Erosion mechanisms in rain impacted flow and their effects and interactions P.I.A. Kinnell

1. Erosion mechanisms in rain impacted flow and their effects and interactions P.I.A. Kinnell University of Canberra Australia Oral Paper 3995 European Geophysical Union General Assembly 2012

2. Soil Erosion • It is well known that raindrop impact is themain driver ofdetachment in rain-impacted flows and isalso involved in determining how detached materialis transported across the soil surface

3. Transport in rain-impacted flow • Detachment by raindrop impact may be followed by • Raindrop induced saltation (RIS) • Raindrop induced rolling (RIR) • Transport in suspension (FS) • Flow driven saltation (FDR) • Flow driven rolling (FDR) Transport Mechanism 1. Raindrop Induced Saltation (RIS) • Detachment and uplift caused by raindrops impacting flow Flow Flow

4. Transport in rain-impacted flow Transport Mechanism 1. Raindrop Induced Saltation (RIS) • Particles move downstream during fall Flow Wait for a subsequent impact before moving again

5. Transport in rain-impacted flow Transport Mechanism 2. Raindrop Induced Rolling (RIR) • Particles move downstream by rolling Flow Wait for a subsequent impact before moving again

6. Transport in rain-impacted flow Transport Mechanism 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

7. Transport in rain-impacted flow Transport Mechanism 4. Flow Driven Saltation (FDS)Transport Mechanism 5. Flow Driven Rolling (FDR) After detachment by drop impactCoarse particles move without raindrop stimulation Flow

8. NB: Both raindrop detachment and flow detachment can operate at thesame time Flowdrivenerosion Raindrop drivenerosion FlowDrivenTransport Change in soil surface(crusting) Flow depth effect on drop energy available for detachment Critical conditions for detachment and transport modes Raindrop detachment only occurs when the raindrop energy exceeds that needed to cause detachment SplashErosion Rain DrivenTransportin Flow Coarse sandRD-RIR Coarse sandRD-FDR Flow detachment only occurs when the shear stress needed to cause detachment is exceeded Flow Energy

9. Factors affecting erosion by rain-impacted flows • Soil: particle size, particle density cohesion and interparticle friction • Rain: raindrop size and velocity rainfall intensity • Flow: flow depth, flow velocity

10. Critical conditions for detachment and transport modes Raindrop detachment only occurs when the raindrop energy exceeds that needed to cause detachment Suspension Rain DrivenTransportin Flow Coarse sandRD-RIR Coarse sandRD-FDR Flow detachment only occurs when the shear stress needed to cause detachment is exceeded Flow Energy

11. The effect of flow velocity Sand moves across the surface by raindrop induced saltation Apparatus enabling control of flow depth and velocity in rain-impacted flow over eroding surfaces

12. The effect of rainfall intensity Data from experiments by Kinnell (1992) using 0.2 mm sand and 2.7 mm drops The rate sediment is discharged when transported by raindrop induced saltation is linearly related to rainfall intensity and flow velocity

13. Particle travel distance The effect of flow velocity • Sediment discharge varies with particle travel distance (X) 2 parallel flows same particles but different flow velocities Only impacts within the distance X of the boundary produce discharge of particles 3 times the discharge than Travel distance varies with flow velocity There are 3 times the number of drop impacts producing discharge when travel distance is X3 than when X1 Impact frequency varies with rainfall intensity

14. The effect of flow depth When rain has a single drop size, the rate sediment is discharged when transported by raindrop induced saltation is linearly related to rainfall intensity (I) and flow velocity (u) Consequently qs(p,d) = kp Id u f[h,d]where kp is a coefficient dependent on p and f[h,d] is a function that accounts for the effect of flow depth for drops of size d

15. The effect of flow depth qs(p,d) = kp Id u f[h,d] • f[h,d] is affected by the mass of material lifted into the flow by each drop impactThat decreases with flow depth as more of the drop energy is dissipated in the flow • f[h,d] is affected by the height particles are lifted in the flow by each drop impact.That depends on how much of the drop energy is dissipated in the flow and how the surface of the flow constrains the uplift

16. Height lifted restricted by energy available from drop impact Height lifted restricted by surface The effect of flow depth Sediment discharge varies directly with particle travel distance

17. The effect of flow depth qs(p,d) = kp Id u f[h,d] Also a decrease in the mass lifted into the flow

18. Loose particles on the surface qs(p,d) = kp Id u f[h,d] Particles that fall back to the bed after being detached and lifted into the flow form a layer of loose particles sitting on the cohesive surface They require energy to move them before detachment from the cohesive layer can occur and this causes the value of kp to vary with time

19. Loose particles on the surface qs(p,d) = kp Id u f[h,d] • kp = kp.M (1 – H) + kp.L H • kp.M is the value of kp when no loose particles are on the surface • kp.L is the is the value of kp when the loose particles on the surface fully protect against detachment (H = 1) • H is the degree of protection provided by the loose particles

20. Loose particles on the surface qs(p,d) = kp Id u f[h,d] • Sediment concentration cs(p,d) = qs(p,d) / qw • For a surface of p sized sand: cs(p,d) / Id = kp.L (f[h,d]/h) because H = 1 = qs(p,d) / (h u) = kp Idu (f[h,d]/h) /u = kp Id (f[h,d]/h) Sediment concentration is the amount of sediment discharged per unit quantity of water without distinguishing the transport mode kpL = 0.1443f[h,d]/h= 1 + 0.0104 h2 + 0.202 h h = depth , u = flow velocity

21. Loose particles on the surface • For soil: cs(p,d) / Id = (kp.M (1 – H) + kp.L H) (f[h,d]/h) Problem is that H is unknown in the experiments with soil so detachment is unknown but declines with time as the amount of loose material builds up

22. Simulation result 3 m Particle size and density • Distance travelled during a saltation event is affected by particle size and density as they influence the time particles are moving in the flow after a drop impact. Rain : 2.7 mm drops at 60 mm/h over 3 m long surface eroding a small area with 50% 0.46 mm sand 50% 0.46 mm coal at the top

23. Response of fine particles travelling in continuous suspension reflects overall change in detachment Simulation result Particle size and density Amount of the slowest moving particle in layer of loose material increases with time so the slowest moving particle controls the time taken to reach the steady state INCLUDES protective effect of loose material 3 m Rain : 2.7 mm drops at 60 mm/h eroding a 3 m long cohesive surface with 50% 0.46 mm sand 50% 0.46 mm coal

24. Particle size and density Amount of the slowest moving particle in layer of loose material increases with time so the slowest moving particle controls the time taken to reach the steady state Experiment 2.7 mm drops falling on 3 m long sandy soil on 0.5% and 5% slopes Time taken to reach the steady state also varies with slope length, slope gradient, runoff and rainfall characteristics

25. Suspension Flow Driven Saltation Critical conditions for detachment and transport modes Raindrop detachment only occurs when the raindrop energy exceeds that needed to cause detachment Suspension Rain DrivenTransportin Flow Coarse sandRD-RIR Coarse sandRD-FDR Flow detachment only occurs when the shear stress needed to cause detachment is exceeded Flow Energy

26. RIS0.46 mm coal FDS0.46 mm coal RIS0.46 mm sand RIS – RDS - RIS Flow velocity in the outflow on 9 % slopes

27. Conclusion It is important to be well aware of the effects and interactions that exist between the detachment and transport mechanisms that operate in rain-impacted flows when interpreting the results of experiments undertaken on sheet and interrill erosion areas