Bedload measurement using a submerged camera module in a laboratory flume

Measuring bedload in natural channels is important. Usually, it is performed by direct measurement devices. Recent studies show the use of several sensor-based indirect devices. However, laboratory calibration of these devices is limited. The present work proposes an optics-based procedure which can be used in controlled laboratory conditions. A high-resolution, wide-angle camera (CAMup) installed vertically upward in a watertight compartment is placed at the downstream end of a laboratory flume. The top surface of the camera module is made of a transparent polycarbonate plate (30×30cm) such that the camera can view the bedload activity. Another camera (CAMdn) with similar specifications is positioned looking downward at an area upstream of the transparent box. Simultaneous views from these two cameras are used to determine bedload sediment discharge and grain size distribution. Two sets of experiments are performed. In the first set of experiments, particles from three different grain size classes (2mm, 4.75mm, and 10mm) from the upstream end of the flume are released. The experiments are conducted in clear water for different flow conditions (10 cm-100 L/s, 10 cm-200 L/s, and 20 cm-250 L/s). It is found that CAMup can detect particles more accurately than CAMdn for all three cases, particularly the smaller particles. In the second experiment, three known grain-size mixtures are fed at a water depth of 10 cm and flow rate of 150 L/s. CAMdn underestimates the number of smaller grains in the mixture significantly. However, CAMup determines the grain size distribution with higher accuracy.