Collective Particle Entrainment Explored With Experimental Data and Coupled Transfer Functions

Abstract Bedload transport in rivers with mixed grain sizes is challenging to predict, with implications for understanding how rivers form and respond to environmental change. Experimental work shows that collective particle entrainment is an important contributing mechanism of bedload transport, and here collective effects to the transport of a sediment mixture are conceptually explored. Two different time series of experimental sediment particle activity (i.e., a measure of the number of particles in motion) for sediments 4–32 mm in diameter are used to indirectly examine the role of collective entrainment. Particle activity was measured at a fixed position using an imaging light table at a time‐averaged resolution of 1 Hz for a duration of approximately 240 min during periods of topographic steady‐state. The two time series reveal a consistent transport behavior within the Fourier domain: activities for smaller particle size populations have increasing power density for decreasing frequency, whereas activities for larger particle size populations have a near uniform power density across all frequencies. Consequently, the activities of smaller particle sizes dominate the power spectra. A set of coupled transfer functions inspired by a probabilistic birth‐death model are developed to conceptually explore the transport behavior evident in the Fourier domain, with two notable results. The transport of smaller particles includes collective entrainment terms that represent mobilization due to both larger and similar particle sizes, whereas larger particles include collective terms limited to similar sized particles. The size‐dependent collective controls on particle entrainment described here offers a testable explanation for further analysis.