Erosion mechanisms in rain impacted flow and their effects and interactions P.I.A. Kinnell

Erosion mechanisms in rain impacted flow Erosion mechanisms in rain impacted flow and their effects and interactionsand their effects and interactions

P.I.A. KinnellP.I.A. KinnellUniversity of CanberraUniversity of Canberra

AustraliaAustralia

Oral Paper 3995 European Geophysical Union

General Assembly 2012

Soil ErosionSoil Erosion

• It is well known that raindrop impact is the main driver of detachment in rain-impacted flows and is also involved in determining how detached material is transported across the soil surface

• Detachment and uplift caused by raindrops impacting flow

FlowFlow

Transport in rain-impacted flowTransport in rain-impacted flow

Transport Mechanism 1. Raindrop Induced Saltation (RIS)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)

• Particles move downstream during fall

FlowWait for a subsequent impact before moving again

Transport Mechanism 1. Raindrop Induced Saltation (RIS)

Transport in rain-impacted flowTransport in rain-impacted flow

Transport Mechanism 2. Raindrop Induced Rolling (RIR)

• Particles move downstream by rolling

FlowWait for a subsequent impact before moving again

Transport in rain-impacted flowTransport in rain-impacted flow

• Small particles remain suspended and

Flow

Large particles

wait

move without raindrop

stimulation

Acts at the same time as RD – RIS/RIR

Transport Mechanism 3. Flow Suspension (FS)

Transport in rain-impacted flowTransport in rain-impacted flow

After detachment by drop impact Coarse particles

Flow

move without raindrop

stimulation

Transport Mechanism 4. Flow Driven Saltation (FDS)Transport Mechanism 5. Flow Driven Rolling (FDR)

Transport in rain-impacted flowTransport in rain-impacted flow

Critical conditions for detachment and Critical conditions for detachment and transport modestransport 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 sandRD-RIR

Coarse sand RD-FDR

SplashErosion

Rain Driven

Transportin Flow

FlowDrivenTransport

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

Factors affecting erosion byFactors affecting erosion by rain-impacted flows rain-impacted flows

• Soil: particle size, particle density cohesion and interparticle friction

• Rain: raindrop size and velocity rainfall intensity

• Flow: flow depth, flow velocity

Critical conditions for detachment and Critical conditions for detachment and transport modestransport 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 sandRD-RIR

Coarse sand RD-FDR

Rain Driven

Transportin Flow

Suspension

The effect of flow velocity The effect of flow velocity

Apparatus enabling control of flow depth and velocity in rain-impacted flow over eroding surfaces

Sand moves across the surface by raindrop induced saltation

The effect of rainfall intensityThe effect of rainfall intensity

The rate sediment is discharged when transported by raindrop induced saltation is linearly related to rainfall intensity and flow velocity

Data from experiments by Kinnell (1992)

using 0.2 mm sand and 2.7 mm drops

• Sediment discharge varies with particle travel distance (X)

ParticlParticle e travel travel distancdistancee

3 times 3 times the the discharge discharge thanthan

2 parallel flows same

particles but

different flow

velocities

There are 3 times the number of drop impacts producing discharge when travel distance is X3 than when X1

Travel distance varies with flow velocity

The effect of flow velocity The effect of flow velocity

Impact frequency varies with rainfall intensity

Only impacts within the distance X of the boundary produce discharge of particles

When rain has a single drop size, the rate sediment is discharged when transported by raindrop induced saltation is linearly related to rainfall intensity (I) and flow velocity (u)

Consequently qs(p,d) = kp Id u f[h,d]

where kp is a coefficient dependent on p and f[h,d] is a function that accounts for the effect of flow depth for drops of size d

The effect of flow depthThe effect of flow depth

1) f[h,d] is affected by the mass of material lifted into the flow by each drop impact

That decreases with flow depth as more of the drop energy is dissipated in the flow

2) f[h,d] is affected by the height particles are lifted in the flow by each drop impact.

