Investigation into the Pebble Ridge at Westward Ho! Essay Sample

Investigation into the Pebble Ridge at Westward Ho! Pages
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The aim of this study is to see whether pebble length changes with distance from the proximal end of the pebble ridge and along transects up the ridge at Westward Ho! Westward Ho! is located at the Taw Torridge Estuary on the north Devon coast shown by figure 1. The supply of material has come from the south with the aid of wind and longshore drift. However, dissipating wave energy at the mouth of the River Taw has ended any further northward movement and an accumulation of material has built up at the distal end of the ridge.

The pebbles are made of hard, fine-grained sandstone, which comes from the cliffs west of Westward Ho! Theory suggests that with distance from the proximal end of the pebble ridge, pebble length should decrease as a result of attrition and the direction of longshore drift. Attrition is when waves cause rocks and pebbles to bump into each other and break up. Longshore drift is the movement of material (ie sand, gravel or shell fragments) along the shore by a current in the area between the high and low water marks known as the surf zone.

I intend to test the following hypothesis:

Longshore drift is occurring when pebble length at three locations along the ridge, the base, the middle and the top decreases with distance from the proximal to the distal end of the pebble ridge.

Figure 1: Location Map of Westward Ho!

In order to test my hypotheses, I shall be recording data at three transects along the pebble ridge at Westward Ho! I shall have one transect at the proximal end with an OS grid reference of approximately 434 298, on at the middle of the ridge with a grid reference of 436 302 and one at the distal end with a grid reference of 437 305. Each transect will be roughly 400m apart. To find if pebble length does decrease with distance I shall measure the length of 10 pebbles every 2m along each transect.

I have decided to use three transects because then I shall get data which will show clearly the extremes in the different locations. I shall take my data recordings at systematic 2m intervals in order to collect enough data to perform sufficient statistical tests to prove or disprove my hypotheses. I am recording the length of 10 pebbles at each site to get a clear mean.

Before beginning data collection, I visited Westward Ho! to perform a risk assessment, the only risk was the high tide so in order to overcome that problem I collected all my data when the tide had gone out.

Analysis and Presentation of Findings

Change in pebble length with distance from the proximal end of the pebble ridge

Figure 2: Mean Axis and Distance at the Three Transects

Transect A

Transect B

Transect C

Distance (m)

Mean Axis

Distance (m)

Mean Axis

Distance (m)

Mean Axis

0

26.68

0

27.73

0

22.8

2

27.32

2

26.06

2

21.29

4

25.96

4

21.18

4

24.08

6

18.77

6

20.45

6

23.17

8

24.65

8

24.63

8

12.78

10

22.21

10

19.04

10

16.23

12

21.84

12

22.03

12

14.78

14

18.71

14

20.54

14

14.61

16

20.72

16

21.23

16

8.97

18

21.48

18

21

18

8.3

20

23.43

20

16.42

20

15.41

22

20.26

22

20.06

22

16.82

24

25.14

24

18.81

24

15.17

26

29.12

26

16.77

26

11.98

28

26.73

28

16.32

28

14.84

30

20.57

30

13.88

30

18.16

32

21.19

32

12.32

32

18.99

34

19.17

34

19.57

34

24.8

36

21.05

36

23.97

36

20.38

38

22.61

38

19.33

38

16.67

40

26.94

40

24.24

Mean of Mean

17.01

42

21.27

Mean of Mean

21.27

44

20.69

46

23.32

48

25.25

50

23.71

52

30.66

54

29.41

Mean of Mean

24.4

Figure 3: A graph of mean pebble length

Figure 3, using the data from figure 2, clearly shows the changing mean of mean pebble length for each transect showing that distance from the proximal end of the pebble ridge does result in decreasing pebble length therefore attrition is occurring which is causing the pebbles to get smaller, also indicating that longshore drift is also occurring at this site.

“There is no significant difference between the lengths of pebbles at transects A, B and C”

To try to disprove this hypothesis I shall perform Chi Squared.

Figure 4: Chi Squared of my three transects

Chi Squared for Pebble Length along the Ridge at Westward Ho!

