Report On the Change In Sediment Size Along the Length and Breadth of Hurst Beach.

REPORT ON THE CHANGE IN SEDIMENT SIZE ALONG THE LENGTH AND BREADTH OF HURST BEACH

IDENTIFICATION OF THE QUESTION

Having spent a day collecting and collaborating data at Chesil Beach, Dorset we decided to make a comparison with Hurst Beach.

Hurst Beach is a shingle, coastal deposit which has formed a spit on Hampshire’s south coast (Fig. 2).

This is much like Chesil Beach (Fig. 1), though on a smaller scale. Having examined Chesil Beach one factor that was observed was the change in the size of sediment laterally along the beach. Using our knowledge of longshore drift we decided to investigate two hypotheses concerning the sediments on Hurst Beach:

The size of sediment increases laterally along Hurst Beach
The size of sediment on Hurst Beach increases further from the sea

DEVELOPMENT OF A STRATEGY

In able to answer our hypotheses the sediments on the spit needed to be sampled. It would be both unnecessary and unreasonable to sample every stone, and so a sampling strategy was developed. It was decided to sample the sediment in three places with increasing distance from the sea at lateral intervals of 100m. These 100m points had been marked out with posts during developments on the spit and so were simple to determine. The sediment samples would then be placed in plastic bags which would be carefully labelled to ensure minimum confusion.

COLLECTION OF DATA

The labour was divided between five groups of students and so eleven sampling sites were set up, with each group sampling at least two of the posts. At each point the width of the beach was measured from the sea to the base of the public track-way, a flat path at the top of the storm bank. This distance was then divided into three sections, thus giving a top sample at the uppermost part of the beach, a middle sample and lower sampling point as close to the water as we could safely get. At each ...

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COLLECTION OF DATA

Once all the sediment samples had been collected, we set about measuring them. A way of sampling the stones had to be devised so as to maintain regularity with our technique. The stones were poured out of their plastic bags and then laid out into a grid. Once this had been completed we used the random number generator on a calculator to choose which number pebbles would be included in our sample. 25 pebbles were chosen in total from each of the three samples at each post. This, we thought, would give us a fair representation of the individual characteristics of the beach.

The stones then needed to be measured. To achieve this one person measured all the stones using a rule with millimetre divisions whilst one person recorded the results.

RESULTS

B measurement mean for stones at lower sampling points: 11.36mm

B measurement mean for stones at middle sampling points: 17.36mm

B measurement mean for stones at top sampling points: 18.73mm

When this data was input into the Spearman’s rank correlation test the following results were produced.

Lower Sample

Middle Sample

Top Sample

DATA ANALYSIS

The bar charts all show varying results. The chart for the top of the beach shows bars that do increase along the beach with only one point, post 12, having a value lower than that of post 8. Yet when this data was analysed using the Spearman’s rank test it produced an insignificant correlation at each significance level. Overall the B measurement rises from 10mm at post 8 to 14mm at post 18 with the value for the B measurement at the lower sampling points being 11.37mm across.

However, the bar graph for the middle sampling points shows a pattern in which the size of the stones seems to decrease along the beach. The post 8 value for the middle sample B measurement is 23mm compared to the post 18 value of 13mm. This would show an overall decrease in the size of sediment laterally along Hurst Beach were it not for the result at post 14 which is greater than post 8 by 1mm. The Spearman’s rank test conducted on this data gave a result showing there to be an insignificant correlation at every significance level.

The data for the top of the beach shows a somewhat different pattern. The graph shows there to be a trend in increasing B measurement along the beach. Though there is not a perfect correlation, the B measurement does rise from 16mm at post 8 to 30mm at post 18. This set of data was also the only set to produce a result of a significant positive correlation using the Spearman’s rank test.

The means for each set of data were calculated from all the values at every post along the beach. The means show that at the lower point of the beach the sediments B measurement was 11.36mm, at the middle point the mean measurement was 17.36mm and at the top the mean measurement was 18.73mm. This shows that the size of sediment does indeed increase with distance from the sea.

SUMMARY AND LIMITATIONS

I do not feel that the results I have gained are sufficient to prove my first hypothesis as the graphs are inconclusive. The data from the top of the beach does show the sediment size to increase along the spit showing evidence of longshore drift. However, the middle and lower data do not show any firm evidence of this.

I do on the other hand feel that I can conclude that the size of sediment does increase further from the sea on Hurst Beach. The data is sufficient to show this and there is geographical reason as larger stones will be pushed up the beach by either constructive waves with weak backwash or storm waves which hurl large sediments to the top of the beach.

The main limitation to the investigation is the stones collected at each point may have been sorted in a non-random way so that when they were picked using a random number generator an unfair representation may have been selected. This could have been the cause for anomalous results.