a* full marks gcse geography coursework (rivers)

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An investigation into how physical channel characteristics change throughout the

course of the River Holford

Name: Nishay Patel

Candidate Number: 8331

Centre Number: 12760

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Contents

Introduction P.3Location P.4Exmoor National Park P.5Land use map P.6Land uses and Employment P.7Facts about the River Holford P.7OS map of River Holford P.8The Geology of Somerset P.9Geology and site map P.10Aim and Hypotheses P.11Sequence of work P.12Methodology P.13Risk Assessment P.14Justification of Methods P.15Techniques to measure data P.16-18Equipment P.19Secondary data and Formulae P.20 Site Descriptions P.21-25 Problems and Limitations P.26Data Presentation P.27Depth graphs P.28-33Cross section graphs P.34-37Cross section base map P.38Width graph P.39Gradient graph P.39Formulae Sheet P.41 Data Analysis P.42Spearman’s Rank P.43Hypothesis 1 P.44-47Hypothesis 2 P.48-51Hypothesis 3 P.52-53Conclusions and Evaluation P.54Summary table and explanations P.55-56Limitations P.57Improvements P.58Acknowledgements P.58Bibliography P.58Appendix-Raw Data P.59

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Introduction

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Introduction

In my introduction I will be talking about where in the UK the River Holford is situated and the describing the relief around it.

I chose to investigate this river study over our four day field trip to Somerset in order to find out the different factors that affect the physical characteristics of the River Holford such as its depth and the channel’s width.

Location

Somerset is a county in the South West of England and borders five other counties including Bristol, Gloucestershire, Wiltshire, Dorset and Devon. The Bristol Channel and the estuary of the River Severn are to the north and west of Somerset. Somerset is a rural county and is where many tourists come to visit the rolling hills including the Quantock Hills. It is also home to Exmoor National park which is known for its scenery and rare wildlife such as the red deer.

Local Map of Somerset

This is a national map of where Somerset is in relation to the rest of England.

The Bristol Channel is located in this area here.

This is a local map of Holford, which is a settlement in Somerset where the river Holford runs through and is named after.

This is a confluence River Holford.

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During our four day field trip we stayed in a mansion called Nettlecombe Court which is located in Nettleton Park. It was originally built with the intentions of it being a manor house until it became a boy’s school in 1963. The house itself is in Exmoor National Park and is set in a valley just north of the Brendon Hills.

A photograph of Nettlecombe Court.

Exmoor National ParkSomerset is home to Exmoor National Park which is 693 km2 and is only one of twelve National parks in the United Kingdom. Within Exmoor National Park there is a site of special scientific interest which is 194 km2 large and a 4 km2 national nature reserve. There is a population of 10,873 people in Somerset and a census taken in 2001 showed that it had increased by 2.1% between 1991 and 2001. 25% of Somerset’s population is over sixty-five and this shows that it is a popular retirement resort. There are 1,011 enlisted buildings in Exmoor National Park and 11.5% of the houses are second homes, which mean that they are old and cheaper to buy. With regards to tourism, over 1.4 million visitors go to Somerset each year and this brings in over £35.5 million.

Land uses

The main land uses in Somerset are, farmland, which is mainly pastoral sheep farming. Woodland, where Deciduous and Coniferous trees grow which would have been ideal for fuel and as building materials when early settlers arrived. Settlements, such as Holford and Kilve, and moor land where vegetation such as heather and bracken grows.

Woodland12.2%

Moorland12.2%

Farmland55.9%

Other19.7%

Land uses in Somerset

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Land Use map

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Land use along the course of the River Holford is important to look at because it could affect the characteristics if the channel. As you can see on the land use map there is a high amount deciduous forest near the source meaning that the trees will intercept rain water and prevent it from reaching the river. This tells us that the sites near the forest will have a shallower depth.

As you can see on the land use map on page 6 the River Holford flows through urban areas such as Kilve. You would expect the river’s width and depth to increase because the concrete surfaces in these areas would increase surface runoff and decrease infiltration. Water from people’s drains run into the river and this causes the depth of the river to increase. In Kilve the channel has been altered by man making them more efficient. This tells us that the wetted perimeter of the river will decrease, meaning that river’s velocity will increase. This is due to the fact that there will be less friction between the water and the banks and bed of the river.

Employment in Somerset

Employment figures show that 15% of the people living in Somerset work in the hotel and catering business because of the huge amounts of tourists who go there each year. Farming plays a major role in Somerset because it does not only take up most of the land space but, 13.9% of the population works in agriculture.

Facts about the River Holford

The River Holford’s source is at Lady's Fountain Spring, Frog Combe which is 250 m above sea level, ands mouth is at Kilve. All together it is 7.4 km long. The upper course of the River Holford is very steep and the lower course has hardly any gradient. The middle course is less steep when compared to the upper course. This tells us that there is a decline in gradient from source to mouth.

This is the source of the River Holford at Lady's Fountain Spring, Frog Combe.

This is the mouth of the River Kilve.

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OS map

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The Geology of Somerset

The River Holford is 7.4 km long and has three main types of geology throughout its course which are Devonian Quartzite, Permo-Triassic Marl and Jurassic Limestone.

1. Devonian Quartzite is found in the upper course of the River Holford where the gradient is steep. It is a hard, impermeable red metamorphic rock and means that the depth will be shallow here as water cannot percolate through it. The width will be narrow here seeing as the river requires more energy to erode the rock. Quartzite was once sandstone and this conversion took place after heat and pressure. Its appearance is red because of the amount of iron oxide and other chemical impurities inside it.

2. Permo-Triassic Marl is also found in the upper course of the river and is a soft, impermeable sedimentary rock which is composed mainly of calcium carbonate. This means that the width will be larger here seeing as more erosion takes place in this part of the river. However, the depth here is deep as less water is lost via percolation.

