HI. Sediment Transport
71
The constant in eq. (21) is based on calibration using flume and field data with
velocities in the range of 0.4 to 1.6 m/s, depths in the range of 0.1 to 25 m mad
mean particle size in the range of 180 to 700 gm (Van Rijn, 1984), Voogt et al.
(1989) showed that eq. (2t) is valid for velocities upto 3 rods. The reference level
1
is assumed to be equal to half the bedform height (a = --A) or equal to the
2
effective bed roughness (a = ks) when the bedform height is unknown. The
reference level should not be chosen smaller than a = 0.01 h for reasons of
accuracy.
2.7
Influence of Non-Uniform Bed Material
Under nattrral conditions the bed material is non-uniform. Observations in flume
and field conditions have shown that the bed material and the suspended
sediment material have different particle size distributions. Usually, the
suspended sediment particles are considerably smaller than the bed material
particles. Basically, it is possible to compute the suspended load for any known
type of bed material and flow condition by dividing the bed material into a
number of size fractions and assuming that the size fractions do not influence
each other.
Van Rijn (1984) has applied the size fraction method to determine a
representative suspended sediment diameter (ds) which accounts for the nonuniformity effects. Using the size fraction method (as proposed by Einstein), the
total suspended load has been computed for various conditions, whereafter the
representative (suspended) particle diameter (ds) was determined by trial and
error gave the same value for the suspended load than according to the sizefraction method.
In all, six computations have been done using two types of bed material with a
geometric standard deviation CSs = (d84 /d50 + dl 6 /d50) = 1.5 and 2.5. The dso
of the bed material was equal to 250 ~tm. The mean flow velocities were 0.5 and
1.5 m/s (1.6 and 4.92 ft./s), The flow depth was assumed to be 10 m (32.81 ft.).
The concentration profile was computed by applying eq. (17) with ~5 = 1 ,
= 1
and K = 0,4. The reference concentration according eq. (21) was applied at a level
of z = 0.05 h. The flow velocity profile was assumed to be logarithmic, eq. (20).
The computed ds-values have been related to c%-parameter and to the
dimensionless shear-stress parameter T, as follows :
71
The constant in eq. (21) is based on calibration using flume and field data with
velocities in the range of 0.4 to 1.6 m/s, depths in the range of 0.1 to 25 m mad
mean particle size in the range of 180 to 700 gm (Van Rijn, 1984), Voogt et al.
(1989) showed that eq. (2t) is valid for velocities upto 3 rods. The reference level
1
is assumed to be equal to half the bedform height (a = --A) or equal to the
2
effective bed roughness (a = ks) when the bedform height is unknown. The
reference level should not be chosen smaller than a = 0.01 h for reasons of
accuracy.
2.7
Influence of Non-Uniform Bed Material
Under nattrral conditions the bed material is non-uniform. Observations in flume
and field conditions have shown that the bed material and the suspended
sediment material have different particle size distributions. Usually, the
suspended sediment particles are considerably smaller than the bed material
particles. Basically, it is possible to compute the suspended load for any known
type of bed material and flow condition by dividing the bed material into a
number of size fractions and assuming that the size fractions do not influence
each other.
Van Rijn (1984) has applied the size fraction method to determine a
representative suspended sediment diameter (ds) which accounts for the nonuniformity effects. Using the size fraction method (as proposed by Einstein), the
total suspended load has been computed for various conditions, whereafter the
representative (suspended) particle diameter (ds) was determined by trial and
error gave the same value for the suspended load than according to the sizefraction method.
In all, six computations have been done using two types of bed material with a
geometric standard deviation CSs = (d84 /d50 + dl 6 /d50) = 1.5 and 2.5. The dso
of the bed material was equal to 250 ~tm. The mean flow velocities were 0.5 and
1.5 m/s (1.6 and 4.92 ft./s), The flow depth was assumed to be 10 m (32.81 ft.).
The concentration profile was computed by applying eq. (17) with ~5 = 1 ,
= 1
and K = 0,4. The reference concentration according eq. (21) was applied at a level
of z = 0.05 h. The flow velocity profile was assumed to be logarithmic, eq. (20).
The computed ds-values have been related to c%-parameter and to the
dimensionless shear-stress parameter T, as follows :