How exposed bedrock morphology modulates discharge-driven runoff partitioning and sediment loss on karst hillslopes

Exposed bedrock is a common structural feature of karst rocky slopes, but its role in runoff and erosion has often been assessed mainly by exposure percentage, with limited attention to morphology. This study examined how exposed bedrock morphology regulates runoff partitioning, hydrodynamic characteristics, and sediment yield on a 15° karst slope under concentrated flow. Four treatments were tested in a flume experiment: bare slope, nearly triangular bedrock (TS), nearly quadrilateral bedrock (QS), and nearly rhombic bedrock (DS). Inflow discharges of 2, 6, and 10 L·min −1 were applied, and runoff pathways, flow velocity, and sediment yield were measured. Bedrock morphology mainly affected runoff partitioning rather than total runoff generation. At 2 L·min −1 , the surface runoff ratio increased from 34.57% on the bare slope to 44.05%, 55.67%, and 58.35% under TS, QS, and DS, respectively, whereas fissure runoff decreased from 32.05% to below 10% under all bedrock treatments. At 10 L⋅min −1 , surface runoff dominated all treatments, accounting for 89.17%–91.55% of total runoff, indicating weaker morphology effects under high discharge. Sediment response varied with morphology and discharge: DS increased sediment yield by 36.9% at low discharge, whereas all bedrock treatments reduced sediment yield at medium and high discharges. Hydrodynamic results indicated that erosion was associated mainly with changes in flow depth, shear stress, and stream power rather than mean velocity alone. Sediment concentration declined from 3.74 to 0.57 g·L −1 as discharge increased, indicating that cumulative sediment yield did not increase proportionally with surface-runoff volume. These findings recast exposed bedrock from a passive cover factor as an active hydro morphological control on karst slope erosion.