Underwater riprap has been commonly used for protection of riverbeds, banks and offshore structures against damage from scouring. Riprap is usually placed by dropping stones from the surface or certain depth of the water. However, in the case of water flow present in the construction site, the trajectory of riprap stones is difficult to predict. In this paper, CFD-DEM coupling simulations are implemented for investigating the drifting trajectory of riprap stones dropped in groups. The results of the simulations show that median value of drifting distance is logarithmic to the flow velocity. The drifting distances of the stones follow normal distribution. The results of the simulated contact points of the stones to the bottom are in good agreement with the reported results from experiments.
Underwater riprap is a common reinforcement approach adopted for protection of river bed and submarine pipelines from scouring damages. During the process of settlement, stones usually drift due to the force applied by the water flow. In addition, interactions between stones (i.e. collision and friction) also affect the drift trajectory of the stones. The drift distance of riprap in flow direction is regarded as an important parameter in during underwater riprap placement. However, as it is influenced by multiple factors, the trajectory of underwater riprap during settlement is difficult to be accurately predicted.
Considering the the morphology of stones used in underwater riprap and complex flow situation in the construction site, various empirical or semi-empirical formulas have been developed to estimate the drift distance of underwater riprap in previous researches. With the average velocity and average settlement velocity as the parameters of a simplified drifting model, Liang (1978) developed the drift distance formula of the underwater riprap by motion balance equation considering the propulsive force of water flow and the inertial force. Zhan(1992) proposed an empirical formula to calculate the drift distance of underwater riprap by assuming the vertical distribution of flow velocity to be exponential.
