USLEM and erosion in grid cells

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Rainfall ErosionDetachmentandTransportSystems
P.I.A. Kinnell University of Canberra
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Soil Erosion

• involves
• the detachment of soil material at some place
• and
• the transport of this material away from the site
  of detachment
• Two linked processes

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Soil Erosion

• Soil loss occurs when particles are detached from
  the surface of the soil matrix and transported
  across some boundary

Loose detached particle
boundary
Deposition
Detachment
Transport

Erosion but no soil loss
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Detachment and Transport on Hillslopes
Onset of rain Raindrop detachment (RD) splash
transport (ST)
covers the whole slope
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Detachment Transport Systems
Raindrop Detachment Splash Transport (RD-ST)

• The detachment and transport system associated
  with Splash Erosion

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Detachment Transport Systems
Raindrop Detachment Splash Transport (RD-ST)

• The detachment and transport system associated
  with Splash Erosion

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Detachment Transport Systems
Raindrop Detachment Splash Transport (RD-ST)

On horizontal surfaces particles splashed back
and forth
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Detachment Transport Systems
Raindrop Detachment Splash Transport (RD-ST)

On horizontal surfaces particles splashed back
and forthand a layer of loose previously
detached particles forms
Previously detached particles
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Detachment Transport Systems
Raindrop Detachment Splash Transport (RD-ST)

• Previously detached particles protect soil
  surface from detachment
• But are splashed

Previously detached particles
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Detachment Transport Systems
Raindrop Detachment Splash Transport (RD-ST)

• Splashed particles come from both soil
  surface and layer of previously detached particles

Previously detached particles
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Detachment Transport Systems
Raindrop Detachment Splash Transport (RD-ST)

• On sloping surfaces more splashed down slope
  than up so more erosion as slope gradient
  increases
• but previously detached particles get thicker in
  down slope direction .

Previously-detached particles
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Detachment Transport Systems
Raindrop Detachment Splash Transport (RD-ST)

• Erodibility susceptibility of eroding surface
  to erosion
• depends on (a) splash of particles immediately
  after detachment AND (b) splash of previously
  detached material

Previously-detached particles
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Detachment Transport Systems
Raindrop Detachment Splash Transport (RD-ST)

• Erodibility kS (1-H) kPDP H

ks erodibility when no PDP
H degree of protection provided by the PDP (0 -
1)
kPDP erodibility when fully protected
kPDP
ks
Previously-detached particles
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Detachment Transport Systems
Raindrop Induced Saltation (RIS)

Occurs when raindrops impact shallow flow
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Detachment Transport Systems
Raindrop Induced Saltation (RIS)

• Uplift caused by raindrop impacting flow

Flow
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Detachment Transport Systems
Raindrop Induced Saltation (RIS)

• Uplift - Fall

Flow
Particles move downstream during the saltation
event
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Detachment Transport Systems
Raindrop Induced Saltation (RIS)

• Layer of previously detached particles
  depth increasing downstream

Flow
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Detachment Transport Systems
Raindrop Induced Saltation (RIS)

• Erodibility kS (1-H) kPDP H

Flow
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Detachment Transport Systems
Raindrop Detatachment Flow Suspension (RD-FS)

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Detachment Transport Systems
Raindrop Detatachment Flow Suspension (RD-FS)

• Uplift

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Detachment Transport Systems
Raindrop Detatachment Flow Suspension (RD-FS)

• Uplift - Suspended gt FS Fall gt RIS
  at low flow velocities

Particles in Suspension
RIS
Particles transported by RIS travel slower than
by FS
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Detachment Transport Systems
Raindrop Detatachment Flow Driven Saltation
(RD-FDS)

• Uplift - Suspended gt FS Fall gt FDS
  at higher flow velocities

Particles in Suspension
FDS
Particles transported by FDS travel faster than
by RIS
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Detachment and Transport on Hillslopes
Once runoff develops
With clay, silt and sand particles 3
transport systems with raindrop detachment RD
splash transport (ST) RD raindrop induced
saltation (RIS) RD unassisted
flow transport (FS FDS)
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Detachment Transport Systems
Flow Detatachment Unassistred Flow Transport
(FD-FT)
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Detachment Transport Systems
Flow Detatachment Unassistred Flow Transport
(FD-FT)

• Uplift results from flow energy

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Detachment Transport Systems
Flow Detatachment Unassistred Flow Transport
(FD-FT)

• Uplift results from flow energyTransport
  Suspended Load Flow Driven Saltation

Particles in Suspension
FDS
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Efficiency of Transportof

• Sand, Silt and Clay particles
• Splash Transport
  Raindrop Induced Saltation
• Flow Driven Saltation
• Flow Driven Suspension

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Detachment Transport Systems
Raindrop Induced Rolling (RIR)largely associated
with gravel particles

• Move downstream by rolling

Flow
Wait for a subsequent impact before moving again
Flow Driven Rolling (FDR) may also follow RD
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Detachment and Transport on Hillslopes
Raindrop detachment (RD) erosion systems RD
splash transport (ST)RD raindrop induced
saltation (RIS)RD raindrop induced rolling
(RIR)RD unassisted flow transport (FT)
(suspension, saltation, rolling)
Flow detachment (FD) erosion systems
FD unassisted FT
(suspension, saltation, rolling)
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Detachment and Transport on Hillslopes
Toposequence
Raindrop detachment (RD) erosion systems RD
splash transport (ST)RD raindrop induced
saltation (RIS)RD raindrop induced rolling
(RIR)RD unassisted flow transport (FT)
(suspension, saltation, rolling)
Flow detachment (FD) erosion systems
FD unassisted FT
(suspension, saltation, rolling)
Toposequence may expand and contract one or more
times during an event
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Sheet Erosion

