An Investigation of Channel and Stream Characteristics For Streams on the Isle of Arran
An Investigation of Channel and Stream Characteristics
For Streams on the Isle of Arran
Aims
To investigate hypothesis associated with increasing distance down stream for streams on the
Isle of Arron.
Objectives
To set up and test hypothesis
To investigate how channel characteristics change with increasing stream order.
Hypothesis
. As the river progresses downstream the bedload decreases in size.
2. The bedload becomes rounder in shape downsream.
3. The width will increase in size as the river goes downstream towards the mouth of the river.
4. The velocity of the river will decrease as it goes downstream.
5. The depth will increase in size as it goes downstream.
6. The discharge will also increase as it goes downstream.
Theory
There are three processes by which material is transported from land to the sea (via rivers)
these are:
i) Weathering-physical and chemical breakdown of rocks.
ii) The slope-the products of wethering are moved down the gravity gradient in mass
movements by the slope wash processes.
iii) Fluid transfer-water, air and ice.
The load, carried by natural streams, can be separated into three compounds. The material
carried by the river is called the rivers load. It comes from river erosion and weathering of the
river banks. Especially when the river is bankfull, considerable quantities of load can be carried.
The total amount of load that a river can carry is called its capacity.
The greatest size of material it can transport is called its conference.
Load is transported in the following compounds:
. as dissolved load-material is transported in suspension, this is the chemically dissolved rock.
2. as wash load-fine particles move steadily in suspension (fine mud floating in the water).
3. as bed-material load-material is transported in saltation. This is coarse sand and small
pebbles bouncing along the river bed. Bed material also moves in traction which is when large
boulders trundle along using smaller stones as ball bearings. The bed load (material of all sizes)
travels slower than water. The bed material can also be transported as suspended load.
Particles move with the main body of water.
In this project it is not possible (given the amount of time) to study each of these processes and
its results. I will be investigating the bed-material load(size and shape). This includes traction and
saltation.
BED MATERIAL LOAD
Unlike the transport of the dissolved and wash load, the rate of bed-material transport is almost
entirely a function of the transporting capacity
of the flow. Having investigated this subject it has become apparent to me that reliable field
measurements are difficult to make and many experiments have been carried out in laboratories
where conditions can be simplified.
Rolling is the primary mode of transport in gravel-bed streams while saltation is found largely where grains
are sand sized.
As circumstances of the river change, such as the slope or the discharge,so does the rate of
particle movements and the size distribution of the bed-material. Two processes are involved,
abrasion and sorting.
Abrasion- this includes the mechanical processes like grinding, impact and rubbing, which chip
and fracture particles during transportation and when the particles vibrate.
Sorting- this is when only a small amount of the bed material is smaller than the threshold size
and this is transported by the flow, sorting also occurs when smaller particles are transported
faster and further than larger ones.
In the past abrasion was thought of as the main process responsible for the size reduction of bed
material downstream but as a result of experiments in abrasion tanks it was realised that the
rates of reduction in size of the bed material were much larger than abrasion could be
responsible for.
Recent work has re-established the first idea about abrasion. Scientists are still not sure of the
truth.
The chemical breakdown of material under certain conditions adds to the changes downstream.
Properties other than size change through dowstream transport can be noticed . Particles
become rounder as a result of abrasion. Bed-material is generally better sorted with distance
downstream, although tributary inflow can disrupt the picture.
To Summarize
Particles are reduced in size and shape by abrasion processes. They are given their position
along the stream by sorting processes.
Further, one should also realise that relief and the widespread impact of human activity distorts
the global pattern of nature. The physical changes that man has made to the natural environment
such as building dams and reservoirs, changing the courses of rivers (diverting rivers) may have
similar effects on the bed material in the rivers as have the many years of climatic change.
Isle of Arran , island, western Scotland, in the Firth of Clyde,part of Strathclyde region. The
island is mountainous, ...
This is a preview of the whole essay
along the stream by sorting processes.
Further, one should also realise that relief and the widespread impact of human activity distorts
the global pattern of nature. The physical changes that man has made to the natural environment
such as building dams and reservoirs, changing the courses of rivers (diverting rivers) may have
similar effects on the bed material in the rivers as have the many years of climatic change.
