Vortex-tube sediment extractors. I: trapping efficiency

The vortex tube is a device for extracting sediment from canals that consists of a tube laid horizontally across the canal bed with an open slit along its top edge. Flow from near the canal bed is collected in the vortex tube and is extracted from the canal. Its trapping efficiency is defined as the proportion of the canal sediment load that is extracted. The present paper provides a theory for predicting the trapping efficiency of vortex tubes that is based on a theoretical sediment flux profile for the canal upstream from the vortex tube. The theory is compared with field measurements of trapping efficiency at six sediment extractors. The ratio of observed trapping efficiency to predicted trapping efficiency has a mean value of 1.0 and a standard deviation of 0.25 over 120 measurements. The present paper also provides quantitative guidance on locating vortex tubes so that local effects, such as canal bends, do not reduce trapping efficiency. (A design method for vortex tubes, which uses trapping efficiency predictions, is presented in a companion paper.)