THE FATE OF ERODED
SOIL
Tony Parsons
University of Sheffield, UK

Thank you. The title of my talk, ‘The Fate of Eroded Soil’ it is what becomesof or what happens to soil when it is eroded.

My first slide is to say that there are two main agencies that are responsible forsoil erosion. One is water and the other is wind. I shall say nothing aboutwind erosion. My entire talk is about the fate of soil that is eroded by water.

Rilling on valley side slopes

Studies that have looked at soil erosion have usually looked at it from two
points of views.

Either they look at places where soil is eroded from and the study is concernedto understand the amounts of soil that are lost from hill slopes such as this one
or this one or this one or the processes that cause soil to be eroded from hillslopes like this.

The focus is on the amounts of erosion and the mechanisms of erosion.

The other group of studies in soil erosion is concerned with what are often
called off-site impacts. That is the impact of soil when it arrives somewhere
else where it is not wanted. The first group of studies is concerned about thefact that soil is being lost from places where it is wanted. The second groupis concerned with studies trying to look at amounts of soil that arrive, in thiscase, in a residential street.

The road is covered in soil or here where the valley floor has been filled withcoarse material off the hill slope and changing character of that valley floor.
or here where the valley floor has been filled with coarse material off the hillslope and changing character of that valley floor.

Probably, one of the most important types of studies of soil erosion where they
are looking at the arrival of soil in places where it is not wanted is in studies
that are looking at sedimentation rates in reservoirs.

You can see thisreservoir, you can see the wall in front is very, very high and you can alreadysee that there is not much depth of water there.

Stone-built dam from 1950s full of sediment

In this slide, the reservoir is now completely useless because all of the water
storage capacity has been filled with sediment. This construction for water
storage no longer serves any useful purpose. We have these two types ofstudies. If you want to understand this in terms of this then generally thathas been a problem.

THE SCALE PROBLEM

The problem can be expressed as a problem of scale that whenever you try to
make measurements of rates of erosion on hill slopes and relate them to rates
of sedimentation in reservoirs, in this case, a small reservoir at the bottom ofa small catchment and this hill slope is located in this small catchment. Thenyou find the two sets of information don’t fit together. But in general, we findthat if you take rates of erosion from measurements on hill slopes like that andyou then multiply those rates of erosion by the size of catchment then theamount of sediment which arrives with the catchment outlet is generally muchless.

(Walling, 1983)

Now, people have tried to reconcile these differences to try and explain whythat if you have a catchment, the amount of sediment that leaves thecatchment is much less than the amount of erosion within the catchment.
That attempt has been expressed in what is called the sediment delivery ratio.
The sediment delivery ratio simply compares how much sediment supposedly
leaves the hill slopes and how much sediment arrives in the outlet of thecatchment and clogs the values [ph] of the two.

You can see that the

sediment delivery ratios are on the whole quite small. This is a log scale. On
this catchment here, only 10% of the sediment that is leaving the hill slopes isarriving in the catchment outlet. There’s a big issue about where the rest of
the sediment goes.

One way, people have tried to express that relationship is in these so-calledcatchment sediment budgets. What people have often done is to measure
the different types of erosion and to measure the sediments at the catchment
outlet. As I say, generally find that this number is much smaller than the sumof these numbers.

What they have assumed very often is that all of thesediment that doesn’t reach the outlets of the catchment is deposited invarious low areas within the catchment so valley floor alluvium, tributary
valley floor alluvium, and colluvium sediment at the bottom of the hill slopes.
There are many things wrong with this diagram. The first thing is that it
cannot possibly work.

If you erode sediment off the hill slopes and it doesn’t

leave the catchment, eventually, the catchment will fill up with sediment.
You can’t do this forever. This diagram might work in the short term. It
might be possible for a short period of time for rates of erosion on the hillslopes to be less than the rates of erosion or rates of sediment lost from thecatchment.

But in the long term, those numbers must be pulled up.

Otherwise, if that were not true, we would have no valleys left in the world.
They would all be filled up with sediment.

