Geography - Ivestigation of the River Colne, Buckinghamshire

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25 Pages

Introduction Pages 3-6

Hypotheses (at top of page): Page 3

Maps of Location of River: Page 4

3D map of river: Page 6

Methodology: Pages 6-8

Data Presentation: Page 8-19

Raw Data Table: Pages 8-9

Photos: Pages 10-14

Volume of flow: Proportional Line map: Page 15

Width, Depth, Velocity and Volume of Flow Graphs: Pages 16-17

Vegetation and Height Above Sea Level Graphs/Charts: Page 18

Differences in Height Above Sea Level Graph: Page 19

Data Analysis: Page 19-22

Hypothesis 1: Pages 19-20

Hypothesis 2: Pages 20-21

Hypothesis 3: Pages 21-22

Conclusion: Page 22

Evaluation: Pages 22-25

Final Conclusion: Page 25

Bibliography: Page 25

I formulated my hypotheses based on my current knowledge:

* I expect the river to get wider and deeper as it flows downstream because it will be joined by tributaries, and other water e.g. from rainfall, so the volume of water in the river will increase, therefore the width and depth of the river must increase to accommodate this extra water. The extra water will also entail more hydraulic action, so the banks of the river will be eroded more, and more water also means more sediment. This sediment will also erode the banks more; rocks carried by saltation or traction will erode the bed, making the river deeper, and alluvium in suspension will erode the bank. In the upper course of most rivers, due to the bedload, there is usually more vertical erosion than lateral (as the river tries to reach its base level), so the river is deep, but narrow, with steep banks. In the middle and lower course, there is progressively less vertical erosion and more lateral erosion, so the river should get a lot wider and a bit deeper as it flows downstream.

Human intervention could change the shape of the river, and indeed the nature of the water itself, and plants and fishes can build up the riverbed with decaying material, or deepen it by foraging for food or growing roots. Finally, the banks themselves are likely to be eroded more easily, because they are less likely to be made of rock, as they are in the upper course, and more likely to be made of soil or formerly deposited sediment, which are easier to erode. It is by this common sense and geographic theory that I know that rivers widen and deepen as they flow downstream.

* I expect the rate of flow will increase because there are fewer obstructions in the river channel. Whereas in the Upper Course of most rivers, the riverbed is uneven and contains many loose stones, e.g. Figure 1, whereas in the Middle and Lower Course of the river, most of this bedload will have been broken down by attrition, abrasion and corrosion, and it is now sediment in suspension or solution, e.g. Figure 2. Therefore, the river will be losing less energy through friction between the bedload and the riverbed, so it will have more kinetic energy and will flow faster. Conversely, because the river is no longer in mountains, the gradient is shallower, which may mean that the river flows slower. On the whole, however, even the steep incline in a river's Upper Course will not compensate for the energy loss through friction and erosion (both erosion of the banks and of the bedload). Furthermore, for the section of river I will be studying, there will be no mountains (I have included a 3D map of the area just to prove this [The River Colne is highlighted in pink]), so the gradient should decrease slightly over the length of my study as it normally would in the middle course of a river.

* I expect the gradient of the river will get shallower because the land itself will get flatter, as the river is no longer near its source in hills or mountains. Also, as the river flows through its Middle Course, it will begin to meander, partly as a result of the decreased gradient in the first place, which means that, in every kilometre the river flows, it will only drop a fraction of the height it would drop if it was flowing in a straight line. Furthermore, as the river flows into its Lower Course, I would expect it to develop a floodplain, so the gradient is naturally flatter than it was upstream

In this case, the Colne doesn't actually have a floodplain as such. Instead, a succession of man-made gravel pits, now lakes, and a canal run alongside it; so the land is naturally flatter anyway. In addition, I have procured a 3D map of the area (next page), so I know that the immediate surroundings of the river are reasonably flat, and that the river's gradient decreases gradually over the 10km I have chosen to study.

Methodology:

Unfortunately (in retrospection), I did not do a pilot study or investigate the River Colne before I did my main data collection day. If I had, I would have changed my preference or rivers and studied another, as the Colne turned out to be too wide and too deep to measure whilst standing in it, leaving me no choice but to measure from bridges, which was no problem in practise because there are many bridges on the river, although it may have affected my readings. However, I did plan where I would collect my data using Memory Map, an Ordnance Survey application, and I further prepared myself by printing off a small scale local map to help me navigate the river Colne, its byways and its access routes on the day I collected my data. Furthermore, because I was unable to borrow a hydro-prop from school, I had to design and make a velocity measuring device myself from a plastic container with a length of string, knotted at 5cm intervals, through its lid. Then I experimented with varying amounts of weight (I used 1 penny coins as weights) in it to make it float in the water rather than on it.

On the 26th of August when I arrived at the river with an assistant, who helped me take readings, I was surprised by its width and depth, as I had not noticed the width while I was planning, nor had I remembered the width from previous visits or made a pilot study to check. Unperturbed, I decided I would study the Colne anyway, at ten sites roughly a kilometre apart so as to gain a fair sense of how the river changed as it flowed downstream. However, I needed to make sure each site had a bridge so that I could take readings there, as the river was too deep to stand in. A good way to check, I found, was to pick sites, looking at the map I had printed out, where roads, tracks or paths crossed the river, whilst still keeping the prospective sites as close to a kilometre apart as possible. Thus, I decided upon my ten sites, and marked them on the printout. I have since marked them on the OS application, as below. Then I proceeded to measure the river where I was, at my first site.

First of all, I measured the width. Unfortunately, unprepared as I was, my tape measure was only 3 metres long. So I measured 3 metres horizontally along the sides of the bridge, from vertically above the bank. When I reached the end of the tape measure, I marked the endpoint, pivoted about it, and then measured another 3 metres. I repeated this process across the bridge to get the entire width in metres. Then I had to measure depth. I thought of attaching a weight, a stone, to the bottom of the tape measure, but then I thought that I could take all the weights out of my velocity measurer and fill it with stones, then measure how far up the string the river came, and hence find the depth. So I dropped it into the river, making sure the string was as taut and vertical as possible, and noted the knot it was at. I counted the knots to the bottom as I pulled it out, and then used the tape measure to find the depth in metres. I repeated this process at three more places equidistant from each other, and then used the tape measure from the bank to get readings for the depth at each bank, giving me six depth readings an equal distance apart. Unfortunately, the current of the river made the string belly a little downstream, so my readings were not, perhaps, as accurate as they could have been. It was important for me to attain measurements of the river's width and depth in order to work out the river's cross-sectional area, and the
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Next, I measured the river's velocity. I took the stones out of the depth measuring device, and put the weights back in and dropped the device into the water at the near bank. I made a mark on the bank level with where it started and timed a minute. Then I made a mark on the bank level with where it was when the minute was up, and measured the distance between the two marks. Then I repeated the procedure at the other bank, and then at the four sites in the middle of the river where I ...

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