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:
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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.
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.
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.
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.
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.
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 was repeated at every point. In this way, I could easily work out the range and average and identify anomalies, which could occur if a large pebble was found near the shore, as opposed to near the cliffs. I selected this technique as it allowed me to accurately measure the pebbles features in a short space of time.
I also took some photos that I have annotated in my project as they allowed me to explain with visual evidence and proof what I was measuring and how it was being done. The photos provided evidence of what the cove looked like and showed the smaller pebbles at the front and the largest pebbles at the back of the cove. The photos immediately give the impression that my hypothesis shall be proved correct, due to the sediment distribution on the beach.
I decided to record all my results in a pre-prepared table as this saved me a lot of time and unnecessary effort. By using this method efficiently I was able to easily calculate the average, range and where necessary, the mode and median. These methods that I chose greatly sped up the time it took me to carry out the investigation and thus allowed me to spend greater time investigating the beach profiles. The tables I then constructed on excel easily allowed me to construct my relevant graphs in a quick and easy manner so that I could concentrate on the writing.
Firstly, we can see that the four profiles show some similarities and links but vary in their width and individual slopes. Transects A-B and E-F were both relatively narrow (15.5m). This may be due to larger sediment, which tends to make the beach steeper and narrower whereas finer sediment tends to make the beach wider and more gently sloping, as it is less solid and condensed than larger sediment. Wave energy may also be important since in the more exposed areas of the cove the higher wave energy would tend to create a wider, flatter beach profile. This suggests that I was to find larger sediment at Transects A-B and E-F and smaller sediment at Transects C-D and G-H, thus giving a fair representation of the whole cove, which is what I set out to do in one of my methods.
Transect A-B was almost flat until at 13.5m where there was a steep gradient until 14.5m and then its gradient lowered again until 15.5m. This can be explained as sediment is larger near to the cliffs and smaller near to the sea and thus the gradient rises when it meets the large rocks near the cliffs that are seldom eroded and well intact. This suggested that up to 13.5m, where the profile was almost flat, I was going to find smaller sediment and then from 13.5m to 14.5m I would find larger sediment as the profile gradient increased greatly.
This was the reason that I chose to collect pebbles at the same points as measuring the profiles as it allowed me to identify the size of sediment I was likely to pick up. Transect E-F had a steeper gradient and an interesting reverse slope at 14.5m possibly because of a storm beach which means all the sediment is thrown up the beach by large destructive waves and larger sediment is concentrated near the top of the beach, but smaller sediment, that is carried further by the waves was dumped just behind it so a reverse slope was formed. This meant that I was to find small sediment unusually placed at the top of the beach (20m), among big sediment.
Transects C-D and G-H were relatively wider (20m) and more uniform in profile possibly because there was less larger sediment and more finer sediment which would mean that they would be wider and more uniform as finer sediment tends to be more compact and more malleable. Thus suggesting, due to the uniform profile, that I was to find smaller sediment at Transects C-D and G-H.
The following graphs help to prove my hypothesis correct:
This graph shows that the closer to the sea, for example at 5m, the smaller the long axis of the pebbles average size (3.8cm). This was probably because there was most attrition in this active zone compared to nearer the cliff where material rarely underwent transport and erosion. There were exceptions due to the random selection of the pebbles in the quadrat so at whatever point up the beach there were always going to be pebbles that did not conform the rule. For example at 5m there was a pebble with a long axis of 10cm, which was longer than on transects A-B and E-F when they were at 20m up the beach. But on the whole we can see that the graph conforms to my hypothesis.
In the graph, we can see a clear pattern emerging as at each point up the beach the graph assumes the same shape. At every point up the beach transect A-B had the smallest long axis, which is odd as its profile suggested that Transect A-B and E-F were to have generally larger sediment, as both those profiles were relatively narrow. Transect E-F had the second smallest long axis at every point. Transect C-D appears to get smaller at each location and so does transect G-H until the 20m point when it rapidly increases from 6.3cm at the 15m mark to 13cm at the 20m mark. This was due to their uniform profiles.
This shows that there is some correlation between the distance up the beach and the long axis. We can see from the graph that for 5m and 20m up the beach the points are spread apart from the mean (range is 2-16cm for 5m and 2.2-17.8cm at 20m). But at 10m and 15m, the points are much more grouped around the mean (the range is 1.8 – 11 cm for 10m and 2.4-10cm at 15m). This is because at 5m up the beach, some pebbles are small, having been eroded, and some are large as they may have come from the cliffs and are not usually found near the sea. At 20m up the beach, most pebbles are large as they are seldom eroded by waves, but some are small as they are carried right up the beach in the swash of the waves, and cannot be taken back due to weak backwashes. At 10-15m up the beach, the points are close to the mean as this is the ‘active site’ where material constantly comes to and from the beach and back in to the sea so the points are bound to be close together.