This does not work in the longterm.
But generally, whenever you see diagrams like this there is no attempt to

provide any time scale over which they’re meant to represent what is going on.
If that diagram said 1950 to 1980, then it might be a reasonable diagram.
But, if it is meant to be an abstract timeless statement then it cannot be true.
The second main thing which is wrong with this diagram is that you will see
how conveniently the numbers all add up.

If you add those numbers andthose numbers, they balance. The reason they balance is generally because
they haven’t all been measured. What happens is that some of these things
are measured and where they don’t measure, they just assume that thebounce must exist.

There is no attempt in this diagram to show any

assessment of uncertainty. These diagrams are really rather misleading interms of what might be going on.
Now, I think what is going on, what is important is that we need to think about
the fate of eroded soil. We need to think where does it go when it’s eroded.
How far does it move? What becomes of it?

What I want to do now is to talk about work that I’ve done over a number ofyears, 20 years now, looking at where sediment goes once it’s eroded. I’ll
take a more or less chronological story in that this slide comes from work done
20 years ago.
What we were doing here was a typical study, if you like, of erosion on the hillslopes. It was not typical in the sense it’s a very large scale study. This hillslope plot measures about 30 meters in length by 18 meters wide. Now thething you need to note is that we measure what comes off the plot but we also
have lines of people on the plot making measurements of what is happening
within the plot. That is unusual in that those people who measure run-off
plots, erosion plots like this, simply measure what comes off. What we wereinterested in was trying to understand what was going on within the plot.
That gave us some interesting insights into the fate of eroded soil.
One of the things we did was to calculate how much sediment passed each ofthese cross sections.

This is actually for a different plot but it’s the same idea. If you stand at thispoint here and you measure the amount of sediment which is moving past thatcross section, then if you assume that nothing leaves the top of the plot
because effectively, there’s almost no sediment transport at the top of the plot,
the amount of sediment here is equal to this area here which must therefore
be that triangle there. You can calculate the rate of change between there
and there roughly if you assume it’s a linear rate of change.
You see that the section sediment loss i.e. the amount of sediment going past
here, past here, past here, past here increases, has a function of distance. If
you measure the sediment at this point and you do the same analysis, you findthat the amount of sediment being added as you go further and further downslope gets smaller and smaller. In fact, the sediment starts to be lost. Whatthat suggests is that sediment being eroded here doesn’t get to here but
travels only some of the distance and then does not make it throughout therest of the way.

Now one of the things we did on that large plot was to look at the movementof marked particles.

The movement in marked particles is a theme that goesright through the 20 years. But, the theme is one of improving technology all
the time. In 1992, there was no way of identifying soil particles really. What
we wanted to know was how far an individual soil particle traveled during astorm event.
We wanted to be able to say, well, if a soil particle started here, where would
it end up at the end of the event? The way we did this was to create artificial
soil particles. What we used was magnetite. What you see here underneath
this tape measure, these black lines are lines of magnetite.

The reason for using magnetite is that you can find it by using a detector.
What you could do is put the magnetite down on the ground and then after therain storm, go and measure the magnetic susceptibility of the hill slope
sectioned down slope and see how much increase in magnetic susceptibility
there has been as a result of the storm event.

What we did was to have a line of magnetite which is the one you saw in thefirst photograph with the tape measure. And then, we had three lines where
we would measure the change in magnetic susceptibility after the storm event.
In addition, you remember those cross-sections that we had on the plot? At
those cross-sections, we have a number of small flumes that can catch waterand sediment passing through. We were able to, at the end of the event,measure the magnetic susceptibility of these four, I suppose we looked at theseed [ph] line as well to see how much magnetite had been lost.
But also, we could use the water samples from these cross-sections to see
whether any of the magnetite had passed through those points.
（次の図の説明の後に、この図にもどって再度説明された。
）
If we go back to this line, what we are saying is that magnetite has been lostfrom there, magnetite is being deposited there but nothing really much has
happened there or there.

This marked sediment doesn’t seem to have

traveled very far at all. This distance is a few centimeters.

