Introducing Finer Grains Into Bedload: The Transition to a New Equilibrium

Abstract

Simplified experiments on fine grain inputs to a coarse bed in mobile equilibrium were undertaken in a small, steep, narrow flume using spherical glass particles to study the influence upon the channel response of the size ratio between the bed (Dc) and the input fines (Df). Size ratios (Dc/Df) between 7.14 and 1.25 were tested, with a constant flow and coarse sediment feed rate and a variety of fine and total feed rates. Transition to a new, two‐size equilibrium occurs through slope adjustment (aggradation/degradation), accompanying a change in sediment mobility created by the addition of the fine material. Previous work has documented superior mobility following a fine grain input; the present experiments identify limits of this behavior related to the fine grain size, the proportion of fines introduced, and the total sediment supply. The mechanistic reasons for these limits are examined with respect to the grain sorting behavior, leading to the development (or not) of a quasi‐static layer of the fine material at the base of the transport layer. Despite the variation in bed slope response depending upon these factors, the slope transitions consistently follow an exponential profile.

Comparing the behaviour of spherical beads and natural grains in bedload mixtures

“…The Hill et al () experiments varied grain size ratio of natural sediment by using a range of coarse grain diameters ( D c ) while the fine feed consistently composed 50% of the total feed. For comparison, the Gilbert () data in Figure includes only results with an approximately 50:50 fine to coarse mixture (see appendix A in Dudill et al, , for details), and only the data from our experiments with a fine feed/total feed proportion of approximately 42% are included. Figure presents two‐size equilibrium slope divided by one‐size equilibrium slope ( S eq / S o ) as a function of fine feed/total feed for the largest grain size ratios available in our dataset alongside results from Gilbert's () experiments with a similar grain size ratio.…”

“…It has been known since the pioneering work of Gilbert (1914) that adding fine material to a gravel bed can increase the total transport rate out of proportion to the increased input. Increasing the proportions of fine sediment in the total feed increases transport of the coarse material until the finer sediment eventually overwhelms and inhibits entrainment of the coarse fraction (Gilbert, 1914;Iseya and Ikeda, 1987;Ikeda and Iseya, 1988;Curran and Wilcock, 2005b;Dudill et al, 2017Dudill et al, , 2018. The effect is in part caused by a smoothing effect of the finer sediment on the coarser bed that increases near-bed velocities and hence lift and drag forces on the coarser fraction (Venditti et al, 2010b).…”

Discrete and continuum modelling of grain size segregation during bedload transport

“…In mountains, steep slopes drive an intense transport of a wide range of grain sizes implying size sorting or segregation (see e.g. Gray 2018), which is largely responsible for our limited ability to predict sediment flux and river morphology (Bathurst 2007;Frey & Church 2011;Dudill et al 2018). Sediment grain interactions can produce vertical, longitudinal or lateral sorting, which lead to very complex and varied morphologies of bed surface and subsurface (such as armouring, bedload sheet, etc.)…”

Identifying the Brazil nut effect in archaeological site formation processes

“…It had previously been assumed that only fluid mechanics control this pattern, where the river water would wash away the finer particles, leaving the larger particles behind. Experimental results, however, suggested that some riverbed armoring may be due to the granular segregation that follows the BNE from below-rather than fluid-driven sorting from above (Ferdowsi et al 2017; and see further Seil et al 2018;Dudill et al 2018).…”