BED LOAD TRANSPORT OF SEDIMENT MIXTURES

Abstract

Hydraulic engineers are interested in the total amount of sediment that is transported by an alluvial channel, for given hydraulic and sediment characteristics. Knowledge regarding the rate of sediment transport and its composition under different flow conditions is of use in analysing the flow in natural watercourses. Problems such as the design of stable channels, design and maintenance of hydraulic structures, aggradation and degradation in rivers can be solved with a greater degree of certainty and refinement with the help of such information. According to the method of transportation, the total load can be subdivided into suspended load, saltation load, and contact load. Contact load is the material rolled or slid along the bed in substantially continuous contact with the bed. Saltation load is the material bouncing along the bed, or moved, directly or indirectly by the impact of bouncing particles. Suspended load is the material moving in suspension in the fluid. In most of the cases of water flow saltation load is very small and furthermore it is difficult to draw line of distinction between saltation load and contact load, so these are combined and designated as bed load. Bed load transport has been the subject of extensive research for several decades. The present-day knowledge of the subject is such that one may hope to compute the bed load transport of uniform sediment to a fair degree of accuracy. Natural sediments are invariably non-uniform and the bed load movement in such sediments is quite complex. The smaller particles are sheltered by the bigger ones and are therefore transported at a relatively smaller rate. On the other hand, the bigger particles experience larger fluid dynamic forces as they are more exposed to flow than they would be if they were a part of uniform sediment bed and consequently transported faster. The effect of the presence of one size on the transport rate of another in case of non-uniform sediment is accounted for through several correction factors. Einstein (1950) proposed the first semi-theoretical model for fractionwise calculation of bed load transport rates. Misri et al. (1984) pointed out the limitation of Einstein's method, viz nonconsideration of exposure effects of coarse fractions and presented the concept of exposure-cum-sheltering parameter for the calculation of bed load transport rates- a concept used by Proffit et al. (1983), Samaga et al. (1986), and Mittal et al. (1990). Bridge and Bennett (1992) presented a semi-theoretical model for bed ii load transport of non-uniform sediments, but it involves estimation of some model parameters like angle of repose, (I), and drag-partition about which there are some uncertainties. Patel and Ranga Raju (1995) checked many of the available relationships including that of Ashida and Michiue (1971), Mittal et al. (1990) and Bridge and Bennett (1992), for their accuracy and proposed an empirical relationship. Checks made on these methods reveal their inadequacies. The critical review on the subject thus suggested the need for an in-depth study related to the bed load transport of non-uniform sediments and the same was taken up during this investigation. The data from the previous investigations were used in the analysis and they covered a range of o-g from 1.73 to 7.43 and da from 0.32 mm to 54 MM.