Measuring velocity of water flow within a gravel layer using an electrolyte tracer method with a Pulse Boundary Model

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Summary Water flow within a gravel layer is of great interest in many studies and applications. The electrolyte tracer method, based on the solution to the governing partial differential equation of solute transport in water flow under a pulse boundary condition, the Pulse Boundary Model, was suggested to measure the velocity of water flow within a gravel layer. Laboratory experiments were made to illustrate the measurement method. The dye tracer method was used to make comparative measurements. Gravel of about 2 cm in diameter was placed to a 5 cm thick layer in a flume of 4 m long, 20 cm wide and 50 cm deep. Flow velocities were measured at locations of 0.3, 0.6, 0.9, 1.2 and 1.5 m from the electrolyte tracer injector, which was 1 m from the water inlet, under 3 flow rates of 3, 6 and 12 L/min and for 3 slope gradients of 4°, 8° and 12°. Highly concentrated KCl solution was injected into the water flow within the gravel layer through a funnel. The solute transport processes were registered by the measurement system, as detected by the ion-sensitive sensors, before being used for flow velocity estimations at different locations. The measured flow velocity increased with slope gradient but was similar under the different flow rates used. The velocity values measured by the Pulse Boundary Model were about 20–50% lower than those estimated by the dye tracer method although they were strongly linearly correlated. The measured velocity values were in about the same order of magnitude as those reported in the literature. Therefore, the Pulse Boundary Model electrolyte tracer method was demonstrated to be rational and applicable for measuring the velocity of water flow within a gravel layer.