Water-Worked Bedload: Hydrodynamic and Mass Transport

Dear Colleagues,

Flow over a streambed that is complex in nature, especially in the near-bed flow zone, has been a topic of continued research interest over several decades due to its practical importance. It is even more complicated due to flow unsteadiness that intensifies transport processes and causes morphological changes in rivers. Our knowledge on how sediment is transported under unsteady flow conditions is still insufficient, which triggers a good deal of experimental, theoretical and computational efforts. The near-bed flow is greatly influenced by a complex, colossal fluid-sediment interaction which gives rise to a spatial flow heterogeneity in addition to an enhanced temporal intermittency in the near-bed flow zone. In a natural river, the flow velocity plays a major role in developing the bed surface topography formed by the roughness elements. To be specific, the sediment particles at the bed surface are in motion if the flow velocity is greater than the threshold velocity of sediment particle motion. Thus, the surface topography of a natural bed exhibits a spatially multifaceted three-dimensional organization, because it is created by the transport processes as a result of the continual deposition and reworking due to fluvial processes. In this way, a so-called water-worked bed is formed in a natural river. On the other hand, in laboratory experimental studies, a simulated streambed is generally prepared depositing randomly mixed sediments to a given thickness into the experimental flume. The streambed surface is then scraped and levelled, forming a screeded bed. Nevertheless, in the laboratory, if the distribution of scaled sediment size is at par with the natural bed sediment size (prototype), the simulated streambed can be deemed acceptable provided that the bed surface characteristics in laboratory flume are analogous to those in the prototype. To be specific, the screeded bed is essentially a mixture of randomly sorted sediments and its statistical distributions in terms of bed surface topography are unable to appropriately represent a water-worked bed, even though the bed surface formed by the sediment has a same particle size distribution. Importantly, the signature of the bed roughness characteristics is essentially transmitted to the characteristics of the time-averaged flow, turbulence parameters and resistance to flow.

Prof. Paweł M. Rowiński
Prof. Subhasish Dey
Guest Editors

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