Analysis

Analysis Efficiency is how well a river can carry water and load. As shown earlier, it is worked out by dividing the wetted perimeter by the cross sectional area. The closer the CSA is to a semi-circular shape, the more efficient it will be. This goes in relation with the wetted perimeter, as the larger it is, the more efficient the river is. These two factors make the river better at carrying water and load. Rivers with more crooked, edged shapes and smaller wetted perimeters, in relation to the CSA, will not be as good at carrying load and water because it will be shallower, and therefore less water and load can get through. There are many factors which change the cross sectional area of the river as it goes downstream. Erosion is one and this can be broken down into smaller sections. Corrosion occurs where the river uses its load to grind against the bed and sides. This erosion deepens the river by vertical erosion and widens it by lateral erosion. Attrition occurs when material in the water collide with one another. They then break and become smaller particles. These particles become smoother and rounded. Also, there is corrosion, where the solvent action of water dissolves soluble materials and carries them away in solution. Lastly, when talking about erosion affecting the CSA of the river downstream, hydraulic action where water travelling at a high speed enters the line of weakness of rock when it hits against these rocks at the side of the channel. This causes the rock to break. Discharge is directly linked to erosion, when talking about the CSA. This is because; the higher the discharge is (WxDxV) the more erosion there will be. As the river goes downstream, we can clearly see in my results that the discharge increases at each site. Higher velocity (creating stronger hydraulic action) and high discharge created more erosion downstream, the majority of the erosion being lateral because the gradient of the river became, generally, less deep from source to mouth. This lateral erosion meant that the river became wider downstream and therefore it was more efficient, transporting more water and load. Greater velocity created more energy, meaning that there would be more erosion from the hydraulic action. At site 4 there is a lot of meandering which is caused by the eroding of the banks by the load smashing into the bank. There are also many factors, which reduce the wetted perimeter in relation to the CSA. The erosion that takes place, which is explained above, smoothes out the riverbed, causing it to decrease and become more like a semi circle. The wetted perimeter was also affected by the vegetation and load amount. At the first site the load amount was very high, and there was also a lot of plants, which affected the WP. This became less of a problem downstream as the load amount decreased and there was less vegetation around the sites, therefore becoming more efficient. The efficiency ratio became greater from source to mouth, multiplying the width, depth and wetted perimeter. This clearly shows that the river became more efficient downstream, which fits perfectly with Bradshaw’s model of a perfect river.

From the data that I collected, we can clearly see that the river Tillingbourne fits Bradshaw’s model of Perfect River. To show this I can use the example of the CSA increasing at every site. This connects to velocity, which makes up the discharge when multiplied together. The higher the discharge the more erosion there was meaning that the wetted perimeter would be smoother and therefore more efficient, allowing more water and load to be carried through. The river Tillingbourne does fit Bradshaw’s model. The efficiency of the river became greater from source to mouth.