This is because in storms all of the material is washed up to the base of cliffs by large destructive waves, which have a strong swash and weaker backwash due to percolation through the large sized sediment. This meant that the small backwash only carried the smaller material back seaward, the large material stayed where it was. This process of sorting the beach sediment meant that the larger beach material stays nearer to the cliffs and the smaller rocks tended to be moved back to the swash zone. Thus suggesting that my hypothesis will be proved correct. Also beach sediment at transect A-B was Greensand and at C-D, E-F and G-H was Purbeck and Portland limestone, which is more resistant than the Greensand which explains why at 5m Transects C-D and G-H have a larger long axis than would be otherwise expected.
I believe that we can expect to see that the bar graph bares close comparisons and linkages with the other graphs as the features of pebbles are usually proportional to one another.
We can immediately compare this graph to the graph of distance up the beach against the long axis. Once again transect G-H seems to have the largest beach material at almost every distance, with the exception at 15 m where transect C-D had a short axis of 5.2 cm and transect G-H had a short axis of 4.7 cm. The two graphs should look so similar as the long axis is often related to the short axis. We can also see that at 5m and 20m up the beach the sediment was very large for transects C-D and G-H. At 10m and 15m up the beach, there was significantly less material. This was because at 20m the large destructive waves threw all the material up the beach with a strong swash, but could not take it back to the sea as the waves had a weaker backwash.
They could only take the sediment back in to the sea which was at 10m and 15m. At 5m, there was a lot of material because it was the active zone, which always had material on it. This explains why there was a lot of material at 5m and 20m up the beach, and less at 10m and 15m up the beach. Again this can be linked to the graph showing distance up the beach against long axis as we can see that generally the graph conforms to my hypothesis (sediment gets larger as I got further up the beach), with the exception of Transects C-D in both graphs, where there is far more sediment than expected, especially at 5m, as this happens to be the active zone for this particular Transect and thus naturally a lot of sediment congregates here as it is constantly brought on and off the beach by the waves swash and backwash.
By looking at the ‘y’ axis we can see that all the pebbles were graded 3 or higher on the roundness scale, which tends to suggest that they have all undergone attrition, but obviously some more than others. Of course according to my hypothesis I would hope that those that are highly graded (5-6) would be found near to the shore and those that are graded around three would be found near to the cliffs.
The pebbles at 5m are all fairly round (3.7-6) and at transects E-F and G-H they are exceedingly round (5.4-6). This is because the sea is continually eroding all the pebbles at only 5m away from the sea. This is good as it proves the first part of this section of my hypothesis correct. We can also see that there is very good linkage between these two Transects and Transects E-F and G-H in the graphs showing distance up the beach against long axis and short axis. This is again unsurprising as usually all the features of the pebble are closely related, as they are proportional to one another.
The pebbles between 10-15m up the beach were also quite rounded as they frequently moved up and down the beach in the ‘active site’ zone thus undergoing attrition as they frequently came in to contact with other pebbles. This helps to prove my hypothesis correct as I expected to find relatively round pebbles at 10-15m up the beach.
The pebbles at 20m up the beach appear to have a surprisingly high level of roundness, all between 4.6 and 5.3 on the roundness scale. This may have been due to problems in the collection of results or high tides may have rounded the pebbles and left them high up the beach. It is even possible that the pebbles were initially rounded down by the shore and then pushed up the beach in a storm. On average though, the pebbles are the roundest at 5m up the beach, which is what I stated in my hypothesis. Surprisingly, the last section of this part of my hypothesis was not proved correct, as the pebbles appeared to disprove my hypothesis.
But I should have expected this as by looking at my profiles we can see that Transect A-B rose in gradient between 13.5m and 14.5m but then lowered again until 15.5m, thus suggesting a clump of smaller pebbles found behind the larger ones. Transect E-F takes this idea a step further as it has a reverse slope, which means that I should have expected to find smaller sediment concealed behind (nearer to the cliffs), the larger sediment. Finally, Transects C-D and G-H were relatively uniform in the first place which suggests that there would be more smaller sediment and less larger sediment all over that particular part of the beach. So by looking at the profiles we can now see how they have effectively predicted the particular size of the pebbles and their whereabouts, thus disproving the final stage of this section of my hypothesis.
Transect G-H is clearly seen as having the thickest pebbles at all given distances except one, where at 10m Transect C-D is thicker (3.7cm). This suggests that cliff erosion had taken place before the sampling and so larger pebbles fell down all over the beach meaning that some of the pebbles chosen in the quadrat were larger than would otherwise be expected, especially at the 5m marks. Especially as the profile of Transect G-H suggested that less large material would be found at any location, as it was more uniform in profile. This appears to disprove my hypothesis as I predicted that I would find thicker pebbles at 20m up the beach as generally less pebble erosion takes place this far up the beach.
Transects A-B and E-F started off at 5m with low figures for thickness, which is expected as they become thoroughly worn due to erosion by the sea. Both transects then have a greater thickness (1.4cm at 5m to 1.9cm at 20m for transect A-B and 1.2cm at 5m to 2.2cm at 20m), further up the beach (which was expected.) This helps to prove my hypothesis correct, as this was exactly what I predicted. I thought that the pebbles would remain small near to the sea due to a lot of erosion and stay thicker near to the cliffs due to less erosion. In this case they have done, thus helping to prove my hypothesis correct.
