Experimental techniques for laboratory investigation of clay colloid transport and filtration in a stream with a sand bed

The exchange of kaolinite clay between a stream and its associated sand bed was investigated in a recirculating flume. Bedforms at the sediment-water interface result in two bed-exchange mechanisms: the bedform shape induces an advective flow through the bed ('pumping'), and dune propagation causes the trapping and release of pore water ('turnover'). Chemical and electrostatic interactions then result in filtration of clay by the bed. In order to allow modeling of chemical effects, all flume materials must have defined chemical parameters. This required improvements in the flume water supply, construction of sand-washing equipment, and the use of defined clay and sand preparation procedures. Flume experimental results indicate that clay is extensively trapped by the bed. Advective pumping tends to carry clay to the deep bed where it can be permanently trapped, while turnover tends to continuously mix the upper layers of the bed, hindering penetration to the deep bed.