HI. Sediment Transport
101
3.3
Particle Size and Fall Velocity of Suspended Sediment
3.3.1
Particle size of suspended sediment
Observations in flume and field conditions have shown that the suspended
sediment particles are not uniformly distributed over the depth in case of a
nonuniform bed material composition. The coarser particles are suspended in the
near-bed region, while the finer particles are suspended in the upper layers. Some
information can be obtained from the measurements of Van Rijn (1987) in a
large-scale wave flume. Sediment concentration profiles were determined by
using a pump sampler to obtain water-sediment samples. The suspended
sediment samples of each profile were put together to get a sufficiently large
sample for analysis. Fall velocities of these samples were measured in a settling
tube with a length of 2 m graphically. The fall velocities were converted to
particle size. The ratio of the median suspended sediment size (d~) and the
median bed material size (d5o) is shown in Fig. 16 as a function of a
dimensionless bed shear stress parameter T. Fig. 16 shows similar data obtained
in the surf zone near the coast of The Netherlands (station Groote Keeten, cf.
Kroon and Van Rijn, 1988). Since the bed material is quite uniform (cr~ = 0.5
(dso/d16 + ds4/dso) < 2) in both cases, the median suspended sediment size d~ is
fairly close to the median bed material size dso,b~d. The data can be reasonably
represented by the following equation :
d
- 1 + 0.011(o-~ - 1)(T- 25)
(46)
ds0,boa
in which :
d50,bed
%
T
= representative particle size of suspended sediment
= median particle size of bed material
= 0.5(050/016 q- dsJd5o) = geometrical standard deviation of bed material*
= bed shear stress parameter,
Equation (45) has been proposed by Van Rijn (1984) to represent the suspended
sediment size in steady uniform fow. Based on the present results (Fig. 16), this
equation is also applicable for oscillatory flow over a movable fine sediment bed.
The symbol cy~ is used in this chapter even though it is not universally
accepted by geostatisticians.
101
3.3
Particle Size and Fall Velocity of Suspended Sediment
3.3.1
Particle size of suspended sediment
Observations in flume and field conditions have shown that the suspended
sediment particles are not uniformly distributed over the depth in case of a
nonuniform bed material composition. The coarser particles are suspended in the
near-bed region, while the finer particles are suspended in the upper layers. Some
information can be obtained from the measurements of Van Rijn (1987) in a
large-scale wave flume. Sediment concentration profiles were determined by
using a pump sampler to obtain water-sediment samples. The suspended
sediment samples of each profile were put together to get a sufficiently large
sample for analysis. Fall velocities of these samples were measured in a settling
tube with a length of 2 m graphically. The fall velocities were converted to
particle size. The ratio of the median suspended sediment size (d~) and the
median bed material size (d5o) is shown in Fig. 16 as a function of a
dimensionless bed shear stress parameter T. Fig. 16 shows similar data obtained
in the surf zone near the coast of The Netherlands (station Groote Keeten, cf.
Kroon and Van Rijn, 1988). Since the bed material is quite uniform (cr~ = 0.5
(dso/d16 + ds4/dso) < 2) in both cases, the median suspended sediment size d~ is
fairly close to the median bed material size dso,b~d. The data can be reasonably
represented by the following equation :
d
- 1 + 0.011(o-~ - 1)(T- 25)
(46)
ds0,boa
in which :
d50,bed
%
T
= representative particle size of suspended sediment
= median particle size of bed material
= 0.5(050/016 q- dsJd5o) = geometrical standard deviation of bed material*
= bed shear stress parameter,
Equation (45) has been proposed by Van Rijn (1984) to represent the suspended
sediment size in steady uniform fow. Based on the present results (Fig. 16), this
equation is also applicable for oscillatory flow over a movable fine sediment bed.
The symbol cy~ is used in this chapter even though it is not universally
accepted by geostatisticians.