the effect of raindrop impacted flow on sediment composition

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The effect of raindrop impacted flow on The effect of raindrop impacted flow on sediment compositionsediment composition

RainRain

Forms of Erosion on a Hillslope

Splash Erosion

RiverRiver

DetachmentTransport Fall

Loose predetached particle

Detachment is the plucking of soil particles from within the soil surface where the particles are held by cohesion and inter-particle friction

Uplift

Raindrop impact is one of the two agents causing detachment in eroding areas

Forms of Erosion on a Hillslopedetachment is the initializing process

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 Relatively inefficient erosion inefficient erosion systemsystem


RainRain


Splash Erosion

Surface RunoffSurface Runoff

Rain-impacted Flow

RiverRiver

1. Raindrop Detachment & Raindrop Induced Saltation (RD-RIS)

• Uplift caused by raindrop impacting flow

Flow

Erosion by Rain-impacted Flow Forms of Erosion on a Hillslope

3 common detachment and transport systems

• Move downstream during fall

FlowWait for a subsequent impact before moving again


1. Raindrop Detachment & Raindrop Induced Saltation (RD-RIS)

2. Raindrop Induced Rolling (RIR)

• Move downstream by rolling

FlowWait for a subsequent impact before moving again


• Raindrops cause uplift in flow

Flow

3. Raindrop Detachment with Flow Suspension (RD-FS)


• Small particles remain suspended and

Flow

Large particles

wait

move without further

stimulation

Acts at the same time as RD – RIS/RIR


3. Raindrop Detachment with Flow Suspension (RD-FS)

Splash Erosion and Erosion by Rain-impacted Flow cause

SHEET EROSION

Loss of soil in a relatively uniform sheet over the area


RainRain


Splash Erosion

Surface RunoffSurface Runoff

Rain-impacted Flow Rill &

Interrill Erosion

RiverRiver

Flow

Rill Erosion and Interrill Erosion

Interrill area

Rill

Rills are channels that can be removed by cultivation

Close up of a piece of a field


Flow Detachment & Flow Driven Transport• Detachment and uplifted using flow energy

Flow

Rill Erosion Forms of Erosion on a Hillslope

• Fine - Transported by flow as suspended load - fast moving

• Coarse - Transported by flow as bed load – fast moving

Rill Erosion

Flow TransportFlow Transport

Raindrop impact not involved in

any way


Flow Detachment & Flow Driven Transport

Critical conditions for detachment and transport Critical conditions for detachment and transport modes for silt and sand and fine particlesmodes for silt and sand and fine particles

Flow Energy

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

- RILL EROSION

Raindrop detachment (RD) only occurs when the raindrop energy exceeds that need to cause detachment

Raindrop impacted flow Dominates sheet and interrill erosion

Most of the time, Most of the time, material leaving material leaving this bare area has this bare area has been been detached detached from the soil from the soil surface by surface by raindrops NOT raindrops NOT the flowthe flow..

Flow detachment is dominant in channels Flow detachment is dominant in channels only when channels are developingonly when channels are developing..

Rain- impacted flow (RD-FS/RIS/RIR)is responsible for loss of nutrient rich soil material from the land that may end up in water supplies etc


Flow

Erosion by Rain-impacted Flow Raindrop Detachment & Raindrop Induced Saltation

(RD-RIS)

Particles transported by raindrop induced saltation move horizontally at velocities that depend on their size, density, and the velocity of the flow because these factors control the distance particles travel after a drop impact (x)

x

Drop Drop impacimpactt

DistancDistance e particle particle traveltravel after a after a drop drop impactimpactOnly impacts within the distance X cause

particles to pass over the boundary

Looking down on an area of soil covered by rain-impacted flow

Positions of drop impacts over some period of time

Erosion by Rain-impacted Flow

• Sediment discharge varies with particle travel distance (X) - varies with flow velocity and particle size and density


DistancDistance e particle particle traveltravel after a after a drop drop impactimpactOnly impacts within the distance X cause

particles to pass over the boundary

Positions of drop impacts over some period of time


DistancDistance e particle particle traveltravel after a after a drop drop impactimpact


• Sediment discharge varies with particle travel distance (X) - varies with flow velocity and particle size and density

3 times faster

Experiments with coal and sand indicate that coal particles move about 2.75 times

faster than sand particles of the same size

Only impacts within the distance X cause particles to pass over the boundary


Mechanistic model of raindrop induced saltation2.7 mm raindrops impacting a 7 mm deep flow

0.46 mm sand 0.46 mm coal

Drop impacts generated randomly in space as with natural rain


- time in flow 0.2 s

- time in flow 0.55 s

Particle travel ratesParticle travel rates

Non erodible 2980 mm

Flow

Erodible : 20 mm long

Rain : 2.7 mm drops at 60 mm/h over 3 m length

Simulation result

Flow velocity = 150 mm/s7 mm

Sand takes 2.75 times as long to reach the end


• Particles of sand can be considered to have times of concentrations that are 2.75 times longer than particles of coal of the same size

• The concept of time of concentration is useful in looking at the effect of rainfall on runoff

• The concept of time of concentration is useful in looking at the effect of rainfall on runoff.