Transect C-D appears to decrease in thickness from 5m and get increasingly smaller further up the beach. This is the exact opposite of what my hypothesis stated. I thought sediment would increase in thickness, as it got further up the beach due to lack of erosion. In this case it appears to decrease in thickness, as it got further up the beach. As Transect C-D tends to be wider in profile, thus producing finer and more compact sediment, it is generally expected to be smaller in sediment size, even at 20m up the beach, than is usually expected, thus explaining why the sediment appears to decrease in size, rather than increase.
Once again we can see excellent linkage between this graph and the graphs showing distance up the beach against long axis, short axis and at 15m and 20m up the beach for the graph showing distance up the beach against roundness, due to the fact that they are all proportional to one another.
How and why the methods could have been improved? How and why data may have been different on other days/times/conditions?
I believe that the data for my graphs were on the whole reliable, although there were some anomalies. Both methods I used to obtain the beach profile and pebble size were accurate as the equipment was reliable and easy to use although I could have used the cailleux index as it is more accurate than the powers index to measure the pebbles angularity. There were a number of ways in which data collection could have been improved. Sample size could have been increased and so greater reliability achieved. If I had always chosen to take the smallest or largest pebbles out of the quadrat at each location, this would have made it a fairer test, as opposed to taking random pebbles. I did however take my readings at four points at what I thought were equidistant around the cove.
I only sampled pebble size for one day. If I stayed in the area and sampled pebble size for a week, I could then have taken more samples and got a more reliable set of results. This would have meant I could have taken an average for the data, worked out the range and also found the median and the mode. If I took results at different times of the day, I could have found that at high tides the pebbles near to the sea might have been the pebbles at 10-15m at low tide. By taking results at different times of the year, I could have found that my results differed dramatically, which would have effected my conclusion. For instance, in the Winter I could have found that the pebbles could have been rounder as there could have been higher tides and more storms resulting in more pebbles becoming more rounded.
In the summer, the pebbles could be less round as the sea is generally calmer and therefore there is less chance of them being severely eroded. . I did not know what the past weather conditions were at Lulworth cove. There could have been a storm, days before I collected my results, which would have thrown the sediment about the beach and meant that small sediment could have been found near the cliffs and the larger sediment found near the sea, which would prove my hypothesis wrong as I would find rocks in extraordinary and unexpected places around the cove.
Presentation: How and why these could have been done differently?
My graphs were presented before the text that analysed them. This made it often hard to look at the writing and compare it to the graphs at the same time. It would have been more convenient to place the graphs in the middle of the block of text. This on the other hand, may have made the text become difficult to follow and thus eventually tiresome to read. I also believe that my results would have looked better if I had placed them in the actual project in a separate section, ‘Observation of Results.’ This would have been easier to see, rather than having them in the Appendix, just to prove that I actually went to Lulworth Cove.
I could have placed some graphs or pie charts on other pieces of paper e.g. putting pie charts showing percentage rock type on a map showing my four Transects. This would have increased the mathematical complexity of my project and presented the same information, but in a clearer fashion. Other than these points I am entirely satisfied with the layout and presentation of my project, as I believe it to be portrayed in a mathematically complex manner, but yet still clear and easy to read and understand.
Interpretation: How valid are the conclusions. Would the findings apply to other times/places?
I believe that my conclusion is valid, as I have generally found that most pebbles conformed to my hypothesis, thus ensuring that one theory stood above them all to prove my hypothesis correct. I believe that I would have found the same theory emerging if I chose to conduct the investigation somewhere else, although it may not have appeared so clearly, as Lulworth cove is a concordant coastline and the more resistant rocks were found at the back of the cove and the weaker rocks were found near the sea.
This meant that the rocks near to the sea were easily eroded and the rocks near the cliffs, being more resistant were less easily eroded and thus bigger. This was a perfect situation for my hypothesis and would have been very different elsewhere at another location. I would probably not have found that another location was a cove shape and thus would not have the more resistant rocks conveniently placed at the back of the cove and the less resistant rocks conveniently placed at the front of the cove.
I believe that I would not have found the same theory emerge at other times. This is because in the past, especially a long, long time ago, the pebbles near the shore would not have been eroded very much by the sea as they would not have been by the shore long enough for a lot of noticeable change in their size or shape. This would make the investigation pointless, as the pebbles near the sea would appear the same size and shape to those near to the cliffs, large and angular, thus completely disproving my hypothesis.
My project was to investigate how beach sediment varied in size and shape up the beach. I have found that in general, sediment is smaller and less angular near the sea due to hydraulic action, attrition, corrosion and corrasion acting on the pebbles. Also, sediment tended to be larger and more angular nearer to the cliffs as these processes seldom acted upon the pebbles. Of course there were some anomalies as no investigation is perfect and so I left them out of my final conclusion, as they would not have helped to prove my hypothesis correct.
I have also noticed that it was very relevant that I sampled pebbles at the same places that I measured the beach profiles as the beach profiles successfully predicted (in most cases) what size of pebble I was likely to find at which point up the beach. From looking at my graphs and explanations, we can clearly see that my hypothesis was overall proved correct as I predicted just what eventually happened.