scholarly journals Comparison of Methods for Determining Erosion Threshold of Cohesive Sediments Using a Microcosm System

2021 ◽  
Vol 8 ◽  
Author(s):  
Hun Jun Ha ◽  
Ho Kyung Ha
Keyword(s):  
Suspended Sediment ◽  
Sediment Concentration ◽  
Surface Erosion ◽  
Intertidal Flat ◽  
Cohesive Sediments ◽  
Mean Values ◽  
Erosion Threshold ◽  
Mud Flat ◽  
Em Method ◽  
Tidal Emersion

Erosion of cohesive sediments is a ubiquitous phenomenon in estuarine and intertidal environments. Several methods have been proposed to determine the surface erosion threshold (τc0), which are still debatable because of the numerous and uncertain definitions. Based on erosion microcosm experiments, we have compared three different methods using (1) eroded mass (EM), (2) erosion rate (ER), and (3) suspended sediment concentration (SSC), and suggested a suitable method for revealing the variation of erodibility in intertidal sediments. Erosion experiments using a microcosm system were carried out in the Muuido tidal flat, west coast of South Korea. The mean values of τc0 for three methods were: 0.20 ± 0.08 Pa (EM); 0.18 ± 0.07 Pa (ER); and (3) 0.17 ± 0.09 Pa (SSC). The SSC method yielded the lowest τc0, due to the outflow of suspended sediment from the erosion chamber of the microcosm. This was because SSC gradually decreased with time after depleting the erodible sediment at a given bed shear stress (τb). Therefore, the regression between SSC and applied τb might skew an x-intercept, resulting in the underestimation (or “not-determined”) of τc0. The EM method yielded robust and accurate (within the range of τb step at which erosion begins) results. The EM method represents how the erodible depth thickens as τb increases and therefore seems better suited than the SSC and ER methods for representing depth-limited erosion of cohesive sediments. Furthermore, this study identified the spatiotemporal variations of τc0 by EM method in an intertidal flat. The τc0 in mud flat was about two times higher than that in mixed flat. Compared to the end of tidal emersion, the sediment was 10–40% more erodible at the beginning stage.

Determination of Settling Velocity of Suspended Sediment in Estuary

2014 ◽  
Vol 670-671 ◽  
pp. 805-808 ◽  
Author(s):  
Hui Ming Huang ◽  
Da Ke Chen ◽  
Wei Na Zhang ◽  
Cheng Chen
Keyword(s):  
Suspended Sediment ◽  
Settling Velocity ◽  
Sediment Concentration ◽  
Fine Sediment ◽  
Cohesive Sediments ◽  
The Changjiang River ◽  
Sediment Particle Size ◽  
Logarithmic Distribution ◽  
Sediment Data

The settling velocity of sediment is a hot issue and a basic problem in study of sediment transport and estuarine engineering. According to field hydrodynamics and sediment data around the South Passage of the Changjiang River in China, this paper detected the characteristics of sediment particle size and vertical distribution pattern of suspended sediment concentration, and further estimated the settling velocity of suspended sediment in three methods. The results show that the sediments including suspended and bed load can be categorized into cohesive sediments and the sediment concentration profile agree well with logarithmic distribution. Furthermore, by comparison, it is found that the Rouse formula is more reasonable for estimating the settling velocity of fine sediment, but the Zhang Ruijin and Stokes formula obviously underestimate the values of settling velocity, caused by do not taking into account the flocculation of fine sediment in estuary.

scholarly journals An Assimilation Simulation Approach to Suspended Sediment Concentration in Inland Lakes Using Hybrid Perturbation Generation Method

2021 ◽  
Author(s):  
Fei Guo ◽  
Jingjia Zhang ◽  
A-xing Zhu ◽  
Zhuo Zhang ◽  
Hong Zhang
Keyword(s):  
Suspended Sediment ◽  
Suspended Sediment Concentration ◽  
Random Perturbation ◽  
Suspended Sediments ◽  
Sediment Concentration ◽  
Hydrological Simulation ◽  
Simulation Accuracy ◽  
Mean Values ◽  
Inland Lakes ◽  
Perturbation Field

