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.