What we observe is that if we take the source line prior to the first experiment,
we have a magnetic susceptibility which is around about 900 and it declines
significantly after the first experiment, not so much after the second one
remained quite a bit, after the second one or after the third one.

If you lookat the transacts we can see that a transact 1, there is a very significant
increase in magnetic susceptibility after the first experiment and not verymuch after the others. Similarly, down here we find no significant increases
in magnetic susceptibility.

But, some magnetite does move and we have data from those cross-sections
showing that there is evidence that amounts of magnetite do get through thesecond cross-section and even down to the third.
What that tells us is that most sediment that is eroded doesn’t go very far.
But some of it might go quite a long way. Not surprisingly, you can fit astatistical distributions list.
distribution.

What you get is a negative exponential

Most things don’t go very far and progressively as you give

greater and greater distance, smaller and smaller amounts of sediment are
moved up quite far.

The next step in this story was to try and understand that in a laboratory
situation and rather than just fitting a statistical distribution to provide some
sort of quantitative understanding of what controls how far sediment moves.
This laboratory set up was a simple flume with a bed of sand. On that bed ofsand, marked particles were placed. The sand couldn’t move because it was
glued into position.

The only thing that could move were the individual

marked particles.
We used two sources of water. Rainfall fell onto this and water also flowedfrom a trough at the top so we could have varying amounts of runoff andvarying amounts of rainfall. The idea was to see how far particles traveled.
Now, one of the constraints on this experiment is that in order for particles tobe identified they have to be quite big. That is a problem now we still have insome degree.
We looked at particles that were between 2.88 millimeters in diameter up to
about 10 millimeters. They’re very big particles compared to the average soil
particle.

What we discovered was that we could produce a relationship that seems towork over all particle sizes.

In this graph, you can see no preferential

position of the small particles compared to the large particles.
The graph shows the degree of scatter and it applies to all the particles. What
this graph shows is if you take the mass of the particle and multiply it by the
distance it’s traveled you can relate that to the product of the rainfall energy
and the flow energy.

What these experiments gave us was an equation thatwould say if you know the size of the particle, you know the rainfall energy,
you know the flow energy you could predict how far that particle will travel asa mean-median travel distance. As I say it works but it works for relatively
large particles.
One of the things we did was then to take that regression equation and simply
project it down to see what it would predict for the particles of magnetite thatwe’d used in the field experiment. The magnetite was 80 microns in diameter,
three orders of magnetite smaller than to the particles we used here. The
answer comes out to be not too bad. Certainly within the same order ofmagnitude has the observations. It suggests that this equation does give areasonable prediction.

What those experiments did was to try and understand how far particles
moved on hill slopes.
But, we can apply the same argument to sediment in river channels. If you
have dry land rivers or in this case relatively small channels, they derive muchof their sediment from the hill slopes. What is happening is sediment is being
washed off the hill slope into the channels and then being moved through thechannel. We can think about the travel distance of particles in the channel as
well.
During a storm event, as the flow magnitude increases so the spatial scourand erosion will take place in that bed. As the storm ends and the discharge
decreases you will get deposition. There is a cycle of scour and fill in thechannel. What my colleague here is holding is two scour chains which can beused to measure how deep the scour and fill has been in the channel. You canthink about this as a volume of sediment that has been picked up from here
during the event and moved downstream.

At the end of the event, it has

been replaced by sediment that has come from upstream. In this particularchannel, the scour and fill match as you would expect in a short term. The
experiments were only for a few years. Essentially, you can think about it asa sort of conveyor belt. The sediment is here. A flow event happens and it’smoved to there.
One of the things we tried to think about was how far does the sediment move.
As you saw in an earlier slide, this is one of the channels in that small
catchment. We have the volume of sediment going into the reservoir or thebottom of the catchment. By knowing how much goes into the reservoir, you
can calculate knowing the depth of scour, how far sediment is moving.
On an annual basis because we could only survey the reservoir once a year
while it was dry, sediment moves through this channel about half a kilometer.
We have some estimates as to how far sediment moves through thesechannels in a year.

