Freeze-thaw effect of bed surface on bed load transport behaviour

High-latitude rivers generally experience seasonal freeze-thaw process, and the impact of freeze-thaw process on riverbanks dynamics and instability has been widely reported. By contrast, bed-load transport behaviours in alluvial rivers under the freeze-thaw action is still unknown. As such, two comparative experimental series were executed to isolate the freeze-thaw impact on bed load transport over uniform and non-uniform sediment beds in the present study, the first series focused on the bed load transport under freeze - thaw - free and normal room temperature (20 °C) condition, and this series is designated as benchmark tests, against which the impact of freeze-thaw action on bedload transport rate and composition can be examined, the second series are the group of comparative experimental tests, within which both uniform and non-uniform sediment beds were worked by sequential events of freeze, thaw, and steady flow at normal room temperature (20 °C) . Our experiments show that bed load transport rates are usually higher for both uniform and non - uniform sediment beds affected by freeze - thaw process if compared with measurements in benchmark tests, accordingly, a ratio g of bedload rates measured within freeze-thaw-impacted sediment bed over their equivalents obtained from freeze-thaw-free bed. A comparison between the ratio £ and normalized shear stress indicates that the ratio £ is higher (i.e. ^>1.0) at lower normalized shear stress and gradually reduced (i.e. g—>1.0) as normalized shear stress increased. For comparative tests using non-uniform sediment bed, the impact of freeze - thaw action on transported grain size is significant at low flows and gradually attenuated by increasing flow magnitude. These findings are important for enhancing our understanding of alluvial bed stability and evolution in cold regions. © 2023 International Research and Training Center on Erosion and Sedimentation and China Water and Power Press. All rights reserved.