Optical Measurement Method for Quantifying Sediment Transport in Physical Experiments

The quantification of sediment transport ṁ_S [kg/s] is a challenge when carrying out physical laboratory and model tests with movable riverbed. In particular, knowledge of the actual sediment transport is of great importance for the quality of the test results in investigations involving large-scale transport processes, such as those occurring in the course of floods in river reservoirs or during reservoir flushing. Numerical calculations can also benefit from reliable time series of sediment transport.

The authors present a measurement method that makes it comparatively inexpensive to carry out continuous measurements of sediment transport and display them in real time with very little computing power. The measurement method is embedded in a system for the continuous diversion or recirculation of sediments discharged from a flume or a laboratory canal. For example, the transport rate of sediment flowing through the weir structure can be measured during investigations of river reservoirs with a movable bed upstream of a river barrage. For this purpose, the discharged sediments are fed to a submersible pump directly after the weir structure and transported via a pipeline to the measuring field.

At the core of the measurement method is a camera-optical gray value comparison of 8-bit grayscale images from a standard industrial camera of a transparent measuring field. The measuring field consists of two transparent glass plates, one above the other, with a thin film of particle-laden liquid flowing between them (e.g. sand grains or plastic granulate in water). The glass plates are illuminated from below so that the sediment particles produce local darkening on their way through the measuring field. The industrial camera observes the measuring field from above and records the shadows of the sediment particles in front of the otherwise white illuminated background as a grayscale image.

For each image, the concentration of the sediment in the liquid at a discrete point in time is first determined with the help of the gray value comparison. The sediment transport ṁ_S is derived from a calibration function, which was determined in advance from long-term measurements with several constant sediment concentrations. The transport load can then be calculated over time by integrating multiple successive images. Comparative validation measurements showed measurement errors of less than 10 percent for sediment concentrations between 0 and 20 percent.