Breaking from the average: Why large grains matter in gravel-bed streams

While the influence of large grains on the morphodynamics of gravel-bed rivers has long been recognized, nothing dominates our collective efforts to model such rivers like the bed surface D-50, which turns up in virtually all the relevant equations. While researchers interested in flow resistance have recognized the relative importance of large grains and have modified flow resistance equations accordingly, there have been few attempts to quantify the effects of large grains on gravel-bed river morphodynamics. However, there is little evidence that D-50 exerts first-order control over the physics occurring along the channel boundary, and its prevalence seems to be primarily based on the untested, a priori assumption that the best description of a distribution is the mean or median value. This commentary questions the long-standing assumption that D-50 is the best choice for characteristic grain size, and uses evidence from previous studies to show that mobilization of the largest grains in the bed likely controls morphological stability, and possibly sediment transport. (c) 2018 John Wiley & Sons, Ltd.