Compare runoff rates (mm/h) over time for a given rainfall event

300 m impervious, n=0.03

Gradient = 3 %

Gradient = 0.5 %


Rainfall rate 50 mm/h


Gradient = 3 %

Gradient = 0.5 %

0

10

20

30

40

50

60

0 10 20 30 40 50 60time (mins)

run

off

ra

te (

mm

/h)

s = 3%

s = 0.5%


Rainfall rate 50 mm/h

Gradient = 3 %

Gradient = 0.5 %

0

2

4

6

8

10

12

0 10 20 30 40 50 60

time (mins)

rati

o r

un

off

s =

3%

to

s =

0.5

%

7.8



Cohesive erodible 3000 mm surface with sand : coal = 1:1 plus fine material

Flow


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

Flow velocity = 150 mm/s

Detention storage of sediment - build up of loose sand and coal particles on the surface protects the surface against detachment and causes fine discharge to decrease

``` ` ` ` ` `7 mm


Cohesive erodible 3000 mm surface with sand : coal = 1:1 plus fine material

Flow


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

Flow velocity = 150 mm/s

Detention storage of sediment - build up of loose sand and coal particles on the surface protects the surface against detachment and causes fine discharge to decrease

``` ` ` ` ` `

Initially much more coal is discharged than sand but over time the two materials tend towards composition in the original erodible surface

0

1

2

3

0 20 40 60 80 100 120

time (mins)

ratio

Coa

l to

San

dXpd coal = 2.75 Xpd sand

7 mm

0

0.5

1

1.5

2

0 20 40 60 80 100 120time (mins)

ER

for 0

.46

mm

coa

l

Enrichment RatiosEnrichment Ratios

s = 3% with s = 0.5%

0.46 mm coal with 0.46 mm sand

0

0.5

1

1.5

2

0 10 20 30 40 50 60

time (mins)

ER

for s

= 3

%

Enrichment ratios ≠ 1.0 only when erosion is not occurring at the

steady state

Enrichment ratios = 1.0 at the steady state

“ER” for flow example

Detention storage of sediment reduces detachment

The ratio of the proportion of the material in the discharge to the proportion of the material in the original

Experimental EvidenceExperimental Evidence

Walker, Kinnell, Green 1978

• 3 m long inclined sand surface• 2 slope gradients: 0.5%, 5%• Events of 1 hour rainfall with uniform drop size• 2 drop sizes : 2.7 mm, 5.1 mm• 3 rainfall intensities: 45, 100, 150 mm/h

Experimental EvidenceExperimental Evidence

5%slope

0.5%slope

150 mm/h45 mm/h

2.7mm drops

Rolling

2 mins

60 mins

Enrichment at 2 mins and 60 mins for 2.7 mm and 5.1 mm drops

Reductionin impact frequency

and flow velocity gives slower developement

Increase in flow depth + reduction in flow velocity gives slower development

Rolling

Lowest erosive stress

Highest erosive stress

Experimental EvidenceExperimental EvidencePalis et al 1990:

Sandy clay loam soil on 0.1 % slope 5.8 m long

100 mm/h using continuous spray

0

1

2

0.01 0.1 1 10size of particles discharged (mm)

enri

chm

ent

rati

o

0.6 min

5 min

15 min35 min

Confounding FactorsConfounding Factors

• Effective particle travel velocities vary for near zero to that of the flow

• Aggregates breakdown may occur during transport of soil material– changes relative travel rates

• Interactions between particles of different sizes and densities

Confounding FactorsConfounding Factors• Model on 10 m long impervious plot inclined at 9 %• Cohesive source has 5 particles sizes equally represented• 50 mm/h rain intensity (2.7 mm drops) • Flow depth and velocity vary down along the slope

0

0.05

0.1

0.15

0.2

0.25

0.3

0 2 4 6 8 10

depth (mm)

de

pth

fu

nc

tio

n f

or

se

dim

en

t d

isc

ha

rge

Height particles are lifted is restricted by height of water above surface

Height particles are lifted is restricted by water absorbing drop energy


0

0.05

0.1

0.15

0.2

0.25

0.3

0 2 4 6 8 10

depth (mm)

dep

th f

un

cti

on

fo

r se

dim

en

t d

isch

arg

e

0 50 100 150

velocity (mm/s)

velo

city

fu

nct

ion

fo

r se

dim

ent

dis

char

ge


0

0.1

0.2

0.3

0.4

0.5

0 20 40 60 80 100

time (hours)

pro

po

rtio

n

0.11 mm sand

0.46 mm coal

0.2 mm sand

0.46 mm sand

0.9 mm sand

Time to reach the steady state controlled by the slowest moving particles

Slower particles affect the discharge of faster ones

Enrichment

Depletion

Confounding FactorsConfounding Factors

Critical shear stress for flow driven saltation

Raindrop impacted flow

Raindrop detachment + flow driven saltation

Confounding FactorsConfounding Factors• Model on 2 - 30 m long plots inclined at 9 %• 2 part high intensity rainfall event• Cohesive source has 5 particles sizes equally represented• Flow depth and velocity vary down along the slope

Enhanced loss of coal when L > 15m, 0.11 sand when L > 20mresulting from short term change from RIS to FDS

Fundamentally, sediment enrichment occurs because

1. All particles do NOT travel laterally at the same rate

2. Erosion of the soil is occurring under non-steady conditions• Time of concentration approach relevant but need to

consider the effect of detention storage of sediment on detachment etc

• Modelling presented here is only qualitative – need to undertake research to determine more effectively how particles of differing sizes and densities actually travel and interact in rain- impacted flows

A% carbon or nutrient in soil

SEDIMENT ENRICHMENT----

more than A% carbonor nutrient in sediment

Rain

Complicating FactorsComplicating Factors

0

0.05

0.1

0.15

0.2

0.25

0.3

0 2 4 6 8 10

depth (mm)

dep

th f

un

ctio

n f

or

sed

imen

t d

isc

har

ge

0 50 100 150

velocity (mm/s)

velo

city

fu

nct

ion

fo

r se

dim

ent

dis

char

ge

Flow depthFlow velocity

Need to be known in experiments on rain-impacted flows

2.7 mm drops Previous research on depth effect

Interrill erosion experiments ?Depth effect not well known

Sprays: complex flow depth effectextrapolation

the effect of raindrop impacted flow on sediment composition

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