Coastal Management in PorlockBay
Coastal Management in Porlock Bay
Aims
Aim 1:
To find out if beach material is moved across Porlock Bay by longshore drift.
Aim 2:
To find out which type of sea defense is best for Porlock bay.
If L.S.D. (long shore drift) were taking place I would expect to find:
a) Deposition at one end of the bay and against any abstractions such as groynes.
b) An increase in pebble roundness in the direction of long shore drift. The process of attrition will erode the pebbles.
c) A decrease of pebble size in the direction of long shore drift, again attrition is involved.
d) Undercutting (active erosion of the cliffs at one end of the bay).
Method (data collection)
If Long Shore Drift were taking place, I would expect to find:
* Deposition of pebbles on one side of any obstruction. This obstruction could be a headland or an artificial barrier such as a groyne.
* A decrease in pebble size in the direction of long shore drift.
* An increase in pebble roundness in the direction of long shore drift.
I visited the two extreme ends of the beach, Gore point at the west and Hurl stone point at the east, at each end I measured:
* The shape of the beach known as the beach profile, using a tape measurer and a clinometer. A clinometer measures the angle of a slope.
* The size and shape of a sample of pebbles. These pebbles were picked out randomly. We took 100 pebbles from each end and measured their length using a ruler and used the shape chart to decide what shape the pebbles were.
First we had gone to gore point to study the area. There we saw a lot of sub angular rocks, so we set up a beach profile experiment.
Then after that we had gone to hurl stone point. There we saw a lot a lot of rounded pebbles, so we then again set up a beach profile experiment.
Introduction
We went on a Geography field trip with our class to Crowcombe in Somerset. Our class along with Mr Hopping went for three days and two nights, and stayed at the Youth Hostel. I studied and investigated Coasts. Which to be a bit more specific was interaction between sea and land. The general weather conditions were sunny with showery intervals.
There are five types of erosion as well as the four types of transport but first I will talk about the types of erosions. There are five different types, and they are corrosion or you could say abrasion, scouring, hydraulic action, solution and attrition.
Corrosion/abrasion is where the breaking waves scoop up stones and rocks fragments and hurl these at cliffs. This has the effect of chipping away at the rock, eventually breaking pieces off.
Scouring is waves that break at the base of a cliff swirl and remove loose rock with the strong current.
Hydraulic action is the pressure exerted by breaking waves and traps and compresses air in cracks. The intense pressure forces open the cracks further so weakening the rock.
Solution is where some rocks have a chemical composition which salt water can dissolve. In chalk and limestone, the calcium carbonate is dissolved, so weakening the rock.
Attrition is when the waves swirl rock fragments about and as they collide they wear down.
The process of longshore drift is when longshore drift is at work when waves break at an angle to the beach. The swash runs up the beach at an angle. However, the backwash flows straight back due to the beach's slope. This process of longshore drift also provides the link between the wearing down and building up of the coastline. The movement of material builds up to form distinctive features including spits, tombolo, beaches and bars.
There are the four different types of transport, which are Solution, Suspension, Saltation, and Traction. Solution is where the salts and minerals are dissolved from the rocks. Then we have suspension, which are finer materials that can be carried in suspension by waves. Saltation is the material, which is just too heavy to be carried in suspension, and that will be bounced along. We finally have traction, and this is heavier material that is rolled along the seabed by the strongest waves.
Formation of Porlock Bay
00,000BP
Bristol Channel
Sandstone Clay Limestone
resistance rock soft rock resistance rock
The rock types are important in explaining how Porlock Bay was formed.
Bristol Channel
Porlock Bay
Sandstone Clay Limestone
Resistance rock soft rock resistance rock
Rocks such as Sandstone and Limestone are resistant to erosion. These rocks will form headlands and cliffs. Soft rocks such as Clay are easily eroded and will form wide beaches called Bays.
Porlock Bay is a 4km wide pebble beach, which is placed in the North-west Somerset, in southwest England. Porlock Bay faces the Atlantic Ocean. When gales blow along the coastline they usually blow from northwest. Porlock Bay faces north.