3. Jurassic Limestone is found in the lower course of the River Holford and is a permeable sedimentary rock which is resistant to erosion. Limestone is mainly made from calcium carbonate. Because Jurassic Limestone is a permeable rock more water is lost via percolation meaning that there is less water in the channel. This tells us that the depth of the river will decrease at the sites where the geology is Limestone.

This is an example of Quartzite. This is a sample of Jurrasic Limestone.

This is an example of Permo- Triassic Marl.

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Geology and site map

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Hypotheses

Aim: An investigation into the changing physical characteristics form source to mouth of the River Holford.

Objectives: We will be looking at a number of different factors that change throughout the course of the river. These will be the width of the river channel, the average depth, velocity, volume and the gradient. When these readings are taken there should be correlations between them.

Hypothesis 1- The width of the river will increase as you move from the source to mouth.

The width of a river channel increases as it goes downstream because tributaries and confluences join into it, thus making it wider because it holds more water. In the upper profile of the river you would find mainly vertical erosion which cuts downwards. In the lower course of the river more lateral erosion takes place which means the river cuts sideways into the valley. The river gets more powerful as you go down its course which means that it has more erosion capability. There is also less energy waste on friction as there is less substrate is on the bed. This is why you would expect a wider channel in the lower profile of the river.

Hypothesis 2- The depth of the river will increase as you go from source to mouth.

Depth is how deep the river is. As the River Holford flows along its course, tributaries and confluences join it which means there is more water in the river than at its source. In the higher profile of the river mainly vertical erosion takes place. This is the rapid down cutting of the bed. In the lower course of the river there is lateral erosion which means that the river erodes the valley sideways. However, as the river continues downstream there is less substrate on the bed which means that there less energy waste on friction. This means that there is more energy to erode the bed.

Hypothesis 3- Gradient will decrease as you move from source to mouth.

Gradient is the elevation of the river along its course. This hypothesis has been made because the contour lines of an OS map are closer together at the source than any other point along the rivers course. The map tells us that the source is 250m high and the mouth is not elevated at all. Rivers cannot flow upwards and always flow at a negative gradient due to gravity.

This is Bradshaw’s model and tells us how different characteristics change as you go from upstream to downstream.

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Sequence of work Start

Firstly we established our aims and hypotheses before embarking on this trip.

Preparation

Suitable equipment was selected in order to measure the factors along the river accurately

Practice

We left school on the 16th October. Shortly after reaching our destination, we went to a nearby river and practised measuring using the different equipment.

Fieldwork

Many measurements such as the width, depth and gradient, were taken at eight different points along the River Holford.

Data Collection

The data which was collected during the fieldwork was analysed using graphs.

Write up

The introduction and the methodology were then written about the background of the area and the different sites at which the different measurements were taken.

This is a somatic diagram of how I prepared for and undertook my river study.

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Methodology

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Methodology

In this section I shall be describing the different sites at which we collected our primary data and methods that were used to collect this data. I will also be looking at how the risks were minimized.

Risk Assessment

Risk How it was minimized

There could have been rat faeces in the river which could have caused diseases if it got into the skin.

Marigold gloves were worn at all times when in contact with the river.

We could have slipped on river substrate while taking the different measurements.

Wellington boots were worn while in the river in order to give us more grip and this would minimize the risk of slipping.

Ticks Long sleeve clothing was worn to cover the whole body and therefore decreasing the chances of getting ticks on the body.

Getting diseases from the river Hands were washed thoroughly with soap after returning from the river study.

Uneven terrain Hiking boots were worn in order to minimize any injuries taking place.

Hypothermia Suitable warm clothing such as, woolly hats and gloves were worn when we were outside on the field trip.

Falling into the river and breaking bones. We were working in groups of 6 so that if someone fell in the river, then a teacher could be called by the remaining people.

Getting wet in the river. Waterproof clothing was worn when we were in the river.

Justification of Methods

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There are three types of sampling methods which could have been used on this river study and are a random method, a systematic method and a stratified method. We chose a stratified sampling method for the sites because we had prior knowledge of the area and so knew where the different parts of geology, the confluences and the urban areas were. This sampling method allowed us to find out the impacts after confluences and if there was a difference in other factors such as depth in areas of different geology along the river’s course. If we used a systematic sampling method, then you might overlook key points along the river and go into private land without knowing. A stratified approach was practical method to adopt because we could measure all along the river’s course in such a limited amount of time, even if it was biased because you chose specific areas in which you wanted to sample. A random sampling method was used to measure the clast size at each site so that we were not biased when collecting the data.

We chose to sample eight sites along the River Holford’s course because this would give us a broader range of results. When all the data was collected and put into graphs, this would tell us the change from the source to the mouth of the different factors that we were measuring. If we only sampled at two sites, then we would not find a trend in the data and find it very hard to interpret it.

These are the different factors that we sampled and how many we chose to sample at each site:

1) Gradient - We chose to sample one gradient at every site because the reading would always be the same.

2) Width - We chose to sample one width reading at every site because the reading would always stay the same.

3) Depth - We chose to take five depth readings at each site as one would not be enough to give an average reading.

4) Velocity - 5 readings were taken at each site to see where the fastest flow was along the width of the river.

5) Clast angularity and size - 15 clasts were picked at random at every site in order to get a broader set of results.

6) Wetted perimeter - One reading was taken at each site as the wetted perimeter would always be the same.

Width

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The width was calculated at each of the eight sites along the course of the river Holford. It took the least amount of time to measure so was therefore taken first. We used a measuring tape in meters to measure the width because it is flexible and very easy to use. In order to measure the width, we had to face upstream and measure from left to right. We put the measuring tape right to the edge of the banks and then read out the reading. It only required two people to measure the width and so the other members of the group were writing down the readings in meters.

Measuring the width of the river.

