Sedimentation Characteristic And Sedimentation Influence On Hydraulic Performance Of Long-throated Flume

Flow measurement is an important basis for irrigation districts to improve water use efficiency and promote water-saving agriculture.There are nearly one hundred large and medium-sized irrigation districts in the Yellow River Basin in China.The Yellow River is famous for its high sediment loads,which causes most channels to transport water as well as sediment.It is important to study the anti-sediment disturbance ability and hydraulic performance of the water measuring devices in sediment-laden flow.There are many kinds of water measuring devices developed,among which the long-throated flume has the advantages of low cost,high accuracy and good flexibility.However,most of the current research on long-throated flumes has been carried out under clear-water flow,how sedimentation influences hydraulic performance is still unclear,and it is necessary to promote the study of sediment-laden flow discharge measurement.This thesis presents a series of experimental and numerical studies to investigate the sedimentation characteristics and hydraulic performance of long-throated flumes in sediment-laden flow,to guide the application of long-throated flumes in sediment-transporting channels.The study was conducted in a rectangular channel,which installed a long-throated flume with variations of contraction,they are side contraction flume,vertical contraction flume,and both side and vertical contraction flume.All three flumes have rectangular control sections and the contraction ratio is 0.4.The contents and results of this thesis are as follows:(1)For clear-water flow,the hydraulic characteristics and accuracy of the long-throated flume were investigated.The modular limit and flow measurement accuracy of long-throated flumes were analyzed.The modular limit is greater than 0.86,and the mean error of water discharge measurement is less than 5%.The backwater height and the energy loss in the side contraction flume are the lowest.(2)For sediment-laden flow,an experimentally investigation was conducted on the sediment deposition progress and hydraulic characteristics of the long-throated flumes.The results showed that sediment mainly accumulates in the approach channel of the flume,there are small amounts of sediment deposition in the long-throated flume,and the control section does not produce sedimentation.Flow characteristics such as flow patterns and surface lines along the channel are similar to the clear-water flow after the sediment is deposited in the approach channel.The fluctuations of water level in the measuring station caused by sedimentation are small,and the flow measurement accuracy of the long-throated flumes was still within 5%with sedimentation.(3)The numerical simulations were carried out to compute the sediment passage capacity of three long-throated flumes with different contraction forms.The influence of sediment deposition on hydraulic characteristics and head-discharge equations was investigated.Compared with the clear-water flow data,the Froude number tends to increase in the upstream approached channel during sediment loading,flow constriction in the control section can create a critical flow condition.The water level only shows a 1～2mm change,and the change in backwater water height and energy loss is nearly zero.For the head-discharge equations of long-throated flumes,the integrated coefficient C of equations tends to increase with sedimentation,the overall change is less than 3.08%,in which the changes in the flow velocity correction coefficient C_v dominate.The sediment deposition rates of three contraction forms of the long-throated flume were compared when the sediment concentration is10 kg/m~3,and the result showed that the vertical contraction flume has the largest sediment deposition rate,both side and vertical contraction flume is the second,and the side contraction flume is the smallest.The following conclusions are drawn from all results.Sediment accumulation in the approach channel does not cause the long-throated flume to fail.The hydraulic characteristics and the head-discharge equation of long-throated flume are less affected by sedimentation and have high accuracy.The long-throated flume is feasible for flow measurement in sediment-laden flow.Among the three contraction forms of the long-throated flume,the side contraction flume has the best overall performance of hydraulic performance and sediment passage capacity,so it is recommended to be used in irrigation districts as a priority.The innovations of this work are as follows.The influence of sedimentation on the head-discharge equation is determined by coefficients C_d and C_v,it is found that C_v dominates the change of the head-discharge equation,and the increase of flow head caused by sediment deposition is the main reason for the change of the head-discharge equation.The sediment deposition features and their influences on the performance of the long-throated flume revealed in this study guide practical applications.The study helps users to comprehensively grasp water measurement,and select suitable water-measuring facilities for irrigation districts.The research will provide technical support for the application of long-throated flumes in irrigation districts of sediment-laden rivers in China,which is of great significance for improving water resources management in irrigation areas and promoting agricultural water conservation.