Large grains matter: contrasting bed stability and morphodynamics during two nearly identical experiments

While the stabilizing function of large grains in step-pool streams has long been recognized, the role they play in gravel-bed streams is less clear. Most researchers have ignored the role of large grains in gravel-bed streams, and have assumed that the median bed surface size controls the erodibility of alluvial boundaries. The experiments presented herein challenge this convention. Two experiments were conducted that demonstrate the significant morphodynamic implications of a slight change to the coarse tail of the bed material. The two distributions had the same range of particle sizes, and nearly identical bulk d(50) values (1.6 mm); however the d(90) of experiment GSD1 was slightly finer (3.7 mm) than that for experiment GSD2 (3.9 mm). Transport rates during GSD1 were nearly four times greater than during GSD2 (even though the dimensionless shear stress was slightly lower), and the channel developed a sinuous pattern with well-developed riffles, pools and bars. During GSD2 the initial rectangular channel remained virtually unchanged for the duration of the experiment. The relative stability of GSD2 seems to be associated with a slightly larger proportion of stable (large) grains on the bed surface: at the beginning of GSD1, 3.5% of the bed was immobile, while almost twice as much of it (6.1%) was immobile at the beginning of GSD2. The results demonstrate that the largest grains (not the median size) exert first-order control on channel stability. Copyright (c) 2017 John Wiley & Sons, Ltd.