The distance at which a wave has travelled is called a fetch.
Long Shore Drift
Long shore drift is the transportation of material along a beach. The prevailing wind causes the waves to break the beach at an angle. Swash carries the material up the beach at an angle. The backwash then brings the material back down. Every time a wave breaks the material is shifted up the beach in the direction of the prevailing wind. So for example, Long shore drift occurred from Gore Point to Hurlstone. The material was eroded from Gore Point and deposited at Hurlstone.
Longshore Drift –
material is travelling from Gorepoint which is on the west to Hurlstone which is in the East.
Gore Point Hurlstone
Coastal Erosion
Erosion is destructive waves, which wears the coastline. This happens when the waves are full with energy. Coastal erosion eventually will erode down landforms and cliffs.
There are four ways in which waves can erode. The following below state:
Hydraulic Action - This is when waves crash against the cliffs causing the water to trap in the cracks. When the waves move back, pressure is released causing the water to expand. Due to the expansion and contraction of the cracks, an explosion happens which leads the rock fragments to break off.
Abrasion - This is when breaking waves hit the rocks and pebbles against the cliffs, which eventually wears away the land.
Attrition - This is when waves smash rock fragments against each other causing the pebbles to become smoother, rounder and smaller. Eventually, the particles will turn into grit and sand.
Soloution - This is when the different chemicals in the seawater such, as limestone and chalk will dissolve or rot rocks that are exposed to the chemicals.
Coastal landforms
Bays and headlands are formed after coastal erosion and they are situated along the coast where there is both hard and soft rock. The soft rock easily erodes where as the hard rock is much harder to erode so it stays there and the rock then is called a headland. Due to the geology at Porlock bay the resistant rock, red marl, is eroded quicker than the non resistant rock which is quartzite.
The headland has faults and cracks due to coastal erosion occurring at all times. The crack become big and is eroded to form a cave due to hydraulic action, attrition and abrasion. The force of the water continues hitting the new cave and the gap widens. An arch is formed. The bottom of the arch gets eroded and the top becomes weak and collapses into the water. The heavy top cliff which collapsed into the water is called a stack. Water erodes the bottom of the stack and a stump.
Geology
The geology of Porlock bay’s area has both hard and soft rock. The hard rock is more resistant to the costal processes and the damage from weathering. Quartzite is an example of a hard rock. However, the soft rock is less resistant to the effects of weathering. Red Marl is an example of a soft rock.
Hypotheses
-
Pebble sizes are going to get smaller going from Gore point to Hurlstone point – due to longshore drift, the pebbles are going to hit the sea bed as it travels from West to East so parts of the pebble will chip off making the pebbles smaller as it gets to Hurlstone.
-
Pebble shape will become more rounded going from west to east – due to longshore drift the pebbles will hit the sea bed as it travels so it will make the pebbles more rounded as the sharp, rough edges will chip off as it smashes against the sea bed.
-
The size of the beach will be bigger at Hurlstone point compared to Gore point due to longshore drift – As the material will all deposit in Hurlstone, the size of the beach will be bigger. The material will be eroded from Gore point which will make the beach smaller and transported by longshore drift to Hurlstone which will make a bigger beach.
Methodology
Equipment we used:
- Calliper – To measure the diameter of the clast
- Powers index card – To class pebble shapes into different categories
- 30m tape – To measure the size of the beach
- Clinometer – To measure the angle of the facets
We arrived at Gore point and we were told to get into groups of 4. Firstly we measured the angle of the facet using the clinometers but we had to calibrate the clinometer. We were told that in order to calibrate the clinometer the shortest person in the group was to use the clinometere to get an accurate measurement. The shortest person had to point the clinometere at the tallest person in the group and point it where ever it read 0 on the clinometere. For example if the clinometere read 0 degrees on the tallest persons forehead, that must be noted and pointed at the persons forehead for every facet which was measured. We then measured the length of the facet using the 30m tape. We recorded all the measurements down. We repeated those steps for every facet in order to create a beach profile.
In order to categorize the pebble shapes we got the 30m tape and opened it down the beach starting from the beach to the sea. We picked up a random pebble at every one metre. We measured the pebbles using systematic sampling. We picked up 30 pebbles in total. We measured the pebble’s length (a-axis) using a pebbleometere.
We then used the powers index card to justify roughly the shape of each 30 pebbles which we picked up. We observed each pebble and matched it to the correct description. This was very subjective as not everyone in the group agreed with the description of the pebble’s shape but we had to choose one.
These are the descriptions which were on the powers index card:
Very Angular - everything about the pebble is sharp and rough.
Angular - most of the pebble is rough, except for one or two rounded corners.