Isle of Arran , island, western Scotland, in the Firth of Clyde,part of Strathclyde region. The
island is mountainous, and has an area of about 433 sq km (169 sq mi) reaching a maximum
elevation of 874 m (2,866 ft), and the wild and rugged breakers of the northern shore attract
both geologists and tourists. The island also is noted for beautiful glens and picturesque
waterfalls. Places of interest include a ruined castle on the northern part of the island. The
industries of the island include fishing, raising sheep and cattle, and growing oats and potatoes.
Data Required
The location of the eight sites we were to visit was given to us (as can be seen from the
following map). At each site the length of the b-axis of the pebbles needed to be recorded. I
had to measure as many pebbles as possible at each site (given the time) so that the nearest
average could be reached and the results would be as accurate and meaningful as possible.
Therefore at least ten pebbles per site had to be measured. The b-axis were measured so that I
could get an idea of the size and shape of the bedload. It is important that the pebbles were
picked randomly and that each stone in the river had an equal chance of being picked. (should
not look at the pebbles and decide which one you would like to measure).
The main hypothesis is that the pebbles decrease in size and become rounder as they travel
downstream, I will try to prove this by showing facts collected and analysing data and find out if
a pattern was actually found.
At each site the width of the river had to be measured. And also the depth had to be
measured. At each site the velocity of the river had to be measured.
Methodology
There are a number of measurements that can be made within the valley and channel of a river
and that can be used in the calculation of several fluvial features such as discharge, and load.
To measure the width of the river
Using a tape measure at right angles across the surface of the river from water edge to water
edge.
To measure the depth of the river
Divide the width by 10 and measure depth at these points. The first and last readings should be
on the river banks.
To measure the speed of the river
The best and easiest method is to use some sort of flow vane which will enable you to obtain
either a direct reading or be able to calculate the speed on a conversion chart. Without a flow
vane it is possible to calculate the speed by using a piece of wood or a cork as a float. The
procedure is as follows:
* measure a ten metre section of the river
* measure the time it takes the float to cover the distance
* do this several times and at various points across the channel
* find the average time - this is the surface speed.
* multiply by 0.8 to find the true speed of the river which will be below the surface and
about the point that you used for your calculations with the flow vane.
Measuring the cross section area (so that that you can find the discharge)
Firstly run a tape across the river along the water line from bank to bank. When you have found
the width divide this by 10. Along this tape calculate the depth of the stream at each of the ten
points. Once calculated, transfer this information onto a piece of graph paper making sure that
you use the same scale on both the vertical and horizontal axes. Having produced the cross
section diagram you simply count the squares and a simply calculation of multiplying number
with size will give the cross sectional area.
To measure the discharge of the river
Once you have found the speed (in metres per second) and cross sectional area (in square
metres) you simply multiply them to give the discharge in cubic metres per second or cumecs.
To measure the bedload size (Length of b-axis)
Pick up a pebble at each depth point measure its b-axis and record its shape avoid bias.
When finding the locations at which point to carry out the investigation we were given the map
on the next page and told that we would be going to the sites one - eight marked on the map.
We therefore did not have to choose the locations for ourselves but were given them.
We can see from the positions of each of the sites in relation to each other. The geology of the
land is also shown. If it is found that there is a large difference in the shape and size of the
pebbles in these different areas we may consider this to be an important reason.
The grid reference for the eight sites taken from an ordnance survey map and the distance from
the source (km) are:-
Site one 951 459 0.25km
Site two 952 465 0.75km
Site three 952 476 2km
Site four 949 483 2.75km
Site five 947 493 3.25km
Site six 945 496 4km
Site seven 943 499 4.30km
Site eight 939 503 5km
Since I was the only person to have shape as a hypothesis I decided to sort the pebbles
according to their shape into four catagues - discs, spheres, rods and blades as drawn below.
Discs Spheres
Blades Rods
The information about the number of discs, spheres, blades and rods at each site was then
displayed in the form of pie charts, one for each site. These enable one to compare easily and
quickly the number of pebbles of each shape that were collected at each site.
I found that this method enables me to clearly appreciate the number of pebbles of each
different shape at each site separately but it was not so good when comparing the sites with
one another.
I then tried to use another graphical method to display this more clearly. I drew eight bar
charts next to one another and found that by shading all the discs in one colour, all the rods in
another, I was able to see clearly any trends in number over all eight sites.
