Fine grain sediment transport over a coarse grain gravel bed

Flume experiments were conducted on a sand and gravel bed to understand the effects of surface sand concentrations on flow properties and examine the effectiveness of bed-load transport for flushing flows. Fine sediment infiltration is a common occurrence in dammed rivers, or rivers with an altered hydrograph, intrusion of fine sediments can be harmful to benthic invertebrates and degrade fish spawning habitat. Experiments were conducted using a 6m flume with a working width of 0.3m. Velocity and turbulence profiles were collected with a Laser Doppler Velocimeter, sediment transport was measured with a trap, the bed was photographed and the photos analyzed to yield the surface sand cover. Results show that near-bed shear stress decreases by half as the open work gravel bed was filled with sand. Two flushing runs were designed with flow levels ranging from those capable of sand transport to those capable of gravel transport. Results show the fully sand covered bed persisted in a steady state until the sediment supply was depleted, after which the bed flushed the surface sand. Flows capable of sand transport flushed 30-40% of the surface sand, flows capable of gravel transport flushed up to 80% of the sand. In all cases bed load transport was not sufficient to fully clean the bed. Additional analysis suggests that flushing flows require stress levels 10-30% higher than taucg to flush the bed to values of less than 30% sand cover. The data also show that higher-discharge flows require significantly less water, than low discharge flows to achieve similar flushing results.