Transcript USLE-M and erosion in grid cells

Rainfall Erosion
Detachment
and
Transport
Systems
P.I.A. Kinnell
University of Canberra
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
Soil Erosion

Soil loss occurs when particles are
detached from the surface of the soil matrix
and transported across some boundary
Detachment
Transport
Deposition
boundary
Loose detached particle
Erosion but no soil loss
Detachment and Transport on Hillslopes
Onset of rain: Raindrop detachment (RD) + splash transport (ST)
covers the whole slope
Detachment & Transport Systems
Raindrop Detachment & Splash Transport (RD-ST)

The detachment and transport
system associated with
Splash Erosion
Detachment & Transport Systems
Raindrop Detachment & Splash Transport (RD-ST)

The detachment and transport
system associated with
Splash Erosion
Detachment & Transport Systems
Raindrop Detachment & Splash Transport (RD-ST)
On horizontal surfaces particles splashed back and forth
Detachment & Transport Systems
Raindrop Detachment & Splash Transport (RD-ST)
On horizontal surfaces particles splashed back and forth
and a layer of loose previously detached particles forms
Previously detached particles
Detachment & Transport Systems
Raindrop Detachment & Splash Transport (RD-ST)
Previously detached particles protect soil surface
from detachment
But are splashed
Previously detached particles
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
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
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
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
Detachment & Transport Systems
Raindrop Induced Saltation (RIS)
Occurs when raindrops impact shallow flow
Detachment & Transport Systems
Raindrop Induced Saltation (RIS)

Uplift caused by raindrop impacting flow
Flow
Detachment & Transport Systems
Raindrop Induced Saltation (RIS)

Uplift - Fall
Flow
Particles move downstream during the saltation event
Detachment & Transport Systems
Raindrop Induced Saltation (RIS)

Layer of previously detached particles –
depth increasing downstream
Flow
Detachment & Transport Systems
Raindrop Induced Saltation (RIS)

Erodibility = kS (1-H) + kPDP H
Flow
Detachment & Transport Systems
Raindrop Detatachment & Flow Suspension (RD-FS)
Detachment & Transport Systems
Raindrop Detatachment & Flow Suspension (RD-FS)

Uplift
Detachment & Transport Systems
Raindrop Detatachment & Flow Suspension (RD-FS)

Uplift - Suspended > FS
Fall > RIS at low flow velocities
Particles in Suspension
RIS
Particles transported by RIS travel slower than by FS
Detachment & Transport Systems
Raindrop Detatachment & Flow Driven Saltation (RD-FDS)

Uplift - Suspended > FS
Fall > FDS at higher flow velocities
Particles in Suspension
FDS
Particles transported by FDS travel faster than by RIS
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)
Detachment & Transport Systems
Flow Detatachment & Unassistred Flow Transport (FD-FT)
Detachment & Transport Systems
Flow Detatachment & Unassistred Flow Transport (FD-FT)

Uplift results from flow energy
Detachment & Transport Systems
Flow Detatachment & Unassistred Flow Transport (FD-FT)

Uplift results from flow energy
Transport: Suspended Load & Flow Driven Saltation
Particles in Suspension
FDS
Efficiency of Transport
of
Sand, Silt and Clay particles
Increasing
Splash Transport
Raindrop Induced Saltation
Flow Driven Saltation
Flow Driven Suspension
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
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)
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
Toposequence may expand
and contract one or more
times during an event
FD + unassisted FT
(suspension, saltation, rolling)
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 Systems
RD - ST
RD - RIS & RIR
RD - FS (& FDS & FDR)

Rill Erosion

Rill erosion refers to erosion in small
channels that can be removed by normal
cultivation.

Detachment & Transport Systems
FD – FS & FDS & FDR
Interrill Erosion

Interrill erosion refers to erosion in
interrill areas

Detachment & Transport Systems
RD - ST
RD - RIS & RDR
RD - FS (& FDS & FDR)
Rill Erosion
Flow Detatachment & Unassisted Flow Transport (FD-FT)

Energy absorbed in transport leaves less energy
for detachment
Flow Suspension
FDS
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 discharge
Tc = transport capacity (max sed. discharge)

(1 - [qs/Tc]) = 0 if qs = Tc
so DF =0
Rill Erosion

DF = erodibility (flow energy) (1 - [qs/Tc])
qs = sediment discharge
Tc = 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
Rill Erosion
.
.
.
.
.
.

