GY2311/GY2312 Lectures 8-10 Sediment transport Modes of motion Suspended sediment transport Bedload transport

1. GY2311/GY2312 Lectures 8-10 Sediment transport Modes of motion Suspended sediment transport Bedload transport DEPARTMENT OF GEOGRAPHY

2. Suspended sediment transport Desert dust storm Muddy river water ‘Clean’ glacial ice

3. Suspended sediment concentrations in rivers River Wharfe, Yorkshire Dales. Sed. Conc. c. 500 mg l-1 Desert, stream , S. Israel. Sed. Conc. c. 30,000 mg l-1

4. Why does sediment suspension occur? Sediment suspension is initiated by the sweep of a turbulent eddy along the bed. Sediment suspension is maintained by vertical turbulent velocity fluctuations.

5. Why does sediment suspension occur? ‘Bursting’ - repetitive ejections of fluid away from the boundary with subsequent high speed in-rushes of fluid toward the boundary that sweep away the low-speed fluid remaining from the ejections

6. The fate of suspended material depends on its fall velocity (wo) The fall velocity (wo – omega nought) of a particle is the maximum velocity that can be attained by a particle falling freely in a fluid. If wo > vertical turbulent velocities – deposition If wo < vertical turbulent velocities - suspension

7. Fd wo Fw velocity time Fall velocity wo reflects the balance between the downward acting weight forces (Fw) and the upward acting fluid drag forces (Fd).

8. Fall velocity the downward acting weight force is determined by i) particle size (D) ii) excess density (rs-rf) rs and rf are sediment and fluid densities respectively the upward acting fluid drag force is determined by i) fluid viscosity (m) ii) particle shape iii) particle roughness

9. Stokes’ Law Fall velocity – Stokes’ Law Stokes’ Law defines the fall velocity of smooth spheres through a still (Newtonian) fluid (e.g. air and water) at low grain Reynolds Numbers (Re* < 1).

10. Flow separation Vortex shedding Turbulent flow (Impact Law) Laminar and turbulent flow around a falling particle Laminar flow (Stokes’ Law)

11. Impact Law Fall velocity - the Impact Law

12. Fall velocity – Stokes’ Law and the Impact Law Rep – Grain Reynolds No.

13. Criterion for suspended sediment transport Vertical turbulence velocities > wo

14. Criterion for suspended sediment transport svt > wo svt = u* > wo A particle will travel in suspension if the shear velocity of the flow is greater than the fall velocity of the particle.

15. Implications Vertical gradients in suspended sediment concentrations and grain size – an example from the Mississippi River

16. Implications Suspended sediment samples need to be integrated over the depth of flow. Sampling suspended sediment concentrations

17. Implications Fall velocity of a 0.3 mm grain: in air = 2 m s-1 in water = 0.04 m s-1 Since the mean flow velocity (u) = c. 20u* then the mean flow velocity required for suspension in air = 40 m s-1 (100 mph) but only 0.8 m s-1 in water – i.e. suspended sediment transport occurs more readily in water than in air. Contrasts between saltation and suspension dynamics in water and air

18. Implications Saltation heights are lower in water than in air because viscous drag is more effective in slowing down the rise of a particle than is gravity. Impact forces are greater in air than than in water due to the greater saltation heights and the lesser cushioning effect of water. Reptation is, therefore, an important aeolian process whereas it is inconsequential for fluvial processes. Contrasts between saltation and suspension dynamics in water and air

19. Modelling the vertical gradient of suspended sediment concentrations and grain size • Suspended sediment concentration and grain size can be modelled as a diffusive process if we assume: • steady uniform flow; • constant time averaged vertical sediment concentrations.

20. Rouse Number Diffusive model for suspended sediment concentration and size Cy is the sediment concentration (mg l-1) at height y above the bed (m) Y is the flow depth (m) w is the fall velocity (m s-1) u* is the shear velocity (m s-1) k is Von Karman’s constant (0.4) Ca is a reference concentration (mg l-1) at a reference height ya above the bed (m).

21. The Rouse Number (wo/u*k) Low values are given by fine particles (low w) and high velocities and turbulence (high u*) and give a near uniform vertical distribution of suspended sediment. High values are given by coarse particles (high w) and low velocities and turbulence (low u*) and give strong vertical gradients in the distribution of suspended sediment.

22. Modelling suspended sediment transport in rivers The suspended load transport at height y is the product of concentration and velocity at that height. The suspended load per unit width is obtained by multiplying the velocity and concentration profiles and integrating over the flow depth. This approach yields the suspended bed material load in steady inform flow since the theoretical velocity and concentration profiles are derived on the assumption of this condition

23. A cautionary note Suspended sediment concentrations (SSC) are often controlled by sediment supply (e.g. hillslope wash load) rather than flow hydraulics – implications for modelling suspended sediment loads. Susp sed conc, mg l-1 SSC in the River Creedy, UK