Bed protections are usually characterized by low-mobility transport conditions and nonequilibrium turbulence profiles. As the present knowledge of the influence of turbulence on stability of cover layer units is minimal, an in-depth investigation was undertaken regarding the influence of turbulence on the stability of rough granular beds. Detailed measurements of (fluctuating) pressures on a bed element are used to evaluate certain concepts that are often used in modeling the entrainment of bed material from hydraulically rough beds. Three pressure transducers are placed in a cube that is part of a rough granular bed under open-channel flow, and velocities are measured using laser Doppler velocimetry. The measurements show that the magnitude of the fluctuating pressure at a certain point of the cube is a function of the exposure relative to the stones upstream of the cube. A quadrant analysis reveals that the drag force is not only directly dependent on the horizontal near-bed velocity, but on the vertica...

A probability density function (PDF) of the instantaneous bed shear stress in a turbulent flow is derived in this paper. It is argued that the shape of the PDF is similar to the PDF of the instantaneous drag forces on bed roughness elements. The influence of the near-bed relative turbulence intensity is included in the PDF. The shape of the distribution compares well with our measurements of the instantaneous drag force on a protruding bed element for a range of turbulence intensities. However, deviations are apparent at high turbulence intensities. The PDF also compares well with measurements of shear stresses on a smooth wall.