Dynamics of Dual‐Mode Bedload Transport With Three‐Dimensional Alternate Bars Migration in Subcritical Flow: Experiments and Model Analysis

Bedload transport often exhibits dual‐mode behavior due to interactions of spatiotemporal controlling factors with the migrating three‐dimensional bedforms (characterized by the fully developed patterns in the bed, such as alternate bars, pools, and clusters). This study explores dual‐mode bedload transport based on experimental measurements and develops Einstein's exponential‐based model to characterize large fluctuations of bedload sediment discharge. The particle waiting time, particle flux, and bed elevation are measured in a series of well‐controlled laboratory experiments. Flume experiments show that the waiting time distribution of sediments gives a bimodal characteristic, two distinct modes can be identified from the measured data. This study encapsulates this dual‐mode bedload transport behavior in a hyperexponential distribution of sediment resting times and introduces it into the continuous time random walk (CTRW) framework. Considering the scaling limit of the thin/heavy‐tailed CTRW processes, a single‐rate mass transfer (SRMT) and fractional‐derivative SRMT (F‐SRMT) models are obtained, and the model parameters are determined from the hyperexponential distribution. Further analyses reveal that the dual‐mode bedload transport behavior is controlled by mass exchange between the mobile and immobile zones, and a dimensionless index η can quantify the intensity of dual‐mode behavior. Applications show that the dual‐mode bedload transport models are much more accurate in characterizing bedload transport in a mixed‐size gravel bed than the traditional exponential‐based model, and the nonlocal movement of bedload sediments is significant in the mixed‐size gravel bed. Further investigations will focus on the applicability test of the dual‐mode models to other flow regimes and conditions. Plain Language Summary Sediment transport caused by particles rolling and sliding as well as saltation on riverbeds is called bedload transport. Bedload transport is highly complex due to the complexity of the flow and bedform in natural rivers. Bedforms are commonly classified in terms of their characteristic longitudinal scales, and a three‐dimensional (3‐D) bedform represents the bed with fully developed patterns such as alternate bars, pools, and clusters. Traditional models in quantifying bedload transport may differ from field data by 1–4 orders of magnitude, especially for the migrating 3‐D bedforms. In this study, flume experiments reveal that the waiting time