That depends on how much of the drop energy is dissipated in the flow and how the surface of the flow constrains the uplift

The effect of flow depthThe effect of flow depthqs(p,d) = kp Id u f[h,d]

Height lifted restricted by energy available from drop impact

Height lifted restricted by surface

The effect of flow depthThe effect of flow depth

Sediment discharge varies directly with particle travel distance

qs(p,d) = kp Id u f[h,d]

The effect of flow depthThe effect of flow depth

0

1

2

0 5 10 15 20

flow depth (mm)

f[h

,d] s

d = 1 mm d = 6 mm

d = 3 mm

Also a decrease in the mass lifted into the flow

Loose particles on the surfaceLoose particles on the surface

Particles that fall back to the bed after being detached and lifted into the flow form a layer of loose particles sitting on the cohesive surface

They require energy to move them before detachment from the cohesive layer can occur and this causes the value of kp to vary with time

qs(p,d) = kp Id u f[h,d]

Loose particles on the surfaceLoose particles on the surface

kp = kp.M (1 – H) + kp.L H

• kp.M is the value of kp when no loose particles are on the surface

• kp.L is the is the value of kp when the loose particles on the surface fully protect against detachment (H = 1)

• H is the degree of protection provided by the loose particles

qs(p,d) = kp Id u f[h,d]

Loose particles on the surfaceLoose particles on the surface

• Sediment concentration cs(p,d) = qs(p,d) / qw

• For a surface of p sized sand: cs(p,d) / Id = kp.L (f[h,d]/h) because H = 1

qs(p,d) = kp Id u f[h,d]

kpL = 0.1443f[h,d]/h= 1 + 0.0104 h2 + 0.202 h

= qs(p,d) / (h u) = kp Id u (f[h,d]/h) /u= kp Id (f[h,d]/h)

Sediment concentration is the amount of sediment discharged per unit quantity of water without distinguishing the transport mode

h = depth , u = flow velocity

• For soil: cs(p,d) / Id = (kp.M (1 – H) + kp.L H) (f[h,d]/h)

Loose particles on the surfaceLoose particles on the surface

Problem is that H is unknown in the experiments with soil so detachment is unknown but declines with time as the amount of loose material builds up

Particle size and densityParticle size and density• Distance travelled during a saltation event is

affected by particle size and density as they influence the time particles are moving in the flow after a drop impact.

Simulation result

Rain : 2.7 mm drops at 60 mm/h over 3 m long surface eroding a small area with 50% 0.46 mm sand 50% 0.46 mm coal at the top

3 m

Particle size and densityParticle size and density

Simulation result

0

5

10

15

20

25

30

35

40

45

0 20 40 60 80 100 120time (mins)

dis

char

ge

(g m

-1 m

in-1

)

fine0.46 mm coal0.46 mm sand

INCLUDES protective effect of loose material

Response of fine particles travelling in continuous suspension reflects overall change in detachment

Amount of the slowest moving particle in layer of loose material increases with time so the slowest moving particle controls the time taken to reach the steady state

Rain : 2.7 mm drops at 60 mm/h eroding a 3 m long cohesive surface with 50% 0.46 mm sand 50% 0.46 mm coal

3 m

Particle size and densityParticle size and densityAmount of the slowest moving particle in layer of loose material increases with time so the slowest moving particle controls the time taken to reach the steady state

Time taken to reach the steady state also varies with slope length, slope gradient, runoff and rainfall characteristics

Experiment 2.7 mm drops falling on 3 m long sandy soil on 0.5% and 5% slopes

Critical conditions for detachment and Critical conditions for detachment and transport modestransport 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 sandRD-RIR

Coarse sand RD-FDR

Rain Driven

Transportin Flow

Suspension

Flow Driven Saltation

Suspension

RIS – RDS - RISRIS – RDS - RIS

RIS0.46 mm

sand

FDS0.46 mm

coal

RIS0.46 mm

coal

Flow velocity in the outflow on

9 % slopes

ConclusionConclusion

It is important to be well aware of the effects and interactions that exist between the detachment and transport mechanisms that operate in rain-impacted flows when interpreting the results of experiments undertaken on sheet and interrill erosion areas