The coast in this area is generally formed by cliff fronting hills, which rise steeply to 400 meters on Exmoor. At Porlock the hills are set back from the coast between Gore Point to the West and Hurlstone Point to the East, a distance of 5 kilometres. Between these points Porlock Bay has formed a shingle ridge to the landward edge, which has provided some protection to the property at Porlock Weir and Porlock Marsh.
Porlock Marsh is a strip of low-lying farmland behind Porlock Bay. Porlock Marsh is less than 10m above sea level. As a result Porlock Marsh floods at some high tides. The submarine forest, which is present at Porlock Marsh, shows that the sea level has risen in the past.
On 28th October 1996 the remains of the Hurricane Lily reached the British Isles and the storms that ensued gave rise to high tides and strong winds, which caused sea water to break through the shingle ridge. Four houses the Ship Inn and three cottages were flooded and ...
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Porlock Marsh is a strip of low-lying farmland behind Porlock Bay. Porlock Marsh is less than 10m above sea level. As a result Porlock Marsh floods at some high tides. The submarine forest, which is present at Porlock Marsh, shows that the sea level has risen in the past.
On 28th October 1996 the remains of the Hurricane Lily reached the British Isles and the storms that ensued gave rise to high tides and strong winds, which caused sea water to break through the shingle ridge. Four houses the Ship Inn and three cottages were flooded and 38 sheep were killed. Not only does this result in human suffering, but also in economical damage, i.e. loss of residential dwellings, farms, and live stock. Porlock is a tourist resort, the harbour and yacht trade attracting people to the area. A terminal Groyne is already in place, at the harbour as a means of preventing flooding there.
During our visit to Somerset we went to Gore Point to investigate the wave processes, which erode the pebbles on the beach. We also visited Minehead to investigate the wave protection measures in place and which ones could be effectively used at both Gore Point and Hurlstone Point. We viewed the terminal Groyne in place at Gore Point harbour and looked at how this affected the long shore drift occurring.
Hypothesis
Hypothesis 1:
An increase in pebble roundness in the direction of longshore drift. The process of attrition will erode the pebbles.
Hypothesis 2:
A decrease of pebble size in the direction of long shore drift, again attrition is involved.
Hypothesis 3:
The depositions of beach materials against any obstruction and at the end of one of the beaches as proof that material are being moved along the beach.
Hypothesis 4:
The groynes will be the best sea defense at Porlock bay.
Method
In order to test the possibility of long shore drift occurring and its direction, we had to visit two sites. The first site was on the Westside and was called Gore point. The second site was on the Eastside and was called Hurlestone point.
To gather accurate data we were split into groups. In my group, which is the classmates who I worked with were Akram and Henry. We all did simple tasks and these were rotated gradually. We did this to avoid errors and increase the accuracy of data. The same information was gathered at both sites.
I visited both ends of the bay and made a beach profile at each one. This was to show the differences in the size and height of the beaches therefore proving that the end with most material would also show the direction of L.S.D. To do this we split up into groups of 3 and used what is known as a clinometer along with a measuring tape to measure the distance/lengths and faces/facets of the beach. How we did this is explained in a step by step guide below:
. All walked to the bottom of the beach, closest to where the sea is and this was depending on the sea level.
2. Place the end of the measuring tape on the ground near the sea and place a heavy rock on top of it to stop it moving.
3. Two of you stay with the end of the tape whilst the other person takes the other person takes the other end and walks backwards with it until he/she reaches an obvious change in slope of the beach. It is here where the person must stop and look towards the other two people. Where this person is standing is known as the facet. One of the two people at the other end of the tape measure must record the length of the first beach face (the distance that the tape measure has been pulled out) whilst the other looks into the clinometer directly at the person with the other end of the tape.
Diagram of person using clinometer
4. The person with the clinometer must look through it and line up the centre line with the person with the other end of the tape. They will notice the scale moving up and down depending on the height they look through at it. When they have lined it up with the other person they must record the number/numbers that are lined up with the centre line. It is these numbers, which are the angle of the beach that you are standing on. The recordings that you make must be labelled facet 1 so that you know where to draw it when completing your beach profile.