Non erodible layer
Rills may often simply act as efficient transport
routes for interrill erosion
Detachment & Transport Systems
Raindrop
Energy
(E)
RAIN WITH
NO RUNOFF
RAIN WITH
RUNOFF
Fine Particles
RD-FS
Silt & Sand
RD-RIS
Clay, Silt &
Sand
RD-ST
Silt & Sand
RD-FDS
Clay, Silt &
Sand
FD-FDS,FS
B
A
Ec
Ec
NO EROSION
E < Ec, Ω < Ω(bound)
0
0
RAIN WITH
NO RUNOFF
τc (loose)
τc (bound)
Flow Shear Stress
(τ)
Diagram summarising the interaction between raindrops and flow
in respect to determining the detachment and transport
Detachment & Transport Systems
Raindrop
Energy
(E)
RAIN WITH
NO RUNOFF
RAIN WITH
RUNOFF
Fine Particles
RD-FS
Silt & Sand
RD-RIS
Clay, Silt &
Sand
RD-ST
Silt & Sand
RD-FDS
Clay, Silt &
Sand
FD-FDS,FS
Critical drop
energy for
detachment
B
A
Ec
Ec
NO EROSION
E < Ec, Ω < Ω(bound)
0
0
RAIN WITH
NO RUNOFF
τc (loose)
Flow Shear Stress
(τ)
τc (bound)
Detachment & Transport Systems
Raindrop
Energy
(E)
RAIN WITH
NO RUNOFF
RAIN WITH
RUNOFF
Fine Particles
RD-FS
Silt & Sand
RD-RIS
Clay, Silt &
Sand
RD-ST
Silt & Sand
RD-FDS
Clay, Silt &
Sand
FD-FDS,FS
Critical drop
energy for
detachment
B
A
Ec
Ec
NO EROSION
E < Ec, Ω < Ω(bound)
0
0
RAIN WITH
NO RUNOFF
τc (loose)
τc (bound)
Flow Shear Stress
(τ)
Critical flow “energy”
for detachment
Detachment & Transport Systems
Raindrop
Energy
(E)
RAIN WITH
NO RUNOFF
RAIN WITH
RUNOFF
Fine Particles
RD-FS
Silt & Sand
RD-RIS
Clay, Silt &
Sand
RD-ST
Silt & Sand
RD-FDS
Clay, Silt &
Sand
FD-FDS,FS
Critical drop
energy for
detachment
B
A
Ec
Ec
NO EROSION
E < Ec, Ω < Ω(bound)
0
0
RAIN WITH
NO RUNOFF
Critical flow “energy” to move
previously detached material
τc (loose)
τc (bound)
Flow Shear Stress
(τ)
Critical flow “energy”
for detachment
Flow Transport
Detachment
(controlled by cohesion)
Transport of
previously detached
material

Critical flow energy for maintaining transport
 Varies with particle size
Detachment & Transport Systems
Raindrop Detatachment & Flow Transport (RD-FT)

Uplift - Suspended > FT
Fall > RIFT at low flow velocities
Flow Transport
RIS
Particles transported by RIS travel slower than by FT
Detachment & Transport Systems
Raindrop Detatachment & Flow Transport (RD-FT)
Flow velocities can increase to above those that favour RIS

Uplift - Suspended > FT
Fall > FT (Bed Load)
Flow Transport
FT
Rainfall Intensity and RIS
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

Particle
travel
distance the
distance
travelled
after lifted
into flow
by a drop
impact
Particles upstream of the “active” zone require
many impacts to move to the active zone
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 )
Rainfall Intensity and RIS
3 parallel
flows same
velocity but
different
particles

Particle
travel
distance
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)
Rainfall Intensity and RIS
0.2 mm
sand
Rainfall Intensity and RIS
In real life a
large
number of
travel
distances
occur at the
same time
in same
flow

Particle
travel
distance
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)
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
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
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
Peter Kinnell
University of Canberra
Canberra ACT 2601
Australia
[email protected]