Preferential transport of carbon materials in rain-impacted flow Peter Kinnell

1. Preferential transport of carbon materials in rain-impacted flow Peter Kinnell University of Canberra Australia

2. A% carbon in load Rain A% carbon in soil more than A% carbon in sediment A% carbon in soil

3. Why does sediment discharged in rain-impacted flows contain proportionately more carbon than the soil ? The answer :Because not all the particles are transported across the soil surface in at the same rate in rain-impacted flows

4. Erosion mechanisms in rain-impacted flowsDetachment is the initializing process Raindrop impact is the dominant agent causing detachment in rain-impacted flows Detachment is the plucking of soil particles from within the soil surface where the particles are held by cohesion and inter-particle friction Loose predetached particle Fall Uplift Transport Detachment

5. Erosion mechanisms in rain-impacted flows3 common transport mechanisms 1. Raindrop Induced Saltation (RIS) • Detachment and uplift caused by raindrops impacting flow Flow

6. Erosion mechanisms in rain-impacted flows3 common transport mechanisms 1. Raindrop Induced Saltation (RIS) • Particles move downstream during fall Flow Wait for a subsequent impact before moving again

7. Erosion mechanisms in rain-impacted flows3 common transport mechanisms 2. Raindrop Induced Rolling (RIR) • Particles move downstream by rolling Flow Wait for a subsequent impact before moving again

8. Erosion mechanisms in rain-impacted flows3 common transport mechanisms 3. Flow Suspension (FS) • Raindrops cause detachment and uplift Flow Acts at the same time as RIS & RIR

9. Erosion mechanisms in rain-impacted flows3 common transport mechanisms 3. Flow Suspension (FS) • Small particles remain suspended and move without raindrop stimulation Flow Large particles wait Acts at the same time as RD – RIS/RIR

10. Particle travel rates • Particles travel at rates that depend on the transport mechanism moving them • Fine suspended material moves at the velocity of the flow • Particles moving by raindrop induced saltation and rolling move at velocities that depend on their size, density, the frequency of drop impacts and the velocity of the flow

11. Particle travel rates Particles moving by raindrop induced saltation have velocities that depend on theirsize and density because these factors control the distance particles move after each drop impact

12. Particle travel rates Distance particle travelafter a drop impact Drop impact Only impacts within the distance X cause particles to pass over the boundary Positions of drop impacts over some period of time Looking down on an area of soil covered by rain-impacted flow

13. Particle travel rates Distance particle travelafter a drop impact • Sediment discharge varies with particle travel distance (X) - varies with flow velocity and particle size and density Drop impact Only impacts within the distance X cause particles to pass over the boundary Positions of drop impacts over some period of time

14. Particle travel rates Distance particle travelafter a drop impact • Sediment discharge varies with particle travel distance (X) - varies with flow velocity and particlesizeanddensity Drop impact Only impacts within the distance X cause particles to pass over the boundary 3 times faster Experiments with coal and sand indicate that coal particles move about 2.75 times faster than sand particles of the same size

15. Particle travel rates Mechanistic model of raindrop induced saltation 2.7 mm raindrops impacting a 7 mm deep flow 0.46 mm sand 0.46 mm coal Drop impacts generated randomly in space as with natural rain

16. 7 mm Simulation result Particle travel rates Rain : 2.7 mm drops at 60 mm/h over 3 m length Flow velocity = 150 mm/s Flow Non erodible 2980 mm Erodible : 20 mm long

17. 7 mm ` ` ` ` ` ` ` ` Simulation result Particle travel rates Rain : 2.7 mm drops at 60 mm/h over 3 m length Flow velocity = 150 mm/s Flow Cohesive erodible 3000 mm surface with sand : coal = 1:1 plusfine material Fine discharge decreases because build up of loose sand and coal particles on the surface protects the surface against detachment

18. ` ` ` ` ` ` ` ` Xpd coal = 2.75 Xpd sand Particle travel rates Rain : 2.7 mm drops at 60 mm/h over 3 m length Flow velocity = 150 mm/s Flow Cohesive erodible 3000 mm surface with sand : coal = 1:1 plus fine material Fine discharge decreases because build up of loose sand and coal particles on the surface protects the surface against detachment Initially much more coal is discharged than sand but over time the two materials tend towards composition in the original erodible surface

19. Particle travel rates • Variations in particle travel rates result in the initial discharge rates being greater for faster moving particles • Particles moving by raindrop stimulated transport processes provide a protective layer on the surface that reduces detachment • The protective layer coarsens over time and this causes the composition of the discharge to become the same as that of the original surface at the steady state if the particles are stable

20. Rain more than A% carbon in sediment A% carbon in soil • Enrichment results from particles containing carbon travelling relatively faster than mineral soil particles of the same size

21. Confounding Factors • Some smaller mineral soil particles travel at or faster than the velocities of particles rich in carbon – enrichment effect not limited to carbon material • Aggregates breakdown may occur during transport – changes relative travel rates • Effective particle travel velocities vary for near zero to that of the flow

22. Enrichment Depletion Confounding Factors • Model on 10 m long impervious plot inclined at 9 % • Cohesive source has 5 particles sizes equally represented • 50 mm/h rain intensity (2.7 mm drops) • Flow velocity varies down along the slope Slower particles affect the discharge of faster ones Issue: Some smaller mineral soil particles travel at or faster than the velocities of particles rich in carbon – enrichment effect not limited to carbon material Time to reach the steady state controlled by the slowest moving particles

23. Experimental Evidence Walker, Kinnell, Green 1978 • 3 m long inclined sand surface • 2 slope gradients: 0.5%, 5% • Events of 1 hour rainfall with uniform drop size • 2 drop sizes : 2.7 mm, 5.1 mm • 3 rainfall intensities: 45, 100, 150 mm/h

24. Rolling Reduction in flow velocity gives slower development Reductionin impact frequency and flow velocity gives slower developement Experimental Evidence Enrichment at 2 mins and 60 mins for 2.7 mm and 5.1 mm drops 150 mm/h 45 mm/h 2.7mm drops 2 mins 5% 60 mins 0.5%

25. Rain more than A% carbon in sediment A% carbon in soil Enrichment occurs because • All particles do NOT travel laterally at the same rate • Erosion of the soil is occurring under non-steady conditions • Results from experiments on the erosion of carbon need to be interpreted given this understanding

27. Critical conditions for detachment and transport modes Raindrop detachment only occurs when the raindrop energy exceeds that need to cause detachment Flow driven saltation & rolling-more efficient than RIS & RIR Coarse sandRD-RIR Coarse sandRD-FDR Flow detachment only occurs when the shear stress needed to cause detachment is exceeded Flow Energy