Study On Velocity Measurement Of Steady Sheet Flow With The Electrolyte Pulse Method And Its Application

The velocity of sheet flow is the basic in the research of soil erosion on slope, but there is no sound method of measuring it so far for the flow is very shallow and sediment contained in the flow has great influences on the measurement. In the conventional tracer method, the flow velocity is determined by the time for the tracer to cover a certain distance, and an arbitrarily chosen empirical correction coefficient. The empirical coefficient is influenced by many factors. Further the empirical coefficient is a function of velocity; therefore inter-dependence of the two arises. The measurement of sheet flow velocity on hillslope needs more theoretical research, for the development of new instrument.On the base of erosion research and related theories, an electrolyte pulse method for measuring velocity of sheet flow is advanced. The detailed outcomes are as follows.(l)The dynamic convective-dispersion transfer theories of electrolyte solute in steady sheet flow on slope were used to analyze the dynamic transport of solute in flowing water. Mathematical models were outlined for the transfer of solutes in sheet flow. The basic assumptions of solute transport were: the flow is one-dimensional with the influences of rainfall and infiltration neglected; the flow is steady or its fluctuation is negligible; and the time interval for solute injection is so short that the input of electrolyte can be treated as a pulse input. The 1-D models, including the governing equation and the initial and boundary conditions were given. Laplace's transformation method was used to obtain the analytical solution ton the convective-dispersion process of solute transport. The analyticalsolution was graphically shown to illustrate the conceptual validation of solution, which describes the main characteristics of solute transport in flowing water. Theoretical analysis of the analytical solution indicates that the velocity of flow can not be calculated with the estimated velocity by the time for the solution peak to cover a distance from the injection location to the measuring point, as used in color or salt tracer method. The conventional tracer methods for velocity determination have their theoretical faults. The dispersion coefficient of solute has significant effect on the velocity measured by the tracer method. The bigger the dispersion coefficient, the larger the time interval between the peak and centroid arriving time. Therefore, the sediment content should be considered and different correction coefficient should be used while measuring the velocity of flow using tracer method.(2)A experimental prototype was developed to measure the velocity of sheet flow, with the integration of electronics technique, compute application knowledge and instrumentation techniques. A Visual Basic based operation and control system were developed for automated data logging, parameters (including velocity) estimation, and file management. The system estimates the velocity with the following procedures. The sensors and data logging system were used to measure the temporal concentration changes of the transporting solute. The least square method was used to fit the analytical solution to the measured solute transport data and the related model parameters, i.e. the velocity and the dispersion coefficient were determined. And the velocity was so \determined. This operation system was found to be easy to use and reliable as well as stable, by numerous laboratory experiments.(3)Taking distribution of solute in the flowing water of sheet flow as a system of particles, the centroid velocity of the system of particles was calculated with kinematics theory and found to be equal to the velocity calculated by the method outlined above. Therefore, it was proved that the analytical mathematical solution of electrolyte transport is correct in the view of kinematics. Under the assumption of no solute loss, the centroid velocity can be used for the averaged flow velocity. The experiments indicated that there are no differences in flow velocity or rates of f...