This diagram shows what you should see when looking through a clinometer.
5. This process must now be repeated so that the rest of the beach can be recorded. To do this, the person on their own with the end of the tape measure must wait until the other two people have wound the tape up and walked up to the same position that he/she is standing in. then the whole process of steps 1 to 4 must be repeated, but this time the point closest to the beach will now be the point where the person was standing on their own. Then we had to record the results as facet 2 and keep repeating the process all the way up to the back of the beach, taking results for each part of the beach and labelling them appropriately.
Beach profiles
If material were being moved from one beach to the other across the bay, we would expect the shape of the beaches to be different. Perhaps the beach gaining material would be wider and deeper as well as stepper. In order to measure this we would need data on beach profiles.
The equipment we needed was a clinometer, measuring tape, recording sheet, pens, pencils, clipboard, paper and waterproof bag.
We were each given a section of the coast and this where we had to make measurements from the sea to the cliff. We had to estimate the break of the slope fact and then we measured the length, the angles were also measured. The way in which we did it was we stretched the 30 metres tape measure from one end to another and then we measured the angle with a clinometer.
Pebble size and pebble shape
The equipment that we needed to complete this activity was a recording sheet, random number sheet, powers roundness index, paper, pens, pencil, waterproof bag, ruler and 30 metres tape measure.
The reason that we believe that the pebbles are rounder and smaller in direction of longshore drift is because if the pebbles were moved, We would expect the waves to bang them together, so that they would get smaller and rounder.
To test
To test we could first of all get into our groups and measure then length of the coast. So each group will have a section of the entire beach. Therefore we could cover the entire beach with all our data put together. We would need a random number sheet so it would show that we are not being biest, and then we needed to have recorded the first reading, sphericity (how round it is) and length. We will need to record the data immediately so we do not forget and we done this 10 times. After, we all gathered our results up at the centre, so all the results were put on to one sheet. This would have proved that this was a fair test and that our results were more accurate, also this would have saved us as a group a lot of time.
Analysis
Aim 1:
To find out whether longshore drift is occurring in Porlock Bay.
Hypothesis 1:
Pebble length will decrease in size from west till east in the direction of longshore drift.
Pebble length in the class results show that the pebbles are smaller at the west side. This is confirmed by the group results. Looking in more detail at the class results it is apparent that there are no pebbles larger than 15cm, whereas in the east 31 were larger than15cm. This suggests that the sea is moving material from Gore point to Hurlestone point. Longshore drift must be the process. As the pebbles are transported they are eroded by attrition. This is the smashing of pebbles by pebbles in rough sea areas. This gradually reduces the size of the pebbles, as they are moved from point to point. My first hypothesis is that pebble length gets smaller from Grove point to Hurlestone point and is proven correct.
Hypothesis 2:
The pebble roundness will increase in the percentage of the total from west to east. The class results show to be true. This is also backed up by my group results. Taking closer looks at the data.... In the west over 50% are very angular to sub-rounded while in the east 75% are rounded to sub-rounded. This backed up my group.
These pebbles are from the same type and source of rock. These are located in the cliffs at the end of the beach. At that stage they are so loose, they can be picked out by hand. Observations made at the site told me they were all uneven, jagged and angular. This suggests again longshore drift is taking place. Hydraulic action has scoured out the angular pebbles during high tide storms. On their journey to Hurlestone point they have been bounced along the rocky seabed and banged into each other. This latter process- attrition takes the pebbles and bangs them together. This proves my second hypothesis and suggests longshore drift is taking place.
Hypothesis 3: Beach profile
An eroded beach will be shallow and a depositional beach, which will be deep as well as steep. What this means is material is being transferred by longshore drift from one beach to another. Therefore I expect the beach at Hurlstone point of Porlock Bay to have a bigger area.
Looking at the profiles we can see that Gore point has a gentle slope and low area while Hurlestone point steeper and has a greater area. What is happening is that material is being moved from one place to another. This proves hypothesis 3 and all that is in the first aim is correct.
