Experimental Study on Riverbed Evolution Characteristics of Boulder Bar Reach in Mountain River

Variable sediment supply and widely graded bed materials modify boulder–bar development, sediment transport and bar evolution in mountain mixed-size reaches. To elucidate the evolution characteristics of bars in boulder-strewn mountain river segments, this study conducted flume experiments. By adjusting parameters including discharge, boulder position, and boulder protrusion height, the paper analyzes bed scour and deposition deformations under various conditions. Results show discharge growth intensifies bed deformation: Both bar area and volume increase with rising discharge, with longer downstream bar extension and a positive correlation between bar length–width ratio and velocity. Higher boulder protrusion height and exposure amplify scour depth, expand bars laterally and reduce scour pit width–depth ratios. Boulders at bar heads migrate furthest downstream; moving boulders toward bar tails increases bar area/volume while bar height peaks then declines. Bar scale follows mid-channel bar head > bar tail > side anabranches. Boulder embedding depth linearly rises with exposure, both parameters positively linked to post-scour bar volume loss. The stable co-evolutionary relationship between scour depth and extent (R2 = 0.849) confirms their synchronized development under varying flow and boulder conditions. Boulders limit downstream bar elongation, flat beds boost scour diffusion, and scour pit width–depth ratio positively correlates with flow velocity. This work offers experimental and mechanistic references for mountain river geomorphic prediction.