1. Some concepts relevant to rainfall erosion research and models Peter Kinnell University of Canberra Australia EGU2014

2. Rainfall Erosion 2 Drivers: Surface Water Flow Raindrop Impact

3. Critical conditions for detachment and transport modes Erosion results from the expenditure of energy associated with both flow and raindrop impact

4. Splash Erosion Critical conditions for detachment and transport modes Flow Energy Flow detachment only occurs when the shear stress needed to cause detachment is exceeded Coarse sand RD-RIR Coarse sand RD-FDR Flow driven erosion Not a 2D (X,Y) graph

5. Rainfall Erosion Channels caused by flow driven erosion Rill Erosion Gully Erosion

6. Bed Load Flow Flow driven saltation and rolling

7. Very fine particles remain suspended in the flowing water Suspended Load Flow Bed load transport processes

8. Splash Erosion Critical conditions for detachment and transport modes Flow Energy Flow detachment only occurs when the shear stress needed to cause detachment is exceeded Raindrop detachment only occurs when the raindrop energy exceeds that needed to cause detachment Coarse sand RD-RIR Coarse sand RD-FDR Raindrop driven erosion Change in soil surface (crusting) Flow depth effect on drop energy available for detachment Flow driven erosion Not a 2D (X,Y) graph NB: Both raindrop detachment and flow detachment can operate at the same time Splash Erosion

9. On sloping surfaces more splashed down slope than up so more erosion as slope gradient increases Raindrop Detachment & Splash Transport (RD-ST) Splash Erosion Transport process limits erosion particularly on low gradient slopes - Relatively inefficient erosion system especially on slopes with low to moderate gradients Raindrop impact driven erosion

10. Splash Erosion Critical conditions for detachment and transport modes Flow Energy Flow detachment only occurs when the shear stress needed to cause detachment is exceeded Raindrop detachment only occurs when the raindrop energy exceeds that needed to cause detachment Coarse sand RD-RIR Coarse sand RD-FDR Not a 2D (X,Y) graph Splash Erosion Flow Driven Saltation and Rolling Suspension in flow FLOW DRIVEN TRANSPORT of material DETACHED by RAINDROPS will occur BELOW the shear stress that is needed to cause detachment by flow Raindrop driven saltation and rolling

11. Detachment and uplift caused by raindrops impacting flow Flow Raindrop impact driven erosion Rain-impacted flow Raindrop Induced Saltation (RIS)

12. Particles move downstream during fall Flow Wait for a subsequent impact before moving again Raindrop Induced Saltation (RIS) Raindrop impact driven erosion Rain-impacted flow X Transport rate varies directly with x

13. Flow Raindrop Induced Saltation (RIS) Raindrop impact driven erosion Rain-impacted flow Height of water surface constrains height to which particles are lifted Dissipation of drop energy in water restricts height to which particles are lifted Q s = k s u f[h,d] s Q s = sed discharge, k s = “erodibility”, u = flow vel, f[h,d] s = function of flow depth and drop size when travelling at V T X depends on height particles are lifted in very shallow flow Peak for any given drop size is related to the maximum height particles are lifted in the flow In most field and laboratory experiments using artificial rainfall flow depths are UNKOWN

14. Raindrop Induced Rolling (RIR) Particles move downstream by rolling Flow Wait for a subsequent impact before moving again Raindrop impact driven erosion Rain-impacted flow

15. Flow Raindrop impact driven erosion Rain-impacted flow Detachment by raindrop impact may be followed by 1.Raindrop induced saltation (RIS) 2.Raindrop induced rolling (RIR) 3.Transport in suspension (FS) 4.Flow driven saltation (FDR) 5.Flow driven rolling (FDR) Some models fail to include raindrop driven transport processes and so underestimate erosion loss

16. Rain Forms of Water Erosion on a Hillslope Splash Erosion Flow energy increasing Rill & Interrill Erosion Rill Interrill Sheet Erosion Surface Runoff River (Gully Erosion) Detachment and transport processes control the development of the various forms of erosion on a hillslope Splash Erosion

17. Sheet erosion on a plot Raindrop driven transport in flow Splash erosion Raindrop driven transport in flow Flow driven transport Transport efficiency Splash transport Raindrop driven transport in flow Flow driven transport Increases Splash and raindrop driven transport in the flow result in loose particles sitting on the surface Flow driven transport can flush loose particles from the surface Increase in runoff rate

18. Sheet erosion on a plot Raindrop driven transport in flow Splash erosion Raindrop driven transport in flow Flow driven transport Increase in runoff rate Flow driven transport flushes loose particles of coal from the surface when slope is 15 m or more Transport of 0.46 mm coal flow driven -------------------- raidrop driven Simulation model results

19. Sheet erosion on a plot Raindrop driven transport in flow Splash erosion Raindrop driven transport in flow Flow driven transport Increase in runoff rate Flow driven transport can flush loose particles from the surface

20. Sheet erosion on a plot Raindrop driven transport in flow Splash erosion Raindrop driven transport in flow Flow driven transport Increase in runoff rate Rilling also produces a flush of soil material when rilling is active In general, most models of rainfall erosion fail to properly take account of the effect of changes in detachment and transport mechanisms in TIME and SPACE 44 m bare fallow on 5% slope E = kinetic energy flux I = rainfall intensity I s = average infiltration rate for storm

21. Sheet erosion on a plot Raindrop driven transport in flow Splash erosion Raindrop driven transport in flow Flow driven transport Increase in runoff rate Rilling also produces a flush of soil material when rilling is active RUNOFF is a factor the needs to be included when modelling event erosion 44 m bare fallow on 5% slope I – I s is used as a surrogate for the runoff rate

22. Sheet erosion on a plot RUNOFF is a factor the needs to be included when modelling event erosion USLE R e = EI 30 USLE-M R e = Q R EI 30 Q R = runoff ratio

23. Sheet erosion on a plot Raindrop driven transport in flow Splash erosion Raindrop driven transport in flow Flow driven transport Increase in runoff rate SCALE is important Experiments on 1 m x 1m plots apply only to INTERRILL EROSION and cannot be used to parameterize models that focus on sheet erosion 44 m bare fallow on 5% slope

24. Conclusion Detachment and transport processes control the development of the various forms of erosion on a hillslope or a plot SCALE is important Experiments on 1 m x 1m plots apply only to INTERRILL EROSION and cannot be used to parameterize models that focus on sheet erosion RUNOFF is a factor the needs to be included when modelling event erosion The effect of FLOW DEPTH on detachment and transport in rain impacted flows means that results produced in many laboratory and field experiments are not readily applied elsewhere