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Does the river Alyn follow Bradshaw's model?

Extracts from this document...

Introduction

Does the river Alyn follow Bradshaw's model? Introduction What is Bradshaw's model? The hydraulic geometry of rivers varies throughout their course. Bradshaw's model is a general model for how different river variables change downstream. Below is a diagram showing Bradshaw's model. Aim of my investigation The aim of my investigation is to see how the river variables change on the river Alyn and how this compares to Bradshaw's model in terms of 2 key questions: * How the velocity of the river Alyn changes along the profile? * How the Discharge of the river Alyn changes along the profile? Hypothesis Following Bradshaw's model I would expect both the river velocity and discharge to increase the further you travel downstream. Theory Velocity is the speed that the water is moving. It is measured in metres per second. The theory behind velocity increasing downstream is that it depends on 3 factors: Channel Gradient, Channel Roughness and Channel Shape. * Channel Gradient - Due to gravity the greater the channel slope the greater the velocity if other factors remain constant. * Channel Roughness - The rougher the channel is (how many rocks and boulders are found in the river channel) the more obstacles there are opposing the waters flow leading to increased friction and decreased velocity. The channel is less rough the further you go downstream and so the river flows faster. * Channel Shape - This is described by hydraulic radius. This is the ratio between the cross sectional area of a river and its wetted perimeter. For example if two rivers have an equal cross sectional area the river with the greater wetted perimeter will have more friction and so the velocity will be less. River A River B Both river A and B have a cross sectional area of 60 m�. However river A has a wetted perimeter of 62 metres where as river B has only has 22 metres of wetted perimeter. ...read more.

Middle

25 1.39 10 0.33 15 5 25 0.74 15 0.72 13 2 4 =294 Cross-sectional Area Rank Velocity Rank D D 2.42 4 0.38 12 8 64 1.34 11 1.1 1 10 100 1 14 0.9 4 10 100 1.94 7 0.57 10.5 3.5 12.25 2.58 2 0.58 9 7 49 2.44 3 0.27 14 11 121 3.19 1 0.65 7 6 36 2.29 5 0.57 10.5 5.5 30.25 1.16 12 0.51 11 1 1 0.50 16 0.34 13 3 9 1.69 9 0.6 8 1 1 1.08 13 0.91 3 10 100 2 6 0.83 6 0 0 1.85 8 0.88 5 3 9 1.39 10 0.24 15 5 25 0.74 15 0.97 2 13 169 = 826.5 Hydraulic Radius Rank Discharge Rank D D 0.214 7 0.92 10 3 9 0.11 12 1.49 5 7 49 0.16 10 0.9 11 1 1 0.283 3 1.1 7 4 16 0.231 5 1.5 4 1 1 0.238 4 0.66 14 10 100 0.355 1 2.07 1 0 0 0.151 11 1.3 6 5 25 0.019 14 0.83 12 2 4 0.07 15.5 0.17 16 0.5 0.25 0.104 13 1.02 8 5 25 0.169 9 0.99 9 0 0 0.225 6 1.66 2 4 16 0.284 2 1.63 3 1 1 0.17 8 0.33 15 7 49 0.07 16.5 0.72 13 3.5 12.25 = 308.5 Hydraulic Radius Rank Velocity Rank D D 0.214 7 0.38 12 5 25 0.11 12 1.1 1 11 121 0.16 10 0.9 4 6 36 0.283 3 0.57 10.5 7.5 56.25 0.231 5 0.58 9 4 16 0.238 4 0.27 14 10 100 0.355 1 0.65 7 6 36 0.151 11 0.57 10.5 0.5 0.25 0.019 14 0.51 11 3 9 0.07 15.5 0.34 13 2.5 6.25 0.104 13 0.6 8 5 25 0.169 9 0.91 3 6 36 0.225 6 0.83 6 0 0 0.284 2 0.88 5 3 9 0.17 8 0.24 15 7 49 0.07 16.5 0.97 2 14.5 210.25 =735 Velocity Rank Discharge Rank D D ...read more.

Conclusion

When calculating the cross-sectional area the measurements we took were very inaccurate. Again a number of factors contributed to this: firstly some measurements were only taken to the nearest 10 cm whilst some were taken to the nearest centimetre or millimetre. Although the odd centimetre may not seem that important it can have a large affect when it is multiplied to find the cross-sectional area and then multiplied again to find the discharge. There were also potential errors with the equipment we used. It is possible that sometimes there might have been some slack in the tape measure giving us an overestimate of the actual widths. Also, the measurements for width may not have been in a straight line perpendicular to the riverbank. If a slightly diagonal route was taken when measuring to the opposite bank this would increase the distance measured. When measuring depth we only took 3 measurements to try and gain an average for the whole of the channel. Although this is better than taking just one measurement it is still gives quite an inaccurate reflection of the actual depth. Other problems with my investigation include the fact that I only tested the area at one time of year. At different times during the year the discharge of the river changes, but we can't tell if the whole rivers discharge would rise or fall by the same proportion. For example in the summer the part of the river nearest the source might lose 10% of its water but further down the river it might lose that 10% and a further 10% due to other tributaries drying up, This means the discharge would change by different amounts at different parts of the river. Although the investigation would have been more accurate if carried out over a larger section of river I did not have suitable resources or equipment for measuring on a large scale. It would have taken a considerable length of time to record data for a large enough sample. ...read more.

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