Depth

After the width was measured we found the depth. The depth of the river was calculated using a ridged meter ruler and it was predicted that as we continued downstream, the river would get deeper. We divided the river’s width by 5 and then used this to take 5 separate readings along the width. While measuring the depth we used the narrow edge of the metre ruler to prevent a build up of water and stood after the metre ruler in order not to affect the results.

Wetted Perimeter

Direction of flow

Direction of flow

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The third reading we took was the wetted perimeter. The wetted perimeter was measured using a measuring tape because it is flexible. Firstly, the measuring tape was spread along the width of the river and left slacking. Then, someone in the group walked in a straight line along the measuring tape and read the measurement out to be recorded. While we were measuring the wetted perimeter we were careful to work upstream and from the left bank to right bank so not to alter our readings.

Measuring the wetted perimeter of the River Holford.

Velocity

The velocity was measured using a hydroprop and an impeller. At each site, we measured 5 velocity readings by dividing the channel’s width by 5. The impeller was placed onto the hydroprop, and then put two thirds of the depth into the river in order to prevent the impeller from touching the bed. A stopwatch was started immediately as the impeller was placed into the water and stopped when the impeller stopped. Some readings were defaulted at 100 seconds because at some sites, the water was too shallow for the impeller to go in.

Measuring the velocity

Angularity and Size of Bed load

Direction of flow

Direction of flow

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First, 15 clasts were picked up from the river bed at random, with our eyes closed so that we were not biased in the selection of the clasts. Then the angularity of the clasts were measured by using the cailleux roundness index chart. Next, the longest axis of the clasts were measured using a rigid meter ruler and then recorded.

Measuring the longest Axis of the clasts. Measuring the angularity of the clasts.

Gradient

The last reading that we had to take at each site was the Gradient. The equipment which was used to find the gradient was an E-ruler and a surveyor’s level. First, one group member went into the river and held the E-ruler straight just above the tip of their boot. Then, another person on land five meters away, downstream looked through the surveyor’s level and read the height on the E-ruler. The readings were then noted for each site.

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Equipment

1) The hydroprop and the impeller were used to measure the velocity of the River Holford. The impeller was attached to the hydroprop and were placed two thirds of the depth of the river in order to stop there being friction between the impeller and the bed. The impeller turned and a stop watch was started, and stopped when the impeller stopped rotating.

2) A 1 meter ruler was used to measure the clast size and the depth. It was ideal for these two measurements because it was rigid and very easy to read.

3) A tape measure was used to measure the wetted perimeter and the width of the river. It was also used to split the river up into 5 parts when we were finding out the gradient.

4) A surveyor’s level was used to measure the gradient. You had to look through the surveyor’s level at an E-ruler and measure the reading on it.

Secondary Data

3

1. Impellor and hydroprop

2. 1 meter ruler

3. Tape measure

4. Surveyor’s level

5. E-ruler

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Secondary data is data that we have to rely on. I used a number of different sources such as, the internet and maps in order to support the primary data collected. An ordnance survey map of the area around the River Holford was used to predict if the gradient was decreasing in order to strengthen the readings.

Our gradient data was mostly secondary data because we did not physically take all of the eight measurements ourselves. We only measured one out of the eight sites.

Weather information of Somerset was looked up on the internet to see if there was rain prior to our study. If there was a lot of rain or no rain at all, then this factor would be taken into consideration when our data was analysed.

Formulae

Gradient (m/m) = Downstream reading (m) – Upstream reading (m) ÷ 10m

Channel Size (m 2 ) = Average depth (cm) ÷ 100 x width (m)

Channel Shape (form ratio) = Average depth (cm) ÷ 100 x width (m)

Bank full CSA (m 2 ) = bank full width (m) x Bank full average depth (cm) ÷ 100

Spearman’s Rank Correlation

Spearman’s rank technique is a statistical test which will be used for each hypothesis to see whether there is a correlation between two variables, and to test the actual strength and the reliability of the relationship.

This is the equation to work out Spearman’s Rank Correlation:

After finding the results from the Spearman’s rank, a significance test will be used to test the significance of the relationship. This will show us whether my results occurred by chance and if we should accept or reject my hypotheses.

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Site Descriptions

Site1- This site was very close to the source of the River Holford. It is in a wooded area with steep sides and there is a very obvious drop in gradient as you continue from the source to site 1. Site 1 is a small stream which had very little depth and width because it had not rained prior to our trip. The clasts were all angular here and the geology was Devonian Quartzite.

Site 2- The river at site 2 was very narrow but was getting deeper as predicted. The geology in this area was Quartzite and the gradient was decreasing. There were still steep “v” shaped sides here just like at source 1.

Direction of flow

Direction of flow

Angular clasts

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Site 3- At this site, the river widened considerably when compared to site 2 but still stayed relatively the same depth. The rock type was still Quartzite and there was non-coniferous woodland around the river.

Site 4- At this site we were near to a settlement called Holford. The river’s width had increased considerably and there was a change in geology from the previous sites and now there was Permo-Triassic Marl.

Direction of flow

Direction of flow

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Site 5- At this site the river’s width, depth and velocity increased slightly. The geology at this site was Marl. The cross sectional area also increased because we were in the middle course of the river.

Site 6- At this site, there was a rapid increase in the velocity of the river. The environment here was very similar to that of site 1 because there were steep sides which were covered with dense trees and vegetation. The geology here was still Marl.

Direction of flow

Direction of flow

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Site 7- This site was in the lower course of the river and was on Jurassic Limestone. The river had calmed down and its velocity decreased. The cross sectional area increased because we were in the lower course of the river. There was very little gradient here and the clasts were decreasing in size and were mostly rounded.

Site 8- This site was located in the lower course of the river and was the closest site to the river’s mouth at Kilve. This part of the river was on Jurassic Limestone and there were vast amounts of vegetation that had grown on the river banks here. The river had become calmer and there was very little change in the gradient.

Direction of flow

irection of flow

Direction of flow

Vegetation on the banks of the river.

