COLLOIDAL PARTICLE EXCHANGE BETWEEN STREAM AND STREAM BED IN A LABORATORY FLUME

The exchange of fine natural particles between the surface and pore waters of a sand-bed stream has been investigated in a laboratory flume. At the beginning of each experiment, kaolinite clay is added to the recirculating surface water flowing over a sand bed with dunes. Transport of clay into the bed is then determined by measuring the decrease of clay concentration in the surface water with time. Two conclusions can be drawn from the experiments done to date. First, although the kaolinite particles are too small to settle appreciably in the surface flow, they can be trapped to a great extent in the bed. Second, the trapped clay can be released when the surface water no longer has a high clay concentration.