Research paper: A visual method for threshold detection of sediment motion in a flume experiment without human interference
Mélanie Vah, Alaa Khoury, Armelle Jarno, François Marin
An experimental investigation of the entrainment threshold of sediment grains conducted in a non-tiltable flume is reported and an original processing method based on image correlation suitable for detection of sediment threshold of motion is presented. Unlike existing visual measurement methods, the proposed method is not affected by the individual judgement of the experimentalists, which may strongly impact the estimation of incipient motion. Three entrainment thresholds were exhibited. The method highlighted a first threshold for very low shear stress conditions that was interpreted as a nascent movement threshold. The other two thresholds of motion were related to distinct stages of surface mobility of the bed: one related to bedload threshold and the other to bedform threshold. Series of tests were first performed to study the sensitivity of the method to different parameters: angle of view of the camera, size of the image and acceleration rate of the linear ramp of flow velocity. A ratio of the image surface and the grain surface equal to 3000 was found to be the minimum size to perform reliable estimations of entrainment thresholds for all the sediments in the range of 0.328–2.648 mm tested. It was the only condition for the method to work. Once the method was validated, tests were performed with homogeneous sands and compared to results based on reference-based and visual methods. The correlation method yields similar values for the bedload threshold to those obtained from reference-based methods. Thus it could be an alternative to other methodologies by the simplicity of its implementation, precision, no requirement for human interference and direct correlation between threshold velocity and bed mobility without extrapolation.