Preferential transport of carbon materials in rain-impacted flow in rain-impacted flow Peter Kinnell University of Canberra Australia

A% carbon in soil A% carbon in load A% carbon in soil more than A% carbon in sediment Rain

Why does sediment discharged in rain-impacted flows contain proportionately more carbon than the soil ? The answer : Because not all the particles are transported across the soil surface in at the same rate in rain-impacted flows

Flow Erosion mechanisms in rain-impacted flows Detachment is the plucking of soil particles from within the soil surface where the particles are held by cohesion and inter-particle friction Raindrop impact is the dominant form of detachment

Detachment Transport Fall Loose predetached particle Erosion mechanisms in rain-impacted flows Detachment is the initializing process Detachment is the plucking of soil particles from within the soil surface where the particles are held by cohesion and inter-particle friction Uplift Raindrop impact is the dominant agent causing detachment in rain-impacted flows

1. Raindrop Induced Saltation (RIS) Detachment and uplift caused by raindrops impacting flow Flow Erosion mechanisms in rain-impacted flows 3 common transport mechanisms

Particles move downstream during fall Flow Wait for a subsequent impact before moving again 1. Raindrop Induced Saltation (RIS) Erosion mechanisms in rain-impacted flows 3 common transport mechanisms

2. Raindrop Induced Rolling (RIR) Particles move downstream by rolling Flow Wait for a subsequent impact before moving again Erosion mechanisms in rain-impacted flows 3 common transport mechanisms

Raindrops cause detachment and uplift Flow 3. Flow Suspension (FS) Acts at the same time as RIS & RIR Erosion mechanisms in rain-impacted flows 3 common transport mechanisms

Small particles remain suspended and Flow Large particles wait move without raindrop stimulation Acts at the same time as RD – RIS/RIR 3. Flow Suspension (FS) Erosion mechanisms in rain-impacted flows 3 common transport mechanisms

Particle travel rates Particles travel at rates that depend on the transport mechanism moving them Fine suspended material moves at the velocity of the flow Particles moving by raindrop induced saltation and rolling move at velocities that depend on their size, density, the frequency of drop impacts and the velocity of the flow

Particle travel rates Particles moving by raindrop induced saltation have velocities that depend on their size and density because these factors control the distance particles move after each drop impact

Drop impact Particle travel rates Distance particle travel after a drop impact Only impacts within the distance X cause particles to pass over the boundary Looking down on an area of soil covered by rain-impacted flow Positions of drop impacts over some period of time

Sediment discharge varies with particle travel distance (X) - varies with flow velocity and particle size and density Drop impact Particle travel rates Distance particle travel after a drop impact Only impacts within the distance X cause particles to pass over the boundary Positions of drop impacts over some period of time

Distance particle travel after a drop impact Drop impact Particle travel rates Sediment discharge varies with particle travel distance (X) - varies with flow velocity and particle size and density 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 Only impacts within the distance X cause particles to pass over the boundary

Particle travel rates Mechanistic model of raindrop induced saltation 2.7 mm raindrops impacting a 7 mm deep flow 0.46 mm sand0.46 mm coal Drop impacts generated randomly in space as with natural rain

Particle travel rates Non erodible 2980 mm Flow Erodible : 20 mm long Rain : 2.7 mm drops at 60 mm/h over 3 m length Simulation result Flow velocity = 150 mm/s 7 mm

Particle travel rates Cohesive erodible 3000 mm surface with sand : coal = 1:1 plus fine material Flow Rain : 2.7 mm drops at 60 mm/h over 3 m length Simulation result Flow velocity = 150 mm/s Fine discharge decreases because build up of loose sand and coal particles on the surface protects the surface against detachment ```````` 7 mm

Particle travel rates Cohesive erodible 3000 mm surface with sand : coal = 1:1 plus fine material Flow Rain : 2.7 mm drops at 60 mm/h over 3 m length Flow velocity = 150 mm/s Fine discharge decreases because build up of loose sand and coal particles on the surface protects the surface against detachment ```````` Initially much more coal is discharged than sand but over time the two materials tend towards composition in the original erodible surface X pd coal = 2.75 X pd sand

Variations in particle travel rates result in the initial discharge rates being greater for faster moving particles Particles moving by raindrop stimulated transport processes provide a protective layer on the surface that reduces detachment The protective layer coarsens over time and this causes the composition of the discharge to become the same as that of the original surface at the steady state if the particles are stable Particle travel rates

Enrichment results from particles containing carbon travelling relatively faster than mineral soil particles of the same size A% carbon in soil more than A% carbon in sediment Rain

Confounding Factors Some smaller mineral soil particles travel at or faster than the velocities of particles rich in carbon – enrichment effect not limited to carbon material Aggregates breakdown may occur during transport – changes relative travel rates Effective particle travel velocities vary for near zero to that of the flow

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 velocity varies down along the slope Time to reach the steady state controlled by the slowest moving particles Issue: Some smaller mineral soil particles travel at or faster than the velocities of particles rich in carbon – enrichment effect not limited to carbon material Slower particles affect the discharge of faster ones Enrichment Depletion

Experimental Evidence Walker, Kinnell, Green 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

Experimental Evidence 5% 0.5% 150 mm/h 45 mm/h 2.7mm drops Rolling 2 mins 60 mins Enrichment at 2 mins and 60 mins for 2.7 mm and 5.1 mm drops Reduction in impact frequency and flow velocity gives slower developement Reduction in flow velocity gives slower development

Enrichment occurs because 1.All particles do NOT travel laterally at the same rate 2.Erosion of the soil is occurring under non- steady conditions Results from experiments on the erosion of carbon need to be interpreted given this understanding A% carbon in soil more than A% carbon in sediment Rain

Critical conditions for detachment and transport modes Flow Energy Flow detachment only occurs when the shear stress needed to cause detachment is exceeded Raindrop detachment only occurs when the raindrop energy exceeds that need to cause detachment Coarse sand RD-RIR Coarse sand RD-FDR Flow driven saltation & rolling - more efficient than RIS & RIR