1. P.I.A. Kinnell University of Canberra Rainfall Erosion Detachment and Transport Systems

2. Soil Erosion involves the detachment of soil material at some place and and the transport of this material away from the site of detachment Two linked processes

3. Soil Erosion n Soil loss occurs when particles are detached from the surface of the soil matrix and transported across some boundary DetachmentTransport Deposition Loose detached particle boundary Erosion but no soil loss

4. Detachment and Transport on Hillslopes Onset of rain: Raindrop detachment (RD) + splash transport (ST) covers the whole slope

5. Detachment & Transport Systems n n The detachment and transport system associated with Splash Erosion Raindrop Detachment & Splash Transport (RD-ST)

6. Detachment & Transport Systems n n The detachment and transport system associated with Splash Erosion Raindrop Detachment & Splash Transport (RD-ST)

7. Detachment & Transport Systems Raindrop Detachment & Splash Transport (RD-ST) On horizontal surfaces particles splashed back and forth

8. Detachment & Transport Systems Raindrop Detachment & Splash Transport (RD-ST) Previously detached particles On horizontal surfaces particles splashed back and forth and a layer of loose previously detached particles forms

9. Detachment & Transport Systems Previously detached particles protect soil surface from detachment But are splashed Raindrop Detachment & Splash Transport (RD-ST) Previously detached particles

10. Detachment & Transport Systems Splashed particles come from both soil surface and layer of previously detached particles Splashed particles come from both soil surface and layer of previously detached particles Raindrop Detachment & Splash Transport (RD-ST) Previously detached particles

11. Detachment & Transport Systems On sloping surfaces more splashed down slope than up so more erosion as slope gradient increases previously detached particles get thicker in down slope direction. but previously detached particles get thicker in down slope direction. Raindrop Detachment & Splash Transport (RD-ST) Previously-detached particles

12. Detachment & Transport Systems Erodibility = susceptibility of eroding surface to erosion depends on (a) splash of particles immediately after detachment AND (b) splash of previously detached material Raindrop Detachment & Splash Transport (RD-ST) Previously-detached particles

13. Detachment & Transport Systems Erodibility = k S (1-H) + k PDP H Raindrop Detachment & Splash Transport (RD-ST) ksks k PDP k s = erodibility when no PDP H = degree of protection provided by the PDP (0 - 1) k PDP = erodibility when fully protected Previously-detached particles

14. Raindrop Induced Saltation (RIS) Detachment & Transport Systems Occurs when raindrops impact shallow flow

15. Raindrop Induced Saltation (RIS) Detachment & Transport Systems n Uplift caused by raindrop impacting flow Flow

16. Raindrop Induced Saltation (RIS) Detachment & Transport Systems n Uplift - Fall Flow Particles move downstream during the saltation event

17. Raindrop Induced Saltation (RIS) Detachment & Transport Systems n Layer of previously detached particles – depth increasing downstream Flow

18. Raindrop Induced Saltation (RIS) Detachment & Transport Systems n n Erodibility = k S (1-H) + k PDP H Flow

19. Raindrop Detatachment & Flow Suspension (RD-FS) Detachment & Transport Systems

20. n Uplift Raindrop Detatachment & Flow Suspension (RD-FS)

21. Detachment & Transport Systems n Uplift - Suspended > FS Fall > RIS at low flow velocities Particles in Suspension RIS Particles transported by RIS travel slower than by FS Raindrop Detatachment & Flow Suspension (RD-FS)

22. Detachment & Transport Systems n Uplift - Suspended > FS Fall > FDS at higher flow velocities Particles in Suspension FDS Particles transported by FDS travel faster than by RIS Raindrop Detatachment & Flow Driven Saltation (RD-FDS)

23. Detachment and Transport on Hillslopes With clay, silt and sand particles: 3 transport systems with raindrop detachment RD + splash transport (ST) RD + raindrop induced saltation (RIS) RD + unassisted flow transport (FS & FDS) Once runoff develops

24. Detachment & Transport Systems Flow Detatachment & Unassistred Flow Transport (FD-FT)

25. Detachment & Transport Systems n Uplift results from flow energy Flow Detatachment & Unassistred Flow Transport (FD-FT)

26. Detachment & Transport Systems n Uplift results from flow energy Transport: Suspended Load & Flow Driven Saltation FDS Particles in Suspension