Sediment Size

A

B

C

O Values

E Values

O – E

(O – E)^2

(O – E)^2/E

O

E

O

E

O

E

Totals

26

26.97

-0.97

0.9409

0.0349

Very Small

<10

26

26.97

5

19.48

33

18.55

64

122

138.37

-16.37

267.9769

1.9367

Small

10 – 19

122

138.37

113

103.78

106

98.84

341

82

87.65

-5.65

31.9225

0.3642

Medium

20 – 29

82

87.65

80

65.74

54

62.61

216

46

26.38

19.62

384.9444

14.5923

Large

30 – 39

46

26.38

12

19.78

7

18.84

65

4

21.62

-17.62

310.4644

14.3601

Very Large

>40

4

21.62

0

1.22

0

1.16

4

5

19.48

-14.48

209.6704

10.7634

Totals

280

210

200

690

113

103.78

9.22

85.0084

0.8191

80

65.74

14.26

203.3476

3.0932

12

19.78

-7.78

60.5284

3.0601

0

1.22

-1.22

1.4884

1.2200

33

18.55

14.45

208.8025

11.2562

106

98.84

7.16

51.2656

0.5187

54

62.61

-8.61

74.1321

1.1840

7

18.84

-11.84

140.1856

7.4408

0

1.16

-1.16

1.3456

1.1600

Degrees of Freedom =

2 x 4 = 8

?=

71.8036

My result falls in the 0.01% certainty that my null hypothesis is true therefore there is a 99.9% probability that my hypothesis is true, pebble length does change with distance from the proximal end of the pebble ridge. This also proves that there is some movement of pebbles along the ridge at Westward Ho! I shall now see whether pebble length changes at the base, top and end of the pebble ridge on the three transects. I found the different locations to record data by using the values from the first 8m and the last 8m of each transect then found the middle and took 8m around the middle of the transect.

Figure 5: The change in mean pebble length at different locations along the transects.

Transect A

Transect B

Transect C

Mean Pebble Length (cm)

Mean of Mean

Mean Pebble Length (cm)

Mean of Mean

Mean Pebble Length (cm)

Mean of Mean

Base

26.68

24.6825

27.73

23.855

22.8

22.835

27.32

26.06

21.29

25.96

21.18

24.08

18.77

20.45

23.17

Middle

25.14

25.39

21.23

19.6775

8.97

12.375

29.12

21

8.3

26.73

16.42

15.41

20.57

20.06

16.82

End

25.25

27.2575

19.57

21.77

18.99

20.21

23.71

23.97

24.8

30.66

19.33

20.38

29.41

24.21

16.67

Figure 5 shows that mean pebble length at the base, middle and end of the ridge profile does decrease with distance from proximal end. This can be better shown by figure 6 shows the decrease in pebble length with distance which follows my hypothesis that sediment length at three locations along the ridge, the base, the middle and the top will decrease with distance from the proximal to the distal end of the pebble ridge. This proves that longshore drift and attrition are working hand in hand to move sediment along the ridge at Westward Ho!

Figure 6: Comparison of Mean Pebble Length at the base, middle and top of the ridge at the three transects

Evaluation and Conclusion

After performing this fieldwork I can say that I have proved my hypothesis to be true. I can be clear of this because figures 2, 3, 4, 5 and 6 all show a decrease in mean pebble length with distance from the proximal to the distal end of the ridge. This therefore shows that longshore drift and attrition are occurring at Westward Ho! The process of attrition is in action here, as pebbles enter the sea then are knocked about by waves making them smaller, as the pebbles become smaller, longshore drift is able to carry the smaller sediment in suspension along the pebble ridge. As the shape of the coastline changes at the Taw Torridge Estuary, the velocity of the wave drops causing the waves to drop the sediment they are carrying therefore causing deposition at the distal end of the spit. This deposition has formed the large sandy end of the ridge at Westward Ho! The direction of longshore drift can also be seen at Westward Ho! as, sediment is moved in that direction. Therefore we can say that at Westward Ho! pebble ridge, longshore drift is moving from south west to north east, which fits in with the way that winds prevail around the UK.

I feel that my data was as accurate as it could have been because I followed the instructions to the letter. I was not rushed to finish quickly so took time over measuring lengths and not making haphazard guesses. However, the reliability of my data could have been improved by collecting data from a greater number of transects and going further towards the mouth of the river. However for this to occur, further hazard analysis would have to be carried out. If I was to do this experiment again, I would record data from a greater number of transects and go further down the pebble ridge to see if I get any further clarity from my findings.

Bibliography

* Processes and Landforms – Alan Clowes and Peter Comfort [1993]

* http://www.devon.gov.uk/geology/NRB.pdf

* http://www.northdevon.gov.uk/

* http://www.multimap.com

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