Does the river Alyn follow Bradshaw's model?

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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 m². However river A has a wetted perimeter of 62 metres where as river B has only has 22 metres of wetted perimeter. In river A the ratio between cross sectional area and wetted perimeter is very low which means the river flows slowly. In river B the ratio is much greater which means the river flows faster. In general the larger the cross sectional area in comparison to the wetted perimeter the faster the river flows. This ratio is usually greater the further you go downstream and so the river flows faster.

Discharge is the volume of water flowing in a river at a given point and is measured in cubic metres per second (cumecs). Discharge increases downstream due to a number of factors. Firstly there are more streams and tributaries entering the river the further you go along a river. Velocity also causes discharge to increase downstream. The faster the velocity the greater the amount of water flowing past a point in a given time span. Another factor that causes discharge to increase downstream is an increase in surface run-off. In the lower part of the profile there are normally more buildings and less vegetation than in the upper part. This means there is less interception from trees and more impermeable surfaces the further you go downstream. Consequently there is a greater surface run-off and a greater amount of water entering the river causing a greater discharge. Seasonal variations in rainfall and temperature mean that varying amounts of water enter and leave the river resulting in a varying discharge. Discharge decreases as more water is taken from the river for domestic purposes. Also water used for irrigation of farmland can lead to a decrease in discharge.

The importance of Hydraulic Geometry

Being able to measure the hydraulic geometry, particularly discharge and velocity in a river is important. Knowing how river variables change can be useful in predicting how the river will react to heavy rainfall which can help with flood prediction and protection. Being able to measure the velocity and discharge at a point on a river is important when building dams so that your dam can provide the optimum amount of hydroelectric power. It is also important that you know the geometry of the river in built up areas so that you can predict how the rivers course, size, or flow might change and take action.

Location

The river on which I am investigating velocity and discharge is the river Alyn. It is found in North Wales approximately 25 miles southwest of Liverpool. Its source is found in Llanferres in the Llanddegla Moors. The particular site I will be working at will be Loggerheads, which is found roughly 2 miles west of a residential area called Mold.

Methodology – Sampling

Sampling is where you select and test part of an area as opposed to sampling the whole thing. Sampling is carried out to test ideas and see if theories are correct or to find out information for the first time so that theories can me made. Sampling is obviously less accurate than testing a whole population but it saves a lot of time and can still help to show general trends and relationships. The larger you can make the sample the more accurately it is likely to reflect the whole population. When sampling, it is important to try not to introduce any bias as this will give misleading results. There are a number of different types of sampling including random, stratified and systematic. We carried out the systematic method of sampling where samples are taken at agreed intervals. In this case we tested the river every 30 metres for 450 metres. This is an unbiased method of sampling but it is possible that we may have introduced bias without knowing it. For example it may have been that by coincidence every 30 metres the river changed completely randomly which would cause biased results. The method used to pick the 4 points on the river where we would sample was biased as these areas were picked to try and show certain trends in pollution levels. The areas I tested, Loggerheads, wasn’t chosen at random which means that there may be some bias in my results.

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Below are some photographs showing the methods we used when sampling.

Data Analysis

Having gathered the data, I need to analyse it in a way that will answer the key questions: ‘Does discharge increase downstream?’ and ‘Does velocity increase downstream?’ Therefore my first graphs plot velocity and discharge against the distance along the profile of the river. If Bradshaw’s model is correct these graphs should show a positive correlation. However, the results of both graphs were completely random and showed no correlation.

For example the graph of discharge against profile had the least ...

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