At this stage in the upper course the river channel is narrow, shallow and v-shaped,
and is often lined with large angular boulders. It is shallow at this stage as there is not a lot of erosion present to dig away at the channel bed yet. The gradient of the river may be quite steep waterfalls and rapids may become of this. The velocity of the river is high at waterfalls and rapids but may be low in other areas where there are rocky beds in the river. This would slow the velocity down, as much energy would be used to overcome the friction of the rocks. At this time the river would be clear and fresh as it has not had time to grind down the boulders into fine sand and silt-sized particles by abrasion and attrition. The human usage of the river in the upper courses would be mainly irrigation for grazing and crop fields.
The vertical erosion in the upper course creates a v-shaped valley. Which is steep-sided and narrow. As the river continues to erode downward to form a v-shaped valley, soil and loose rock on the valley sides are moved downhill by slope wash or soil creep. The river also winds its way around interlocking spurs of hard rock. This is not a meander.
In the upper courses the wetted perimeter is small and v-shaped. This changes when the river goes down stream. It gets bigger and a higher gradient bank is formed.
Middle Course
As the river flows downstream to the middle course, the gradient of the riverbanks are less steep and the river is not as high above its base level. The river will still carry on to erode vertically, but lateral erosion becomes present. This changes the shape of the riverbeds to make a more of a u-shape.
Another big factor, which is changed from the upper course, is that in the middle course there are meanders. The river begins to meander in order to surplus energy. Some of the rivers energy is used in transporting the eroded material down stream by four different ways.
- These are, Traction- large boulders roll along the riverbed.
- Saltation- smaller pebbles bounce along the riverbed. They are picked up and then dropped as the river flow changes.
- Suspension- where the finer materials in the waters load such as sand and slit are carried in the rivers flow. This then gives the river a brown murky appearance.
- Solution- is where minerals are dissolved from the rocks in the river. Such rocks as limestone and chalk.
Lower Course
In the Lower course of the river the channel becomes wider and deeper. At this time the velocity is greater than in the upper course. This is because the channel is more efficient with less friction. The channel shape is semi-circular and much smoother because of the deposits of sand and mud. As mentioned before, in the lower course the river has solution and suspension. the Deposition of sand and silt is called alluvium. Alluvium is found is great thickness, especially in flood plains. This leads to the formation of levees. Deposition is the cause of many features:
A big change we find in the Lower course is ox-bow lakes. This is where the meander bends become larger and more vigorous as the river flows down stream. The continued erosion on the outer bends and deposition on the inside bends will eventually lead to the formation of Ox-bow Lake. The neck of the meander narrows as erosion continues on the outside bends. Over time of this erosion the neck is broken. This creates a straight channel. Eventually the Ox-bow Lake dries up, forming a meander scar.
A flood plain is the wide, flat area either side of the river in the lower courses. A flood plain is formed by deposition and erosion. The erosion on the slip off slopes provides sediment to build up the valley floor. This added in floods to the surrounding lands, which are flood plains. As the water from the river floods in to the flood plain there is more friction created. The water is shallow and the river’s velocity falls. The load then precipitates out the surrounding flood plain as alluvium.
Methodology:
The aim of our course was to note the change from the source of the river to the mouth. Completing many tests in the three stages, the Upper course, Middle course and the Lower course will do this. At each stage we did fifteen tests. Which included Sediment sizes, shape of the channel (this is achieved by completing many small tests which include measuring the river width then dividing it by five and seeing the depth at each point enable us to see the shape of the riverbed.
Hypothesis:
- There is no relationship between wetted perimeter, and velocity.
- The river gets wider as it nears the river mouth.
- The river gets deeper as it nears the mouth.
- The velocity gets slower as it nears the mouth.
In order to find the width and depth of the river we placed two people each side of the riverbank with a tape measure end on each side. We measured the width across from the very edge of the river cliffs. This measurement was then divided by five and then we measured the depth at these five equal divided points using a ruler stick. Then these five measurements gave us the shape of the channel when plotted as a cross section.
To find the velocity of the river we placed a meter stick above the flow of the river and then squirted red (so that we could see it easily) food colouring then timing the amount of time it took until the first bit of dyed water passed the meter stick. This measurement was then divided by one (as it was measured over one meter). This then gave us the velocity in meters per second (m/s) of the water.
In order to find the sediment we randomly picked five rocks and measured them using a calliper. Then placed the five results on a spreadsheet and then added them all up and divided by five to find our average sediment size.
The land use of the surrounding areas was observed, we took photos and notes. We noticed that there were many factors effecting the natural flow of the river, e.g. in the upper courses the land were used for grazing with heavy livestock which compresses the ground and affects the ability for natural run off. Also the fact there was man made channel banks would affect the rate of natural erosion therefore making the velocity higher.
Also the weather of previous weeks can vary the velocity. If more rainy periods were present it would quicken the rate of erosion. If there were little rain, the effects would be opposite.
Presentation of Results:
The following graphs show us;
- Depth/width wetted perimeter (Pythagoras theorem)
- Velocity
- Sediment size
- Cross section of river
- Spearmans rank.
Data Analysis:
In graph number one (wetted perimeter) we can see that the river changes in width and depth throughout its journey. These changes can be noticed in the graphs above. These are that the further down stream a river travels the wetted perimeter increases. An example of this is at Radipole 7 the wetted perimeter is 573.085cm and at Broadway (river mouth) the wetted perimeter is 1701.65cm. This is just from the Middle Course to the river mouth. At Radipole 7 the shape of the channel is what we would expect at this stage of the river (Middle course). Again at Radipole 8 the river channel is more or less what we can expect of a near lower course section. But at Radipole Lake we have abnormality. This is that one depth doubles in depth than all the other measurements of the section. This can be to several reasons, there could be a man made object penetrating into the ground (example - a bridge or a tower).
By looking at the velocity graph we can see that the velocity at Radipole sites five and six there is abnormality. At these points the velocity almost doubles. This has no linkage with the sediment size. This could be due to the linkage with the depths and the width at this stage.
The chart showing sediment size suggests that as the river flows downstream erosion happens faster, as in the graph it shows a correlation between the site number and the sediment size.
In the cross sections of the river we can see that at each point downstream closer to the river mouth, the river channel gets wider deeper and of a greater wetted perimeter.
Evaluation:
Overall out of all my findings and testing I believe I have carried out a fair test. So in total I accept my Hypothesis. There were many things that could or could not affect my findings these are as follows.
On testing for the velocity of the river, the way in which we stand could interfere the flow of the water. This could be greatly improved by getting an electronic such as a speedometer device so a fair test is completed. The only other factor that can change is the sediment size. To find the average sediment size I randomly picked five sediments that I came across. This may not be a representative sample as the size can vary greatly. Maybe sectioning of an area of the channel and picking a larger number such as 50 sediments to complete a fairer test could improve this.
I think my results are valid. But there are many changeables, such as the amount of precipitation the week before could greatly affect the river in the areas of velocity, width and depth of the river if there is more water running though the river the deeper and wider it will be and therefore faster. But the erosion will not be greatly affected this is a change which happens over greater periods of time.