Sediment Transport by Water

1. Sediment Transport by Water • Theory • Processes • Rainsplash • overland flow transport • Rilling and gullying • Mass movements • Weathering limited versus supply limited • The extreme event

2. Theory • Mechanics of flow • Stream energy • Entrainment

3. Mechanics of flow • Water is subject to two forces: • gravity (Wa = g sin ) • friction • Defines ability of water to erode and transport sediment

4. Types of flow in open channels

5. Laminar Flow • Each fluid element moves along a specific path with no significant mixing between layers • Boundary layer in contact with the bed has no forward velocity • Each layer can slip past each other

6. Turbulent flow • At a critical velocity or depth laminar flow becomes unstable and the parallel streamlines are destroyed • Adjacent layers mix, transferring momentum by large scale eddies • Velocity more evenly distributed with depth • Steeper near bed velocity gradient

7. Reynolds Number (Re) • Re =  h u/ • where  = fluid density • h = flow depth • u = fluid viscosity •  = viscosity • larger values, larger turbulence

8. Entrainment • Movement of material depends on its physical properties; grain size shape density structual arrangement • Basic distinction; cohesive (silt-clay size) non-cohesive

9. Shear stress • Causes initial movement • Shear stress = estimate of force exerted on the bed by the fluid

10. cr  D • but doesn’t include lift forces • Lift due to: • eddies • difference in velocity at top and bottom of grain

11. Critical shear stress

12. Shields (1936) • Dimensionless critical shear stress • Plot against particle Reynolds no. (ratio of grain size to thickness of laminar sublayer)

13. use of average  or  spatial variability over bed channel size irregularity of eddies degree of exposure pivot angles imbrication degree of packing grain shape microtopography Factors producing scatter

14. Erosion • Entrainment/detatchment • Transport

15. Rainsplash Weathering Tillage Trampling Runoff Rainsplash Overland flow Rill flow Gully flow Detatchment vs Transport

16. Rainsplash • varies with rainfall intensity • varies with land cover • varies with slope • varies with % of area which is rilled • varies with lithology • crusting?

17. Surface Wash • particles detatched and transported by surface flowing water • force = velocity x mass (i.e. Q) • controls relate to character of materials, especially ability to produce rainfall excess

18. Resistance to detatchment • non-uniform • varies with particle size • cyclic variation with season • sand/silt clay ratio • stoniness

19. Rills • Impermanent channels • vary in lateral position year to year • develop once threshold exceeded in a single event

20. Gullies • permanent incised X-sectional form • develop once threshold exceeded over longer term average conditions • may be discontinuous • gully / arroyo / donga

21. Mass Movement • possibly only important in extreme events • directly contribute to load or rills/gullies • 4 main types • shallow slides • slab failure • rockfalls • deep seated slides

22. Soil Erosion Soil loss = R K L S P C R = rainfall erosivity K = erodibility of soil L = slope length s = slope angle P = coefficient of cultivation methods C = crop management factor

23. Weathering Limited When unlimited capacity for transport occurs, removal of material is limited by the rate at which material is detatched.

24. Transport Limited When there is an abundant supply of material and erosion depends on the efficiency of forces transporting the material away.

25. Equilibrium condition Removal of material = supply of material • Contionuous range between extremes • Occurs over different timescales: • Cyclic • Graded • Steady-state

26. Transport vs Weathering

27. Timescales • Cyclic • period over which an effective change in basin elevation can be measured • Graded (equilibrium) • a change in any factor will cause a displacement of the equilibrium in a direction which will absorb the effect of change • Steady state • a measurement can be taken and the system assumed to be in a constant condition

28. Timescales

29. Magnitude-Frequency concept • Wolman and Miller, 1960 • majority of ‘work’ carried out by events which occur on average 1 or 2 times per year • basin characteristics adjusted to these events

30. Different in semi-arid channels: • stress-strain reln more complicated • large spatial variation • morphology adjusted to extreme events

31. Extreme events Do majority of work because: • larger particle size • transmission losses • poor sorting • vegetation

32. Themes of dryland floods • Get scour and fill in times of extreme floods but channels restore themselves afterwards • Average sediment yields before a flood are exceeded for sometime afterwards • Work done during a flood is poorly related to flow volume or total ppt