The effect of raindrop impacted flow on sediment composition
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Transcript of 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
Forms of Erosion on a Hillslope
RainRain
Forms of Erosion on a Hillslope
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
Erosion by Rain-impacted Flow Forms of Erosion on a Hillslope
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
Erosion by Rain-impacted Flow Forms of Erosion on a Hillslope
• Raindrops cause uplift in flow
Flow
3. Raindrop Detachment with Flow Suspension (RD-FS)
Erosion by Rain-impacted Flow Forms of Erosion on a Hillslope
• Small particles remain suspended and
Flow
Large particles
wait
move without further
stimulation
Acts at the same time as RD – RIS/RIR
Erosion by Rain-impacted Flow Forms of Erosion on a Hillslope
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
Forms of Erosion on a Hillslope
RainRain
Forms of Erosion on a Hillslope
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
Forms of Erosion on a Hillslope
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
Forms of Erosion on a Hillslope
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
Forms of Erosion on a Hillslope
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
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
Positions of drop impacts over some period of time
Erosion by Rain-impacted Flow
DistancDistance e particle particle traveltravel after a after a drop drop impactimpact
Drop Drop impacimpactt
• 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
Erosion by Rain-impacted Flow
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
Erosion by Rain-impacted Flow
- 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
Particle travel ratesParticle travel rates
• 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.
Particle travel ratesParticle travel rates
Compare runoff rates (mm/h) over time for a given rainfall event
300 m impervious, n=0.03
Gradient = 3 %
Gradient = 0.5 %
Particle travel ratesParticle travel rates
Rainfall rate 50 mm/h
300 m impervious, n=0.3
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%
Particle travel ratesParticle travel rates
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
300 m impervious, n=0.3
Particle travel ratesParticle travel rates
Cohesive erodible 3000 mm surface with sand : coal = 1:1 plus fine material
Flow
Rain : 2.7 mm drops at 60 mm/h over 3 m length
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
Particle travel ratesParticle travel rates
Cohesive erodible 3000 mm surface with sand : coal = 1:1 plus fine material
Flow
Rain : 2.7 mm drops at 60 mm/h over 3 m length
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
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)
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
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
0.1
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