The next stage of the story comes to Tsukuba where I’ve been doing work withProfessor Omda [ph] since 2009. In 2009, we did some experiments. The
experiments were undertaken in the large rainfall simulator.

Again, thequestion was can we try to understand and can we predict how far sedimentwill move? We constructed a hill slope and then moved the rainfall simulator
over it and ran an experiment.

Now, the experimental design was that within this plot we have a line of
sediment which you can see is a different color from the rest. The color isn't
important because that is not going to help us identify it.

But, what is

important is that this sediment is very rich in Cesium-137 whereas this is not.
The experiment was to see where this cesium enriched sediment moved
during the experiment and then to see if we could develop a model to predict
behavior.

This is a photograph of the experiment. You can see the plot was designed to
be steeper on this side than on this side.

The idea was that flow would thenconcentrate on this side and we would have areas with no flow on the outside.
We wanted that because we wanted to be able to see what would happen to
bits of the sediment where the only mechanism for moving the sediment was
raindrop impact.
Here we just have raindrop impact. Here we have raindrop impact and a zone
where the flow is not strong enough to pick up the sediment but will transport
the sediment if it’s been picked up by the rainfall. The rainfall will splash up
sediment and the flow will then transport it. We got splash only, splash andflow transport and on this side, you can see these clearly defined challenges.
We’ve got detachment by the flow. The experiment allowed us to look at
three types of process causing sediment transport.

左の図This is the cesium measurements after the experiment.
Prior to the experiment, every cell along this line had the same amount of
cesium enriched sediment in it. There’s a uniform amount and there wasnone in this area here. We have a map of each of these cells.

Each of thecovered cells has the surface straight off and then the cesium content of thatcell was measured. We have to thank the Master students of Professor Omda
who diligently worked by collecting all of the data for those cells andprocessing.

Analytical scaling basedon sediment fluxx+ L

ϕ ( x + L) =∫ E (ξ ) . d ξ

x + L( x)ϕ ( x + L) =−2 u
∫ K1 e

4/9

du

x

x

0.8

1 mm
2 mm
4 mm

sediment flux g s-1

0.7
0.6
0.5
0.4

Measured on interrill plots at Walnut
Gulch (Parsons et al., 2006)

0.3
0.2
0.1
0
0

5

10

distance
m
15

20

25

30

Theoretical curves for interrill erosion based on travel distance (Parsons et al.,2004)

Now, coming out of our previous work on transport distance, we have beenable to produce a model of certain transport which is based on the particle sizeand how far it moves. What we end up with is a relationship like this thatsays the sediment flux first of all increases and then slowly decreases with
distance down slope. For the case where sediment is moved by raindrops
and flow operating together, no flow detachment, all of the detachment isachieved by the raindrops but the flow transport is there.
We have a model that is based upon transport distance.

That graphillustrates a part of the model. The model as a whole covers the three typesof processes that we talked about; splash only splash transport, flow transport,
and flow detachment.

That theoretical model was developed and then tested

on plots in Arizona.

The lines here show you the results from theexperiments to test them all. As you can see the model seems to give us areasonable prediction of the behavior of the plots.

Wainwright et al. (2008a) ESPL

MAHLERAN

That’s just a summary of how the model works so you could calculate raindrop
detachment. And then, if there is no flow then the transport is only by rainsplash.

You can calculate the deposition by splash.

there is transport by a concentrated flow.
concentrated flow and so on?

If there is flow thenThen, the question again is there

The model is a comprehensive model ofdifferent types of sediment transport.

Existing Approach
MAHLERAN: cell based

左の図

What it will allow us to do is for this laboratory plot to calculate the amount of
sediment transport.

This is a DEM [ph] of that plot which illustrates the sideslope.
中央の図This is the predictive flow path and you can see these strong flow paths downin the middle on the right hand side and the smaller flow paths were also
visibly photographed.
右の図This also predicts total sediment loss from the plot.

（上図は動画の一部）
But the problem with total sediment loss is – what that does is tells you theamount of sediment.

