Chapter 2: Method.
During our day out in the Brecon Beacons (11th July, 2006), the class split up into groups and went off to measure two different rivers. The first river my group and I measured was the Afon Tawe in Craig-y-nos, and the second was Cwm Haffes. This is a picture of the Afon Tawe. The Cwm Haffes is a tributary to the Afon Tawe.
At the Afon Tawe, the first thing my group and I did, was to pick a part of the river to measure. To do this we considered the safety and speed of the river. After we had finally chosen a typically safe spot, we got on with the measuring.
Measuring the River.
The four aspects of the that we were measuring were its width, depth, speed and gradient.
Width.
We measured the width using a 30metre tape measure. To do this, one member of the group put a pair of wellies on, took one end of the tape and went off to the opposite bank of the river. The person left holding the other end of the tape measure laid it down on the bed of the river and pulled it tight. I thought this was very reliable because it was easy and simple to read.
Depth.
To measure the depth, we divided the width into four equal sections. We did this so that the depth would be spread out and taken in unbiased locations. We then took a metre ruler and placed it side on to the tape measure. We did this side on so that the water from the river does not rise, if it rises it could affect the accuracy of the result. We then ensured that the bottom of the ruler was flat on the bottom of the river. Then, we read off the number where the river surface marked the ruler. I think that this is not very reiable because of stones or rocks placed at the bottom of the river. However, is this method reliable for different sized rivers? The reliability for small rivers is higher than the reliability for larger rivers. This is because the gaps in smaller rivers have smaller distances then the gaps in bigger rivers. See diagram below.
Speed Using a Flowmetre.
To measure the speed we used a flowmetre. To measure the speed using a flowmetre, you need to count the number of spins the propeller makes in a minute. For this to happen, we needed to place the flowmetre into the river and wait for the propeller to start turning. When the propeller started to turn, we then started counting the number of turns made whilst someone else timed the minute. I think that this is not as reliable because you could lose track of time. In addition, you need to take more than one measurement on a section of river because the water flows at different speeds at different parts of the river. For example, when measuring a meander. It is fast on one side whilst slow on the other.
Speed Using a Float.
There is a second way to measure the speed. This is to have one member of the group to again take the tape measure 10metres down the riverbank. Then get an item, which will float, drop it at one end of the tape measure and time how long it takes to approach the opposite end of the tape measure. You would then repeat this to get an accurate result. My group and I did not use this technique. However, I assume that if we did use this technique, then we would have found some faults. I assume that these faults would include:
- An object getting in the way,
- If there was a current then it might carry the object we are using in another direction therefore resulting in inaccurate results.
Gradient.
Finally, we measured the gradient. To do this we kept 10m of tape measure along the riverbank, and then used a clinometre. One person from the group went down to the far end of the 10m. Next I put the clinometre to my eye and looked into the other person’s eye. I then waited for the wheel on the clinometre to stop spinning and then read off the number that it stopped on. I think this is not very reliable because you need two persons of the same height at either end of the 10m otherwise the result can be effected.
After we ad measured the first river, my group and I then went to Cwm Haffes and repeated the same techniques. We measured both rivers on the same day so that we could fairly compare our results to allow us to complete our coursework.
