preferential transport of carbon materials in rain-impacted flow peter kinnell
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Preferential transport of carbon materials in rain-impacted flow Peter Kinnell University of Canberra Australia. A% carbon in load. Rain. A% carbon in soil. more than A% carbon in sediment. A% carbon in soil. - PowerPoint PPT PresentationTRANSCRIPT
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)
Erosion mechanisms in rain-impacted flows3 common transport mechanisms
2. Raindrop Induced Rolling (RIR)
• Particles move downstream by rolling
FlowWait for a subsequent impact before moving again
Erosion mechanisms in rain-impacted flows3 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 flows3 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 flows3 common transport mechanisms
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
Particle travel ratesParticle travel rates
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
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
Positions of drop impacts over some period of time
DistancDistance e particle particle traveltravel after a after a drop drop impactimpact
Drop Drop impacimpactt
Particle travel ratesParticle 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 ratesParticle travel rates
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
Particle travel ratesParticle 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/s7 mm
Particle travel ratesParticle 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
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
Particle travel ratesParticle 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
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
Particle travel ratesParticle 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 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
more than A% carbon in sediment
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