New Analysis

Efficiency is how well a river can carry its water and its load, is worked out by dividing the rivers wetted perimeter by the cross-sectional area (CSA). The larger the wetted perimeter and the more semi-circular the CSA is, the more efficient the river will be. The efficiency of the river is determined by its shape, if a river’s bed and banks are uneven it has edged shapes, has eroded parts to it or the CSA or wetted perimeter is small or un-semi-circular the water will be more shallow and, therefore, it will have a poor efficiency. There are many things that can change the cross-sectional area or the river as it flows downstream. Erosion occurs when the rivers load grinds the river bed down vertically, and grinds the sides. This affects the river largely as it deepens it by vertical erosion and makes it wider by lateral erosion towards the mouth. There is also erosion that happens when small materials such as small stones collide with one another in the water and break into smaller particles and they then become smooth and rounded, this erosion is called attrition. And finally, erosion that affects the CSA of the river downstream is hydraulic action when the water is moving at a high speed and hits into weak rock at the side of the channel causing it to break up. When talking about the CSA of a river, there is a link with discharge as the higher the discharge the greater the erosion. In my results it is clearly seen that the discharge increases from site to site and downstream. As we followed the river downstream it became clear that the water was moving at a higher velocity. This meaning a stronger hydraulic action is created causing the higher amount of discharge to erode the banks and river bed. However the majority of the erosion that occurred was lateral as the river became a lot wider than it did downstream. With the rivers discharge eroding the bed and banks the river is made more efficient and was able to carry more water and load. Therefore the river became a lot faster therefore more hydraulic action causing more erosion. Also meandering occurred at site 4 caused by the discharge occurred from the river crashing on the banks. Also, there are other factors that reduce the wetted perimeter of a river, the hydraulic action causing the river bed to become more smooth and causing it to become more semi-circular therefore in effect making it more efficient. The wetted perimeter was also affected by the load amount, at the first site there was a lot of load, clogging up the river, however, as we moved further downstream this became less of a problem as there was less vegetation and making it more efficient. The river Tillingbourne’s efficiency ratio from source to mouth became greater, with the width, depth and wetted perimeter all multiplying. This fits in with Bradshaw’s idea that all rivers should become more efficient as they go downstream. From my data it is clear that the river Tillingbourne fits in with Bradshaw’s idea of a perfect river, as the CSA increased at every site, the velocity mostly increased at most sites and the wetted perimeter increased at most sites, therefore, making the river more efficient and also allowing the river to be able to carry more water and load

As you go down stream, the friction became less and therefore made the river Tillingbourne more efficient

The Load size gets smaller as you go down stream, you can tell this as on my data table 2 as there is a lot more stones in the first to sites, and stones were the largest load size as they are larger than pebbles, gravel and sand. The recordings I took down for the load size on site 4 show that the amount of stones within the load size was 0.5% when in site 1 it was 22.5% There was less friction as the river Tillingbourne went downstream as there was less load and therefore not causing the river to slow.

Conclusion As the river moves from source to mouth, the river becomes more efficient. The reason being there was a decrease in load size and the Cross-sectional Area become more semi-circular. With the river more semi circular and the decrease in load size there is less friction, meaning the water flows more freely and therefore making the river more efficient. Overall Evaluation Timing- as we went during autumn, the river was neither high, because of rainfall, nor low because of low rainfall so it is hard to tell if our results will be accurate all year round. This also links with seasons. Although we went at autumn we cannot be sure that the night before there was a heavy downpour of rain. Also I do not feel we had much time when it came to the analyzing itself and with more time I thing we could have more accurate results. Accuracy- I think overall, the data my group collected was quite accurate considering our equipment was not perfect and our timing was not as good as it could have been and we did not spend very long analyzing the river. Also we are not very experienced at this sort of thing and we did not spend very long learning how to do it. Validity – The data that was collected gives a clear picture of the river. It also fits Bradshaw’s model. Reliability – I feel that our results could prove to be quite helpful to the local council as despite our results not being a 100% accurate they give a good indication of how the river develops downstream. I feel with more time and better equipment I could get a lot more accurate results.