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Mouth- This is the river Holford’s mouth at Kilve beach. Although this was not a site and we did not sample here, we can tell that the clasts are smooth and that there is a downward gradient as the water goes into the Bristol Channel.

The water percolates through the limestone and came back up on the beach into the Bristol Channel.

Water emerging from underground

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Problems and Limitations encountered in Data Collection

There were many problems that we encountered on our river study. Some we could not change but others we could.

Physical Factors

One physical problem was that there was debris in the river such as tree branches and clasts. This affected measurements such as depth and velocity, because there was a build up of water due to this river substrate. This meant that many readings were difficult to measure accurately.

There was a lack of rain before we went on this field trip which meant that the river in some parts was very shallow and could have limited the reliability of the data.

At some points, the impeller could not turn due to the fact that the river was too shallow and this could have also been because there was a lack of rain. This meant that, especially in the upper course sites that the readings had to be defaulted at 100 seconds.

Human factors

One human problem was that the angularity of the clasts had to be decided by eye. This meant that it was the group member’s opinion on whether the clast was rounder or angular. Instead of measuring the clasts size, we could have measured its volume in order to make it more accurate.

Debris such as leaves, rocks and tree branches blocking the flow of the river.

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Another human limitation was that we could have taken ten measurements for every factor per site. This would have meant a wider range of results to interpret. The reason that this was not undertaken was because of the limited amount of time we had to sample the river. Accessibility of land was a human limitation because part of the river was blocked because it was in private land. This meant that we could not sample in this area and therefore we could not tell if the trends of the previous sites would continue.

Data Presentation

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In this section I shall be presenting my data as graphs and describing the basic trends that are seen in them. I will also be justifying why I used a particular type of graph as oppose to another one.

Graphs to show the depth of the river along all eight sites

I have chosen to represent the depth of the river as a bar chart because it is very easy to visualize the depth of the river in this way. You can also tell where the deepest part of the river was along each of the eight sites. Bar charts are used when the change is large.

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Site 1 Depth

Depth

Number of Readings taken

Dept

h (m

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Overall, all the depth readings at this site were what was expected to be seen at this stage along the river’s course because the river at this stage was very shallow. Measurements 1 and 2 are the same at 0.015m and this was correct because the channel was very narrow here. However, measurement 4’s depth was only 0.001m and this is incredibly low when compared to the other depth readings taken at this site.

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Site 2 Depth

Depth


Dept

h (m

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The readings at site 2 tell us that the river was increasing in depth from an average of 0.0122m at site 1 to 0.1246m. Reading 5’s depth was 0.51m which was very far away from the trends that the other readings taken at this site showed. This reading could be seen as an outlier. An explanation of why this outlier occurred may have been because there were many objects such as tree branches in the river which could have caused a build up of water where we were measuring the depth. Reading 3 was quite low when compared to the other readings at 0.005m.

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Site 3 Depth

Depth


Dept

h (m

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Reading 1 was the shallowest depth and was 0.037m. The depth then increased by 0.03m at reading 2 and continued to increase at reading 3 where it was 0.074m. The depth then steadily increased by 0.009m until reading 5.

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Site 4 Depth

Depth


Dept

h (m

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At Site 4 the average depth was 0.093m which was larger then at the previous site. The depth stayed the same at readings 1 and 2 and was 0.105m. It then increased slightly by 0.005m at reading 4 until the depth decreased by 0.065m at reading 5.

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Site 5 Depth

Depth


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h (m

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At site 5 the five readings taken across the width of the channel were quite varied. Reading 2 was the lowest measurment that was taken and was 0.0102m. This may lead us to believe that this was an outlier seeing as none of the other results are this low. The depth then increased after reading 2 until 4 where it was at its highest of 0.17m. The depth then decreased until reading 5 where it was 0.07m.

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Site 6 Depth

Depth


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h (m

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The depth at site 6 generally decreased from 0.099m at site 5 to 0.0812m which came as a surprise because the depth of the river is supposed to increase as you get closer to its mouth. Reading 1 had a depth of 0.055m which was expected because it was the nearest to the bank of the river. The depth then deepened at reading 2 where it was 0.107m until reading 3 where it was 0.064m. At readings 4 and 5 the readings increased to 0.091m.

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Site 7 Depth

Depth


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At reading 1 the depth was 0.043m and was the lowest measurement because it was the closest reading taken to the bank. The depth then increased at reading 2 where it was 0.096m and then deepened further by 0.008m until reading 3. The river’s deepest point in the channel was at reading 4 where the depth was 0.127m. The depth then decreased at reading 5 by 0.074m.

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Site 8 Depth

Depth


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h (m

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At site 8 our readings showed us that this was overall the deepest part of the river. The depth at reading 1 was 0.092m which was the shallowest reading because it was the closest one taken to the bank. The river increased in depth at site 2 where it was 0.14m and continued to increase until reading 3 which was 0.195m and was the deepest point in the channel. The river’s depth then decreased at sites 4 and 5 by 0.045m.

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Average Depth vs Distance downstream

Depth

Site number

Dept

h (m

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Site Number Average Depth (m)

1 0.0122

2 0.1246

3 0.0686

4 0.093

5 0.099

6 0.0812

7 0.0876

8 0.152

Overall, there is an increase in depth as you go along the river’s course. This can be seen at site 1 where the average depth was 0.0122m, and then at site 8 where the depth was 0.152m. There is a 0.1398 m difference between the two and shows that the depth of the river increases as you go from its source to its mouth.

Graphs showing the Cross sectional of each site

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A x-y plot was chosen to represent the cross section of the river because it is easy to see how the river changed as we progressed through the sites. It was used to determine relationships between the width and depth.We did not use a bar graph to represent this data because they are used to track changes over time.

At site 1 the cross sectional area was 0.008418 m2, which is the lowest result out of all the sites and was what we expected. The channel’s width and depth were very small here. There was an unexpected drop in the depth of the river at the 5th plot where it was 0.001m, and means that this reading was unexpected.

