DATA ANAYLSIS
Velocities against Cross sectional area of river.
The trend above occurred because as the cross sectional area increases, the area of which there is minimal friction between the water, riverbed and surface increases. This limit in friction increases velocity as velocity can occur more easily and more efficiently.
The anomalies on this graph is situated at…
Site 1 – The velocity reading is higher than expected at 0.527 m/sec, with a C.S.A reading of 0.2m2
Suggested reasons for this anomaly (although naturally upstream the water has more energy due to the velocity), is the gradient of the land where the reading was taken may have been steeper than usual giving the water an increase in kinetic energy therefore increasing the velocity reading unexpectedly.
Site 3 – The velocity reading is lower than expected at 1.85 m/sec with a C.S.A reading of 0.418m2.
Due to the farmers attempt to straighten the river at site 3 the river eroded back to return to its original state of a meander by lateral erosion, pushing the riverbanks back causing the stonewall to collapse into the river and stay there. The river is too small and weak to transport these boulders so the river has been shaping them ever since. Meanwhile the processes attrition & corrasion are shaping the boulders until they are small enough to be transported, the big boulders have disturbed the flow of water by refracting and deflecting the water off the stones, creating more friction (which slows down the water) which then consequently disturbs the velocity reading.
The overall velocity reading may have been hugely effected by the data taken in the inside of the meander where energy levels in the river are at there lowest and processes such as deposition occur (not erosion or transportation) therefore reflecting this on the graph.
These patterns above occur because the riverbed becomes smoother as you go downstream therefore creating less friction along the wetted perimeter, which in return increases the rivers energy and likewise its velocity. Consequently the riverbed becomes smoother because of…
→ River erosion processes such as attrition due to the sediment carried in the river, → Corrasion because of the acidic water due to the surrounding acidic coniferous trees,
→ Hydraulic action the pure force of the river itself eroding the riverbed
All these processes smoothen the riverbed and become more influential and dominant downstream due to the increase of discharge of the water from joining tributaries.
At Site 2, the Manning’s ‘n’ reading is higher than expected. This is because of glacial deposits of ice that have melted in the valley leaving boulder deposits of which are sitting in the river. Also weathering processes such as freeze thaw break boulders off the valley sides into the river, of which the river is not efficient enough to transport them so they consequently increase the Manning’s ‘n’ of the site.
Site 3 velocity readings on graph are generally lower than expected this may be due to the boulders from the farmers previous stonewall and glacial deposits, which increase roughness and friction and as a result, lower the velocity. Also the overall result may have been largely influenced by the inside of the meander readings where sediment deposits are high and energy levels at there lowest.
Also at this site a lot of water was flowing beneath the streambed, which could not have been measured. This has reflected on the velocity readings as the water was shallower than expected, and therefore the velocity of the water below the riverbed could not have been measured consequently lowering velocity readings of the whole site.
The general trend of graph is as velocity increases, the gradient of the riverbed decreases. Also on graph you can see the higher the gradient the lower the velocity. These patterns are unexpected, as I would have expected velocity to decrease as the gradient does. I would have thought that the lack of gravity upon the river would have decreased velocity as you go downstream, but that is not the case. If you look at graph you can see that there is a steeper gradient nearer the source. This is due to deposition of glacial deposits and sediment of the valley, which raise the riverbed but this does not affect velocity. Velocity still increases because of other dominant factors i.e. joining tributaries mounting the discharge and energy of water.
The anomalies found on both graphs where the gradient measurement is slightly higher than expected. This is probably due to limitations of using the equipment.
The reason why velocity still increases downstream as the land becomes less steep is because the other dominant characteristics of the river such as a reduction of friction, override the factor of gradient.
This trend occurs because as you move downstream, the drainage basin of the river extends outwards which increases the discharge of the water due to the joining tributaries and this therefore leads to an increase of water energy of which lateral erosion widens the river. The explanations of the shapes are as follows:
Site 1 shows a highly undulated shape with signs of huge boulders forming the shape of the bed. These boulders come from the riverside where weathering processes would have an effect and break off rocks into the river. The river is too weak to transport these boulders downstream, as it only has a small catchments area, so instead river adjusts around them.
