Experimental Study on Initiation of Motion and Bed Load Transport of Unimodal and Bimodal Sediments

Knowledge of initiation of motion of sediment particles over river bed and its transport characteristics are required in solving many engineering problems, viz. design and maintenance of hydraulic structures, protection against erosion and scour, prediction of river bed level variation, stable channel design, site selection for water intakes, assessment of aquatic habitats etc. Natural river beds are invariably consisting of nonuniform sediments with either unimodal or bimodal grain size distributions. Initiation of motion of unimodal sediments have been explored with reasonably good accuracy by various investigators in the past (Egiazaroff, 1965; Ashida and Michiue, 1971; Hayashi et al., 1980; Parker et al., 1982; Patel and Ranga Raju, 1999; Wu et al. 2000). However, very few investigators (Kuhnle, 1993; Wilcock, 1993; Sambrook Smith, 1997; Almedeij et al., 2006) reported the investigations on bimodal sediments which are typical characteristics of coarse gravel bed streams (Garde and Ranga Raju, 2000). In present study, series of experiments have been under taken with unimodal and bimodal sediments on fractional bed load transport and initiation of motion at Advanced Hydraulic Laboratory of Civil Engineering Department, SVNIT-Surat (INDIA). The grain size distribution of transported bed load follows the grain size distribution of bed material for finer modes, typically at higher flow rates, for unimodal and weakly bimodal sediments. However, the situation is not true for extremely bimodal sediments wherein the grain size distributions of fractional bed load transport rates are completely different from the corresponding bed material grain size distributions. The fractional bed load transport data have been used to compute critical tractive stress (CTS) of individual size fractions using two approaches, viz. reference transport method (RTM) and largest grain method (LGM). Further, estimated CTS from the data using both the approaches, i.e. LGM and RTM, have been used to verify the performance of CTS relationship developed recently for unimodal and bimodal sediments (Patel et al., 2013). The CTS values estimated from observations have been found in good agreement with aforesaid relationship.