At site 2 the cross sectional area was 0.28035 m2 which was higher than at the previous site. Plot 6 was an unexpected rapid increase in the depth which was an outlier because it was outside the range of the other results.

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At site 3 the cross sectional area was 0.155036 m2 which shows that there was an unexpected drop from this reading to the one at site 2. The depth of river increased until plot 6 where it started to decrease until it reached the river’s bank. The width of the river stayed the same as the previous site which was unexpected because the width was supposed to increase as you travel from source to mouth.

At site 4 the cross sectional area was 0.33852 m2 which was an increase from the previous site. The depth at plots 2-5 were very close to each other, until there was a decrease in the

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depth at plot 6 until the river bank. The width of the river increased by 1.38m from the previous site which showed that the river was increasing in size.

At site 5 the cross sectional area was 0.18216 m2 which was lower than the previous site. The depth increased and then decreased at plot 3, where the result was an outlier. The depth then increased until plot 5, (where the water was at its deepest), and then decreased until the river bank. The width of the river decreased by 1.8m from the previous site and this was an unexpected reading.

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Site’s 6 cross sectional area was 0.23142 m2 which was an increase from the last site. The depth increases until plot 3, until plot 4 where there is a decrease. The width increased by 1.01m from the previous site, and this was expected.

At site 7 the cross sectional area was 0.219 m2 which was a decrease from site 6.The depth increased until plot 5 and then it decreased. The depth was at its highest near the middle of the river at plot 5 which was 0.127 m. The width of the river had unexpectedly decreased by 0.35m from the previous site.

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Site’s 8 cross sectional area was 0.42256 m2 which was the highest reading of all the eight sites and this was expected. The depth increases until plot 4 where it is 0.2 and then starts to decrease until the bank. The width of the river increased from 2.50m at site 7 to 2.78m at this site which was expected in the hypotheses.

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CSA base map

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Width along the eight sites of the river Holford

I have chosen to represent my data for width as a lateral bar chart because it is very easy to see the trends and where the river was at its narrowest and widest. Bar graphs are used to track changes over each site.

Site Number Width of River (m)

1 0.69

2 2.25

3 2.26

4 3.64

5 1.84

6 2.85

7 2.50

8 2.78

1

2

3

4

5

6

7

8

0 0.5 1 1.5 2 2.5 3 3.5 4

Width vs Distance downstream

Width

Width (m)

Site

Num

ber

The width of the river starts to increase as we go from site1to site 4 by 2.95m. At site 4 the river was very wide which was unexpected because the widest part of the river was supposed to be the nearest to the mouth. There was an unexpected drop of 1.8m in the width at site 5. The width then increased until site 7 where there was a slight decrease, and then there was a final increase at site 8. Overall, we can see that the river increased in width from 0.69m – 2.78m as you went from its source to its mouth.

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Gradient along the eight sites of the river Holford

1 2 3 4 5 6 7 80

0.010.020.030.040.050.060.070.080.09

0.1

Gradient of the river

Gradient

Site number

Gra

dien

t (m

/m)

I have chosen a line graph to show the gradient of the river Holford because it is very easy to see if the gradient was increasing or decreasing. You can also see the rate at which the river is decreasing in gradient with this type of graph. A line graph was used instead of a bar chart because when smaller changes exist, line graphs are better to use than bar graphs.

The gradient of the river starts to decrease rapidly as you go from site1 to site 2. Then the river starts to decline at a slower rate until site 4 where it is 0.008 m/m. After this, there was an unexpected increase in the gradient of the river until site 5. The river then decreased from site 5 to site 8. Overall, there is a 0.08 m/m downward trend in the data which means that gradient decreases as you go from the source to the mouth.

All raw data that was collected is in the appendix on page 59.

Site Number Gradient of River (m/m)

1 0.086

2 0.034

3 0.021

4 0.008

5 0.029

6 0.021

7 0.021

8 0.006

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Formulae Sheet

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Data Analysis

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In this section I will be analysing my results to see if my hypothesis were correct and using Spearman’s Rank correlation to show the reliability of my results. Scatter graphs will be used to show the possibility of a positive correlation between two variables, such as width and the distance from the source. Simply looking at the line of best fit will tell us if there is a strong positive relationship, no relationship, or a negative relationship between the two variables.

Spearman’s rank

Spearman’s Rank is a statistical technique which is used to see if there is a correlation between the results and to see the reliability of this relationship between them. This technique tells us whether the correlation is really mathematically significant or if it could have been the result of chance alone.

Method of finding R 2

1) State if there is a relationship between your two sets of readings.2) Make a table and rank both sets of measurements, for example distance from

source and depth, with the biggest number having a ranking of 1, and the smallest having a ranking of 8.

3) Take away the two sets of rankings away from each other in order to get the difference between the two ranks (d).

4) Then square the difference between the two values to remove the negative values and get Sigma difference2 (∑d2).

5) Next, calculate the coefficient (r2) by using the formula below. The answer will always be between 1.0 (a perfect positive correlation) and -0.1 (a perfect negative correlation).

This is the Spearman’s Rank formula written in mathematical notation:

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My hypotheses were:

1. Width will increase as you go from source to mouth.2. Depth will increase as you go from source to mouth.3. Gradient will decrease as you go from source to mouth

Hypothesis 1: Width

Scatter graph to show the change in width of the River Holford from source to mouth.

0 1000 2000 3000 4000 5000 6000 7000 8000 90000

0.5

1

1.5

2

2.5

3

3.5

4

Width from source to mouth along the River Holford (m)

distance from source (m)

wid

th (m

)

It can be easily seen from this scatter graph that the width increased as you went form sites 1-8. This supports my hypothesis because the graph indicates that there is a correlation which is that, as the distance from the source increases, the width also increases. Nearly all the widths, except sites 1 and 4, are near to the line of best fit and this shows us that the correlation is quite good. Now a statistical test called Spearman’s Rank correlation must be undertaken to see the strength of this correlation.

