Flow velocity is a key hydraulic variable in the exploration of rill erosion and is usually estimated by multiplying the surface flow velocity of runoff (measured with the dye tracer method) by the flow correction factor (a). However, there are differences among different experimental conditions, and the selection of the right value of a has become critical for accurately estimating the mean flow velocity. There has been little research on velocity correction factors for hyperconcentrated flows on steep slopes. In this study, gravel‐laden sediment (mass fraction of gravel in the sample ranging from 0% to 70%, corresponding to a median diameter of 0.08–2.95 mm) was used as the test material, and different slopes (18%–84%) and unit flow discharges (1.11–4.44 × 10−3 m2 s−1) were considered to investigate the effects of gravel‐laden sediment particle characteristics on runoff a and to elucidate the mechanism of the effects of different hydrodynamic parameters on runoff a. Under the experimental conditions, the value of a ranged from 0.285 to 0.690. a increases with increasing flow discharge and slope, with flow discharge having a greater effect than slope. With increasing gravel content and median diameter (d50), a decreased initially but then stabilised. Additionally, a decreased with increasing sediment content but increased with increasing Reynolds number (Re). Based on the results of this experiment, 0.37, 0.49 and 0.60 are recommended as the correction factors of surface flow velocity for laminar flow (Re ≤ 500), transitional flow (500 < Re ≤ 2000) and turbulent flow (Re > 2000), respectively. Equation (16), which is based on the hydraulic parameters and sediment particle characteristics, has the best accuracy (Nash–Sutcliffe efficiency coefficient [NSE] > 0.9). The research results quantified the impact of sediment particle characteristics on a, contributing to the advancement of hydrodynamic studies on rill flow.
