrients, sediment, etc. were discharged into Lake Pontchartrain, and caused a lot of environmental problems. To evaluate the environmental impacts of the flood water on lake ecosystems, a two-dimensional numerical model was developed based on CCHE2D and applied to simulate the flow circulation, sediment transport and algal biomass distribution in Lake Pontchartrain. The effect of sediment concentration on the growth of algae was considered in the model. The numerical model was calibrated using field measured data provided by USGS, and then it was validated by the BCS Opening Event in 1997. The simulated results were generally in good agreement with filed data and satellite imagery. The field observation and numerical model show that during the spillway opening for flood release, the sediment concentration is very high, which greatly restricts the growth of algae, so there is no algal bloom observed in the lake. After the closure of BCS, the sediment concentration in the lake reduces gradually, and the nutrient concentration of the lake is still high. Under these conditions, numerical results and satellite imagery showed that the chlorophyll concentration was high and algal bloom might occur.

The adsorption/desorption of phosphorus in freshwater sediments from buffer zones: the effects of sediment concentration and pH.

Riparian buffer zones adjacent to reservoirs and lakes protect aquatic ecosystems from polluted surface runoff. Sediments, collected from the buffer zones of Danjiangkou Reservoir (SR) and Honghu Lake (SL) in an ecologically fragile region in central China, were evaluated to reveal their phosphorus-adsorbing/desorbing properties and storage capacities. A nonlinear regression method was used to fit the pseudo-second-order kinetic and the modified crossover-type Langmuir isotherm models to the experimental data. It is shown that the adsorption of phosphorus onto the studied sediments followed the pseudo-second-order kinetic expression. The modified crossover-type Langmuir isotherm model was found to be a suitable method for describing adsorption/desorption processes in the experimental sediments. The maximum adsorption capacities (Q m), partitioning coefficients (K p), native adsorbed exchangeable phosphorus (NAP), and equilibrium phosphorus concentration (EPC0) were subsequently obtained for the experimental sediments. The effects of sediment concentration and pH were also investigated by batch experiments and Fourier transformation infrared and scanning electron microscopy analyses. The adsorption/desorption characteristics of different phosphate species on the sediments from reservoir and lake buffer zones were identified.

English

The effects of sediment concentration and season on coral recruitment algal abundance and benthic community structure were studied in Kenyan coral reef lagoons to determine their potential influence on coral recovery. Nutrient levels and recruit numbers were higher during the southeast monsoon (SEM) than during the northeast monsoon (NEM) season and in sediment-exposed compared to non-sediment exposed reefs. Mean algal biomass also exhibited the same seasonal trend (except at one site), but was higher in the non-sediment exposed reef compared to the other reefs. Corals in the sediment exposed reef exhibited morphological differences relative to the other reefs: fewer corymbose and plate-like but more branching, massive and solitary forms and increased colony and corallite sizes. However, sediments did not suppress coral recruitment rates. These morphological changes coupled with the interaction between biological and physico-chemical characteristics have important ecological and geological implications: by potentially modifying calcium carbonate production and ameliorating the adverse effects of climate induced stress events, this may minimize coral mortality and enhance reef recovery. Key words: Algal biomass, coral recruitment, hydrodynamics, coral morphology, seasonality, sediments.

Effect of self-stratification on sediment diffusivity in channel flows and boundary layers: a study using direct numerical simulations

Abstract. Sediment transport in nature comprises of bedload and suspended load, and precise modelling of these processes is essential for accurate sediment flux estimation. Traditionally, non-cohesive suspended sediment has been modelled using the advection–diffusion equation (Garcia, 2008), where the success of the model is largely dependent on accurate approximation of the sediment diffusion coefficients. The current study explores the effect of self-stratification on sediment diffusivity using suspended sediment concentration data from direct numerical simulations (DNS) of flows subjected to different levels of stratification, where the level of stratification is dependent on the particle size (parameterized using particle fall velocity Ṽ and volume-averaged sediment concentration (parameterized using shear Richardson number Riτ. Two distinct configurations were explored, first the channel flow configuration (similar to flow in a pipe or a duct) and second, a boundary-layer configuration (similar to open-channel flow). Self-stratification was found to modulate the turbulence intensity (Cantero et al., 2009b), which in turn was found to reduce vertical sediment diffusivity in portions of the domain exposed to turbulence damping. The effect of particle size on vertical sediment diffusivity has been studied in the past by several authors (Rouse, 1937; Coleman, 1970; Nielsen and Teakle, 2004); so in addition to the effect of particle size, the current study also explores the effect of sediment concentration on vertical sediment diffusivity. The results from the DNS simulations were compared with experiments (Ismail, 1952; Coleman, 1986) and field measurements (Coleman, 1970), and were found to agree qualitatively, especially for the case of channel flows. The aim of the study is to understand the effect of stratification due to suspended sediment on vertical sediment diffusivity for different flow configurations, in order to gain insight of the underlying physics, which will eventually help us to improve the existing models for sediment diffusivity.

Concentration distribution in an open channel flow by observational approach

This study presents empirical models for two different types of suspended sediment concentration distribution in vertical direction for straight rectangular open channel flow. Initially, asymptotic solutions for inner and outer suspension regions are proposed by observing the data patterns; and finally, analytical models are developed by applying Guo’s logarithmic matching method. As a consequence, it is found that the volumetric sediment concentration is best expressed by a logarithmic law for type I profile (maximum concentration appears at the channel bottom) and by a power law for type II profile (maximum concentration appears at some distance above channel bottom) throughout the flow depth. Models are compared with existing models and experimental data and fairly good agreement is obtained in each case. The effects of sediment concentration on model parameters are also studied, and their variations are expressed by empirical relations.

