Rainfall Erosion
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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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P i a kinnell university of canberra

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 ris3

Rainfall Intensity and RIS

0.2 mm sand


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


P i a kinnell university of canberra

Peter Kinnell

University of Canberra

Canberra ACT 2601

Australia

[email protected]


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