Mobilization of coarse surface layers in gravel‐bedded rivers by finer gravel bed load

Abstract

[1] Additions of sand to gravel beds greatly increase the mobility and flux of gravel. However, it is not known how additions of finer gravel to coarser gravel beds will affect the mobility of bed material. Here we examine the effect of fine gravel pulses on gravel bed material transport and near-bed flow dynamics in a series of flume experiments. Bed material refers exclusively to sediment in the channel prior to the pulse introduction. The observations indicate that fine sediment pulses tend to migrate downstream in low-amplitude waves. As the waves pass over the gravel bed, the interstitial pockets in the bed material surface fill and coarse gravel particles are entrained. This increases bed material transport rates and causes a distinct shift from a selective mobility transport regime where particles coarser than the bed material median (8 mm) make up <30% of the load to an equal mobility transport regime where bed materials coarser than 8 mm and finer than 8 mm are transported in equal proportions. The only possible source for this coarser bed load material is the sediment bed, suggesting that portions of the coarse surface layer are being mobilized. Observations of near-bed velocity and turbulence suggest that fine gravel pulses cause fluid acceleration in the near-bed region associated with a reduction in the level of turbulence produced at the sediment boundary. This accelerated fluid at the bed increases drag exerted on coarse surface layer particles, promoting their mobilization. Our findings suggest that, in general, finer bed sediment (not just sand) can mobilize coarser sediment and that expressions for the influence of sand on bed mobility need to be generalized on the basis of grain ratios.

Gravel threshold of motion: a state function of sediment transport disequilibrium?

“…As described in Sect. 1.1, the effect of finer grains on thresholds of motion is not limited to sand sizes alone (Venditti et al, 2010;Sklar et al, 2009;Johnson et al, 2015). In the Johnson et al (2015) experiments, the finest grain size class has D 50 = 2.4 mm, D 16 = 2.0 mm, D 84 = 2.8 mm.…”

“…(4) are not unique to sand sizes only. Thresholds of motion for intermediate surface diameters (e.g., D 50 ) can similarly be reduced by grains substantially smaller than the bed surface but larger than 2 mm (Venditti et al, 2010;Sklar et al, 2009;Johnson et al, 2015). Mechanistically, the addition of sand or finer gravels smooths the bed surface by preferentially filling local topographic lows, which can affect pocket geometries (making it easier for larger grains to rotate out of a stable position), and also reduce local hydraulic roughness, increasing near-bed velocity and increasing drag on protruding grains.…”

Morpho-textural implications to bedload flux and texture in the sand-gravel ephemeral Poveda Gully

“…9A, 9C), thereby maximizing the transport of clast-supported gravel particles while sand filled interparticle voids (Wilcock, 1988;Venditti et al, 2010b).…”

Influence of consecutive sediment replenishment on channel bed morphology

“…This does not allow small replenished grains to filter into the armoured channel bed. It is known from literature that augmentation of fine grains and filtering of such into channel bed increases the mobilization of bed material (Venditti et al 2010). Therefore it can be assumed that changes in channel bed topography and resulting deposition heights would be enhanced in case of a mobile bed.…”