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This article introduces the SVG (salt-velocity gauge), a novel automated technique for measuring flow velocity by means of salt tracing. SVG allows a high measuring rate (up to one every 2 seconds), short control section length (down to 10 cm), high accuracy (+[sol ]-15 cm s-1), and unbiased calculation of the mean velocity in experimental conditions with turbulent, supercritical flow.A few cubic centimetres of saturated salt solution (NaCl) are injected into the flow at regular time intervals using a programmable solenoid valve. The tracer successively passes two conductivity probes placed a short distance downstream. The transformation of the signal between the two probes is modelled as a one-dimensional diffusion wave equation. Model calibration gives an estimation of the mean velocity and the diffusion for each salt plume.Two implementations of the SVG technique are described. The first was an outdoors simulated rainfall experiment in Senegal (conductivity probes at 40 cm apart, 8 Hz measurement rate, salt injections at 10 second intervals). Mean velocity was estimated to range between 01 and 03 m s-1. The second was a laboratory-based flume experiment (conductivity probes at 10 cm apart, 32 Hz, salt injections at 2 second intervals). Another SVG with probes at 34 cm apart was used for comparison. An acoustic Doppler velocimeter (ADV) was also used to give an independent assessment of velocity. Using the 10 cm salt gauge, estimated mean velocity ranged from 06 to 09 m s-1 with a standard deviation of 15 cm s-1. Comparisons between ADV, 10 cm SVG and 34 cm SVG were consistent and demonstrated that the salt-tracing results were unbiased and independent of distance between probes. Most peaks were modelled with r2> 90 per cent.The SVG technology offers an alternative to the dye-tracing technique, which has been severely criticized in the literature because of the wide interval of recommended values for the correction factor a to be applied to the timings. This article demonstrates that a fixed value of a is inappropriate, since the correction factor varies with velocity, diffusion and the length of the control section. Copyright 2005 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Present-day understanding of rill dynamics is hampered by a lack of detailed data on velocity distributions in rills. The latter are difficult to collect with traditional techniques due to the very low water depths and the relatively high flow velocities in rills. The objectives of this paper were to investigate the feasibility of miniaturized acoustic Doppler velocimeter (mADV) measurements in rill flow and to explore longitudinal variations in flow velocities and their relationship with rill bed morphology. Detailed data on longitudinal flow velocity were required to achieve these objectives. A 1·8 m long rill was formed freely in a flume at 5° slope and 0·001 m3 s-1 discharge. Rill topography was characterized by an alternation of steps and pools. The flume surface was then fixed to preserve rill roughness. A topographical scanning of the entire flume surface was made. Velocity was measured with a mADV along the rill, and at different depths. Flow depth in a longitudinal direction was also measured using an elevation gauge. A strong relationship exists between rill topography and flow hydraulics. Over steps, flow was unidirectional and rapidly accelerating until a threshold Froude number (Fn) value between 1·3 and 1·7 was reached and a hydraulic jump occurred leading to the formation of a pool. In the pool, the flow pattern was multidirectional and complex. The flow was subcritical when leaving the pool and accelerated over the next step until the threshold Froude number value was again reached. Energy loss in the rill was concentrated in the pools, mainly due to the action of a hydraulic jump. This mechanism of energy dissipation appeared to be an essential factor in rill formation and bedform evolution. Copyright © 2004 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]