P.I.A. Kinnell University of Canberra
Rainfall Rainfall ErosionErosion
DetachmentDetachmentandand
TransportTransportSystemsSystems
Soil ErosionSoil Erosion
involvesinvolves
thethe detachmentdetachment of soil material at some placeof soil material at some place
andand
thethe transporttransport of this material away from the of this material away from the site of detachmentsite of detachment
Two linked processesTwo linked processes
Soil ErosionSoil Erosion
Soil lossSoil loss occurs when particles are occurs when particles are detacheddetached from the surface of the soil matrix from the surface of the soil matrix and and transportedtransported across some boundary across some boundary
Detachment Transport Deposition
Loose detached particle
bou
nd
ary
Erosion but no soil Erosion but no soil lossloss
Detachment and Transport on HillslopesDetachment and Transport on HillslopesOnset of rain: Raindrop detachment (RD) + splash transport (ST)
covers the whole slope
Detachment & Transport Detachment & Transport SystemsSystems
The detachment and transport system associated with Splash Erosion
Raindrop Detachment & Splash Transport (RD-ST)
Detachment & Transport Detachment & Transport SystemsSystems
The detachment and transport system associated with Splash Erosion
Raindrop Detachment & Splash Transport (RD-ST)
Detachment & Transport Detachment & Transport SystemsSystems
Raindrop Detachment & Splash Transport (RD-ST)On horizontal surfaces particles splashed back and On horizontal surfaces particles splashed back and forthforth
Detachment & Transport Detachment & Transport SystemsSystems
Raindrop Detachment & Splash Transport (RD-ST)
Previously detached particles
On horizontal surfaces particles splashed back and On horizontal surfaces particles splashed back and forthforthand a layer of loose previously detached particles and a layer of loose previously detached particles formsforms
Detachment & Transport Detachment & Transport SystemsSystems
Previously detached particles protect soil surface from detachment
But are splashed
Raindrop Detachment & Splash Transport (RD-ST)
Previously detached particles
Detachment & Transport Detachment & Transport SystemsSystems
Splashed particles come from both soil surface and Splashed particles come from both soil surface and layer of previously detached particleslayer of previously detached particles
Raindrop Detachment & Splash Transport (RD-ST)
Previously detached particles
Detachment & Transport Detachment & Transport SystemsSystems
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 previously detached particles get thicker in down slope direction . direction .
Raindrop Detachment & Splash Transport (RD-ST)
Previously-detached particles
Detachment & Transport Detachment & Transport SystemsSystems
Erodibility = susceptibility of eroding surface to erosion
depends on (a) splash of particles immediately after detachment AND (b) splash of previously detached material
Raindrop Detachment & Splash Transport (RD-ST)
Previously-detached particles
Detachment & Transport Detachment & Transport SystemsSystems
Erodibility = kS (1-H) + kPDP H
Raindrop Detachment & Splash Transport (RD-ST)
ks
kPDP
ks = erodibility when no PDP H = degree of protection provided by the PDP (0 - 1)
kPDP = erodibility when fully protected
Previously-detached particles
Raindrop Induced Saltation (RIS)
Detachment & Transport Detachment & Transport SystemsSystems
Occurs when raindrops impact shallow flow
Raindrop Induced Saltation (RIS)
Detachment & Transport Detachment & Transport SystemsSystems
Uplift caused by raindrop impacting flowUplift caused by raindrop impacting flow
Flow
Raindrop Induced Saltation (RIS)
Detachment & Transport Detachment & Transport SystemsSystems
Uplift - FallUplift - Fall
Flow
Particles move downstream during the saltation event
Raindrop Induced Saltation (RIS)
Detachment & Transport Detachment & Transport SystemsSystems
Layer of previously detached particles – Layer of previously detached particles – depth increasing downstream depth increasing downstream
Flow
Raindrop Induced Saltation (RIS)
Detachment & Transport Detachment & Transport SystemsSystems
Erodibility = kS (1-H) + kPDP H
Flow
Raindrop Detatachment & Flow Suspension (RD-FS)
Detachment & Transport Detachment & Transport SystemsSystems
