Evaluation of two conflicting paradigms for cohesive sediment deposition

Laboratory experiments using steady flows and simulated tidal flows were conducted to evaluate two conflicting paradigms for cohesive sediment transport: mutually exclusive vs. simultaneous erosion and deposition. Due to the secondary flow and skewed distribution of bed shear stress (τb), the deposition space is mostly confined to the inner wall of an annular flume. The area of deposition (or the length of deposition because of the axially symmetrical flow) provides an indicator to determine when deposition at the sediment–water interface starts. The direct observation of deposition stemming from the inner corner suggests that a critical shear stress for deposition (τcd = 0.03–0.06 Pa) does exist. Changes in deposit length and depth-averaged suspended sediment concentration (SSC) under simulated tidal cycles demonstrate that deposition can happen only in tidal decelerating phases when local τb is less than τcd. The exclusive paradigm incorporated with the correct erosion threshold (τce) profile and erosion behavior could explain changes in laboratory- and field-observed depth-averaged SSC under all tidal regimes.