Acceleration-induced laminarization of sheet flow over smooth steep slopes

Sheet flows descending along steep slopes have traditionally been interpreted as macro-scale hydrological phenomena, such as rainfall runoff and slope erosion. However, their micro-scale characteristics—particularly the susceptibility of the entire flow depth to bottom and free-surface shear due to the extremely shallow depth—have received limited attention. This study experimentally demonstrates the occurrence of laminarescence (laminarization induced by acceleration) in sheet flows over a smooth steep slope. Measurements of velocity profiles and turbulence intensity reveal nearly vertical (plug-like) distributions, showing clear deviations from the log-law. Furthermore, the acceleration parameter K exceeds the order of 10−6 throughout the measurement domain. These quantitative features consistently support the emergence of a laminarescent state. Notably, this state persists over a flow distance exceeding 50 times the mean flow depth, overturning conventional expectations regarding turbulence regeneration in shallow open-channel flows. These findings point to a distinctive turbulence-suppression mechanism that cannot be fully explained by conventional parameters such as Reynolds number, Froude number, or acceleration parameter.