The effect of raindrop impacted flow on sediment composition

1. The effect of raindrop impacted flow on sediment composition http://www.ozemail.com.au/~pkinnell

2. Rain Forms of Erosion on a Hillslope Splash Erosion River

3. Forms of Erosion on a Hillslopedetachment is the initializing process Raindrop impact is one of the two agents causing detachment in eroding areas Detachment is the plucking of soil particles from within the soil surface where the particles are held by cohesion and inter-particle friction Loose predetached particle Fall Uplift Transport Detachment

4. Forms of Erosion on a Hillslope Splash Erosion Raindrop Detachment & Splash Transport (RD-ST) On sloping surfacesmore splashed down slope than up so more erosion as slope gradient increases Transport process limits erosion particularly on low gradient slopes -----Relatively inefficient erosion system

5. Rain Surface Runoff Rain-impacted Flow Forms of Erosion on a Hillslope Splash Erosion River

6. Forms of Erosion on a Hillslope Erosion by Rain-impacted Flow 1. Raindrop Detachment & Raindrop Induced Saltation (RD-RIS) • Uplift caused by raindrop impacting flow 3 common detachment and transport systems Flow

7. Forms of Erosion on a Hillslope Erosion by Rain-impacted Flow 1. Raindrop Detachment & Raindrop Induced Saltation (RD-RIS) • Move downstream during fall Flow Wait for a subsequent impact before moving again

8. Forms of Erosion on a Hillslope Erosion by Rain-impacted Flow 2. Raindrop Induced Rolling (RIR) • Move downstream by rolling Flow Wait for a subsequent impact before moving again

9. Forms of Erosion on a Hillslope Erosion by Rain-impacted Flow 3. Raindrop Detachment with Flow Suspension (RD-FS) • Raindrops cause uplift in flow Flow

10. Forms of Erosion on a Hillslope Erosion by Rain-impacted Flow 3. Raindrop Detachment with Flow Suspension (RD-FS) • Small particles remain suspended and move without further stimulation Flow Large particles wait Acts at the same time as RD – RIS/RIR

11. Forms of Erosion on a Hillslope Splash Erosion and Erosion by Rain-impacted Flow cause SHEET EROSION Loss of soil in a relatively uniform sheet over the area

12. Rain Surface Runoff Rain-impacted Flow Forms of Erosion on a Hillslope Splash Erosion Rill & Interrill Erosion River

13. Rill Interrill area Forms of Erosion on a Hillslope Rill Erosion and Interrill Erosion Close up of a piece of a field Flow Rills are channels that can be removed by cultivation

14. Forms of Erosion on a Hillslope Rill Erosion Flow Detachment & Flow Driven Transport • Detachment and uplifted using flow energy Flow

15. Forms of Erosion on a Hillslope Rill Erosion Flow Detachment & Flow Driven Transport • Fine- Transported by flow as suspended load - fast moving • Coarse - Transported by flow as bed load – fast moving Flow Transport Raindrop impact not involved in any way Flow Transport

16. Critical conditions for detachment and transport modes for silt and sand and fine particles Dominates sheet and interrill erosion Raindrop impacted flow Raindrop detachment (RD) only occurs when the raindrop energy exceeds that need to cause detachment Flow detachment (FD) only occurs when the shear stress needed to cause detachment is exceeded - RILL EROSION Flow Energy

17. Forms of Erosion on a Hillslope Flow detachment is dominant in channels only when channels are developing. Most of the time, material leaving this bare area has been detached from the soil surface by raindrops NOT the flow. Rain- impacted flow (RD-FS/RIS/RIR)is responsible for loss of nutrient rich soil material from the land that may end up in water supplies etc

18. Flow x Erosion byRain-impacted Flow Raindrop Detachment & Raindrop Induced Saltation (RD-RIS) Particles transported by raindrop induced saltation move horizontally at velocities that depend on their size, density, and the velocity of the flow because these factors control the distance particles travel after a drop impact (x)

19. Erosion byRain-impacted Flow Distance particle travelafter a drop impact Drop impact Only impacts within the distance X cause particles to pass over the boundary Positions of drop impacts over some period of time Looking down on an area of soil covered by rain-impacted flow

20. Erosion byRain-impacted Flow Distance particle travelafter a drop impact • Sediment discharge varies with particle travel distance (X) - varies with flow velocity and particle size and density Drop impact Only impacts within the distance X cause particles to pass over the boundary Positions of drop impacts over some period of time

21. Erosion byRain-impacted Flow Distance particle travelafter a drop impact • Sediment discharge varies with particle travel distance (X) - varies with flow velocity and particlesizeanddensity Drop impact Only impacts within the distance X cause particles to pass over the boundary 3 times faster Experiments with coal and sand indicate that coal particles move about 2.75 times faster than sand particles of the same size

22. Erosion byRain-impacted Flow Mechanistic model of raindrop induced saltation 2.7 mm raindrops impacting a 7 mm deep flow - time in flow 0.55 s - time in flow 0.2 s 0.46 mm sand 0.46 mm coal Drop impacts generated randomly in space as with natural rain

23. 7 mm Simulation result Particle travel rates Rain : 2.7 mm drops at 60 mm/h over 3 m length Flow velocity = 150 mm/s Flow Non erodible 2980 mm Sand takes 2.75 times as long to reach the end Erodible : 20 mm long

