Aim:
In this investigation the aim was to compare the characteristics of a river that we visited (Ashes Hollow) to the Bradshaw Model. This is a good river to test certain theories and other predictions, as the river is easily measured and fits into the upper and middle course of a river which is helpful to me. It is also relatively near to me so is very convenient.
Hypothesis:
My hypothesis is looking for a relationship between two features. They were: Velocity increases further down the stream, as the rivers gradient decreases, the rivers width increases and as velocity increases, so does bed-load size.
Location:
Ashes Hollow stream is located in Shropshire, which is near Church Stetton. The valley where the river is is a V-shaped valley. This valley gets steeper the further up the river. There are a few minor waterfalls located in the upper and middle course, and there is a large amount of vegetation all along the river with decreasing amounts nearer the top.
To investigate the river I chose 8 sights along the river. I selected these sights by working out how far from the bottom of the river it was to a point near the top and then dividing the number by 8. Working this out gave me 8 sites that were exactly the same length apart.
Methods:
Width – To measure the rivers width I used a tape measure. I did this by placing the start end on one side of the river bank at water level, and then I moved the measure along to record the width. When taking this measurement I kept in mind that the moving water of the stream so that it didn’t differ the actual measurement by pulling the tape. I took down the results in centimetres and then repeated this at each of my eight sights.
Gradient – To measure the gradient of the river I used a theodolite. Using one other person who was roughly the same height as me, I placed them 5 metres down the river and looked through the theodolite at the person’s eyes and took down the gradient by pressing the trigger. I did this at each of my eight sights.
Velocity – To measure the velocity of the river I dropped an orange into the river and timed how long it would take to travel 5 metres downstream. I took this measurement at the same place I measured the gradient. I dropped the orange half a metre from the start of the 5 metres to let it catch speed, and got a person to stop it when it reached the 5 metres. I recorded the time with a stop watch, And repeated this 3 times and divided them by 5 to get the velocity. I then added the 3 readings together and divided by 3 to get the average velocity. I did this at each of my eight sites.
Wetted Perimeter – The wetted perimeter is the width of the river including all of the bed load and material. I took this measurement at the same place I measured the width at each site so width and wetted perimeter could be compared and to improve accuracy. I took the measurement by placing the tape measure at one river bed and ran the measure over all the of the river bed. I repeated this at each of my eight sites.
Depth – to measure the depth I used a ruler metre since the river was never more than a metre deep, and recorded the measurement in centimetres. I took this measurement at the same place I took the width to improve the reliability and accuracy. I divided the width of the river by 10 then measured the depth with the ruler at 10 points which where all the same distance from each other. When measuring I took into account any rocks that were sticking out of the river or where close enough to the surface and recorded those measurements as anomalies. When I had the 10 depths I added them up and divided by 10 to work out the average depth and did this for each of my eight sites.
Bed load size – To measure the bed load size I used the ‘two handed grab method’. This method involves putting my hands into the river and picked out 10 pebbles. If I didn’t pick up 10 the first time I would do it again until I did. The problem with this method is that I couldn’t pick up the smaller rocks, or the bigger rocks with just my hands. When I had 10 rocks I would lay them out in order of size, and then using the ‘ninth sampling’ I picked out the second largest rock or the ninth smallest. I measured the longest length of the rock and then compared it to a chart to see what category it came in (for example semi rounded).
Graph Analysis
*FIND PICTURE OF BRADSHAW MODEL*
Conclusion
Looking back at my hypothesis, I can now see if they are correct or wrong. For one of my hypothesis I said that as the gradient of the river decreases, the width of the river increases. According to my graphs, as the mean gradient decreases, the mean width increases. This is a general observation though and does not completely answer my hypothesis. Because of this I decided to carry out the Spearman’s Rank on these results.
Spearman’s Rank for River Gradient and Width
The result of 0.99 shows that the relationship is as my hypothesis said, as the river gradient decreases, the river width increases since the Spearman’s Rank shows an almost perfect negative correlation between the two variables. Although there are a few results that seem to be outstanding, I did not remove them as anomalies as the width is the average of 10 measurements so is reasonably reliable. And the river gradient would have had some differing results as there were many small waterfalls and drops within the rivers channel that we measured. This ensured me that I didn’t need to remove any of the results as anomalies and the result of the Spearman’s Rank proved my hypothesis correct.
Looking at my other hypothesis, the velocity increases the further down the stream, the graph shows that it is correct. Since my sites followed the stream from the higher part of the course going downwards, site 1 moves downstream to site 8. The results I gathered from the trip show that the velocity does increase the further down the stream. There isn’t a perfect correlation in the graph, but this could be because of differing gradients and other variables affecting the velocity. For instance in site 4, there could have been a steeper gradient and faster velocity because the stream was leading up to a small waterfall or drop in the gradient of the riv
