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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.

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slide1

Rainfall ErosionDetachmentandTransportSystems

P.I.A. Kinnell University of Canberra

soil erosion

Soil Erosion

involves

thedetachmentof soil material at some place

and

thetransport of this material away from the site of detachment

Two linked processes

soil erosion1
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

detachment and transport on hillslopes
Detachment and Transport on Hillslopes

Onset of rain: Raindrop detachment (RD) + splash transport (ST)

covers the whole slope

detachment transport systems
Detachment & Transport Systems

Raindrop Detachment & Splash Transport (RD-ST)

  • The detachment and transport system associated with Splash Erosion
detachment transport systems1
Detachment & Transport Systems

Raindrop Detachment & Splash Transport (RD-ST)

  • The detachment and transport system associated with Splash Erosion
detachment transport systems2
Detachment & Transport Systems

Raindrop Detachment & Splash Transport (RD-ST)

On horizontal surfaces particles splashed back and forth

detachment transport systems3
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

detachment transport systems4
Detachment & Transport Systems

Raindrop Detachment & Splash Transport (RD-ST)

Previously detached particlesprotect soil surface from detachment

But aresplashed

Previously detached particles

detachment transport systems5
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 systems6
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

butpreviously detached particles get thicker in down slope direction .

Previously-detached particles

detachment transport systems7
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 systems8
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 systems9
Detachment & Transport Systems

Raindrop Induced Saltation (RIS)

Occurs when raindrops impact shallow flow

detachment transport systems10
Detachment & Transport Systems

Raindrop Induced Saltation (RIS)

  • Uplift caused by raindrop impacting flow

Flow

detachment transport systems11
Detachment & Transport Systems

Raindrop Induced Saltation (RIS)

  • Uplift - Fall

Flow

Particles move downstream during the saltation event

detachment transport systems12
Detachment & Transport Systems

Raindrop Induced Saltation (RIS)

  • Layer of previously detached particles – depth increasing downstream

Flow

detachment transport systems13
Detachment & Transport Systems

Raindrop Induced Saltation (RIS)

  • Erodibility = kS (1-H) + kPDP H

Flow

detachment transport systems14
Detachment & Transport Systems

Raindrop Detatachment & Flow Suspension (RD-FS)

detachment transport systems15
Detachment & Transport Systems

Raindrop Detatachment & Flow Suspension (RD-FS)

  • Uplift
detachment transport systems16
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 systems17
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 hillslopes1
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 systems18
Detachment & Transport Systems

Flow Detatachment & Unassistred Flow Transport (FD-FT)

detachment transport systems19
Detachment & Transport Systems

Flow Detatachment & Unassistred Flow Transport (FD-FT)

  • Uplift results from flow energy
detachment transport systems20
Detachment & Transport Systems

Flow Detatachment & Unassistred Flow Transport (FD-FT)

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

Particles in Suspension

FDS

efficiency of transport of

Increasing

Efficiency of Transportof

Sand, Silt and Clay particles

Splash TransportRaindrop Induced Saltation

Flow Driven Saltation

Flow Driven Suspension

detachment transport systems21
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 hillslopes2
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 hillslopes3
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

sheet erosion
Sheet Erosion
  • Sheet erosionrefers 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 erosion
Rill Erosion
  • Rill erosion refers to erosion in small channels that can be removed by normal cultivation.
  • Detachment & Transport SystemsFD – FS & FDS & FDR
interrill erosion
Interrill Erosion
  • Interrill erosionrefers to erosion in interrill areas
  • Detachment & Transport SystemsRD - STRD - RIS & RDRRD - FS (& FDS & FDR)
rill erosion1
Rill Erosion

Flow Detatachment & Unassisted Flow Transport (FD-FT)

  • Energy absorbed in transport leaves less energy for detachment

Flow Suspension

FDS

rill erosion2
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
rill erosion3
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
rill erosion4
Rill Erosion

....

.

.

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

Non erodible layer

detachment transport systems22

RAIN WITHNO RUNOFF

RAIN WITHRUNOFF

Raindrop Energy (E)

Fine Particles

RD-FS

Silt & Sand

RD-RIS

Silt & Sand

RD-FDS

Clay, Silt & Sand

RD-ST

Clay, Silt & Sand

FD-FDS,FS

B

A

Ec

Ec

NO EROSION E < Ec, Ω < Ω(bound)

0

τc (loose)

0

τc (bound)

RAIN WITHNO RUNOFF

Flow Shear Stress (τ)

Detachment & Transport Systems

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

detachment transport systems23

RAIN WITHNO RUNOFF

RAIN WITHRUNOFF

Raindrop Energy (E)

Fine Particles

RD-FS

Silt & Sand

RD-RIS

Silt & Sand

RD-FDS

Clay, Silt & Sand

RD-ST

Clay, Silt & Sand

FD-FDS,FS

B

A

Ec

Ec

NO EROSION E < Ec, Ω < Ω(bound)

0

τc (loose)

0

τc (bound)

RAIN WITHNO RUNOFF

Flow Shear Stress (τ)

Detachment & Transport Systems

Critical dropenergy for detachment

detachment transport systems24

RAIN WITHNO RUNOFF

RAIN WITHRUNOFF

Raindrop Energy (E)

Fine Particles

RD-FS

Silt & Sand

RD-RIS

Silt & Sand

RD-FDS

Clay, Silt & Sand

RD-ST

Clay, Silt & Sand

FD-FDS,FS

B

A

Ec

Ec

NO EROSION E < Ec, Ω < Ω(bound)

0

τc (loose)

0

τc (bound)

RAIN WITHNO RUNOFF

Flow Shear Stress (τ)

Detachment & Transport Systems

Critical dropenergy for detachment

Critical flow “energy” for detachment

detachment transport systems25

RAIN WITHNO RUNOFF

RAIN WITHRUNOFF

Raindrop Energy (E)

Fine Particles

RD-FS

Silt & Sand

RD-RIS

Silt & Sand

RD-FDS

Clay, Silt & Sand

RD-ST

Clay, Silt & Sand

FD-FDS,FS

B

A

Ec

Ec

NO EROSION E < Ec, Ω < Ω(bound)

0

τc (loose)

0

τc (bound)

RAIN WITHNO RUNOFF

Flow Shear Stress (τ)

Detachment & Transport Systems

Critical dropenergy for detachment

Critical flow “energy” for detachment

Critical flow “energy” to move previously detached material

flow transport
Flow Transport

Detachment (controlled by cohesion)

  • Critical flow energy for maintaining transport

Transport of

previously detached

material

  • Varies with particle size
detachment transport systems26
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 systems27
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
Rainfall Intensity and RIS

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

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

rainfall intensity and ris1
Rainfall Intensity and RIS

Particle travel distance

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

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

rainfall intensity and ris2
Rainfall Intensity and RIS

Particle travel distance

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

3 parallel flows same velocity but different particles

Travel 3 times faster than

  • and drop impact frequency (varies with rain intensity)
rainfall intensity and ris4
Rainfall Intensity and RIS

Particle travel distance

In real life a large number of travel distances occur at the same time in same flow

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

Travel 3 times faster than

  • and drop impact frequency (varies with rain intensity)
modelling rainfall erosion
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 erosion1
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
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

slide53
Peter Kinnell

University of Canberra

Canberra ACT 2601

Australia

[email protected]

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