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Site Number

Distance from

Source (m)

Rank Width

(m)

Rank Difference Between Ranks

(d)

Difference2

d²

1 20 8 0.69 8 0 0

2 230 7 2.25 6 1 1

3 480 6 2.26 5 1 1

4 560 5 3.64 1 4 16

5 1104 4 1.84 7 -3 9

6 2987 3 2.85 2 1 1

7 4200 2 2.50 4 -2 4

8 7727 1 2.78 3 -2 4

Total: 36

Spearman’s rank correlation

Below is how we found r2 using the Spearman’s rank equation:

6 x 36 = 1 - 216

83-8 504

= 1- 0.428 r 2 = 0.572

The closer r2 is to +1 or -1, the stronger the likely correlation. A perfect positive correlation is +1, while a perfect negative correlation is -1. My r2 value is 0.572 which is quite close to +1, and therefore suggests to us that there is a fairly strong positive relationship.

Significance Testing

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A further technique is now required to test the significance of the relationship. The r2value of 0.572 must be looked up on the Spearman Rank significance table which is below. First you have to work out the “degrees of freedom”, which is the number of pairs in your sample minus 2. For our test this was 6 (8-2). The result was then plotted on the significance table below in order to be assessed. If the reading is below 5%, then you must reject your hypothesis because your reading was a product of chance. If your reading is above 0.1, then we can be 99.9% confident that the correlation did not occur by chance.

The plot of 0.572 above gives a significance level of under 5% which tells us that it is possible that my result my have been the product of chance and that the hypothesis must be rejected. This means that the probability of the relationship that has been found being a chance event is nearly 99%. However, we did only take 8 samples along the river’s course, so if more measurements were taken, then the results would be more reliable. From the significance test above my hypothesis is very likely to be wrong.

Possible Explanations for width results

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There are many possible explanations for why the width of the river followed the trend which can be seen in the graph.

One explanation of why the width decreased at some sites but then unexpectedly increased at others was that there were three types of geology along the river’s course, (which can be seen in the geology base map on page 10).The geology at the first three sites was Quartzite which is a hard, impermeable red metamorphic rock which means that water cannot percolate through it. This meant that we would not expect a large channel because the rock is resistant so is hard to be eroded and weathered away. The next three sites widths could have been affected because the geology was Marl, which is a soft, impermeable sedimentary rock. This meant that the channel was expected to be wider because more erosion takes place in this part of the river, and this is why there was an increase in the channel’s width at sites 4-6. The geology at the last two sites was limestone which is a permeable sedimentary rock and is resistant to erosion. Limestone’s permeability meant that water in the channel was going to percolate through it, and would mean that there would be less energy in the river so the river may narrow again.

Another explanation of why the width readings were not as the hypothesis had predicted was because of the gradient (which is seen by the contour lines on the OS map on page 8). In the upper course of the river, we expected the river to have more potential energy which meant that the river was deeper rather than wider. As we progressed to the lower course of the river, we found that the river was wider because there was more lateral erosion taking place, which meant the widening of the channel.

Where we were along the drainage basin could have changed the width of the river. An example of this is at site 3 where we were near a confluence. The river would have had more energy to erode the banks and therefore widen the river.

At the first three sites there was dense forest to both sides of the river, (which can be seen on the land use map on page 6). This meant that interception by this canopy layer could stop rain water from reaching the ground and therefore decreasing surface runoff. Surface runoff is very quick and is the movement of water in any form from the atmosphere to the ground. This meant that the width of the river was expected to be very small in the forested areas. On the other hand, the sites which were in open farm land were expected to have a wider channel because there would be little or no interception from trees and vegetation.

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Hypothesis 2: Depth

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0 1000 2000 3000 4000 5000 6000 7000 8000 9000

dept

h (m

)


Depth from source to mouth along the River Holford (m)

It can be easily seen from this scatter graph that the depth increases as you go further out from the source and therefore supports my hypothesis. There is an overall positive correlation which can be seen in the graph which is that, as the distance from the source increases, the depth also increases. However, some of the depths, such as at sites 1 and 2 were not near the line of best fit which could weaken the correlation. Now a statistical test called Spearman’s Rank correlation must be undertaken to see the strength of this correlation.

Site Number

Distance from

Rank Average Depth

Rank Difference Between

Difference2

(d²)

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Source (m)

(m) Ranks

(d)

1 20 8 0.0122 8 0 0

2 230 7 0.1246 2 5 25

3 480 6 0.0686 7 -1 1

4 560 5 0.093 4 1 1

5 1104 4 0.099 3 1 1

6 2987 3 0.0812 6 -3 9

7 4200 2 0.0876 5 -3 9

8 7727 1 0.152 1 0 0

Total: 46



6 x 46 = 1 - 276

83-8 504

= 1- 0.547 r 2 = 0.453

My r2 value is 0.453 which is relatively close to +1, and therefore suggests to us that there is a fairly good positive correlation between the depth and the distance from the source.

Significance of Relationship Testing

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This Spearman’s rank correlation graph test tells us that 0.453 is under a significance level of 5% and means that it was possible that my results was the product of chance and therefore must be rejected. However, we could have compared our data with the other groups completing the same study to see if they got mainly the same results as us, and this would improve the reliability of the sample.

Possible Explanations for depth results

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One reason why the depth increased at some sites and then decreased at others was because of gradient, (which can be seen on page 8). In the upper course of the river we expected the river to be deep because the gradient was steep and this meant that there would be vertical erosion with more potential energy. At the later sources we expected more lateral erosion, so the river would not have been as deep.

At some sites we were very near to confluences and this could have affected our results. This is because at a confluence the river has more energy and power to erode, resulting in a larger, deeper channel.