Detachment & Transport Detachment & Transport SystemsSystems
UpliftUplift
Raindrop Detatachment & Flow Suspension (RD-FS)
Detachment & Transport Detachment & Transport SystemsSystems
Uplift - Suspended > FS Uplift - Suspended > FS Fall > RIS at low flow velocities Fall > RIS at low flow velocities
Particles in Suspension
RIS
Particles transported by RIS travel slower than by FS
Raindrop Detatachment & Flow Suspension (RD-FS)
Detachment & Transport Detachment & Transport SystemsSystems
Uplift - Suspended > FS Uplift - Suspended > FS Fall > FDS at higher flow velocities Fall > FDS at higher flow velocities
Particles in Suspension
FDS
Particles transported by FDS travel faster than by RIS
Raindrop Detatachment & Flow Driven Saltation (RD-FDS)
Detachment and Transport on HillslopesDetachment and Transport on Hillslopes 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)
Once runoff developsOnce runoff develops
Detachment & Transport Detachment & Transport SystemsSystems
Flow Detatachment & Unassistred Flow Transport (FD-FT)
Detachment & Transport Detachment & Transport SystemsSystems
Uplift results from flow energyUplift results from flow energy
Flow Detatachment & Unassistred Flow Transport (FD-FT)
Flow Detatachment & Unassistred Flow Transport (FD-FT)
Detachment & Transport Detachment & Transport SystemsSystems
Uplift results from flow energyUplift results from flow energyTransport: Suspended Load & Flow Driven SaltationTransport: Suspended Load & Flow Driven Saltation
FDS
Particles in Suspension
Efficiency of TransportEfficiency of Transportofof
Sand, Silt and Clay particlesSand, Silt and Clay particles
Splash TransportSplash Transport Raindrop Induced SaltationRaindrop Induced Saltation
Flow Driven SaltationFlow Driven Saltation
Flow Driven SuspensionFlow Driven Suspension
IncreasiIncreasingng
Raindrop Induced Rolling (RIR)largely associated with gravel particles
Move downstream by rollingMove downstream by rolling
FlowWait for a subsequent impact before moving again
Detachment & Transport Detachment & Transport SystemsSystems
Flow Driven Rolling (FDR) may also follow RD
Detachment and Transport on HillslopesDetachment and Transport on HillslopesRaindrop 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 HillslopesDetachment and Transport on HillslopesRaindrop 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)
ToposequenceToposequence
Toposequence may expand and contract one or more times during an event
Sheet ErosionSheet 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)
Rill ErosionRill Erosion
Rill erosion refers to erosion in small channels that can be removed by normal cultivation.
Detachment & Transport SystemsFD – FS & FDS & FDR
Interrill ErosionInterrill Erosion
Interrill erosion refers to erosion in interrill areas
Detachment & Transport SystemsRD - STRD - RIS & RDRRD - FS (& FDS & FDR)
Rill ErosionRill Erosion
Energy absorbed in transport leaves less energy Energy absorbed in transport leaves less energy for detachmentfor detachment
Flow Suspension
FDS
Flow Detatachment & Unassisted Flow Transport (FD-FT)
Flow Detatachment & Unassisted Flow Transport (FD-FT)
Rill ErosionRill Erosion
Energy absorbed in transport leaves less energy for Energy absorbed in transport leaves less energy for detachmentdetachment
Process based models – eg WEPPProcess based models – eg WEPP DDFF = erodibility (flow energy) (1 - [q = erodibility (flow energy) (1 - [qss/T/Tcc])])
qqss = sediment discharge = sediment discharge
TTcc = transport capacity (max sed. discharge) = transport capacity (max sed. discharge)
(1 - [q(1 - [qss/T/Tcc]) = 0 if q]) = 0 if qss = T = Tcc so D so DFF =0 =0
Rill ErosionRill Erosion
DDFF = erodibility (flow energy) (1 - [q = erodibility (flow energy) (1 - [qss/T/Tcc])])
qqss = sediment discharge = sediment dischargeTTcc = transport capacity (max sed. discharge) = transport capacity (max sed. discharge)
Water and sediment flows from interrill areas to rills.Water and sediment flows from interrill areas to rills.Interrill erosion contributes to qInterrill erosion contributes to qss and reduces D and reduces DFF
Rills may often simply act as efficient transport Rills may often simply act as efficient transport routes for interrill erosionroutes for interrill erosion
Rill ErosionRill Erosion
.
