There are 2 types of waves Destructive and Constructive. Constructive waves are low energy waves. They deposit materials and build beaches. Destructive waves are high energy waves. They have more power and can remove the sand from a beach very quickly. The most destructive waves occur during storms. These waves erode in four ways:
These waves carry deposition which is then dumped on land by constructive waves. This happens when the waves have less energy. Groynes are made so deposition builds up against these wooden barriers which make the beach wider and so it absorbs the wave’s energy. This slows down the process of cliff erosion. There are other types of sea defence such as Sea walls which are walls with a curve inside it that reflects the wave and Boulder barriers which absorb the wave energy and the beach material in place.
Bays such as Porlock Bay which the course work is set on forms when there are 2 hard rock headlands between a soft rock headland. Over time waves erode this soft rock headland and leave an arch like shape which is the bay. The 2 hard rock headlands which are outside Porlock Bay have been eroded away by destructive waves. The deposition has been ended up at Gore Pt which by LSD has transported it to Hurlestone Pt. Our investigation of LSD is to do with these pebbles (deposition) we have to collect data from both Gore Pt and Hurlestone Pt from pebble’s size and shape.
Hypotheses
My investigation is to investigate longshore drift in Porlock bay. I have to do this by examining the pebble’s size and shape and the size of the beach
at Gore Pt and Hurlestone Pt. From these 2 points I will collect the data from the pebbles and justify my collecting by differences and comparing the data. We firstly have to collect data from Gore Pt. I expect the pebble’s size to be big and angular as these pebbles have not yet been eroded. This is because the headland is right next to the pebbles and as the wave is moving in Hurlestone’s direction. After we have to collect data from Gore Pt we have to get it from Hurlestone Pt. I expect the pebble’s size to be small and round as abrasion and attrition has eroded these pebbles away. Longshore drift moves the pebbles from Gore Pt to Hurlestone Pt pounding rocks and pebbles against cliffs wearing the land and pebbles away having an abrasion affect. Also as LSD moves the pebbles to Hurlestone Pt the waves smash rock fragments against each other making pebbles smoother, rounder and smaller which is the attrition affect. In conclusion I think the pebbles are going to be big and angular at Gore Pt and small and round at Hurlestone Pt.
The size of the beach is going to be taken by measuring the angle of the facet and the length of it. We are going to do this at both points. I expect the beach at Gore Pt to be small and have lots of facets because this is were all the material has been gathered up from the beginning and it is were the deposition is coming from. I expect the beach at Gore Pt to be small and the beach at Hurlestone Pt to be big as Longshore drift keeps moving the deposition to Hurlestone Pt so the beach is were the transportation is dumping the deposition every day so it will get big. From the collection of data from the beach we have to create a beach profile. An example of a beach profile is shown below:
A cross section of a beach-beach profile.
Method
Equipment we used:
- Pebbleometer
- Powers index
- 30m tape
- Clinometer
- Ranging pole
Our day has come to collect data for our investigations on longshore drift. We firstly arrived at Gore Pt and got into small groups of 3 with exceptions of 4. We firstly measured the angle of the first facet using a clinometer. When we arrived at Gore Pt we were told that the smallest person in the group was going to use the clinometer so we could get an accurate angle. For this to happen the smallest person in the group has to point it at the tallest person in the group and we point it up or down the tallest person so were ever the 0 comes on the person say for example the forehead we have to remember that. The small person in our group pointed the clinometer on me to an accurate angle. We then measured the length of the facet using a 30m tape. After collecting the measurements we recorded them down. We did this for all the facets to come up with a full set of results to make a beach profile we mentioned earlier. After the collecting the data we had lunch and a bit of rest. After lunch we started to collect the pebble data. To do this we firstly got the 30m measuring tape and spread it down the beach starting from the edge of the first facet. We started at the bottom of the beach picking up a pebble at random every meter. In total we picked up 30 pebbles. When we picked up a pebble we measured its a-axis and checked it on the powers index if it was angular or round. After collecting the results of the pebbles we went off to Hurlestone Pt. We did the same for this point measuring the beach and pebbles. We are going to put the results together as a class to get a more reliable result.
Safety and Risk assessment:
- Always be 6m away from the sea
- Avoid being below cliffs as it can erode and drop its load on you
- Do not throw pebbles as someone can get hurt
- Take appropriate waterproofs as it can get wet
- Take lunch and water
- Wear sensible shoes that are comfortable as we will be tackling different conditions.
Results
We have put the results of the shape of the pebbles together to get a class total. This makes the results more reliable as a sample as this gives us a bigger number of pebbles.
Mirror bar graph of Gore Pt and Hurlestone Pt
Beach profile for Gore Pt and Hurlestone Pt.
Data Analysis
From the data I have found out various results. The data shows us that the pebbles shape has been evenly distributed throughout the 6 size categories at Gore Pt. It also shows us that the least amount of pebbles found in one category is the very angular category. The most amount of pebbles found in one category is the round category. The main amounts of pebbles have been found in the Sub category area. The data shows us that the pebbles shape has been poorly distributed almost clustered throughout the 6 size categories at Hurlestone Pt. It also shows us that the least amount of pebbles found in one category is the very angular category. The most amount of pebbles found in one category is the very rounded category. The main amounts of pebbles have been found in the Round category area. For both points the least amount of pebbles has been found in the very angular category. Approximately 150 out of the 180 pebbles have been found in 3 size categories.
Discussion
We have found out from the analysis that the pebbles were well distributed at Gore Pt and not at Hurlestone Pt. This is the case because the pebbles arrived on the beach by the headland next to Gore Pt due to landslides. The landslides occurred as approximately 10,000 years ago they were frost shattered. Frost shatter occurs when water gets into the cracks of the cliff. This water freezes and then expands making the crack bigger this then leads to landslides. Frost shattering leaves angular pebbles of a wide range of sizes. This is one of the reasons the pebbles are evenly distributed. Another reason why it is evenly distributed is because over time some get eroded by the waves and attrition leaving the pebbles all different sizes. The pebbles at Hurlestone Pt were poorly distributed because the pebbles, when at a certain size, were transported by longshore drift it had eroded the pebbles by attrition and abrasion leaving them round. The pebbles would have been eroded with no doubt leaving them in the “round” groups.
Conclusion
I have found a series of results regarding the pebbles. From the results and analysis I have come to a conclusion that my hypothesis was indeed right. My hypotheses stated that the pebbles at Gore Pt will be angular and round at Hurlestone Pt. I have found out that there is longshore drift occurring in Porlock bay. The pebbles have indeed eroded along the way. The pie charts have clearly shown that erosion has taken place as it shows differentials between the pebbles sizes.
Evaluation
The investigation went according to plan. There were many things we could have improved such as being very accurate in putting the pebbles in different sizes according to the powers index and we could have been more precise in measuring the a-axis of the pebbles. We could also have collected more to give us more precise, reliable results. The equipment could have been more accurate such as the tool used to measure the a-axis of the pebbles. To make the results more accurate we should of collected the pebbles at the same time at both Gore Pt and Hurlestone Pt to avoid tidal differences. However this could have had a minimum impact on our study as the time difference was relatively small.