It doesn’t tell you where your marked cesium enrichedsediment was. The difficult task is to know what happens to this.

In the firsttime step you can make this a simple task because any sediment that isremoved from this band will be cesium enriched sediment. But in the second
time step that sediment will have moved to here where it is now mixed withsediment which is not cesium enriched.
sediment in that cell.

It is a proportion of the total

The question is, what is the chance that the cesium

enriched sediment will be picked up compared to the background sediment?
How do you calculate that in terms of knowing what is the depth of soil towhich you mean to apply the model?

Our task then was to develop andconstruct this model essentially to move these marked particles.
（以下の説明の途中で、上図に重ねた、粒子の移動を示す動画有り。
）

What this model then does is to show what becomes of these particles during
an event.
What you see first of all is there are very distinct lines where the sediment
moves relatively quickly. There is an area over here where not very muchhappens. This is the area of splash. You can see part of it is moving around.
Slowly, therefore, you can build up a pattern of where the cesium ought to be
in relation to the experimental observation.

Sediment Runoff

We then developed a model which is based on Mahleran Model.

This

produces sediment loss that was observed. In the experiment, we measured
the sediment coming off.
reasonable approximation.

This is the simulated sediment loss which is aSpatial PerformanceNash-Sutcliffe = 0.97
Syrjala p-value = 0.12

This is the spatial performance of cesium coverage. This tells us where thecesium was detected. This tells us where the model predicts that it should be
detected. You can see a reasonable match in terms of the splash either side
here and the very distinct flow line comes from here and so on.
Obviously, this is more detailed because it’s easy in the model to tell you what
happens in every cell or else this only tells you what happens in a portion of
the cell. We can see that breakthrough of a very low cesium content there ispicked up by the model.

Temporal Performance
Sediment runoff at bottom of plot

If we compare when cesium is first detected in the outflow it’s there and our
prediction is only very slightly…

AT THE RAINDROP SCALE
AWhat we’ve been able to do with that is to develop a model that allows us tosee where sediment is going to and which seems to predict reasonably well.
We’re also interested in trying to understand these processes at other scales.
What I’ll talk about now is some work that’s been going on over the last year
or two. All of the issues is what happens to sediment when it’s detached byraindrops.

Dynamics of Runoff And
Erosion Modelling (DRÆM)200 – 212 μm dry sandtime

2.5 mm droplet impacting at 5.94 m s-1

Trajectories of splashed particles
200 – 212 μm dry sandImage sequence at 3000
fps
Sequence duration 0.11 s

3.5 mm droplet impacting at 6.13 m s-1

For this, we’ve been using particle imaging velocimetry which enables us totrack individual particles as you can see the images are sequenced at 3000
frames per second.

From that, you can track where each droplet goes.

Because the water droplets and the segment rich droplets behave differently,
you can then separate out the sediment particles from the water particles.

Dynamics of Runoff And
Erosion Modelling (DRÆM)Raw Image(pixels)

Interaction of single water droplet with a flat surface – Particle Tracking
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Centre locationsof identified
particles

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Displacement (mm)Particle tracks andassociated velocity
profile

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Displacement (mm)
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Velocity (m s-1)

3000 fps, 1024 x 512 pixels (64.6 mm x 32.4 mm), 0.063 mm/pixel resolution
Water diameter = 2 mm, Sand Dia. < 212 µm, Height 0.5 m, Impact velocity = 2.8 m s-1

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Long et al. (forthcoming)

We have this distribution of where the sediment particles go.

This shows

what becomes of them. That’s the raw image that we work with. That’s
what comes out of the photography. The centers of individual particles are
identified. Because of the frequency of the photography, it’s relatively easy
to say that there aren’t that many options where this particle will be in thenext photograph.
You can assume it has a parabolic trajectory.
certain places.

Therefore, you can look for it inThis particle is not suddenly going to go over there.

It’sgoing to go slightly in that direction. But because you can predict what sortof behavior it is, you can then map all of those centers through time to get this
distribution of where the particles have traveled. What the layout [ph] thenallows us to do is to see both how far particles move and how quickly they
move and what angles they are rejected from the surface?

