Double-averaged turbulence properties in rough bed open-channel flow: A review

The three-dimensional heterogeneous flow over rough beds plays a significant role in fluvial geomorphology and river management. This article deals with the problem of understanding the turbulent flow properties over rough beds as encountered in open-channel flows. Literature suggests that the time-averaged turbulence characteristics are inadequate to characterize the three-dimensional heterogeneity within the roughness influenced flow zones over rough beds. The present study particularly focuses on the changes in double-averaged (DA) (time and space) turbulence characteristics governed by the double-averaged Navier–Stokes equations. In this review article, the characterization of turbulence parameters for different rough beds is presented. Moreover, several studies that have examined the mechanisms behind the changes in DA turbulence flow characteristics, focusing on both natural rough beds and artificial rough bed in laboratory flume experiments under controlled conditions, are compared. These investigations highlight how changes in DA turbulence parameters influence the roughness elements and assesses the usefulness of various turbulence properties for describing three-dimensional flow physics in different flow layers owing to the flow-roughness interaction. The review also points out limitations in current approaches, stressing the importance of standardizing methods to allow meaningful comparisons between studies. Furthermore, the study proposes recommendations for future inquiries and adds important perspectives to the overall understanding of three-dimensional flow physics. Finally, it outlines possible future research directions and aims to explore how turbulence parameters can be effectively characterized at the roughness boundary layer.