Depth-averaged velocity and bed shear stress in unsteady open channel flow over rough bed

Abstract

Very few studies have been carried out in the past in estimating depth-averaged velocity and bed shear stress in unsteady flow over rough beds. An experiment is thus conducted to investigate the vertical and lateral velocity profiles under unsteady flow conditions in a rough open channel for various flow depths. One hydrogram is repeatedly passed through the rectangular flume with a fixed rigid grass bed. Using micro Pitot tube and Acoustic Doppler Velocimeter (ADV), the flow patterns are investigated at both lateral and longitudinal positions over different cross-sections. For two typical flow depths, the velocities in both the rising limb and falling limb are observed. Hysteresis effect between stage-discharge (h ~ Q) rating curve between rising and falling limbs is illustrated. Lateral distribution of depth-averaged velocity and bed shear stress are plotted at three different cross sections and compared with the steady flow conditions. In falling limb of an unsteady flow case, both depth-averaged velocity and bed shear stress distribution in the central region is higher than that of steady flow case. However, in the rising limb, the bed shear stress of unsteady flow is less than that of steady flow case. Further, in an unsteady flow, the magnitude of depth-averaged velocity is found to increase towards the downstream sections. Along the downstream positions, bed shear stress values increase for lower flow depths and decrease for higher flow depth cases.

Turbulence Characteristics in a Rough Open Channel Under Unsteady Flow Conditions

“…Two turbulence characteristics, i.e., mean kinetic energy and turbulent kinetic energy will be calculated and analyzed for the same flow conditions. The variations of these two turbulent characteristics along the three given sections have been demonstrated (Khuntia et al 2018a(Khuntia et al , 2019(Khuntia et al , 2020.…”

“…Lab.) of Civil Engineering, National Institute of Technology Rourkela (Khuntia et al 2018a(Khuntia et al , 2019 A downstream tail gate is provided to maintain uniformity of flow for this experiment. The tailgate was fixed at a particular height to achieve the given flow depths in steady uniform cases.…”