Dynamics of Runoff And
Erosion Modelling (DRÆM)Surface-profile measurement taken using
calibrated images of the sand surface

Long et al. (forthcoming)

We could also use the photography to see the fall of the ground surface thatthe raindrop creates. You can see that after the raindrop impact there was acrater. That crater is then surrounded by an area of sediment and further out
there are individual particles which do not obviously create a topographic
expression.

We’re now able to see what happens during the impact of asingle raindrop.

USE OF RFIDs

Related to that is more work that we’ve been doing in Tsukuba. These are
RFID tagged particles. These are an updated version of the particles that I
was using in the laboratory a decade ago.
The difference is that a decade ago, all I could do is produce colored particles
and see where the colored particles were. The RFID technology allows us to
identify each particle separately and see therefore, where an individual
particle is gone. This work is again done with students of Professor Omda.

We’ve been able to map where the displacement of those individual particles
during a series of events

USE OF RFIDs

and then to produce a distribution curve for how far particles traveled. Again,
we’re able to start thinking how far do individual particles move under aparticular process.

USE OF RFIDs

With Mr. Noguchi, we’ve been looking at particles in the field of Fukushima.
Here our RFIDs on this plot located in various positions particularly so lines
across and down slope.

USE OF RFIDs

The preliminary analysis of those results shows that with different distances
down slope and different storm intensities we get different spatial patterns as
you might expect that as the storm intensity increases there’s more runoff.
There’s a stronger relationship with particle travel distance and distance downslope simply because there is more run-off and therefore more power totransport these particles.

We are beginning by using this technology tounderstand yet more about how sediment moves.

AT THE CONTINENTAL
SCALE

Finally, if we think about sediment transport at a continental scale, then again
there is an accepted view. The accepted view is this that if you want to know
how fast continents erode then all you really need to worry about is suspended
sediment because for the time you get to the downstream end of large rivers,
almost all of the sediment which is being transported is being transported insuspension. By measuring suspended sediment load we can estimate rates
of continental erosion.

For continental erosion, theassumption that suspended
sediment travels at the same
speed as the water,
determines our estimates of
the rates of continental
erosion.

Now, the way this is done is to attain a rating curve between discharge andsediment. You go along in four different stage levels of the river. You takesamples of water and that gives you a way of rating the amount of sediment
for a given discharge. Then, if you have a continuous recorded dischargethen what people have done is to simply apply this equation. You know thesuspended sediment concentration from the rating curve, you measure thewater discharge and by multiplying those two together you get the amount of
sediment. That equation assumes that sediment travels at the same speed
as the water. If you were to imagine doing this analysis and you had fish in
the ocean that swam upstream and you took a rating curve of the numbers offish in the water and then you multiplied that by the discharge, you would get
an apparent loss of fish from the land to the ocean.
By taking those two things and assuming the sediments traveling at the same
speed or indeed in the same direction can lead you to the wrong conclusion.

There’s a good reason why suspended sediment should not travel with thesame speed as the water because the reason the sediment is in suspension is
because of turbulence. Turbulent structures typically look like this. You’regoing to have suspended sediment brought down to the bed and then swept
off of it. It’s more likely that suspended sediment will spend its time travelingvertically and reaching the bed and then being picked up off the bed. There’s
a good chance that once you get down here you will have to wait to be
re-entrained.
We can think about suspended sediment moving with the water, being
deposited on the bed, picked up and moved again.

It has what you can

describe as a virtual velocity which is not the same as the velocity of the water.
That was our idea.

We thought it’s very difficult to test this idea.

Suspended sediment
A first approximation for the travel
distance of particles suspended inwater flows as:

Ls , ϕ = 7.28 × 10 2 e (7.33 × 10 ω ) e
−3

where

(−6.127 Dϕ )Ls, ϕis the median travel distance in suspension for a particle of size
ϕ [m ]

ωis stream power [J m-2 s-1], calculated asD

is the dimensionless particle size

ω = g hu S

But, if you look at the wind erosion literature, they think suspended sedimenthas a finite travel distance but the fluvial literature does not.