Abstract Suspended sediments, as one of the most important factors affecting the water environment of inland lakes, are closely related to the various pollutants migration and interaction. Thus, the simulation and prediction of suspended sediment concentration is important. Existing studies show that the prediction accuracy of suspended sediment concentration can be effective predicted based on assimilation methods coupled with hydrodynamic models. However, in the process of assimilation of hydrological simulation, the existing perturbation generation methods consider that the perturbation error is a random Gaussian distribution, which does not consider the spatial variation characteristics of errors. In this paper, a new method to generate the perturbation field for assimilation simulation was proposed. This method uses hybrid error to generate perturbation field for assimilation simulation instead of using random error. The proposed approach was validated through its application to assimilation simulation of suspended sediment concentration of Taihu Lake in China, and five assimilation experiments was conducted. The proposed method was compared with existing methods for perturbation field generation. After three days and 72 time steps of assimilation simulation based on hybrid perturbation generation, we found that the proposed assimilation method provided results that were more consistent with buoy-measured data. The accuracy of the two assimilation methods based on hybrid perturbation is improved. Compared with the assimilation method based on random perturbation, the mean values of RMSE(root mean square error) decreased from 9.56 to 8.70 and from 9.55 to 8.68, respectively. The results show that the proposed hybrid perturbation generation method has higher simulation accuracy than other methods. This study shows that the method is effective and provides a new idea for the assimilation simulation of suspended sediment concentration in inland lakes.

FRACTAL MODEL FOR SURFACE EROSION OF COHESIVE SEDIMENTS

Fractals ◽  
2014 ◽  
Vol 22 (03) ◽  
pp. 1440006 ◽  
Author(s):  
YONGFU XU ◽  
HAO JIANG ◽  
FEIFEI CHU ◽  
CHUANXIN LIU
Keyword(s):  
Erosion Rate ◽  
Aggregate Size ◽  
Fractal Model ◽  
Surface Erosion ◽  
Physical Interaction ◽  
Cohesive Sediments ◽  
Erosion Threshold ◽  
Median Diameter ◽  
Excess Density ◽  
The Relationship

On the basis of the physical interaction of turbulent bursts with cohesive sediments, a fractal model is proposed for the erosion threshold and the erosion rate of the surface erosion. The existed models for the surface erosion of cohesive sediments in general are lacking in considering the aggregate size distribution in cohesive sediments. In this paper, the aggregate structure within cohesive sediments is described by the fractal model. The relationship is developed for the erosion threshold and the erosion rate as functions of the median diameter and excess density from the fractal model. A stochastic formulation for the surface erosion of cohesive sediments is incorporated in the proposed relations. The proposed relations from the fractal model for aggregates are validated against a bulk number of experimental data. The proposed relations have a series of advantages over other methods of direct measuring erodibility.

scholarly journals SUSPENDED SEDIMENT CONCENTRATION IN THE SURF ZONE

1988 ◽  
Vol 1 (21) ◽  
pp. 123 ◽  
Author(s):  
Makoto Ifuku ◽  
Tadao Kakinuma
Keyword(s):  
Suspended Sediment ◽  
Suspended Sediment Concentration ◽  
Surf Zone ◽  
Diffusion Theory ◽  
Sediment Concentration ◽  
Finite Amplitude ◽  
Reference Level ◽  
Sand Particle ◽  
Phase Lag ◽  
Mean Values

The water particle velocity,the pressure fluctuation and suspended sediment concentration near the bottom were measured in the surf zone. Data were obtained on the mean sediment concentration, phase lag between onshore velocity and suspended sediment concentration. The near-bottom velocity distributions under finite amplitude waves were calculated on the basis of the turbulent boundary layer theory by using timeindependent/ dependent eddy viscosities and Prandtl mixing length theory. The concentrations at reference level were estimated from the Kalkanis' theory using the velocity value at the top of a sand particle and the distributions of suspended sediment concentration were calculated on the basis of the turbulent diffusion theory by using time-independent/ dependent diffusion coefficients. The computed mean values of the suspended sediment concentration agree with those observed by Noda, the authors and Nielsen and those measured by Deigaard et al.. The computed phase lag agrees well with observed by the authors.

scholarly journals The Role of Surges During Periods of Very Shallow Water on Sediment Transport Over Tidal Flats

2021 ◽  
Vol 8 ◽  
Author(s):  
Qian Zhang ◽  
Zheng Gong ◽  
Changkuan Zhang ◽  
Jessica Lacy ◽  
Bruce Jaffe ◽  
...  
Keyword(s):  
Sediment Transport ◽  
Shallow Water ◽  
Suspended Sediment ◽  
Sediment Concentration ◽  
Tidal Flats ◽  
Sediment Flux ◽  
Intertidal Flat ◽  
Flood Duration ◽  
Suspended Sediment Flux