Site 2 shows a cross section of the valley, which has a higher discharge than site 1. There however is an anomaly at interval 6 where the riverbed is higher than expected. These are probably glacial deposits that are too big for the river to transport so again it adjusts around it.
Site 3 shows a river of where a meander is present. The meander was caused by lateral erosion undercutting the river and helicoidally flow which causes the crescent shape of the meander. An anomaly occurred at interval 6 where the riverbed is higher than expected this is probably caused by the remains of the farmers stonewall or glacial deposits which have resulted in this shape.
The trend above occurs because if you look back at figure which shows the river Caerfanell catchment’s area, you can see there are many tributaries joining the main river moving downstream therefore increasing the discharge and hence the waters energy. This widens and deepens the river as there is less friction around the wetted perimeter, and as a result it increases velocity downstream.
The only anomalies of the graph are found at site 1 interval 3, and site 2, interval 4 of where the velocity readings are higher than expected.
The pattern for velocity should have risen rapidly in site 1 and 2 and started to level off or show a little decrease at site 3. Instead the velocity reading at site 1 interval 3 is higher than expected, this may be because the velocity reading was taken after the small waterfall present at site 1 of which velocity would have increased due to a boost in gravity which in return increases the hydraulic radius of the river. This smoothens the riverbed quicker increasing velocity by creating less friction.
At site 2, interval 4, velocity reading is lower than expected. This is because of the glacial deposits of the big boulders could have had an effect on the flow of the water consequently slightly decreasing velocity.
Site 3 velocity readings are generally lower than expected, this may be because the process glacial moraine, actual river deposits and water flowing beneath the stream bed hence having a lowering effect on the velocity.
You can clearly see there is no correlation between gradient with neither roughness nor C.S.A. This pattern occurs because none of the factors contribute towards each other results so hence none affect each other’s results.
Spearman’s rank also proves statistically that there is no relationship between these variables and that there is no correlation between them.
This graph shows all factors I am studying on one simple graph.
From this graph you can see all four variables correlations on the same graph. We can clearly see that cross sectional area, roughness, and velocity all increase as you move downstream. The only factor that doesn’t is gradient, which instead generally decreases downstream. There is an explanation of why these trends occur earlier on in this part of data presentation.
CONCLUSION
In this part of the project I will be answering my previous key questions and summarizing all data to draw a conclusion.
- How does velocity change downstream?
As seen on graph _ _ you can see that velocity increases downstream with site 1 with a value of 0.287 m/s, site 2 with 0.378 m/s and site 3 with 0.457 m/s, so my hypothesis was proved correct. From graph you can see there is a general increase. As mentioned previously the reasons for this trend are due to many joining tributaries as you move downstream which increases the discharge and the kinetic energy of the water, which therefore increases the velocity of the river.
Reason for this pattern as explained before is due to the reduction of friction downstream due to the more efficient channel shape and size, which in turn increased the energy of the water thus increasing the velocity readings.
You can see on graph that there is a steady increase in velocity readings between site 1 and 2 but then it begins to level off at site 3. As explained before this is because of water flowing underneath the river bed of those velocity I was unable to measure therefore making the river’s overall velocity lowered than expected.
- How does channel shape and roughness change downstream?
As seen on graph you can see channel shape dramatically increases downstream in width and depth and you can also see that the cross sectional area increases rapidly downstream. Also if you look back at graph you can see that roughness decreases downstream. This data proved my hypothesis correct.
With the factor channel shape you can see at site 1 how highly undulated the channel shape is. This is due to huge boulders from weathering processes from the edges falling into the river, building up the riverbed into the form of what is in the present day. You can also see with a rating of Manning’s ‘n’ at 0.800 the roughness is rated high.
At site 2, a clear shape of a valley appears with signs of glacial deposits forming anomalies on the graph. The Manning’s ‘n’ rating is 0.065 which describes the channel bed profile as undulating. The cross sectional area and overall discharge is bigger than site 1. This is mainly due to the reduction in friction downstream so the channel shape could widen to support the capacity of water from joining tributaries.
