FINE GRAIN SEDIMENT TRANSPORT OVER A COARSE GRAIN GRAVEL BED: FLUSHING THE FINES

Flume experiments were conducted on a bimodal sediment mixture of sand and gravel, to better understand the effects of surface sand concentrations on flow properties and to examine the effectiveness of bedload transport for flushing flows. Fine sediment infiltration is a common occurrence in dammed rivers, or rivers with an altered or diminished hydrograph. Intrusion of fine sediments can be harmful to benthic invertebrates and degrade fish spawning habitat. Flushing surface sand deposits is an important goal in many river restoration programs. This study experimentally establishes the effects of surface sand on velocity, shear stress and bedload transport. 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 and transported sediment was collected in a trap at the tail of the flume. The bed was continuously photographed during the two runs and photos were analyzed to yield the percent sand cover of the bed. Initially, the open work gravel bed (D50=8mm) was incrementally filled with sand (D50=1mm) until completely sand covered, with velocity and turbulence profiles collected at each increment. After the bed was filled the flume was run at flows levels sufficient to transport sand then increased to transport gravel, while sediment fluxes, sand concentration photos and velocity profiles were recorded. Results show that as sand fills the interstices basal shear stress and roughness height decrease linearly. During the flushing experiments the fully sand covered bed persisted in a steady state until the sediment supply was reduced, after which the bed quickly flushed much of the surface sand. Sand was removed until the roughness height was great enough to raise the reference shear stress and effectively end transport. In all cases, even those with flows sufficient to easily transport the gravel, the bed surface could not be completely cleaned of sand, with 17%-30% of the surface still covered.