24. Particle travel rates • Particles of sand can be considered to have times of concentrations that are 2.75 times longer than particles of coal of the same size • The concept of time of concentration is useful in looking at the effect of rainfall on runoff

25. 300 m impervious, n=0.03 Gradient = 3 % Gradient = 0.5 % Particle travel rates • The concept of time of concentration is useful in looking at the effect of rainfall on runoff. Compare runoff rates (mm/h) over time for a given rainfall event

26. 300 m impervious, n=0.3 Gradient = 3 % Gradient = 0.5 % Particle travel rates Rainfall rate 50 mm/h

27. Gradient = 3 % Gradient = 0.5 % Particle travel rates 300 m impervious, n=0.3 Rainfall rate 50 mm/h 7.8

28. 7 mm ` ` ` ` ` ` ` ` Simulation result Particle travel rates Rain : 2.7 mm drops at 60 mm/h over 3 m length Flow velocity = 150 mm/s Flow Cohesive erodible 3000 mm surface with sand : coal = 1:1 plusfine material Detention storage of sediment - build up of loose sand and coal particles on the surface protects the surface against detachment and causes fine discharge to decrease

29. 7 mm ` ` ` ` ` ` ` ` Xpd coal = 2.75 Xpd sand Particle travel rates Rain : 2.7 mm drops at 60 mm/h over 3 m length Flow velocity = 150 mm/s Flow Cohesive erodible 3000 mm surface with sand : coal = 1:1 plus fine material Detention storage of sediment - build up of loose sand and coal particles on the surface protects the surface against detachment and causes fine discharge to decrease Initially much more coal is discharged than sand but over time the two materials tend towards composition in the original erodible surface

30. Enrichment ratios = 1.0 at the steady state Enrichment ratios ≠ 1.0 only when erosion is not occurring at the steady state Enrichment Ratios The ratio of the proportion of the material in the discharge to the proportion of the material in the original “ER” for flow example s = 3% with s = 0.5% Detention storage of sediment reduces detachment 0.46 mm coal with 0.46 mm sand

31. Experimental Evidence Walker, Kinnell, Green 1978 • 3 m long inclined sand surface • 2 slope gradients: 0.5%, 5% • Events of 1 hour rainfall with uniform drop size • 2 drop sizes : 2.7 mm, 5.1 mm • 3 rainfall intensities: 45, 100, 150 mm/h

32. Rolling Rolling Increase in flow depth + reduction in flow velocity gives slower development Reductionin impact frequency and flow velocity gives slower developement Experimental Evidence Enrichment at 2 mins and 60 mins for 2.7 mm and 5.1 mm drops 150 mm/h 45 mm/h 2.7mm drops 2 mins 5%slope 60 mins Highest erosive stress 0.5%slope Lowest erosive stress

33. Experimental Evidence Palis et al 1990: Sandy clay loam soil on 0.1 % slope 5.8 m long 100 mm/h using continuous spray 0.6 min 15 min 35 min 5 min

34. Confounding Factors • Effective particle travel velocities vary for near zero to that of the flow • Aggregates breakdown may occur during transport of soil material– changes relative travel rates • Interactions between particles of different sizes and densities

35. Confounding Factors Model on 10 m long impervious plot inclined at 9 % Cohesive source has 5 particles sizes equally represented 50 mm/h rain intensity (2.7 mm drops) Flow depth and velocity vary down along the slope Height particles are lifted is restricted by water absorbing drop energy Height particles are lifted is restricted by height of water above surface

36. Confounding Factors Model on 10 m long impervious plot inclined at 9 % Cohesive source has 5 particles sizes equally represented 50 mm/h rain intensity (2.7 mm drops) Flow depth and velocity vary down along the slope

37. Confounding Factors Model on 10 m long impervious plot inclined at 9 % Cohesive source has 5 particles sizes equally represented 50 mm/h rain intensity (2.7 mm drops) Flow depth and velocity vary down along the slope Enrichment Depletion Slower particles affect the discharge of faster ones Time to reach the steady state controlled by the slowest moving particles

38. Confounding Factors Raindrop detachment + flow driven saltation Raindrop impacted flow Critical shear stress for flow driven saltation

39. Confounding Factors • Model on 2 - 30 m long plots inclined at 9 % • 2 part high intensity rainfall event • Cohesive source has 5 particles sizes equally represented • Flow depth and velocity vary down along the slope Enhanced loss of coal when L > 15m, 0.11 sand when L > 20mresulting from short term change from RIS to FDS

40. SEDIMENT ENRICHMENT---- more than A% carbonor nutrient in sediment Rain A% carbon or nutrient in soil Fundamentally, sediment enrichment occurs because • All particles do NOT travel laterally at the same rate • Erosion of the soil is occurring under non-steady conditions • Time of concentration approach relevant but need to consider the effect of detention storage of sediment on detachment etc • Modelling presented here is only qualitative – need to undertake research to determine more effectively how particles of differing sizes and densities actually travel and interact in rain- impacted flows

42. Complicating Factors extrapolation Flow depth Flow velocity Sprays: complex flow depth effect Previous research on depth effect 2.7 mm drops Interrill erosion experiments ?Depth effect not well known Need to be known in experiments on rain-impacted flows