BGU Clay Sand Experiments: Clay Addition Increment

Hyporheic exchange flux (HEF) plays an important role in the transport of nutrients and sediments in stream ecosystems. Deposition of fine suspended sediment particles can clog the streambed, reduce permeability, and lead to a reduction in HEF, resulting in impairment of various ecological processes. However, the dynamics of fine particle deposition and streambed clogging are still not well understood, especially when the bed is in motion. We conducted flume experiments to study the effects of coupled sand-kaolinite dynamics on HEF. Three experiments with a mobile sand bed and constant discharge were conducted in a laboratory flume through repeated kaolinite injection pulses at a fixed increment. HEF and participating porewater volume were assessed using salt and dye tracer tests. Kaolinite deposition rates were inferred from turbidity measurements while deposition patterns were measured using core samples. We found that fine sediment primarily accumulated within a layer below the bedform scour zone, and that this layer was thicker when kaolinite was added in larger pulses. This low-permeability layer led to an overall reduction in HEF, which declined linearly regardless of the pulse increment concentration. However, the rate of reduction in participating pore volume was higher for larger kaolinite addition increments, because faster deposition of kaolinite protected the deposits from scour and resuspension. These results indicate that clogging occurs not just during and after high-flow events, but also under constant flow conditions in which clay particles and hyporheic exchange lead to the formation of a low-conductivity layer in the bed.