Athough this method was clear I thought that if I drew a line graph, one line for the number
of discs at each site, another line for blades, and so on, it may be easier to spot the patterns.
RESULTS OF THE VELOCITY OF THE RIVER AT EACH OF THE SITES
Site one - 33.76 cm/s
Site two - 15.58 cm/s
Site three - 159.68 cm/s
Site four - 74.08 cm/s
Site five - 12.32 cm/s
Site six - 31.68 cm/s
Site seven - 12.92 cm/s
Site eight - 20.25 cm/s
RESULTS OF THE WIDTH OF THE RIVER AT EACH OF THE SITES
Site one - 1.49m
Site two - 4.6m
Site three - 4.87m
Site four - 5.90m
Site five - 7m
Site six - 7.6m
Site seven - 7.2m
Site eight - 8.6m
RESULTS OF THE DEPTH OF THE RIVER AT EACH OF THE SITES
Site one - 4.52 cm
Site two - 12.25 cm
Site three - 11.09 cm
Site four - 21.73 cm
Site five - 6.86 cm
Site six - 2.55 cm
Site seven - 11.73 cm
Site eight - 17.55 cm
RESULTS OF THE BEDLOAD SIZE (LENGTH OF B-AXIS) AT EACH OF THE
SITES
Site one - 10.60 cm
Site two - 5.77 cm
Site three - 20.59 cm
Site four - 12.02 cm
Site five - 16.82 cm
Site six - 23.50 cm
Site seven - 14.27 cm
Site eight - 7.78 cm
RESULTS OF THE SLOPE ANGLE OF THE RIVER AT EACH OF THE SITES
Site one - 5
Site two - 4
Site three - 1
Site four - 5
Site five - 6
Site six - 5
Site seven - 2
Site eight - 2
Analysis
When studying the graphs plotted I can see that that the number of discs is almost stable for all
of the sites (averaging to be just under 5). This is excluding site 4 which has a sudden very
large decrease in numbers. This was unexpected and noted unusual. The number of spheres
decrease as the river travels downstream although there is a small rise in numbers at site one.
There is only one rod at site one and an increase in number follows at site two. There is
then an extremely large increase at four. It then drops back at the next three sites and once
again site four stands out with very unusual results (as it does for discs).
The number of blades does not change drastically at any of the sites and has an average
of 1.375 although again site four has a higher value than the other sites (and seems unusual).
The pie charts I drew display the proportion of discs, spheres, rods and blades at each
site. They make it easy to look at each site individually but are not ideal for comparing sites. (a
bar chart displays this information better).
The first pie chart shows that at site one over 1/3 of the pebbles were discs, there were
almost as many spheres and that there were only a small number of blades and rods.
The second pie chart shows similiar results at site two. Discs were clearly still the most
common shape followed closely by spheres, there were quite a large number of rods and if I
refer back to the chart of site one I can appreciate a large increase. The number of blade is still
comparatively low.
The chart of site three re establishes a similiar pattern to that of site 1 and 2 where discs
were the dominant shape followed by spheres. The number of rods was still quite high but
blades had declined in number once again.
The pie chart of site four, which I have already established as being unusual continues to
emphasise this point. It shows a sudden dramatic increase in the number of rods so that they
now take up over 1/3 of the chart. there was still a reasonable no. of spheres compared with
sites 1 and 2. The number of blades has increased and the most noticeable point is that the
area covered by the discs has decreased enormously.
There was no unexpected change at site six, the number of discs decreased slightly. This
allowed the number of spheres and rods to rise and blades stayed the same as site five.
It is noticeable from the pie chart of site seven that the number discs had risen to over half
of the total number of pebbles (at this site). This meant that the number of spheres, blades and
rods decreased.
The pie chart of site eight shows that almost 2/3 of the pebbles were discs, and there
were no blades at this site.
Pie charts are not the best way to display this information in order that one could easily
compare the sites to one another to find a trend. I then tried a bar chart to see if this was any
clearer.
From the bar chart I was able to see that the trends were as follows. Discs are normally the
most common shape found in the bedload at each site with the exception of site four which
was very unusual, and I will try to give possible explanations why in pages to follow. Blades,
normally very few are found making up the bedload material. The number of rods is usually
reasonably low although it shoots up at site four ( which I have already said acted strangly).