27. Efficiency of Transport of Sand, Silt and Clay particles Sand, Silt and Clay particles Splash Transport Raindrop Induced Saltation Splash Transport Raindrop Induced Saltation Flow Driven Saltation Flow Driven Saltation Flow Driven Suspension Flow Driven SuspensionIncreasing

28. Raindrop Induced Rolling (RIR) largely associated with gravel particles n Move downstream by rolling Flow Wait for a subsequent impact before moving again Detachment & Transport Systems Flow Driven Rolling (FDR) may also follow RD

29. Detachment and Transport on Hillslopes Raindrop detachment (RD) erosion systems RD + splash transport (ST) RD + raindrop induced saltation (RIS) RD + raindrop induced rolling (RIR) RD + unassisted flow transport (FT) (suspension, saltation, rolling) Flow detachment (FD) erosion systems FD + unassisted FT (suspension, saltation, rolling)

30. Detachment and Transport on Hillslopes Raindrop detachment (RD) erosion systems RD + splash transport (ST) RD + raindrop induced saltation (RIS) RD + raindrop induced rolling (RIR) RD + unassisted flow transport (FT) (suspension, saltation, rolling) Flow detachment (FD) erosion systems FD + unassisted FT (suspension, saltation, rolling) Toposequence Toposequence may expand and contract one or more times during an event

31. Sheet Erosion n n Sheet erosion refers to erosion where a portion of the soil surface layer over a relatively wide area is removed somewhat uniformly. n n Detachment & Transport Systems RD - ST RD - RIS & RIR RD - FS (& FDS & FDR)

32. Rill Erosion n n Rill erosion refers to erosion in small channels that can be removed by normal cultivation. n n Detachment & Transport Systems FD – FS & FDS & FDR

33. Interrill Erosion n n Interrill erosion refers to erosion in interrill areas n n Detachment & Transport Systems RD - ST RD - RIS & RDR RD - FS (& FDS & FDR)

34. Rill Erosion n Energy absorbed in transport leaves less energy for detachment Flow Suspension FDS Flow Detatachment & Unassisted Flow Transport (FD-FT)

35. Rill Erosion n Energy absorbed in transport leaves less energy for detachment Process based models – eg WEPP Process based models – eg WEPP n D F = erodibility (flow energy) (1 - [q s /T c ]) q s = sediment discharge T c = transport capacity (max sed. discharge) n (1 - [q s /T c ]) = 0 if q s = T c so D F =0

36. Rill Erosion n D F = erodibility (flow energy) (1 - [q s /T c ]) q s = sediment discharge T c = transport capacity (max sed. discharge) n Water and sediment flows from interrill areas to rills. Interrill erosion contributes to q s and reduces D F n Rills may often simply act as efficient transport routes for interrill erosion

37. Rill Erosion... n Rills may often simply act as efficient transport routes for interrill erosion Non erodible layer

38. Detachment & Transport Systems Diagram summarising the interaction between raindrops and flow in respect to determining the detachment and transport RAIN WITH NO RUNOFF RAIN WITH RUNOFF Fine Particles RD-FS Silt & Sand RD-RIS Silt & Sand RD-FDS Clay, Silt & Sand RD-ST Clay, Silt & Sand FD-FDS,FS NO EROSION E < E c, Ω < Ω (bound) A B 0 EcEc EcEc 0 τ c (bound) τ c (loose) Raindrop Energy (E) Flow Shear Stress ( τ ) RAIN WITH NO RUNOFF

39. RAIN WITH RUNOFF Fine Particles RD-FS Silt & Sand RD-RIS Silt & Sand RD-FDS Clay, Silt & Sand RD-ST Clay, Silt & Sand FD-FDS,FS NO EROSION E < E c, Ω < Ω (bound) A B 0 EcEc EcEc 0 τ c (bound) τ c (loose) Raindrop Energy (E) Flow Shear Stress ( τ ) RAIN WITH NO RUNOFF Detachment & Transport Systems Critical drop energy for detachment

