Study On Sediment Incipient Motion And Suspended Concentration Online Measurement In Sediment-laden Flow

Estuaries and coastal areas are rich in natural resources,where economies are relatively developed generally with densely populated areas.With the development of marine economy,human beings have carried out more and more resource exploitation and production activities in estuaries and coastal areas,such as port construction,waterway regulation,large bridge construction,tidal flat exploitation and utilization,land reclamation and so on.The main hydrodynamic factors encountered in estuaries,coastal areas for such projects is the waves and currents(tidal currents,runoffs,wave currents,etc).In sandy and silty coastal areas,these hydrodynamic forces would cause sediment movement.The motion of sediments would produce such problems as erosion and deposition of shoreline,deposition of ports and waterways,scouring of the foundation of hydraulic structures,etc.,which must be carefully considered before the construction of the projects.Therefore,it is very important to understand the law of sediment laden flow.At present,the study on sediment motion in sediment laden flow needs supplements,which may include the following aspects:The calculation error of the formula of sediment incipient velocity in unidirectional flow needs to be further reduced,especially for the case of high water depth relative to the grain size;Because the influence of sediment viscosity on sediment incipient motion is very complex in such circs of waves and tides,etc.,the formulae of incipient conditions of sediment motion are mostly empirical,and the rheological properties of cohesive sediments need to be further considered;The accurate measurement of near bottom suspended sediment concentration is crucial for the analysis of sediment suspension mechanism and calculation of sediment transport rate,but it is difficult to accurately measure the time series of suspended sediment concentration with the current measurement technology.Therefore,it is necessary to study the sediment incipient problem and the online measurement of suspended sediment concentration in sediment laden flows.In this paper,cohesive sediments and granular sediments are respectively used as the experimental materials.According to the sediment rheology.Based on the contributions of domestic and foreign scholars,the incipient velocity of granular sediment in unidirectional uniform and steady flow,the incipient condition of cohesive sediments in oscillatory flow and the method for measuring suspended sediment concentration online based on optical fiber technology are respectively studied by means of laboratory experiments and theoretical analysis.The main research contents and achievements of this paper are as follows:1.The incipient velocity of granular sediment in unidirectional flowThe experimental study on the incipient velocity of granular sediment with different particle sizes in unidirectional flow is carried out in an open flume.Based on the test data in the present study,the flow conditions in the tests and the natural are analyzed by assuming that the unidirectional flow velocity distribution satisfies the logarithmic flow velocity distribution.By introducing the momentum thickness of the boundary layer,the formula for the incipient velocity of granular sediments in unidirectional flow is established,which is compared with the formula of Wuhan Institute of Water Resources and Electric Power.The results show that when the ratio of water depth to particle size is greater than 10~4,the proposed formula for incipient velocity of granular sediment is closer to the actual situations than the formula of Wuhan Institute of Water Resources and Electric Power.2.The incipient conditions of cohesive sediments in oscillatory flowThe experimental study on the incipient conditions of cohesive sediments with different particle sizes and water contents in oscillatory flow is carried out by the U-type oscillatory flow tunnel.The incipient velocity and bed shear stress of cohesive sediments with different yield stresses in oscillatory flow are measured.The volume center of cohesive sediment particle cell is selected as the action point of bed shear stress on the cohesive sediment cell when the cohesive starts to move in oscillatory flow.The bed shear stress and yield stress are taken into account respectively,rather than the drag force and viscous force.The incipient velocity of cohesive sediment in oscillatory flow is analyzed by using the torque balance,and then the formula is proposed for the incipient velocity of cohesive sediments in oscillatory flow.The calculation formula of Shields parameter in oscillatory flow is obtained by fitting the experimental data,which is divided into a rheological term and a gravity term.The results show that the incipient conditions of cohesive sediments in unidirectional flow are not applicable to the oscillatory flows,and the critical Shields parameter of cohesive sediments in oscillatory in oscillatory flows is mainly related to the yield stress and particle size of sediment.3.The method for online measurement of suspended sediment concentrationA kind of suspended sediment concentration measurement equipment based on optical fiber technology is developed,and then the suspended sediment concentration is measured by means of this equipment.The measurement mechanism based on optical fiber detection of turbidity is analyzed according to the light dissipation principle and Mie scattering principle.The selection of the reflection distance is analyzed from the aspects of reflected light intensity,transmission of light,stability of measurement results and suspended particle size as well.Then,the formula of the turbidity based on the voltage value,reflection distance and particle size is established.The results show that the fiber-based turbidimeter has good stability and reliability for the measurement of suspended sediment concentration in stagnant or running flows.When the distance between the transmitting fiber and the incident fiber is within 1mm to 3mm,the measurement range of the optical fiber-based turbidimeter is only related to the suspended sediment particle size.The above achievements provide support for improving the design accuracy of sediment engineering and further promoting the basic theoretical research of sediment transport.