Sediment Transport Suspended Load - Khalid Arafat .pptx

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 .

1. SEDIMENT TRANSPORT SUSPENDED LOAD

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 .

3. Modes Of Transportation

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. 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*).

6. 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:

7. The fluid velocity distribution over the water depth

8. General Consideration Suspended Load Transport rate can be defined mathematically as : or

9. 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  

10. Rouse number ( z or R o ) 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.

11. Suspended Load Formulas Lane and Kalinske's Approach ( 1941 ) q sw Suspended Load Transport rate in terms of weight. D = depth of water Figure ( 1 )

12. Einstein's Approach ( 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 : ( 2d 65 )

13. Figure 3 Figure 4

14. Chang, Simon and Richardson’s (1965)

15. Example

16. From Figure ( 1 ) PL = .065

17. Einstein's Method ( 1950 )

18. Chang, Simon and Richardson’s (1965) I 1 = 0.04 I 2 = 0.025