Study on the characteristics of bed forms developed under unsteady flows

The main objective of this study is to investigate the morphological response and variability in the wavelength, height, steepness of bed forms developed over a quasi-uniform, coarse-sand bed under unsteady flows, with zero sediment feed condition specified at the upstream boundary. A series of smooth, bell-shaped hydrographs are generated within a titling flume, each defined parametrically by their shape (η), unsteadiness (Γ_HG) and total water work (W_k). By contrast, the bed forms generated in the sediment bed are quantified through their wavelength (λ), height (D) and steepness (ψ). Experimental results indicate that different categories of bed forms, such as dunes, alternate bars, or transitional structures, developed on the bed surface over the hydrograph duration. A close examination of relative importance of three hydrograph's parameters (i.e. η, Γ_HG and W_k) on bed-form dimensional descriptors (i.e. λ, D, ψ) reveals that the flow unsteadiness and total water work appear to be the primary and second-order control on bed-form category developed and their overall magnitude, whereas the hydrograph shape (η) appears to have a minimal impact. Based on this knowledge, a constitutive model to describe the effect of unsteady flow hydrographs on the bed-form categories and magnitude is developed from the results in the present study.