Periods of very shallow water (water depth in the order of 10 cm) occur daily on tidal flats because of the propagation of tides over very gently sloping beds, leading to distinct morphodynamical phenomena. To improve the understanding of the characteristics of velocity and suspended sediment concentration (SSC) surges and their contribution to sediment transport and local bed changes during periods of very shallow water, measurements of near-bed flow, and SSC were carried out at two cross-shore locations on an intertidal flat along the Jiangsu coast, China. Furthermore, the role of surges in local resuspension and morphological change was explored. Results indicate that flow and SSC surges occurred at both stations during very shallow water periods. On the lower intertidal flat, flood surges were erosive, while weaker surges on the middle intertidal flat were not. Surges on lower intertidal flats resulted in local resuspension and strong turbidity, contributing up to 25% of the onshore-suspended sediment flux during flood tides, even though they last only 10% of the flood duration. When surges travel across the flats, conditions change from erosional to depositional. Velocity surges on the middle intertidal flat were too weak to resuspend bed sediment, and the associated SSC surges were produced by advection.

Neural networks approached for modelling river suspended sediment concentration due to tropical storms

2013 ◽  
Vol 11 (4) ◽  
pp. 457-466
Keyword(s):  
Neural Network ◽  
Neural Networks ◽  
Suspended Sediment ◽  
Suspended Sediment Concentration ◽  
Time Series Data ◽  
Water Discharge ◽  
Sediment Concentration ◽  
Series Data ◽  
Generalized Regression Neural Network ◽  
Event Based

Artificial neural networks are one of the advanced technologies employed in hydrology modelling. This paper investigates the potential of two algorithm networks, the feed forward backpropagation (BP) and generalized regression neural network (GRNN) in comparison with the classical regression for modelling the event-based suspended sediment concentration at Jiasian diversion weir in Southern Taiwan. For this study, the hourly time series data comprised of water discharge, turbidity and suspended sediment concentration during the storm events in the year of 2002 are taken into account in the models. The statistical performances comparison showed that both BP and GRNN are superior to the classical regression in the weir sediment modelling. Additionally, the turbidity was found to be a dominant input variable over the water discharge for suspended sediment concentration estimation. Statistically, both neural network models can be successfully applied for the event-based suspended sediment concentration modelling in the weir studied herein when few data are available.

APPLICATION OF ACOUSTIC METHODS IN RESEARCHES OF THE BOTTOM SEDIMENT DYNAMICS

2018 ◽  
Author(s):  
Б. Дивинский ◽  
B. Divinskiy ◽  
И. Грюне ◽  
I. Gryune ◽  
Р. Косьян ◽  
...  
Keyword(s):  
Suspended Sediment ◽  
Sediment Concentration ◽  
Irregular Waves ◽  
Single Frequency ◽  
Contactless Measurement ◽  
Measurement Means ◽  
Uniform Field ◽  
Wave Flume ◽  
Acoustic Methods ◽  
Back Scattering

Acoustic methods belong to contactless measurement means, possess high spatial and time resolution. Thus, the use of multifrequency allows directly profile both concentration and granulometric structure of the suspended substances. In 2008 in the Big Wave Flume (Hanover, Germany) by efforts of the Russian and German scientists there have been carried out the experiment on studying the bottom material suspension laws under the influence of irregular waves. The Aquascat 1000 acoustic back scattering sensor (ABS) manufactured by British company Aquatec (www.aquatecsubsea.com), equipped by a three-frequency transmitter with frequencies 1,0, 2,0 and 3,84 MHz, has been set on distance of 0,75 m from the bottom and 111 m from wave generator at the total depth of 3,2 m. Several dozen series of measurements at various parameters of surface waves have been carried out. The general picture of suspension is so that the external dynamic influence (currents, wave movements, turbulence, gravitation forces) creates a non-uniform field (gradient) of the suspended particles and in most cases due to this the average size of particles undergoes to the spatial-time variations. For this reason while defining the mass concentration of suspended sediment, using the single frequency transmitter there is necessity for numerous definition of the suspension granulometric structure what by isn’t always possible. If two and more frequencies are used the observed results comparison can give the information on average diameters of particles and on that basis the calculation of suspended sediment concentration is possibleLet's emphasize the basic advantages of back scattering acoustic gauges usage: – Obtaining the particles sizes and concentration distribution profiles is possible; – The initial granulometric structure of bottom sediments can be unknown (at use of several frequencies). The following can be referred to some lacks of the device: – The system should be calibrated in laboratory conditions; – In a positive feedback conditions the iterative computing process can converge to zero or to infinity. In this case experiments with a variation of carrier frequencies chosen for the analysis allow partially solve the problem (say experiments with different frequencies pairs, as 2/1 of MHz or 4/2 MHz).

Sign in / Sign up

Export Citation Format

Share Document