.
.
. . .
Rills may often simply act as efficient transport Rills may often simply act as efficient transport routes for interrill erosionroutes for interrill erosion
Non erodible layer
Detachment & Transport Detachment & Transport SystemsSystems
Diagram summarising the interaction between raindrops and flow in respect to determining the detachment and
transport
RAIN WITHNO
RUNOFF
RAIN WITHRUNOFF
Fine Particles
RD-FSSilt & Sand
RD-RISSilt & Sand
RD-FDSClay, Silt &
SandRD-ST
Clay, Silt & Sand
FD-FDS,FS
NO EROSION E < Ec, Ω < Ω(bound)
AB
0
EcEc
0 τc (bound)τc (loose)
Raindrop Energy (E)
Flow Shear Stress (τ)
RAIN WITHNO
RUNOFF
RAIN WITHNO
RUNOFF
RAIN WITHRUNOFF
Fine Particles
RD-FSSilt & Sand
RD-RISSilt & Sand
RD-FDSClay, Silt &
SandRD-ST
Clay, Silt & Sand
FD-FDS,FS
NO EROSION E < Ec, Ω < Ω(bound)
AB
0
EcEc
0 τc (bound)τc (loose)
Raindrop Energy (E)
Flow Shear Stress (τ)
RAIN WITHNO
RUNOFF
Detachment & Transport Detachment & Transport SystemsSystems
Critical dropCritical dropenergy for energy for detachmentdetachment
RAIN WITHNO
RUNOFF
RAIN WITHRUNOFF
Fine Particles
RD-FSSilt & Sand
RD-RISSilt & Sand
RD-FDSClay, Silt &
SandRD-ST
Clay, Silt & Sand
FD-FDS,FS
NO EROSION E < Ec, Ω < Ω(bound)
AB
0
EcEc
0 τc (bound)τc (loose)
Raindrop Energy (E)
Flow Shear Stress (τ)
RAIN WITHNO
RUNOFF
Detachment & Transport Detachment & Transport SystemsSystems
Critical dropCritical dropenergy for energy for detachmentdetachment
Critical flow “energy”Critical flow “energy” for detachment for detachment
RAIN WITHNO
RUNOFF
RAIN WITHRUNOFF
Fine Particles
RD-FSSilt & Sand
RD-RISSilt & Sand
RD-FDSClay, Silt &
SandRD-ST
Clay, Silt & Sand
FD-FDS,FS
NO EROSION E < Ec, Ω < Ω(bound)
AB
0
EcEc
0 τc (bound)τc (loose)
Raindrop Energy (E)
Flow Shear Stress (τ)
RAIN WITHNO
RUNOFF
Detachment & Transport Detachment & Transport SystemsSystems
Critical dropCritical dropenergy for energy for detachmentdetachment
Critical flow “energy”Critical flow “energy” for detachment for detachment
Critical flow “energy” to move previously detached material
Flow TransportFlow Transport
Critical flow energy for maintaining transportCritical flow energy for maintaining transport
Detachment Detachment (controlled by (controlled by cohesion)cohesion)
Transport ofTransport ofpreviously previously detacheddetachedmaterialmaterial
Varies with particle size Varies with particle size
Raindrop Detatachment & Flow Transport (RD-FT)
Detachment & Transport Detachment & Transport SystemsSystems
Uplift - Suspended > FT Uplift - Suspended > FT Fall > RIFT at low flow velocities Fall > RIFT at low flow velocities
Flow Transport
RIS
Particles transported by RIS travel slower than by FT
Raindrop Detatachment & Flow Transport (RD-FT)
Detachment & Transport Detachment & Transport SystemsSystems
Uplift - Suspended > FT Uplift - Suspended > FT Fall > FT (Bed Load) Fall > FT (Bed Load)
Flow Transport
FT
Flow velocities can increase to above those that favour RIS
Rainfall Intensity and RISRainfall Intensity and RIS
Particles upstream of the “active” zone require Particles upstream of the “active” zone require many impacts to move to the active zonemany impacts to move to the active zone
Particle Particle travel travel distance distance - - the the distance distance travelled travelled after after lifted lifted into flow into flow by a drop by a drop impactimpact
Drop Drop impacimpactt Particles must be within a Particles must be within a
distance from a boundary that is distance from a boundary that is less than the travel distance in less than the travel distance in order to pass across that order to pass across that boundaryboundary
Rainfall Intensity and RISRainfall Intensity and RISParticle Particle travel travel distancdistanceeDrop Drop
impacimpactt Particles must be within a Particles must be within a
