Author(s): Prakash Agarwal; Yee-Meng Chiew
Linked Author(s): Yee Meng Chiew
Keywords: Grade control structures; Sediment transport; Particle entrainment; Entrainment probability; Rectangular cavity
Abstract: Winnowing of river bed material from scour protection works, such as grade control structures, can result in devastating consequences to the integrity of the foundation of hydraulic structures, such as bridge piers, as repeatedly witnessed in Taiwanese rivers during typhoon-induced floods. The entrainment of stones from between the gaps of the large concrete blocks used in these control structures have been proven to be a dominant factor associated with their failure (Lai et al. ,2010) .Although published studies have attempted to model such an entrainment process by using a deterministic framework to delineate conditions leading to entrainment and no-entrainment; the present study has instead employed a frequentist probabilistic approach. Laboratory experiments are conducted in a 14 m x 0. 49 m x 0. 6 m open-channel flume with single particle entrainments from a rectangular cavity under different test conditions. Results from these experiments depict a well defined stochastic aspect of the entrainment process, with probability curves following an S-shaped trend. The data debunk the existence of a single threshold value for defining the entrainment criterion. The entrainment occurring at random time intervals is found to be the result not only of the random velocity fluctuations prevailing in the cavity flow field but also due to the flow field undergoing major changes with changing cavity depth. The experiments also show that the Shields parameter is not the most accurate indicator of the entrainment time observed in this study, probably because it does not take into account the cavity features, such as its dimensions, shape, etc. ,which also play an important role in particle entrainment.
Year: 2015