some concepts relevant to rainfall erosion research and models peter kinnell
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Some concepts relevant to rainfall erosion research and models Peter Kinnell University of Canberra Australia. Rainfall Erosion. 2 Drivers: Surface Water Flow Raindrop Impact. Critical conditions for detachment and transport modes. - PowerPoint PPT PresentationTRANSCRIPT
Some concepts relevant to rainfall erosion research and models
Peter Kinnell
University of Canberra
Australia
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
Critical conditions forCritical conditions for detachment and transport modes detachment and transport modes
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
Critical conditions forCritical conditions for detachment and transport modes detachment and transport modes
Flow Energy
Flow detachment only occurs when the shear stress needed to cause detachment is exceeded
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
Not a 2D (X,Y) graph
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
Critical conditions forCritical conditions for detachment and transport modes detachment and transport modes
Flow Energy
Flow detachment only occurs when the shear stress needed to cause detachment is exceeded
Raindrop detachment only occurs when the raindrop energy exceeds that needed to cause detachment
Coarse sand
RD-RIR
Coarse sand RD-FDR
Not a 2D (X,Y) graph
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
Raindrop Induced Saltation (RIS)
Raindrop impact driven erosionRaindrop impact driven erosionRain-impacted flow
X
Transport rate varies directly with x
Flow
Raindrop Induced Saltation (RIS)
Raindrop impact driven erosionRaindrop impact driven erosionRain-impacted 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
Raindrop impact driven erosionRaindrop impact driven erosionRain-impacted flow
FlowFlow
Raindrop impact driven erosionRaindrop impact driven erosionRain-impacted flow
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
Sheet erosion on a plotSheet erosion on a plot
Raindrop driven
transportin
flow
Splash erosion
Splash erosion
Raindrop driven
transportin
flow
Flowdriven
transport
Increase in runoff rate
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
Sheet erosion on a plotSheet erosion on a plot
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
Sheet erosion on a plotSheet erosion on a plot
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
44 m bare fallow on 5% slope
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
Sheet erosion on a plotSheet erosion on a plot
Raindrop driven
transportin
flow
Splash erosion
Splash erosion
Raindrop driven
transportin
flow
Flowdriven
transport
Increase in runoff rate
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
44 m bare fallow on 5% slope
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