The fluvial

literature thinks it stays in suspension forever but the aeolian literature thinks
differently. There’s a reason for thinking that perhaps things aren’t as thefluvial geomorphologists think.

Here again Tsukuba comes to the rescue. You have still just the finest flumein the world. We thought this flume is long enough possibly to test the ideathat suspended sediment does not just travel with the water.

That if westarted water flowing down this flume [Unclear] sediment that would go into
suspension, some of it would be left behind when we turned off the water.

And what about vegetation?

We did an experiment.

We set the flume going with a discharge and we

added some fine sediment well above the threshold for suspension.

We

previously sprayed this sediment with fluorescent paint so that we could see if
it’s still in the flume and it was.

These are the results of how much sediment is left in the first 145 meters of
flume. The last few meters were affected by the – we are at the bottom so
you get a backwater effect. Also, this is a log scale because these numbers as
you can see are very, very small unless in the graph whereas these numbers
are several hundred grams. To get a graph to show something, we have useda log scale. Important point is that if you sum all of this, 18% of the sediment
we put into the flume was still there at the end of the experiment.
What this graph gives us is the tail end of a distribution curve. We know thatthe tail of a distribution curve that starts here and goes up and then at somepoint reaches a maximum value and then will come down here. We also know
the total area under that distribution curve because we know the weight of thesediment we put in.

And so, we can fit the curve that will have the right total

area and is a reasonable fit to these data.
If we do that we find that the mean travel distance of the sediment is aroundabout 170-180 meters. In the same time, the water would have traveled 350
meters. What we calculate is that the velocity of the sediment is about half
the velocity of the water on average. The surprising thing is that if you lookat these three bars for the three sediment sizes, they are as what you might
expect the one we are [Unclear]. The reason we studied the sediment inthese three sizes is we expected to find that the travel distances of thecoarsest sediment would be the shortest. As we got to the finer and finer
sediment, it traveled further and further but our results are the opposite ofthat; 40% of the sediment between half and 1 millimeter is deposited in flume
but only 12% of the sediment between 1.4 or 2 millimeters.
There’s an interesting result is the fact that the finer sediment doesn’t travel
as far as the coarser sediment in this experiment. I think that has to do withthe difference in rest times. Once fine sediment is deposited, it’s actually
very difficult to get it re-entrained. It probably travels further in an individualstate but has to wait longer before being re-entrained.

CONCLUSIONS• All eroded soil travels a finite distance
before being deposited• Understanding of these travel
distances is fundamental to spatial
scaling rates of erosion• Travel distances are important forunderstanding the fate ofcontaminants in the environment
To conclude, all eroded soil travels a finite distance. Understanding thesetravel distances is fundamental to the idea of scaling erosion maps. How wematch what we leaves catchments with what we measure on hill slopes canonly really be understood if we think about how far sediment travels. That’s
important for understanding the fate of contaminants in the environment.
How long contaminants stay in the environment will depend on their travel
distance time, their travel distances, and how far they move in a particulartime. Understanding travel distance is crucial to the fate of eroded soil.

以下質疑Male Participant
Dr. Parsons I will start. You have the sediment delivery ratio as first in yourtalk and then you measured that each particle has a finite travel distance. I
would like to know the relation between the sediment delivery ratio and theactual travel distance.

Do you have any idea to combine?

Tony Parsons
The sediment delivery ratio is based on the idea that you measure sediment
loss and you multiply it by the area of catchment.

But, if the sedimentleaving the catchment has only traveled a short distance then you havemultiplied by the wrong number. Most sediment that leaves the catchment if
you measure that coming out has come from a relatively small area close to
the channel. If you think about the sediment at the top of the hill slope that
isn't going to come out of the catchment for many, many years.

You cannot

assume that the area of the catchment matters.
Some years ago, I was at a conference and I was talking about measuring soil
loss. I said, what we need to measure is not sediment loss from an area but
sediment flux across a line.

