Interpretation of why average velocity changes with distance downstream

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Interpretation of why average velocity changes with distance downstream

Hypothesis - I predict that average velocity will increase with distance downstream

                   

In order to prove my hypothesis I will use the above graphs and my Spearmans rank result for average velocity, which was +0.63.        

        As velocity is the speed of the water passing a point in one second I will need to firstly look at the way in which the water can speed up. The main factors that will affect this are the cross sectional and the wetted perimeter. As we go further down stream both the width and depth increase, and therefore the cross sectional area will also increase, (as the cross sectional area is calculated by multiplying the width by the depth) and if the cross sectional area increases the wetted perimeter will also increase. As there is a larger wetted perimeter there will be a larger pocket of fast moving in the centre of the river that is not experiencing friction from the bed and banks of the river.

Therefore as the rivers wetted perimeter increases with distance downstream the average velocity should also increase with distance downstream.        

        Another factor would be the angle of the riverbed. If I was to ask you would a ball roll faster down a steep slope or down a gentle slope you would you say a steep slope, and yes you would be right. It works exactly the same way for a river; the steeper the bed the faster the water will move over it. Also the smoother the river bed is the less friction there will be and therefore the water will more over this bed faster (the river bed becomes smoother due to erosion.)        

        Another reason for the change in velocity is the landuse. Toward the source of the river Roe there is the Glenshane forest and the land is boggy or peaty, while the mouth and lower section has been drained by the surrounding towns or urban areas. The big difference in the two is overland flow and interception. As the source has trees to intercept and slow down the speed in which water reaches the river and the peat to act like a sponge and hold the water this also cuts out and slows the water reaching the river via overland flow. Therefore there will be a lower level of water entering the river at that point. The lower section however has no tree to intercept the water and as there are impervious surfaces such as tarmac and concrete. The water which falls will then all run as overland flow into a drainage system which will take it to the river much faster than it would get there naturally. Thus causing a large amount of water entering the river at that point        

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We can see clearly from the graphs that the velocity increases with distance downstream but there are a few anomalies, which were stops 2+4 (velocity graph).

        The reason behind stop 2 is quite simple, the velocity increase is caused by the joining of a tributary(the            ) just upstream of are second stop. This excess water creates a larger cross sectional area and thus a larger pocket of fast moving water. It would also cause increased erosion of the riverbed. Stop 4 has increased velocity due to human interference in that dredging has occurred at ...

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