The Role of Turbulence on the Initiation of Sediment Motion

Abstract

The present study examines the role of turbulence on the incipient motion of sediment. For this purpose, well-controlled experiments are performed at the laboratory in a tilting flume. In these tests glass beads of the same size and density are used as the testing material to isolate the role of turbulence. State of the art equipment are used during the course of this study. Specifically, a 3-D Laser Doppler Velocimetry system is employed to measure the instantaneous velocity components at different points near the vicinity of a ball while the ball motion is monitored with a video camera. An image analysis program is developed here to analyze the motion of the particles within a test area.

To examine the importance of the different stress components in the entrainment of sediment, five tests of different packing configuration are performed. Specifically three different roughness regimes are examined namely, the isolated, the wake interference, and the skimming flow. The results reveal that the instantaneous normal stress in the streamwise direction is the most dominant component of the instantaneous stress tensor.

The backbone of this study is the development of a methodology to link the effects of turbulence with the commencement of sediment motion. It is considered that the metastable bursting cycle (i.e. sweeps, ejections, inward and outward interactions) is responsible for the sediment entrainment. And that the sediment entrainment, if any, occurs within a bursting period.

The main concept behind the determination of the critical conditions is that the probability of the entrainment of sediment (effect) is equal to the probability of occurrence of these highly energetic turbulent events that have magnitude greater than the critical (cause). The probability of sediment entrainment is computed by means of the image analysis tool.

The balance of moments is obtained here to determine the minimum moment that is required for the commencement of sediment motion. The balance of moments yields the deduction of a new variable that is used to describe the probability of occurrence of the different turbulent events. This variable is the summation of the instantaneous normal stresses in the streamwise and vertical direction. It is shown here that a two-parameter gamma density function describes quite well the statistical behavior of this variable.

The results that are obtained from the existing model suggest that the present methodology can adequately describe the commencement of sediment motion. It is shown here that the traditionally used shear stress term uw may not be the appropriate measure for the determination of the critical conditions.