Pressure fluctuations and gravel entrainment in rivers

Near-bed, sub-bed and uplift pressure fluctuations measured in New Zealand gravel bed rivers were found to have near Gaussian frequency distributions. At measurement sites the standard deviation of near-bed pressure fluctuations was approximately 3 times the local time-averaged bed shear stress. Pressure structures in the flow decayed rapidly. For the measured rivers, the near-bed pressure decorrelation time was approximately half the flow depth to shear velocity ratio. Measurements made in the middle of a flat plate in the plane of the bed and beneath the bed showed that advecting pressure fluctuations can create a vertical pressure differential in the bed surface layer that is sufficient to entrain gravel particles. Additional pressure fluctuations can be generated within mobile beds.