Sub Angular - most of the pebble is smooth and rounded with one or two sharp corners.
Sub rounded - few bumps but mostly round and smooth.
Rounded - mostly smooth and round with one or two bumps.
Very Rounded - very smooth with no bumps, usually egg shaped.
Results
Clast Size in areas, Gore point and Hurlstone point
I predicted that the pebble size at Hurlstone point will be smaller than at Gore point. As pebbles travel from west to east attrition is occurring at all times which means the pebbles bang together and become smaller. I have presented my class results on a bar graph.
Pebble shape – Power Index
I predicted that the pebbles will become smoother and rounder at Hurlstone point than at Gore Point due to attrition. These are my class results which i have presented in a pie chart.
Safety and Risk Assessment
To make our trip to Somerset safe we were given safety rules which we had to follow. Below is a list:
1 ) We were told to keep away from the edge of the sea. This is because it was very dangerous as there was a very strong sea current.
2) We were told not to throw pebbles. This is because it is dangerous as if the stones hit someone, they could be seriously injured.
3) We were also told to keep away from big cliffs. This is because falling rocks from weak cliffs could fall at any point.
4) If we had any medical condition, then we were supposed to put the medication in the right pocket of our coat. This is so the teachers would have easy access without wasting any time.
5) Do not run because the surface underneath could be slippery which could cause an fatal injury.
6) Traffic along the roads can be dangerous because the roads in the country side are much narrower, which means less margin for error.
Analysis
I will be explaining the coastal processes on Porlock Bay in this section, using the data collected.
Beach size
According to my data I collected and the graphs I drew which show the facets of Hurlstone and Gore Point, you can see long shore drift occurring. You can see that both Hurlstone and Gore Point both have the first highest facet. Any coast lines have the highest facet at the top due to destructive waves forcing the material up to the top.
I compared Hurlstone facet length to Gore point and i found that Hurlstone has longer facet lengths. The average facet length at Gore point is 3.75cm compared to Hurlstone point which is 8.3cm. This concludes that the facet sizes are bigger at Hurlstone Point due to long shore drift which will build up the facet with the material transported.
Clast Size
I measured 30 pebbles in total, picked 1 pebble at every one meters for 30 meters. The sizes of pebbles at Hurlstone Point are smaller than the pebbles at Gore Point. This is due to attrition and abrasion breaking fragments of pebbles as they travel from West to East due to the process of long shore drift. The average pebble size showed me that the average pebble at Gore Point is bigger than at Hurlstone Point. This is due to attrition breaking of parts of pebbles to make it rounder, smoother and smaller at Hurlstone while long shore drift transports the pebbles.
The pebbles are distributed evenly at Gore Point due to abrasion breaking up bigger rocks to form smaller ones. Hydraulic action also breaks off angular rocks of the cliff which is gathered on to the beach.
Clast Shape
The last bit of my investigation was to find out if the pebble shape becomes rounder as it goes from the West to East. As the pebbles from Gore Point are moved to Hurlstone Point by long shore drift, erosion is happening. Attrition and abrasion erosion makes the pebbles rounder and smaller as it travels through its journey from west to east. The pebbles that are being moved collide with other rocks in the sea resulting with the pebbles becoming rounder and smoother as it gets deposited at Hurlstone; this erosion process is called attrition. Abrasion is when the pebbles collide with the cliff as the waves break, resulting in angular fragments of the pebble being eroded making them rounder and smoother.
Conclusion
I conclude that my investigation went successful and i found out there are costal processes occurring at Porlock bay.
As to my hypothesis of pebble sizes that they are going to get smaller as they travel from Gore Point to Hurlstone, this prediction was correct. In my investigation, i found out that that the smaller pebbles are at Hurlstone Point. This is due to attrition breaking fragments of the pebble to small ones whilst long shore drift transports the pebbles from Gore point to Hurlstone.
Evaluation
In my investigation, I could have improved the way i collected the data or how i could have further investigated coastal processes at Porlock Bay. There were many limitations of the study which I am going to talk about. The limitations prevented us to investigate in our study further; we could have got a wide range of results if we investigated further and got accurate result.
I could have improved my investigation into coastal processes at Porlock bay by using more precise measuring equipment. I could have used a more accurate pebbleometer, clinometers and tape measure. This could have improved my investigation as the results I would get is more accurate. We could have used a ranging pole instead of clinometeres which could have given a more accurate result of the facet gradient.
Another way to make my investigation more accurate i could have taken measurements from more than just two points at Porlock bay. If i took measurements from points between Gore Point and Hurlstone, I would have got a more wide range of results which would make my results more accurate.
