An investigation into how beach material varies in shape and size up the beach.

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Tom Owens

  An investigation into how beach material varies in shape and size up the beach

I decided to do ‘An investigation into how beach material varies in size and shape up the beach’ as this topic has always interested me and it should be easy to do in the allotted time slot. I am glad I now understand why pebbles are where they are and how there shape is affected as a result.

The aim of my project was to investigate how beach sediment varied in size and shape up the beach and to investigate variations in the angle of the beach profile and beach width. To do this I collected results in the tables I had made earlier under the following headings:

Long axis- the length of the pebble in cm

Short axis- the width of the pebble in cm

Radius- half of the diameter (long axis.)

Roundness- on a scale of 1-6, how round was the pebble?

Thickness-how thick was the pebble when lying on its side?

The data for my project were collected at Lulworth cove, which is situated, on the south coast of England, Dorset. Lulworth cove is a typical example of a concordant coastline and is such a shape due to weaker rocks (Wealden Clay) being eroded and more resistant rocks (Portland limestone) not being eroded at the mouth of the cove. Thus the cove shape is formed. This is significant because it suggests that many of the pebbles I will find will be made largely of Portland limestone, which is still intact. Any other pebbles I find that are made of different materials e.g. Wealden Clay will probably be found near the sea (at 5m), as they are weaker and therefore quickly denuded.

Hypothesis:

I predicted that the closer to the shore, the smaller the pebble size and the less the angularity, i.e. a smaller long axis, short axis, thickness, radius and lower angularity. This is because the pebbles by the shore undergo more hydraulic action and attrition, which is when the sediment becomes rounded in contact with other pebbles and the sea. The pebbles nearer to the cliffs undergo little hydraulic action or attrition as the sea seldom reaches high enough for them to be eroded. They are consequently more angular and have a bigger long axis, short axis and radius. This also meant that they had a larger thickness and thus I predicted that the pebbles near to the cliffs would be larger.

Risk assessment:

Before approaching the cove I assessed the risks and noted the following five points:

Slipping – There is soft and permeable sliding clay so it is easy to injure oneself.

Drowning - There is a risk of drowning due to the undercurrent. 

Falling cliffs, collapse – It was important to avoid the base of cliffs and wear hard hats so if rocks did fall you were protected.

Sinking in the clay- The clay was wet and saturated so care had to be taken.

Risk of being cut off by the tide - When taking results it was necessary to keep an eye on the tide, as you would have to wait a long time for the tide to go out again to be able to leave the cove.

Stair hole:

At Stair Hole, there were three major rock groups. The most resistant and seaward rock, Portland limestone, was still largely intact and was dipping to the north which suggested, along with its high resistance that a lot of the pebbles I was to pick up would be made of limestone. The Purbeck limestone also dipped to the north but was layered and therefore had undergone a lot of folding, due to weakness, which suggested that I would be less likely to find many pebbles made of Purbeck limestone. The Wealden clay was very weak and incompetent and was quickly denuded so if I found some Wealden clay I would expect to find it near the sea (at 5m) and expect it to be very small. The clay underwent mass movement, otherwise known as rotational slumping. There were two other major rock groups, which could be seen in the cove itself, not in the Stair hole. These were Greensand, which was a very narrow outcrop and was of medium resistance. Chalk was found at the back of the cove and was resistant but not as strong as the limestone so if I were to find chalk pebbles, they would have been situated near the cliffs, (15-20m up the beach) and may have been more rounded due to the lower resistance.

Methods:

I used a variety of methods that were simple, quick and effective. I selected four sample locations by trying to find four points equidistant around the cove. I did this, as I wanted to make sure that the results gave a fair representation of the whole cove.

The sequence I followed was firstly, to look at the cove to get a good representation of it. I then drew a cross section of the cove ‘view east’ so that I could successfully choose four equally spaced apart Transects and this also this allowed me to get a much better picture of what the cove looked like. Having found my four Transect points I drew another map showing where they were exactly. This was also helpful as it allowed me to pin point the Transects exact location. Then, at each Transect

I measured the profile of the beach at each location using a 25metre tape, two 1.5m measuring poles, and a clinometer. The first reading was taken from the sea to the first significant change of slope angle up the beach. This process was continued up the beach until the base of the cliff. Thus, I could find out the varying angle of that particular section of beach. I selected this technique as it was very easy to carry out and could be done in a quick fashion. It was also accurate as the clinometer gave a good reading of the angle. Having done this I measured the pebbles features at the same location that I measured he beach profiles. This was because it allowed me to accurately predict what size and shape of sediment I was likely to measure at each point for each of the four Transects.

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I measured each pebbles long axis, short axis, radius, roundness and thickness at the same four locations that I measured the profile of the beach. At each location, I took four readings up the beach. These were taken at every five metres. To do this I put a quadrat at every point and sampled five random pebbles in the quadrat. A pebbleometer was used to measure the long axis, short axis, radius and thickness and each pebble was graded on its roundness on a Powers index scale of 0-6. (0 being very angular and 6 being well rounded.) This ...

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