We were undertaking this study in the autumn and so there were many rotting leaves around and inside the river. A result of this was that Humic acid was formed and dissolved the river bed and thus created deeper channels. This is an example of solution. Solution is an erosional process where acids in the river dissolve the banks and the bed which results in a deeper channel. Sites 4, 5 and 6 would have been the most affected by this because the geology there was marl which is a soft rock composed of Calcium carbonate.

Near the mouth of the River Holford, (the Bristol Channel), we expected the river to be slightly shallower than at the previous sites because it was losing its energy and was depositing its load, (sediment).

The geology, (which is seen on page 8), at the first 3 sites was Quartzite which is a hard impermeable rock. This meant that we expected the river to be shallow at these first sites because the rock was too resistant. The geology at sites 4-6 was Marl which is an impermeable, soft rock which meant that there would be more erosion taking place here. This is why we expected the river’s depth to increase at these sites. The geology at the last two sites was Limestone which is a hard, permeable rock. Limestone is well jointed which means that the rock has vertical and horizontal bedding planes. This meant that groundwater flow would occur here, where the water moved underground through these pores and joints. This particular type of limestone was resistant to erosion so we expected the river to get narrower here.

Site 4 was near an urban area, (Holford), which meant that there was concrete and tarmac which formed impermeable surfaces. This meant that there would be more overland flow here which is the movement of water in any form from the atmosphere to the ground.

There had been little rain in Somerset before we went on our river study. This meant that the river would not have been as deep as it normally should have been and this could have affected our results.

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Hypothesis 3

Gradient form source to mouth along the River Holford (m/m)

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

0 1000 2000 3000 4000 5000 6000 7000 8000 9000


Gra

dien

t (m

/m)

The gradient decreased from source to mouth and this can be seen in this scatter graph. There is a negative correlation and this can be seen, as the distance from the source increases, the gradient decreases. Nearly all the gradient readings are near the line of best fit which means that the correlation is strong. However, some of the gradient readings, such as at sites 1 and 4 are not near the line of best fit, and this may weaken the strength of the correlation.

River’s can only flow in a downward gradient due to gravity and this is why we got these set of results. At site 5, the gradient was seen to increase and this could be an incorrect measurement.

Site Number

Distance from

Source

Rank Gradient Rank Difference Between Ranks

Difference2

(d²)

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(m) (m/m) (d)

1 20 8 0.086 1 7 49

2 230 7 0.034 2 5 25

3 480 6 0.021 5 1 1

4 560 5 0.008 7 -2 4

5 1104 4 0.029 3 1 1

6 2987 3 0.021 5 -2 4

7 4200 2 0.021 5 -3 9

8 7727 1 0.006 8 -7 49

Total: 139



6 x 139 = 1 - 834

83-8 504

= 1- 1.654 r 2 = -0.654

My r2 value is -0.654 which is relatively close to -1, and therefore suggests to us that there is a fairly good negative correlation between the gradient and the distance from the source.

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Conclusions and Evaluation

In this section I will be seeing if my hypotheses were correct overall using data from my graphs. The limitations of the data collection will be explored and the changes that I would make if I were going to undertake this river study again.

Summary table

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Visually Statistically

Hypotheses 1 Accepted Rejected



This is a summary table showing if my hypotheses were accepted visually, by looking at my data graphs, and statistically, by using Spearman’s Rank correlation.

My aim was to investigate the physical channel characteristics along the course of the River Holford which were the width, the depth and the gradient.

My first hypothesis was that the width of the channel would increase as you went from the source to the mouth. As you can see from the table above, this hypothesis was visually accepted by looking at width graphs. This can be seen in the width graph on page 39 which supported my hypothesis by showing a general increase from the source to the mouth. This can be seen at site 1 where the width was 0.69 m and then at site 8 where the width was 2.78 m. The width increased by 2.09 m and this shows us that there was an increase in width from the source to the mouth of the river. However, at site 4, the width decreased by 1.38m from the previous site. This decrease however can be explained because there was a geology change from Quartzite to Marl. Overall, visually, my hypothesis, that the width will increase as you go from source to mouth, is accepted.

Hypothesis 1 was rejected when it came to assessing the data using Spearman’s rank correlation technique and significance testing. My significance test for width can be seen on page 46 and shows that my significance level, which was 0.572 was below the line marked 5%. This tells us that my results were a product of chance and that I must reject my hypothesis.

My second hypothesis was that the depth of the river would increase as you went from the source to the mouth. As you can see from the table above, this hypothesis was visually accepted by looking at depth graphs. An average depth graph can be seen on page 33 which supported my hypothesis by showing an increase as you got close to the mouth. An example of this can be seen at site 1 where my depth was 0.0122m and then at site 8 where my depth was 0.152m. The depth increased by 0.1398m and this tells us that there was an increase in depth as you go from the source to the mouth. Overall, visually, my hypothesis, that the depth will increase as you go from source to mouth, is accepted.

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Hypothesis 2 was rejected when it came to analysing the data using Spearman’s rank correlation technique and significance testing. My significance test for depth can be seen on page 50 and shows that my significance level, which was 0.453was below the line marked 5%. This tells us that my results were a product of chance and that I must therefore reject my hypothesis.

My third hypothesis was that gradient would decrease as you went from source to mouth. As you can see from the table above, my hypothesis was accepted visually by looking at the gradient graph which can be found on page 40. The graph shows that the gradient decreased as you went from source to mouth and can be seen when a site 1 the gradient was 0.086 m/m and then at site 8 where the gradient was 0.006 m/m. There is a 0.08 m/m difference between the two readings and this tells us that my hypothesis is supported by the graph.

Hypothesis 3 was rejected when it came to the data being analysed using Spearman’s Rank correlation and significance testing. My Spearman’s rank table for gradient can be seen on page 53 and shows that my significance level, which was -0.654 was below the line marked 5%. This tells us that my results were a product of chance and that I must therefore reject my hypothesis.

