Experimental and Numerical Studies of Grain Scaled Bed-Load Transport

Abstract

Accurately calculating bed load transport rate has been a challenge in hydraulic engineering for decades. Bed load transport depends on the interaction between flow and sediment particles. Analyzing the characteristic of sediment particles motion and determining the velocity of sediment particles on a river bed is essential to quantify the transport rate. Therefore, this dissertation reports on the methods and results of a three-phased investigation to analyze the bed-load transport at the grain scale. The first research phase focused on the experimental study of bed-load transport using particle motion tracking. A series of experiments were conducted in a flume to study bed-load transport. The motion of bed-load particles was captured by a series of images taken by a high-speed camera. A novel particle motion tracking method was developed to automatically detect all the moving particles and calculate the instantaneous particle velocities. The instantaneous bed load transport rate was calculated based on particle velocity and the volume of moving particles. To verify this method, bed load transport rate based on the image processing technique was compared to the manually measured ones as well as data from other experiments. Results showed that the new technique made it possible to quantify the spatial and temporal variations of bed load transport rate at the individual particle scale. The second research study focused on the theoretical study of bed load particle velocity and its distribution. A theoretical equation was derived for calculating the particle velocity and distribution at the equilibrium transport state. It was found the mean particle velocity is a function of average bed shear stress, and the instantaneous velocity of a bed load particle is dependent on the instantaneous bed-shear stress. The PDFs of particle velocity and bed shear stress both vary with the turbulence intensity. Results showed that the newly derived theoretical equation accurately predicted the average particle velocity. The PDF of particle velocity is a log-normal function at high Reynolds number, while it is close to an exponential distribution at low Reynolds number. The third research study focused on the numerical investigation of bed-load transport at the grain scale. In detail, this study was carried out mainly on sediment transport around a bridge pier. Bridge scour is commonly calculated based on the steady flow assumption. In practice, the peak discharge of a 100-year event is used for calculating the bridge scour depth. This will overestimate the scour depth, especially in arid and semi-arid region where the typical storms are high-peak and short duration flash floods. Therefore, the numerical test for simulating sediment transport around a bridge pier in unsteady condition was conducted by using the Smooth Particle Hydrodynamics (SPH) model. The simulation obtained by the DualSPHysics showed the scour process around bride pier in dam break flow. The results showed the local scour depth is affected by the large sediment load accompanying the dam break flow. The maximum scour depth was reached quickly, but only lasted for a few seconds before being back-filled by sediment. The maximum scour depth occurring under unsteady flow is much smaller than the calculated value using peak-flow discharge. In other words, using the peak-flow discharge for designing can overestimate the maximum scour depth in comparison to the actual conditions under a flash flood or any unsteady hydrograph.