Sediment is fragmental material, primarily formed by the physical and chemical desintegration of rocks from the earth's crust. Such particles range in size from large boulders to colloidal size fragments and vary in shape from rounded to angular. They also vary in specific gravity and mineral composition, the predominant material being quartz. Once the sediment particles are detached, they may either be transported by gravity, wind or/and water.
The most common modes of sediment transport in rivers are bedload and suspended load. As bedload, sediment particles saltate, roll, and slide, but always staying close to the bed. As suspend load, sediment is carried by the fluid turbulence up in the water column. In the case of river, the volume concentration of solids in the water column tends to be rather dilute even during large floods .
2. Introduction
Sediment is fragmental material, primarily formed by the physical and chemical
desintegration of rocks from the earth's crust. Such particles range in size from large
boulders to colloidal size fragments and vary in shape from rounded to angular. They
also vary in specific gravity and mineral composition, the predominant material
being quartz. Once the sediment particles are detached, they may either be
transported by gravity, wind or/and water.
The most common modes of sediment transport in rivers are bedload and
suspended load. As bedload, sediment particles saltate, roll, and slide, but always
staying close to the bed. As suspend load, sediment is carried by the fluid turbulence
up in the water column. In the case of river, the volume concentration of solids in
the water column tends to be rather dilute even during large floods .
4. When the value of the bed-shear velocity just exceeds the critical value for
initiation of motion, the bed material particles will be rolling and/or sliding in
continuous contact with the bed. For increasing values of the bed-shear
velocity the particles will be moving along the bed by more or less regular
jumps, which are called saltations.
When the value of the bed-shear velocity begins to exceed the fall velocity of
the particles, the sediment particles can be lifted to a level at which the
upward turbulent forces will be of comparable or higher order than the
submerged weight of the particles and as a result the particles may go into
suspension.
The suspended load may also include the fine silt particles brought into
suspension from the catchment area rather than from the streambed material
(bed material load) and is called the wash load. A grain size of 50 mm is
frequently used to make the separation between bed material load and wash
load. Sometimes a value of 63 mm is used (USA). Another method of
discrimination is given by Vlugter (1941) and Bagnold (1962).
5.
6.
7. Definition sketch of suspended sediment transport
Observations show that the suspended sediment concentrations decrease with distance up
from the bed. The rate of decrease depends on the ratio of the fall velocity and the bed-
shear velocity (ws/u*).
8. Fluid and sediment properties
Basic dimensionless parameters of sediment transport processes are:
Bed-shear stress
In steady, uniform river flow with depth h and energy gradient S, the bed-
shear stress is defined as:
9. The fluid velocity distribution over the water depth
11. The Rose Equation
Under Stead equilibrium Condition , downward movement of sediment due to fall
velocity must be balanced by the net upward movement of sediment due to
turbulent fluctuations
Suspension Parameter
12. Rouse number ( z or Ro ) It is a ratio between the sediment fall velocity and the
upwards velocity on the grain as a product of the von Kármán constant and the shear
velocity
It also is used to determine how the particles will move in the fluid. The required Rouse
numbers for transport as bed load, suspended load, and wash load, are given below.
13. Suspended Load Formulas
Lane and Kalinske'sApproach ( 1941 )
qsw Suspended Load Transport rate in terms of weight.
D = depth of water
Figure ( 1 )
14. Einstein'sApproach (1950 )
Einstein is assumed that β= 1 and k = 0.4. Einstein approach is based by shear
velocity and grain roughness or grain size of the suspended load. The
equation of Einstein approach :
( 2d65 )