Bradshaw’s model is generalised in the way that it tells you what is expected to happen along a river with the same factors throughout its course. Some of the model’s predictions did not work for our particular river that we were studying because there were a lot of changes throughout its course that affected our readings, such as the geology and confluences along its course.

Evaluation

Overall, our river study on the River Holford was very successful because we took a lot of measurements which were taken very accurately.

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Limitations

There were some limitations when we were measuring our data and the methods we used to find the sites.

One limitation was that a stratified sampling strategy had been used for the sites because of the little time we had to carry out this study. This meant that our results were biased because we chose specific areas in which we wanted to sample, such as near confluences and on different geology types.

It was hard to get an exact depth reading because the water was always moving. This meant that we were not measuring the depth of the river accurately and weakens our results and conclusion.

At the first 3 sites when we were measuring the velocity, using a hydroprop and an impeller, the water was just too shallow. This meant that the impeller could not turn due to the fact that it was coming into contact with the river bed. As a result of this limitation some of the velocity readings were defaulted to 100 seconds. At all the sites throughout the course of the river, all the groups were trying to collect data at the same time which resulted in them pushing water downstream at a faster rate than normal. This meant that the impeller turned faster then normal and therefore altered our results.

There were lots of objects such as tree branches in the river. These could have affected our depth results because there would have been a water build up in these areas. This would have also affected the velocity data because the flow of the river would have been altered.

Improvements

If I were to repeat this river study again there would be some changes that I would make in order to improve my data.

One of these changes is that I would have taken ten depth readings at each site rather than five. This is because it would broaden our results and make then more accurate. You would also be able to see if the new depth readings followed the same trends as the data before it.

Ten sites could have been looked rather than eight along the river’s course. This would have given us more results to analyse, and to see if my data trends continued at these extra sites.

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We could have compared data with other groups taking part in this study to see if we all got similar results and trends. This would strengthen our conclusion and make it more reliable.

A systematic sampling method could have been used rather than a stratified one if we had a larger period of time in which to carry out this study. This would have meant that our data would not have been biased because we would not take readings near confluences, but all through the river’s course.

We used the cailleux roundness index chart and a ruler when measuring the clast size. To improve this method I feel that we could have just worked out the volume of the clasts. This would mean that there would not be someone’s own opinion on if the clasts were rounded or angular.

Overall this study along the River Holford was highly successful in that we collected good data to be analysed, and taught me more about factors that affect a river’s physical characteristics. These all led me into producing a good piece of coursework.

Acknowledgements

I would like to thank Mr Matthews, Mr Orme, and Mr Parker for coming with us on this trip and offering us guidance along the way. I would also like to thank the staff at Nettlecombe Court for accommodating us and helping us with our river study along the River Holford.

Bibliography

Picture of Quartzite on page 9:http://maurice.strahlen.org/minerals/pics/quartzite-shoksha.jpg

Picture of Limestone on page 9: www.m.godden.btinternet.co.uk/oolite.jpg

Picture of Marl on page 9: en. wikipedia .org/ wiki /River_Holford

Appendix

Raw data: Primary and Secondary:

http://maurice.strahlen.org/minerals/pics/quartzite-shoksha.jpg

http://maurice.strahlen.org/minerals/pics/quartzite-shoksha.jpg

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Site 1 Site 2 Site 3 Site 4 Site 5 Site 6 Site 7 Site 8

Width (m) 0.69 2.25 2.26 3.64 1.84 2.85 2.5 2.78

Wetted Perimeter (m) 0.74 2.4 2.48 2.08 2 3.13 2.84 2.92

Depth (m) 1 0.015 0.045 0.037 0.105 0.105 0.055 0.043 0.092Depth (m) 2 0.015 0.027 0.067 0.105 0.0102 0.107 0.096 0.14Depth (m) 3 0.014 0.005 0.074 0.11 0.14 0.064 0.119 0.195Depth (m) 4 0.001 0.036 0.082 0.105 0.17 0.091 0.127 0.183Depth (m) 5 0.016 0.51 0.083 0.04 0.07 0.089 0.053 0.15

Average Depth0.012

2 0.1246 0.0686 0.0930.0990

4 0.0812 0.0876 0.152

Flow Rate (s) 1 100 54.44 100 12.52 7.6 7.33 100 100Flow Rate (s) 2 100 43.06 58.46 18.62 10.4 23.35 100 61.49Flow Rate (s) 3 100 45.7 17.06 15.9 22.69 18.97 43.99 100Average Flow Rate (s) 100 47.733 58.507 15.68 13.565 16.55 81.33 87.163

Velocity (m/s)0.060

5 0.0964 0.0838 0.2369 0.2696 0.2259 0.068 0.0653

Gradient 0.086 0.034 0.021 0.008 0.029 0.021 0.021 0.006

Clast Analysis

Clast Shape - Enter the number of each shape found at each site

Very Angular 3 4 0 0 0 0 0 0Angular 7 9 2 1 2 1 0 0Sub Angular 4 2 8 4 7 5 1 1Sub Rounded 1 0 5 7 6 5 8 7Rounded 0 0 0 2 0 4 3 6Very Rounded 0 0 0 1 0 0 3 1

Clast Size (cm) 1 9 11.5 7.9 15.5 16 2.2 6.9 7Clast Size (cm) 2 12.5 11.2 10.7 5.4 11.2 8.4 8.2 8.1Clast Size (cm) 3 8 7.2 7 7.1 13.3 0.5 11.5 7.6Clast Size (cm) 4 5 9.7 7.3 13.5 9.6 8 12.4 4.8Clast Size (cm) 5 14.7 8.4 10.3 9.9 13.1 7.8 16.7 9.2Average Clast Size (cm) 9.84 9.6 8.64 10.28 12.64 5.38 11.14 7.34

a* full marks gcse geography coursework (rivers)

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