Investigation on the incipient motion of nouninform sediment mixtures

Abstract

The present analysis aims at further improving the knowledge of incipient motion of nonuniform scdiment. Four sediment mixtures with different gradations are used in this experiment. Sediment mixtures were made by adding different size fractions with varying proportions. A reference transport method is used to define the beginning of bed material movement. Experiments were conducted at the Hydraulics and River Engineering laboratory of BUET. The experiments demonstrate that the incipient motion of individual size fractions within a mixture is largely controlled by their relative size with respect to median size. A consistent trend in the 'f;; vs d; curve is found. It appears that the critical d50 shear stress of individual fractions in sediment mixtures can be described principally by the relative variation of fractions. The ~;; vs ~ for the experimental sediment 'fc50 d50 mixtures fall into a single line, from which it may be inferred that all fractions move pr~dominantly at the same shear stress. From the <; vs Re; curve it is apparent that the finer fractions move at almost the same Reynolds number. Functions being developed for the calculation of critical shear stress of different sizes in sediment mixtures have close proximity with the previous methods. The critical shear stress of median sized sediment of the present investigation was found to be slightly smaller than the stress computed from the Shields' diagram. The value of critical shear stress was found to be dependent on its absolute size and mixture sorting. A semi - empirical method has been developed for the computation of critical shear stress of median sized sediment by rearranging the dimensionless parameters of grain Reynolds number and mixture sorting. By regression with the experimental data sets the derived parameters results in functions of the form of R• 009[ lidX ~(s -l)g ]1.05 d. d3 00 S( d3 ).08 T fu. c50 =. ag 50 V an Lc50 50 = . 1 ag 50 • hese two nctlOns can be a useful tool in determining critical shear stress of median sized sediment for nonuniform sediment mixtures. Previous investigators advocated either Shields' threshold curve or a particular value for the critical shear stress of median sized sediment in case of nonuniform sediment mixtures. The functions developed here will make the calculation of critical shear stress for other sediment sizes more simple and representative. Results of other investigators using different data sets were compared with the results of the present investigations that have used different sediment mixtures. The results show close agreement with the previous experimental results. Methods available in the literature also were compared with the experimental sediment mixtures. Some of them matched closely.