Some concepts relevant to rainfall erosion research and models Peter Kinnell University of Canberra Australia EGU2014.

Some concepts relevant to rainfall erosion research and models

Peter KinnellUniversity of CanberraAustralia

EGU2014

Rainfall ErosionRainfall Erosion

2 Drivers:

• Surface Water Flow

• Raindrop Impact

Critical conditions forCritical conditions for detachment and transport modes detachment and transport modes

Erosion results from the expenditure of energy

associated with both flow and raindrop impact

SplashErosion


Flow Energy

Flow detachment only occurs when the shear stress needed to cause detachment is exceeded

Coarse sand

RD-RIR

Coarse sand RD-FDR

Flowdrivenerosion

Not a 2D (X,Y) graph

Rainfall ErosionRainfall Erosion

Channels caused by flow driven erosion

Rill Erosion

Gully Erosion

Bed LoadBed Load

Flow

Flow driven saltation and rolling

Very fine particles remain suspended in the flowing water

Suspended LoadSuspended Load

Flow

Bed load transport processes

SplashErosion


Flow Energy


Raindrop detachment only occurs when the raindrop energy exceeds that needed to cause detachment

Coarse sand

RD-RIR

Coarse sand RD-FDR

Raindrop drivenerosionChange in

soil surface(crusting)

Flow depth effect on drop energy available for detachment

Flowdrivenerosion


NB: Both raindrop detachment and flow detachment can operate at thesame time

SplashErosion

On sloping surfaces more splashed down slope than up so more erosion as slope gradient increases

Raindrop Detachment & Splash Transport (RD-ST)Splash Erosion

Transport process limits erosion particularly on low gradient slopes

- Relatively inefficient erosion system especially on slopes with low to moderate gradients

Raindrop impact driven erosionRaindrop impact driven erosion

SplashErosion


Flow Energy


Raindrop detachment only occurs when the raindrop energy exceeds that needed to cause detachment

Coarse sand

RD-RIR

Coarse sand RD-FDR


SplashErosion

Flow DrivenSaltation and Rolling

Suspension in flow

FLOW DRIVEN TRANSPORT of material DETACHED by RAINDROPS will occur BELOW the shear stress that is needed to cause detachment by flow

Raindrop driven saltation and rolling

• Detachment and uplift caused by raindrops impacting flow

FlowFlow

Raindrop impact driven erosionRaindrop impact driven erosionRain-impacted flow

Raindrop Induced Saltation (RIS)

• Particles move downstream during fall

FlowWait for a subsequent impact before moving again



X

Transport rate varies directly with x

Flow



Height of water surface constrains height to which particles are lifted

Dissipation of drop energy in water restricts height to which particles are lifted

Qs = ks u f[h,d]s

Qs = sed discharge, ks = “erodibility”, u = flow vel, f[h,d]s = function of flow depth and drop size when travelling at VT

X depends on height particles are lifted

in very shallow flow

Peak for any given drop size is related to the maximum height particles are lifted in the flow

In most field and laboratory experiments using artificial rainfall flow depths are UNKOWN

Raindrop Induced Rolling (RIR)

• Particles move downstream by rolling

FlowWait for a subsequent impact before moving again


FlowFlow


Detachment by raindrop impact may be followed by

1.Raindrop induced saltation (RIS)

2.Raindrop induced rolling (RIR)

3.Transport in suspension (FS)

4.Flow driven saltation (FDR)

5.Flow driven rolling (FDR)Some models fail to include raindrop driven transport processes and so underestimate erosion loss

RainRain

Forms of Water Erosion on a Hillslope

Splash Erosion

Flow energy increasing

Rill & Interrill Erosion

Rill

Interrill

Sheet Erosion

Surface RunoffSurface Runoff

River

(Gully Erosion)

Detachment and transport processes control the development of the various forms of erosion on a hillslope

Splash Erosion

Splash Erosion

Sheet erosion on a plotSheet erosion on a plot

Raindrop driven

transportin

flow

Splash erosion

Splash erosion

Raindrop driven

transportin

flow

Flowdriven

transport

Transport efficiency

Splash transport

Raindrop driven transport in flow

Flow driven transport

Increases

Splash and raindrop driven transport in the flow result in loose particles sitting on the surface

Flow driven transport can flush loose particles from the surface

Increase in runoff rate


Raindrop driven

transportin

flow

Splash erosion

Splash erosion

Raindrop driven

transportin

flow

Flowdriven

transport


Flow driven transport flushes loose particles of coal from the surface when slope is 15 m or more

Transport of 0.46 mm coal

flow driven

--------------------

raidrop driven

Simulation model results


Raindrop driven

transportin

flow

Splash erosion

Splash erosion

Raindrop driven

transportin

flow

Flowdriven

transport

Increase in runoff rate Rilling also produces a flush of soilmaterial when rilling is active

In general, most models of rainfall erosion fail to properly take account of the effect of changes in detachment and transport mechanisms in TIME and SPACE

44 m bare fallow on 5% slope

E = kinetic energy flux I = rainfall intensityIs = average infiltration rate for storm


Raindrop driven

transportin

flow

Splash erosion

Splash erosion

Raindrop driven

transportin

flow

Flowdriven

transport

Increase in runoff rate Rilling also produces a flush of soilmaterial when rilling is active

RUNOFF is a factor the needs to be included when modelling event erosion


I – Is is used as a surrogate for the runoff rate

Sheet erosion on a plotSheet erosion on a plotRUNOFF is a factor the needs to be included when modelling event erosion

0.01

0.1

1

10

100

0.01 0.1 1 10 100

observed soil loss (t/ha)

pre

dic

ted

us

ing

Ae =

K E

I 30

1:1

Bare fallow plot at Morris, Minnesota

USLERe = EI30

USLE-MRe = QREI30

QR = runoff ratio

0.01

0.1

1

10

100

0.01 0.1 1 10 100

observed soil loss (t/ha)

pre

dic

ted

usi

ng

Ae =

k Q

RE

I 30

1:1

Bare fallow plot at Morris, Minnesota


Raindrop driven

transportin

flow

Splash erosion

Splash erosion

Raindrop driven

transportin

flow

Flowdriven

transport


SCALE is importantExperiments on 1 m x 1m plots apply only to INTERRILL EROSION and cannot be used to parameterize models that focus on sheet erosion


ConclusionConclusionDetachment and transport processes control the development of the various forms of erosion on a hillslope or a plot

SCALE is importantExperiments on 1 m x 1m plots apply only to INTERRILL EROSION and cannot be used to parameterize models that focus on sheet erosion

RUNOFF is a factor the needs to be included when modelling event erosion

The effect of FLOW DEPTH on detachment and transport in rain impacted flows means that results produced in many laboratory and field experiments are not readily applied elsewhere

Some concepts relevant to rainfall erosion research and models Peter Kinnell University of Canberra Australia EGU2014.

Documents