40. RAIN WITH NO RUNOFF RAIN WITH RUNOFF Fine Particles RD-FS Silt & Sand RD-RIS Silt & Sand RD-FDS Clay, Silt & Sand RD-ST Clay, Silt & Sand FD-FDS,FS NO EROSION E < E c, Ω < Ω (bound) A B 0 EcEc EcEc 0 τ c (bound) τ c (loose) Raindrop Energy (E) Flow Shear Stress ( τ ) RAIN WITH NO RUNOFF Detachment & Transport Systems Critical drop energy for detachment Critical flow “energy” for detachment Critical flow “energy” for detachment

41. RAIN WITH NO RUNOFF RAIN WITH RUNOFF Fine Particles RD-FS Silt & Sand RD-RIS Silt & Sand RD-FDS Clay, Silt & Sand RD-ST Clay, Silt & Sand FD-FDS,FS NO EROSION E < E c, Ω < Ω (bound) A B 0 EcEc EcEc 0 τ c (bound) τ c (loose) Raindrop Energy (E) Flow Shear Stress ( τ ) RAIN WITH NO RUNOFF Detachment & Transport Systems Critical drop energy for detachment Critical flow “energy” for detachment Critical flow “energy” for detachment Critical flow “energy” to move previously detached material

42. Flow Transport n Critical flow energy for maintaining transport Detachment (controlled by cohesion) Transport of previously detached material  Varies with particle size

43. Raindrop Detatachment & Flow Transport (RD-FT) Detachment & Transport Systems n Uplift - Suspended > FT Fall > RIFT at low flow velocities Flow Transport RIS Particles transported by RIS travel slower than by FT

44. Raindrop Detatachment & Flow Transport (RD-FT) Detachment & Transport Systems n Uplift - Suspended > FT Fall > FT (Bed Load) Flow Transport FT Flow velocities can increase to above those that favour RIS

45. Rainfall Intensity and RIS n Particles upstream of the “active” zone require many impacts to move to the active zone Particle travel distance - the distance travelled after lifted into flow by a drop impact Drop impact Particles must be within a distance from a boundary that is less than the travel distance in order to pass across that boundary

46. Rainfall Intensity and RIS Particle travel distance Drop impact Particles must be within a distance from a boundary that is less than the travel distance in order to pass across that boundary n Sediment discharge varies with particle travel distance (X varies with flow velocity & particle size )

47. Rainfall Intensity and RIS n Sediment discharge varies with particle travel distance (X varies with flow velocity & particle size ) Particle travel distance and drop impact frequency (varies with rain intensity) Travel 3 times faster than 3 parallel flows same velocity but different particles

48. Rainfall Intensity and RIS 0.2 mm sand

49. Rainfall Intensity and RIS Particle travel distance Travel 3 times faster than In real life a large number of travel distances occur at the same time in same flow n Sediment discharge varies with particle travel distance (X varies with flow velocity & particle size ) and drop impact frequency (varies with rain intensity)

50. Modelling rainfall erosion n Knowledge of the 4 detachment and transport systems essential to interpreting the results of experiments n However, so called process-based models do not usually deal with the complexities to any large extent – leads to difficulty when parameterisation is based on experiments

51. Modelling rainfall erosion Interrill erodibility evaluated experimentally - approx 65 mm/h intensity - soil loss after 15 mins, 25 mins, 35 mins + used to produce single erodibility value for each soil Interrill erodibility evaluated experimentally - approx 65 mm/h intensity - soil loss after 15 mins, 25 mins, 35 mins + used to produce single erodibility value for each soil Dominated by RD – RIFT and RD – FT Dominated by RD – RIFT and RD – FT n n Interrill Erodibility = k S (1-H) + k PDP H n n k S, k PDL, and H all unknown n n Difficulty in relating erodibility to soil properties WEPP Interrill Model

52. Some References KINNELL, P.I.A. (2005). Raindrop impact induced erosion processes and prediction. Hydrological Processes (in press) KINNELL, P.I.A. (1994). The effect of predetached particles on erosion by shallow rain-impacted flow.Aust. J. Soil Res. 31(1), 127-142. KINNELL, P.I.A. (1993). Sediment concentrations resulting from flow depth - drop size interactions in shallow overland flow.Trans ASAE 36(4), 1099-1103. KINNELL,P.I.A. (1990). The mechanics of raindrop induced flow transport.Aust. J. Soil Res. 28,497-516

53. Peter Kinnell University of Canberra Canberra ACT 2601 Australiapeter.kinnell@canberra.edu.au