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

Abstract

AbstractSimplified 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.