At site 3, there is a clear shape of a meander bend, which shows the largest cross section of the three sites. This is because site 3 is near the mouth and therefore has the largest drainage basin and as a result receives the largest discharge of water. The meander as previously mentioned is caused by helicoidally flow. The Manning’s ‘n’ rating is lowest at 0.025 which represents a uniform bed profile. It is not a completely smooth bed because of deposits from the farmers previous stonewall and old glacial materials.
- Does channel shape and roughness effect velocity?
Looking back at graph you can see there is a positive correlation between C.S.A and velocity and a negative correlation between roughness and velocity, which means that as channel shape increases in size and becomes smoother downstream, velocity increases. This data has proved my hypothesis correct.
The reason why velocity increases is because there is less friction around the wetted perimeter and the area where there is minimal friction and maximum energy in the river itself becomes larger. Also as you move downstream there is a greater quantity of water, which increases the effect of erosion of the riverbed, which smoothens it, increasing velocity. Spearman’s rank on both occasions doesn’t support that there is a relationship between the C.S.A or roughness with velocity therefore supporting the null hypothesis however spearman’s rank does support the negative correlation between velocity and roughness and positive correlation between velocity and channel shape.
- Does gradient affect velocity?
From graph you can see generally that as gradient decreases, velocity increases. This shows as a negative correlation. This is not expected and proves my hypothesis incorrect. I would have predicted a decreasing force of gravity from a declining slope would reduce velocity readings. This is not the case because this factor is not powerful enough to influence the velocity readings as more dominant factors such as a reduction in friction and an increase in C.S.A overrules the gradient factor. The reason why this occurs is because the more gentle the slope of the channel; the smoother it is which inevitably increases velocity.
You can see at site 1 on graph that velocity is at its lowest with the high slope angle of 4.3o. This is because the channel is rougher and steeper near the source.
There is an anomaly at site 2, with an increase of a slope angle to 4.5o and I would assume that is an error from the method. This was probably caused by group 3’s higher than usual reading of 6.0o, which inevitably averaged the overall result to be higher than expected.
Site 3 follows the trend with the lowest slope angle of 4.0o and the highest velocity reading of 0.457m/s. Statistical data – spearman’s rank states that there is no relationship between the two factors proving the null hypothesis but it does support the stated negative correlation with a value of - 0.24.
- Does gradient affect channel shape or roughness?
From graphs & you can see that there is no correlation between these two factors. All the results are random and have no structure or identifiable pattern. This is because none of these factors rely or have a contribution towards each other’s results. It is also proven statistically by spearman’s rank that there is no relationship between the two variables.
My main question was:
“Is there a correlation between velocity, channel shape, gradient?”
As answered in question 3, you can see that there is a positive correlation with channel shape and velocity, as the factors depend upon each other. Velocity depends upon channel shape to be able to increase as you move downstream as more water equals more energy. But without the energy from water velocity the channel shape of the river wouldn’t be able to hold the capacity of water as it increases downstream.
Also as answered in question 4, there is a negative correlation with velocity and gradient, but theses two factors do not necessarily depend on each other, as I would have expected velocity to decrease as gradient did. We now know that other factors i.e. a decrease in friction have a greater affect on velocity so the fact that there is a gentler slope does not mean that this factor was the reason for the increase in velocity.
There is no correlation between both variables that represent channel shape (C.S.A and roughness) and gradient. This is because none of these factors rely or affect the others shape or results.
If you look at all the factors in this question as a whole as shown on graph you can see that there are correlations of a positive view between C.S.A and velocity, negative correlations between roughness and gradient against velocity and no correlation between channel shape and gradient.
EVALUATION
This project’s data is far from perfect as I am not a professional geologist who knows how to use the equipment to a perfectly accurate standard. Most of the limitations of this project are mentioned in the methodology apart from the general strengths and weaknesses which is stated in the table below: