Visualization and analysis of temporal trends of sand infiltration into a gravel bed

Abstract

Sand infiltration into gravel frameworks affects a wide range of ecological, geomorphic, and engineering processes. Four flume experiments were conducted with tracer materials to examine how a sand pulse infiltrates into a gravel bed. These experiments were primarily designed to test two hypotheses: (1) that vertical gradational trends of interstitial deposits are due to differential transport of finer sand in suspended load (hydraulic sorting) and (2) that the formation of a bridge layer (a thin layer of infiltrated sediments that become lodged in shallow pore throats) precludes subsequent infiltration into a gravel framework. Several sand colors were sequentially introduced into a flume containing a gravel substrate. After the experiments were conducted bed cores were collected and separated into vertical layers including surface layers composed primarily of sand that was transporting as bed load before the experiment was terminated and interstitial deposits in the gravel framework. Sand from each layer was sieved and measured. The color distribution of each grain class of each vertical layer of each core was measured to determine the temporal provenance of the interstitial deposits. Results supported the occurrence of hydraulic sorting. Older (finer) sand particles were disproportionately prevalent in interstitial deposits when compared to bed load samples. The experiments did not support the second hypothesis. Substantial secondary infiltration occurred after the initial formation of a bridge layer. More secondary infiltration was measured for systems with higher d15Gravel/d85Sand ratios and when bed shear was sufficient to mobilize the gravel.