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

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

3. Erosion but no soil loss
Soil Erosion
Soil loss occurs when particles are detached from the surface of the soil matrix and transported across some boundary
Loose detached particle
boundary
Deposition
Detachment
Transport
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
Raindrop Detachment & Splash Transport (RD-ST)
The detachment and transport system associated with Splash Erosion

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

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)
On horizontal surfaces particles splashed back and forth and a layer of loose previously detached particles forms
Previously detached particles

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

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

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

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

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

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

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

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

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

18. Detachment & Transport Systems
Raindrop Induced Saltation (RIS)
Erodibility = kS (1-H) + kPDP H
Flow

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

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

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

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

23. Detachment and Transport on Hillslopes
Once runoff develops
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)

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

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

26. Detachment & Transport Systems
Flow Detatachment & Unassistred Flow Transport (FD-FT)
Uplift results from flow energy Transport: Suspended Load & Flow Driven Saltation
Particles in Suspension
FDS

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

28. Detachment & Transport Systems
Raindrop Induced Rolling (RIR) largely associated with gravel particles
Move downstream by rolling
Flow
Wait for a subsequent impact before moving again
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
Toposequence
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 may expand and contract one or more times during an event

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

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

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

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

35. Flow Detatachment & Unassisted Flow Transport (FD-FT)
Rill Erosion
Flow Detatachment & Unassisted Flow Transport (FD-FT)
Energy absorbed in transport leaves less energy for detachment
Process based models – eg WEPP
DF = erodibility (flow energy) (1 - [qs/Tc]) qs = sediment discharge Tc = transport capacity (max sed. discharge)
(1 - [qs/Tc]) = 0 if qs = Tc so DF =0

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

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

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

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

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

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

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

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

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

45. Rainfall Intensity and RIS
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
Particles upstream of the “active” zone require many impacts to move to the active zone

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
Sediment discharge varies with particle travel distance (X varies with flow velocity & particle size )

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

48. Rainfall Intensity and RIS
0.2 mm sand

49. Rainfall Intensity and RIS
Particle travel distance
In real life a large number of travel distances occur at the same time in same flow
Travel 3 times faster than
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
Knowledge of the 4 detachment and transport systems essential to interpreting the results of experiments
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
WEPP Interrill Model
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
Interrill Erodibility = kS (1-H) + kPDP H
kS, kPDL, and H all unknown
Difficulty in relating erodibility to soil properties

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),
KINNELL, P.I.A. (1993). Sediment concentrations resulting from flow depth - drop size interactions in shallow overland flow.Trans ASAE 36(4),
KINNELL,P.I.A. (1990). The mechanics of raindrop induced flow transport.Aust. J. Soil Res. 28,

53. Peter Kinnell
University of Canberra
Canberra ACT 2601
Australia