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P.I.A. Kinnell University of Canberra

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

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P.I.A. Kinnell University of Canberra

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  1. Rainfall ErosionDetachmentandTransportSystems P.I.A. Kinnell University of Canberra

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

  3. 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 forthand a layer of loose previously detached particles forms Previously detached particles

  9. Detachment & Transport Systems Raindrop Detachment & Splash Transport (RD-ST) Previously detached particlesprotect soil surface from detachment But aresplashed 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 butpreviously 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 energyTransport: Suspended Load & Flow Driven Saltation Particles in Suspension FDS

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

  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 erosionrefers to erosion where a portion of the soil surface layer over a relatively wide area is removed somewhat uniformly. • Detachment & Transport SystemsRD - STRD - RIS & RIRRD - FS (& FDS & FDR)

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

  33. Interrill Erosion • Interrill erosionrefers to erosion in interrill areas • Detachment & Transport SystemsRD - STRD - RIS & RDRRD - FS (& FDS & FDR)

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

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

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

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

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

  42. Flow Transport Detachment (controlled by cohesion) • Critical flow energy for maintaining transport Transport of previously detached material • 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 • Particles upstream of the “active” zone require many impacts to move to the active zone 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 • Sediment discharge varies with particle travel distance (X varies with flow velocity & particle size ) 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

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

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