p.i.a. kinnell university of canberra rainfall erosion detachment and transport systems

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)


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



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


Previously detached particles protect soil surface from detachment

But are splashed




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




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 .


Previously-detached particles


Erodibility = susceptibility of eroding surface to erosion

depends on (a) splash of particles immediately after detachment AND (b) splash of previously detached material




Erodibility = kS (1-H) + kPDP H


ks

kPDP

ks = erodibility when no PDP H = degree of protection provided by the PDP (0 - 1)

kPDP = erodibility when fully protected


Raindrop Induced Saltation (RIS)


Occurs when raindrops impact shallow flow



Uplift caused by raindrop impacting flowUplift caused by raindrop impacting flow

Flow



Uplift - FallUplift - Fall

Flow

Particles move downstream during the saltation event



Layer of previously detached particles – Layer of previously detached particles – depth increasing downstream depth increasing downstream

Flow



Erodibility = kS (1-H) + kPDP H

Flow

Raindrop Detatachment & Flow Suspension (RD-FS)



UpliftUplift



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



Uplift - Suspended > FS Uplift - Suspended > FS Fall > FDS at higher flow velocities Fall > FDS at higher flow velocities


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


Flow Detatachment & Unassistred Flow Transport (FD-FT)


Uplift results from flow energyUplift results from flow energy




Uplift results from flow energyUplift results from flow energyTransport: Suspended Load & Flow Driven SaltationTransport: Suspended Load & Flow Driven Saltation

FDS


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


Flow Driven Rolling (FDR) may also follow RD

Detachment and Transport on HillslopesDetachment and Transport on HillslopesRaindrop detachment (RD) erosion systems


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 + 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)


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)


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


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


.

.

.

. . .

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


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


AB

0

EcEc


Raindrop Energy (E)


RAIN WITHNO

RUNOFF


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


AB

0

EcEc


Raindrop Energy (E)


RAIN WITHNO

RUNOFF



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


AB

0

EcEc


Raindrop Energy (E)


RAIN WITHNO

RUNOFF



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)


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)


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 ))



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


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


• 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

p.i.a. kinnell university of canberra rainfall erosion detachment and transport systems

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