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

Preferential transport of Preferential transport of carbon materialscarbon materials

in rain-impacted flowin rain-impacted flow

Peter KinnellUniversity of CanberraAustralia

A% carbon in soil

A% carbon in load

A% carbon in soil

more than A% carbon in sediment

Rain

Why does sediment discharged Why does sediment discharged in rain-impacted flows contain in rain-impacted flows contain proportionately more carbon proportionately more carbon

than the soil ?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

DetachmentTransport Fall

Loose predetached particle

Erosion mechanisms in rain-impacted flowsDetachment 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 flows3 common transport mechanisms

• Particles move downstream during fall

FlowWait for a subsequent impact before moving again

1. Raindrop Induced Saltation (RIS)


2. Raindrop Induced Rolling (RIR)

• Particles move downstream by rolling

FlowWait for a subsequent impact before moving again


• Raindrops cause detachment and uplift

Flow

3. Flow Suspension (FS)

Acts at the same time as RIS & RIR


• 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)


Particle travel ratesParticle 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


Particles moving by raindrop induced saltation have velocities that depend on theirsize and density

because these factors control the distance particles move after each drop impact

Drop Drop impacimpactt

Particle travel ratesParticle travel rates DistancDistance e particle particle traveltravel after a after a drop drop impactimpactOnly 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


Particle travel ratesParticle travel rates DistancDistance e particle particle traveltravel after a after a drop drop impactimpactOnly impacts within the distance X cause

particles to pass over the boundary

Positions of drop impacts over some period of time

DistancDistance e particle particle traveltravel after a after a drop drop impactimpact



• 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


Mechanistic model of raindrop induced saltation2.7 mm raindrops impacting a 7 mm deep flow

0.46 mm sand 0.46 mm coal

Drop impacts generated randomly in space as with natural rain


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/s7 mm


Cohesive erodible 3000 mm surface with sand : coal = 1:1 plus fine material

Flow


Simulation result

0

5

10

15

20

25

30

35

40

45

0 20 40 60 80 100 120time (mins)

dis

char

ge

(g m

-1 m

in-1

)fine0.46 mm coal0.46 mm sand

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


Cohesive erodible 3000 mm surface with sand : coal = 1:1 plus fine material

Flow


0

5

10

15

20

25

30

35

40

45

0 20 40 60 80 100 120time (mins)

dis

char

ge

(g m

-1 m

in-1

)fine0.46 mm coal0.46 mm sand

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

0

1

2

3

0 20 40 60 80 100 120

time (mins)

ratio

Coa

l to

San

dXpd coal = 2.75 Xpd 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


• Enrichment results from particles containing carbon travelling relatively faster than mineral soil particles of the same size

A% carbon in soil


Rain

Confounding FactorsConfounding 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 FactorsConfounding 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

0

0.1

0.2

0.3

0.4

0.5

0 20 40 60 80 100

time (hours)

pro

po

rtio

n

0.11 mm sand

0.46 mm coal

0.2 mm sand

0.46 mm sand

0.9 mm sand

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 EvidenceExperimental 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

Experimental EvidenceExperimental Evidence

5%

0.5%

150 mm/h45 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

Reductionin 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


Rain

Critical conditions forCritical conditions for detachment and transport modes 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 sandRD-RIR

Coarse sand RD-FDR

Flow driven saltation & rolling

-more efficient than RIS & RIR

Documents

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