• Sheet erosion refers to erosion where a portion
  of the soil surface layer over a relatively wide
  area is removed somewhat uniformly.
• Detachment Transport SystemsRD - STRD - RIS
  RIRRD - FS ( FDS FDR)

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Rill Erosion

• Rill erosion refers to erosion in small channels
  that can be removed by normal cultivation.
• Detachment Transport SystemsFD FS FDS FDR

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Interrill Erosion

• Interrill erosion refers to erosion in interrill
  areas
• Detachment Transport SystemsRD - STRD - RIS
  RDRRD - FS ( FDS FDR)

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Rill Erosion
Flow Detatachment Unassisted Flow Transport
(FD-FT)

• Energy absorbed in transport leaves less energy
  for detachment

Flow Suspension
FDS
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Rill Erosion
Flow Detatachment Unassisted Flow Transport
(FD-FT)

• Energy absorbed in transport leaves less energy
  for detachment
• Process based models eg WEPP
• DF erodibility (flow energy) (1 - qs/Tc)qs
  sediment dischargeTc transport capacity (max
  sed. discharge)
• (1 - qs/Tc) 0 if qs Tc so DF 0

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Rill Erosion

• DF erodibility (flow energy) (1 - qs/Tc)qs
  sediment dischargeTc transport capacity (max
  sed. discharge)
• Water and sediment flows from interrill areas to
  rills.Interrill erosion contributes to qs and
  reduces DF
• Rills may often simply act as efficient transport
  routes for interrill erosion

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Rill Erosion

• .. ..
• .
• .
• Rills may often simply act as efficient transport
  routes for interrill erosion

Non erodible layer
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Detachment Transport Systems

Diagram summarising the interaction between
raindrops and flow in respect to determining the
detachment and transport
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Detachment Transport Systems

• Critical dropenergy for detachment

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Detachment Transport Systems

• Critical dropenergy for detachment
• 
  Critical flow energy
  for detachment

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Detachment Transport Systems

• Critical dropenergy for detachment
• 
  Critical flow energy
  for detachment

Critical flow energy to move previously
detached material
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Flow Transport
detachment
Transport of previously detached material

• Critical flow energies for uniform material

• Vary with particle size

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Detachment Transport Systems
Raindrop Detatachment Flow Transport (RD-FT)

• Uplift - Suspended gt FT Fall gt RIFT
  at low flow velocities

Flow Transport
RIFT
Particles transported by RIFT travel slower than
by FT
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Detachment Transport Systems
Raindrop Detatachment Flow Transport (RD-FT)
Flow velocities can increase to above those that
favour RIFT

• Uplift - Suspended gt FT Fall gt FT
  (Bed Load)

Flow Transport
FT
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Rainfall Intensity and RIS
Particle travel distance - the distance travelled
after lifted into flow by a drop impact
Drop impact
Particles must be within a distance from a
boundary that is less than the travel distance in
order to pass across that boundary

• Particles upstream of the active zone require
  many impacts to move to the active zone

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Rainfall Intensity and RIS
Particle travel distance
Drop impact
Particles must be within a distance from a
boundary that is less than the travel distance in
order to pass across that boundary

• Sediment discharge varies with particle travel
  distance (X varies with flow velocity particle
  size )

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Rainfall Intensity and RIS
Particle travel distance
3 parallel flows same velocity but different
particles
Travel 3 times faster than

• Sediment discharge varies with particle travel
  distance (X varies with flow velocity particle
  size )

• and drop impact frequency (varies with rain
  intensity)

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Rainfall Intensity and RIS
0.2 mm sand
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Rainfall Intensity and RIS
Particle travel distance
In real life a large number of travel distances
occur at the same time in same flow
Travel 3 times faster than

• Sediment discharge varies with particle travel
  distance (X varies with flow velocity particle
  size )

• and drop impact frequency (varies with rain
  intensity)

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Modelling rainfall erosion

• Knowledge of the 4 detachment and transport
  systems essential to interpreting the results of
  experiments
• However, so called process-based models do not
  usually deal with the complexities to any large
  extent leads to difficulty when
  parameterisation is based on experiments

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Modelling rainfall erosion
WEPP Interrill Model

• Interrill erodibility evaluated experimentally-
  approx 65 mm/h intensity- soil loss after 15
  mins, 25 mins, 35 mins used to produce single
  erodibility value for each soil
• Dominated by RD RIFT and RD FT
• Interrill Erodibility kS (1-H) kPDP H
• kS, kPDL, and H all unknown
• Difficulty in relating erodibility to soil
  properties

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Some References

• KINNELL, P.I.A. (2005). Raindrop impact induced
  erosion processes and prediction. Hydrological
  Processes (in press)
• KINNELL, P.I.A. (1994).The effect of predetached
  particles on erosion by shallow rain-impacted
  flow.Aust. J. Soil Res. 31(1), 127-142.
• KINNELL, P.I.A. (1993).Sediment concentrations
  resulting from flow depth - drop size
  interactions in shallow overland flow.Trans ASAE
  36(4), 1099-1103.
• KINNELL,P.I.A. (1990).The mechanics of raindrop
  induced flow transport.Aust. J. Soil Res.
  28,497-516

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• Peter Kinnell
• University of Canberra
• Canberra ACT 2601
• Australia
• peter.kinnell_at_canberra.edu.au