distance from a boundary that is distance from a boundary that is less than the travel distance in less than the travel distance in order to pass across that order to pass across that boundaryboundary
Sediment discharge varies with particle travel distance Sediment discharge varies with particle travel distance (X varies with (X varies with flow velocityflow velocity & & particle size particle size ))
Rainfall Intensity and RISRainfall Intensity and RIS
Sediment discharge varies with particle travel distance Sediment discharge varies with particle travel distance (X varies with (X varies with flow velocityflow velocity & & particle size particle size ))
Particle Particle travel travel distancdistancee
• and drop impact frequency (varies with rain intensity)
Travel Travel 3 3 times times faster faster thanthan
3 parallel flows same
velocity but
different particles
Rainfall Intensity and RISRainfall Intensity and RIS
0.2 mm sand
Rainfall Intensity and RISRainfall Intensity and RISParticle Particle travel travel distancdistancee
Travel Travel 3 3 times times faster faster thanthan
In real life a large
number of travel
distances occur at the same time in
same flow
Sediment discharge varies with particle travel distance Sediment discharge varies with particle travel distance (X varies with (X varies with flow velocityflow velocity & & particle size particle size ))
• and drop impact frequency (varies with rain intensity)
Modelling rainfall erosionModelling rainfall erosion
Knowledge of the 4 detachment and Knowledge of the 4 detachment and transport systems essential to interpreting transport systems essential to interpreting the results of experimentsthe results of experiments
However, so called process-based models However, so called process-based models do not usually deal with the complexities to do not usually deal with the complexities to any large extent – leads to difficulty when any large extent – leads to difficulty when parameterisation is based on experimentsparameterisation is based on experiments
Modelling rainfall erosionModelling rainfall erosion
Interrill erodibility evaluated experimentallyInterrill erodibility evaluated experimentally- approx 65 mm/h intensity- approx 65 mm/h intensity- soil loss after 15 mins, 25 mins, 35 mins +- soil loss after 15 mins, 25 mins, 35 mins + used to produce single erodibility value used to produce single erodibility value for each soil for each soil
Dominated by RD – RIFT and RD – FTDominated 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
WEPP Interrill ModelWEPP Interrill Model
Some ReferencesSome References
KINNELL, P.I.A. (2005). KINNELL, P.I.A. (2005). Raindrop impact induced erosion processes and prediction.Raindrop impact induced erosion processes and prediction. Hydrological Processes (in press) Hydrological Processes (in press)
KINNELL, P.I.A. (1994).KINNELL, P.I.A. (1994).The effect of predetached particles on erosion by shallow rain-The effect of predetached particles on erosion by shallow rain-impacted flow.Aust. J. Soil Res. 31(1), 127-142.impacted flow.Aust. J. Soil Res. 31(1), 127-142.
KINNELL, P.I.A. (1993).KINNELL, P.I.A. (1993).Sediment concentrations resulting from flow depth - drop size Sediment concentrations resulting from flow depth - drop size interactions in shallow overland flow.Trans ASAE 36(4), 1099-interactions in shallow overland flow.Trans ASAE 36(4), 1099-1103. 1103.
KINNELL,P.I.A. (1990).KINNELL,P.I.A. (1990).The mechanics of raindrop induced flow transport.Aust. J. Soil The mechanics of raindrop induced flow transport.Aust. J. Soil Res. 28,497-516Res. 28,497-516
Peter KinnellPeter Kinnell
University of CanberraUniversity of Canberra
Canberra ACT 2601Canberra ACT 2601
AustraliaAustralia
[email protected]@canberra.edu.au