Experimental Study of Sand Sediments Effects on Submerged Hydraulic Jump Properties

Abstract. In this study, the effect of sediment concentration on submerged hydraulic jump (SHJ) characteristics such as jump length, submerged depth on the gate and the energy dissipation is investigated. Experiments were carried out in a flume of 46 cm depth, 12 m length. The width of the flume changes from 10 cm (at the entrance) to 60 cm (at the exit). Sediment load and flow concentration have an influence on submerged hydraulic jump characteristics including submerged depth on the gate, jump length and relative energy dissipation. It is shown that at high Froude numbers increasing the suspended sediment concentration to 28.7 gr/l leads to a reduction in the submerged depth on the gate up to 6% and jump length up to 10%. Also, the energy dissipation of the submerged hydraulic jump increases by 4% and turbulence resulting from the jump leads to upright distribution of concentration at the end of the jump. Also in concentrations higher than 30 gr/l, flow is not able to carry the whole sediments and subsequently leads to their deposition in subcritical area and behind the sluice gate.

Characteristics of energy dissipation in hyperconcentrated flows

An equilibrium equation for the turbulence energy in sediment-laden flows was derived on the basis of solid–liquid two-phase flow theory. The equation was simplified for two-dimensional, uniform, steady and fully developed turbulent hyperconcentrated flows. An energy efficiency coefficient of suspended-load motion was obtained from the turbulence energy equation, which is defined as the ratio of the sediment suspension energy to the turbulence energy of the sediment-laden flows. Laboratory experiments were conducted to investigate the characteristics of energy dissipation in hyperconcentrated flows. A total of 115 experimental runs were carried out, comprising 70 runs with natural sediments and 45 runs with cinder powder. Effects of sediment concentration on sediment suspension energy and flow resistance were analyzed and the relation between the energy efficiency coefficient of suspended-load motion and sediment concentration was established on the basis of experimental data. Furthermore, the characteristics of energy dissipation in hyperconcentrated flows were identified and described. It was found that the high sediment concentration does not increase the energy dissipation; on the contrary, it decreases flow resistance.

Effects of Sediment Concentration and Initial Phosphorus Loading on Phosphate Adsorption in the Chongqing Reach of the Yangtze River

The results are presented from a laboratory and theoretical study to investigate the effects of sediment concentration and initial phosphorus loading on adsorption isotherm and capacity. The tested aquatic sediment was collected from the Chongqing reach of the Yangtze River. The adsorption isotherm of dissolved phosphate was approximated by the Langmuir equation. Using mass conservation and Langmuir adsorption kinetic equations, the authors derived formulas for calculating both the particulate and dissolved phosphate concentration. Kinetic experiments on fine sediment samples (<0.008 mm) show that the adsorption process is time dependent and takes place in three stages, namely, an initial fast stage followed by a gradual adsorption, leading to the eventual equilibrium. The results indicate that larger initial phosphate loading causes longer duration, higher adsorption rate, and less adsorption percentages on both the first and second stages. The sediment concentration mainly influences the adsorption rate of the first stage, in which the adsorption rate significantly increases with the increasing sediment concentration.

One-dimensional explicit finite-volume model for sediment transport

A 1D finite-volume model has been established to simulate the nonequilibrium transport of nonuniform sediment with transient flows, such as dam-break flow and overtopping flow, over movable beds. The effects of sediment transport and bed change on the flow are considered in the flow continuity and momentum equations. An explicit algorithm is adopted to solve the governing equations. The model has been tested preliminarily in several experimental cases, including an experiment on wedge development due to sediment overloading under transcritical flow conditions, an experiment on dam surface erosion due to overtopping flow, and two experiments on dam-break flow over movable beds. The model performs quite well in the cases of wedge development and overtopping flow, but significantly under-predicts the bed erosion due to dam-break flow. A modification has thus been made by considering the effects of sediment concentration on sediment settling and entrainment in the case of dam-break flow over movable beds. The present model has been compared with the traditional movable-bed “clear-water” model that ignores the effects of sediment on the flow. It has been found that the present model provides more reasonable predictions, and the “clear-water” model has larger errors when sediment transport is stronger and may even fail in the case of dam-break flow.

One-Dimensional Modeling of Dam-Break Flow over Movable Beds

A one-dimensional model has been established to simulate the fluvial processes under dam-break flow over movable beds. The hydrodynamic model adopts the generalized shallow water equations, which consider the effects of sediment transport and bed change on the flow. The sediment model computes the nonequilibrium transport of bed load and suspended load. The effects of sediment concentration on sediment settling and entrainment are considered in determining the sediment settling velocity and transport capacity. In particular, a correction factor is proposed to modify the Van Rijn formulas of equilibrium bed-load transport rate and near-bed suspended-load concentration for the simulation of sediment transport under high-shear flow conditions. The governing equations are solved by an explicit finite-volume method with the first-order upwind scheme for intercell fluxes. The model has been tested in two experimental cases, with fairly good agreement between simulations and measurements. The sensitivities of the model results to parameters such as the sediment nonequilibrium adaptation length, Manning's roughness coefficient and the proposed correction factor have been verified. The proposed model has also been compared to an existing model and the results indicate the new model is more reliable.