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Some concepts relevant to rainfall erosion research and models Peter Kinnell

Some concepts relevant to rainfall erosion research and models Peter Kinnell University of Canberra Australia. Rainfall Erosion. 2 Drivers: Surface Water Flow Raindrop Impact. Critical conditions for detachment and transport modes.

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Some concepts relevant to rainfall erosion research and models Peter Kinnell

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

  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. Critical conditions for detachment and transport modes Flowdrivenerosion SplashErosion Coarse sandRD-RIR Coarse sandRD-FDR Flow detachment only occurs when the shear stress needed to cause detachment is exceeded Flow Energy 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. Suspended Load Very fine particles remain suspended in the flowing water Flow Bed load transport processes

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

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

  10. Critical conditions for detachment and transport modes Flow DrivenSaltation and Rolling FLOW DRIVEN TRANSPORT of material DETACHED by RAINDROPS will occur BELOW the shear stress that is needed to cause detachmentby flow 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 SplashErosion SplashErosion Suspensionin flow Coarse sandRD-RIR Coarse sandRD-FDR Raindrop driven saltation and rolling Flow Energy Not a 2D (X,Y) graph

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

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

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

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

  15. Raindrop impact driven erosion 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) Flow Flow Some models fail to include raindrop driven transport processes and so underestimate erosion loss

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

  17. Increase in runoff rate Splash erosion Raindrop driventransportin flow Flowdriven transport Sheet erosion on a plot Transport efficiency Splash transport Raindrop driven transport in flow Flow driven transport Splash erosion Raindrop driventransportin flow 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

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

  19. Increase in runoff rate Splash erosion Raindrop driventransportin flow Flowdriven transport Sheet erosion on a plot Rilling also produces a flush of soil material when rilling is active Splash erosion 44 m bare fallow on 5% slope Raindrop driventransportin flow E = kinetic energy flux I = rainfall intensityIs = average infiltration rate for storm 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

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

  21. USLERe = EI30 USLE-MRe = QREI30 QR = runoff ratio Sheet erosion on a plot RUNOFF is a factor the needs to be included when modelling event erosion

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

  23. Conclusion Detachment and transport processes control the development of the various forms of erosion on a hillslope or a plot SCALE is importantExperiments on 1 m x 1m plots apply only to INTERRILL EROSION and cannot be used to parameterize models that focus on sheet 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 RUNOFF is a factor the needs to be included when modelling event erosion

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