Preferential transport of
carbon materials
in rain-impacted flow
Peter Kinnell
University of Canberra
Australia
A% carbon in load
A% carbon in soil
Rain
A% carbon in soil
more than A%
carbon in sediment
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
Erosion mechanisms in rain-impacted flows
Detachment is the initializing process
Raindrop impact is the dominant agent
causing detachment 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
Loose predetached particle
Uplift
Detachment
Transport
Fall
Erosion mechanisms in rain-impacted flows
3 common transport mechanisms
1. Raindrop Induced Saltation (RIS)
• Detachment and uplift caused by
raindrops impacting flow
Flow
Erosion mechanisms in rain-impacted flows
3 common transport mechanisms
1. Raindrop Induced Saltation (RIS)
• Particles move downstream during fall
Flow
Wait for a
subsequent
impact before
moving again
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
3. Flow Suspension (FS)
• Raindrops cause detachment and uplift
Flow
Acts at the same time as RIS & RIR
Erosion mechanisms in rain-impacted flows
3 common transport mechanisms
3. Flow Suspension (FS)
• Small particles remain suspended and move
without
Flow
raindrop
stimulation
Large
particles
wait
Acts at the same time as RD – RIS/RIR
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
Particle travel rates
Drop
impact
Only impacts within the distance X cause
particles to pass over the boundary
Distance
particle
travel
after a
drop
impact
Positions of drop impacts
over some period of time
Looking down on an area of soil covered by rain-impacted flow
Particle travel rates
Drop
impact
Only impacts within the distance X cause
particles to pass over the boundary
Distance
particle
travel
after a
drop
impact
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 rates
Drop
impact
Only impacts within the distance X cause
particles to pass over the boundary
Distance
particle
travel
after a
drop
impact
3 times faster
• Sediment
discharge
varies
travel
Experiments
with
coalwith
andparticle
sand indicate
distance
(X) - particles
varies withmove
flow velocity
and times
particle
that coal
about 2.75
sizefaster
and density
than sand particles of the same size
Particle travel rates
Mechanistic model of raindrop induced saltation
2.7 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 rates
Rain : 2.7 mm drops at 60 mm/h over 3 m length
7 mm
Flow velocity = 150 mm/s
Non erodible 2980 mm
Erodible : 20 mm long
Simulation result
Flow
Particle travel rates
Rain : 2.7 mm drops at 60 mm/h over 3 m length
7 mm
Flow velocity = 150 mm/s
`
`
`
`
`
`
Flow
`
`
Cohesive erodible 3000 mm surface with sand : coal = 1:1 plus fine material
Simulation result
45
fine
0.46 mm coal
0.46 mm sand
40
discharge (g m-1 min-1)
Fine discharge decreases
because build up of loose
sand and coal particles on
the surface protects the
surface against detachment
35
30
25
20
15
10
5
0
0
20
40
60
time (mins)
80
100
120
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
Initially much more coal is
discharged than sand but
over time the two materials
tend towards composition
in the original erodible
surface
45
-1
discharge
)
m-1 min
ratio
Coal(gto
Sand
Fine discharge decreases
because build up of loose
sand and coal particles on
the surface protects the
surface against detachment
40
3
Xpd coal =
35
2
30
fine
2.75
Xpd coal
sand
0.46 mm
0.46 mm sand
25
1
20
15
0
10
0
5
20
40
20
40
0
0
60
80
100
120
time 60(mins)
80
100
120
time (mins)
Particle travel rates
• 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
Rain
more than A%
carbon in sediment
A% carbon in soil
• Enrichment results from particles containing
carbon travelling relatively faster than mineral
soil particles of the same size
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
0.5
Slower particles
affect the discharge
of faster ones
0.11 mm sand
0.46 mm coal
0.2 mm sand
0.46 mm sand
0.9 mm sand
Enrichment
0.4
Issue:
proportion
•
•
•
•
Some smaller mineral soil particles travel at or faster
than the velocities of particles rich in carbon
Time –to reach
0.2
enrichment effect not limited to carbon material
the steady state
0.3
controlled by the
slowest moving
particles
0.1
Depletion
0
0
20
40
60
time (hours)
80
100
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
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
Reduction
in impact
frequency
and
flow velocity
gives slower
developement
Rolling
2 mins
5%
60 mins
Reduction in
flow velocity
gives slower
development
0.5%
Rain
more than A%
carbon in sediment
A% carbon in soil
Enrichment occurs because
1. All particles do NOT travel laterally at the same
rate
2. Erosion of the soil is occurring under nonsteady conditions
•
Results from experiments on the erosion of
carbon need to be interpreted given this
understanding
Critical conditions for
detachment and transport modes
Raindrop
detachment
only occurs
when the
raindrop
energy
exceeds that
need to cause
detachment
Coarse sand
RD-RIR
Coarse sand
RD-FDR
Flow Energy
Flow driven
saltation & rolling
more efficient
than RIS & RIR
Flow
detachment
only occurs
when the shear
stress needed
to cause
detachment is
exceeded