Spheres decrease in number as the river travels downstream.
The same information was plotted on a line graph which made it easier and clearer to
compare sites. It confirmed the idea that there must have been either an error on my part (in
recording and taking my results) at site four or possibly a natural reason for such peculiar
findings. From this graph it is possible to compare the beginning of the river(site one)to the
end (at site eight) I noticed that the number of discs at the beginning and end was almost the
same. The number of spheres drops a great deal by the time the river reached site eight. The
number of both rods and blades increased downstream.
Finally I found the average of the b- axis for each site and ploted this on a line graph. This
simply showed that the average length, width and breath all decreased downstream. It also
shows that it did not decrease evenly because it went up at site four (which I would expect
knowing that this site behave differently to the others). It also rose at site six which I did not
expect.
I tried to find out if the results were due to the processes involved or if the geology of the
land played a dominant role in the size and shape of the bedload.
I then tried to find out if the velocity of the river at each of the sites would make any
difference to the size and shape of the pebbles as it would mean that the river may possibly
have had more energy if it was travelling at a higher velocity and would therefore turn and chip
the pebbles and make them rounder and smaller. I plotted the information in the form of a bar
chart and found that sites five and six had reasonably small velocities, sites one, four and seven
had slightly higher velocities and site two had an extremely high velocity.
Interpretation of data
It was important to find out if processes or the geology of the land played an important role in
the size and shape of the bed material. I found that I could not really find any evidence from
my results to suggest that the suspended load in the riverhad effected the bedload because I
had an unpredictable rise at site five.
One clear pattern found was that the average length for each of the three axes decreased
downstream. This meant that the pebbles got smaller. I expected this to result. A possible
reason for this is that it was due to processes such as abrasion which is when particles grind
and rub each other so that they become chipped and fractured. They therefore become
smaller and smoother. Another process is sorting which occurs because smaller particles and
pebbles are transported faster and further than larger ones.
There was no obvious pattern concerning the shape of the pebble. If I exclude site four
from my results I can find that, discs are the most common shape there are also many spheres,
both these shapes are very round and the blades are rods, which are longer and thinner, were
not very common at all.
I thought that perhaps the velocity of the river at each site might have had a noticeable effect
on the size and shape of the bed material. When comparing my results I could not find any
patterns. In my bar chart showing the number of discs, blades, rods and spheres at each of the
sites, sites two and five are reasonably similar whereas on the bar chart showing velocity these
two sites are extremely different. Site five has a velocity of 35.7 cm/s (the lowest of all the
sites) whereas site two has a velocity of 67.8 cm/s (the highest of all the sites.) Also on the bar
chart of velocity site four shows a normal velocity (compared with the other sites) .
I must try to explain why site four produced such unexpected and unusual results. One
possibility is that I made an error whilst taking measurements or that I did not choose at
random but favoured particular pebbles. This is quite possible although there is no reason why
I should have been more biased at site four than at any of the other sites.
Another possible explanation is that bed material is generally better sorted with distance
downstream although tributary inflow can disrupt the picture.
I have considered various explanations why site four did not follow the pattern of the
others. Without further tests I am unable to give any definite reasons.
Conclusion
My first hypothesis was that as the river progressed downstream the bedload decreased in size.
I proved that this hypothesis was in fact correct. I found that the average length of the b-axis
decreased downstream.This meant that the pebbles became smaller. This showed clearly that
the size of the pebbles decreased downstream.
My second hypothesis was that bedload becomes rounder downstream. This was not
really proved or disproved because it was found that discs and spheres which are both round
shapes (compared with rods and blades which are long and thin) dominated the bedload for all
eight sites and did not really increase or decrease dramatically at either end of the river. I found
out that there was no relationship between the geology of the land and the results I collected.
I also found that on this occasion the velocity of the river did not directly affect the size and
shape of the bedload.
Criticisms
From other research investigated I would have expected my hypothesis to have been proved
correct. This was not conclusive. Any number of the following could explain why this is so.
* My hypothesis could have been wrong.
* My methods or measurements may have been inaccurate. The measuring equipment
may have led to inaccuracy.
* We may have visited exceptional sites. In addition, I was unable to find sufficient
explanations indicating why such results were obtained.
I would have liked the opportunity to have returned to Isle of Arran to check these results which
were out of line with the other, in particular site four.