Then, a few years later I went back to theconference and said, in fact that’s what we’ve been measuring all the time but
we have called it sediment loss for an area, but when we measure sediment
coming out of a plot, we’re measuring something crossing a line.

It doesn’ttell us what happened up there. Maybe the amount of sediment crossing theline at the bottom is not related to the area at all. I think that’s why if wemeasure travel distance we can understand how much lives the catchment as
a function of travel distance of sediment. That’s a long answer.
Male Participant
But still maybe lots of things to do?
Tony Parsons
Yes, still lots of things to do but I think understanding travel distance is crucial
to do.

Male Participant
Thank you very much for your very nice presentation. I think you focusedmore about some larger particles, I’m working on some model with thenuclear transport in a catchment. In that model we consider the sedimenttransport also because basically [Unclear] for us maybe basically transport isthe sediment. In our model, I think always when we go to calculate we foundalways that the simulated sediments are higher than [Unclear] always but
after your presentation I found data because we assumed that all thesediments flows same as the water. I think as you mentioned that maybe ourassumptions are wrong. We should revise…?
Tony Parsons
I think so. Yes, a lot of the work is done with large particles for the reasons
I explained but initially they were doing ones we could see now that we use
RFIDs even though the RFID itself is quite small [Unclear] to make it look likea soil particle it’s still quite big. But, the magnetite was soil sized. It was 80
microns and the dominant size of raindrop detached soil is 100 microns. The
problem with magnetite is it’s a lot heavier than soil. The reason we ground
it down to 80 microns was to make it have the same mass as the soil particleof 100 microns. We have the same mass but it also has a small area. It had
this small target to the raindrops. There is an issue. With the cesium richsoil, we are looking at real soil particles. I think that is very promising. We
can now model reasonably successfully I think where real soil particles aregoing.
Male Participant
Do you have any paper on this…?
Tony Parsons
About?
Male Participant…journal papers or?
Tony Parsons

On all of this?
Male Participant
Yeah.
Tony Parsons
Everything up to the cesium experiments are in the [Unclear].

That paper

came out last year. The only things that aren’t published are the RFID work
which we haven’t quite started and we will finish soon. The flume work whichis the same, the paper is half written and I hope will be distributed in a monthor so. But everything else has been published.
Male Participant
Maybe I can make one question [Unclear] from your conclusion, the particle
size and the weight of this eroded soil particle materials are being broken
down for understanding the contaminants.

I can understand that your study

might be important to understand the sediment as a physical contaminant but
I’m not clear about how much helpful is your study in understanding thesediment as a chemical contaminant. Is there any?
Tony Parsons
Yeah.

The only way in which it’s relevant to the chemistry is if thecontaminants are adsorbed on to soil particles. As an example, if cesium
adsorbs on the clay particles and therefore to a large extent will travel where
the clay particles go to. It’s not entirely true because some of the cesium will
be desorbed from the soil particles by plant roots and taken up into the plants.
We can’t simply say where the soil goes, the cesium goes. But we can if we
understand the mechanisms of adsorption and desorption of contaminants
onto soil particles, then we can use that to understand, if we understandwhere soil particles go. We can understand something of where chemicallyattached contaminants go. The story is more complicated but that is part ofa bigger story.
Male ParticipantYeah because I didn’t see any chemical composition in [Unclear] soil particle.

Tony Parsons
Yes.
Male Participant
What is inside that so that maybe helpful to understand.
Tony Parsons
Yeah. Now, I’m interested in where soil particles go. The reason we did thecesium experiment was because that was a way of identifying particular soil
particles. We could say these soil particles can be detected because we canmeasure the cesium activity in a soil sample. It’s the same approach as using
the magnetite saying this is something we can find and is like a soil particle oris a soil particle in the case of cesium. What we’re trying to do is understandwhere soil goes. How things are adsorbed onto soil particles is a separate
issue.

Maybe we’ll get to investigate that as well in the next few years.

Thank you.
Male Participant
Thank you very much.
Tony Parsons
Thank you.
END