Aim 2:
To see which Sea defense (s) is/are the most effective for Porlock Bay.
Conclusion
Aim 1:
Form looking at the results I have gained from this investigation along with my analysis I have proved my hypothesis correct, in that longshore drift does occur in Porlock Bay and moves beach material form east to west. I can tell this as the pebbles decrease in size and round off as they travel across the bay. It is also evident from different sizes of beach as shown in my beach profiles. This is because longshore drift has moved material from Gore point to Hurlstone point, making Gore point far smaller than it should have been. All three signs of longshore drift that I predicted would be present and would occur in the Porlock Bay. For example:
) One end is bigger then the other, as longshore drift moves material making one beach depleted.
2) Stones are smaller at one end than they are at the other.
3) Stones are rounder at one end than they are at the other.
As all these three signs of longshore drift shows the area in my results. I have proven that longshore drift occurs in the bay. I was also right when I predicted that the direction of longshore drift would be from west to east due to the onshore winds and long-fetch waves traveling across the Atlantic.
Conclusion
Aim 2
From analyzing the situation in Porlock Bay as best as I could, using background information, acquired knowledge, results from both ends of the bay and beach profiles and come up with the solution that beach nourishment is the best solution.
I have decided upon this scheme, as it isn't all that costly and will save the land behind the bay from turning it in salt marsh. The scheme will also preserve the areas natural beauty and will therefore give it a tourist appeal. If tourist were to start visiting the area, money generated from this industry could be used to continue depositing material on the beach. As material is moved from west to east along the Bay, I feel as though new material need only to be placed at Gore Point, as this needs the most protection. As you already know, road entrance to the bay is not a good method of entry to large trucks etc.... I think that material should be either pumped from below the low tide mark or dumped onto Gore point near the harbor. This will also avoid problems to do with access to the area as no vehicles will be needed to move new material around the beach, as longshore drift will do this for you. As the main material in the bay is stone, the beaches should be nourished with stones, however the new stones should be fairly large so that it takes a long time for longshore drift to move them along the bay. Another idea that would work out cheaper is to load up a ship with excess stones from Hurlstone point and then dump them off at Gore point. This recycling of the beach material would stop the headland at Hurlstone from getting any bigger as well as eliminate costs for the importation of new stones. This solution should work out at far less than the original quote of £400,000 and this leaves excess cash for the costs of renewing the defenses.
Limitations
Measurements
I am extremely confident in our measurements of pebbles and even more confident in our measurements of the beach facets. I think these are as reliable as we could get them to be, as we practiced using all equipment before we went out and used the most precise equipment we could get hold of. I personally feel that the pebble shape classifications could have been better if we had used a more defined system than the powers index. This is as I had to record many stones that did not really fit any of the descriptions given in the index but had to put them into one anyway. This could have been easily solved if we were to come up with our own identification method and used more class divisions to give a better graphical representation of the pebbles. This would have also given us a better idea of the stone shapes at each end of the bay and more precise results.
Even though I am happy with my results I feel as though some of the pebble measurements may have been rushed and slightly inaccurate due to out limited time at each end of the bay. Using a micrometer rather than a centimeter ruler could however have compensated this. One of these would have given readings accurate, down to the nearest millimeter when measuring the pebble width.
I could have also used a digital clinometer to give us beach facet angles, precise to the nearest degree. Even though this is digital and more precise equipment could have been used, it still would have proved that longshore drift was occurring just as the original equipment did.
Time
Without a doubt if we had more time at the bay we would of as group gathered more reading and therefore we would have better results. For example, if we had time to recorded information of 200 stones at each of the bay, we would have been given a far better representation of the entire beach. If I had some idea of how many stones there were at each end of the bay I would have been able to take a stratified sample of the stones and therefore give a true representation of the whole beach.
Time of the year
I also feel that the time of the year that we visited the bay varied in my results. This is as the position of the earth and moon effects the tides and will therefore change the shape of the beach and the amount of material at each end. Also the temperature would affect the rate of erosion and the tide levels, and their changes are what my beach profile would have looked at. I am sure of this because I have seen a beach profile from a group that carried the investigation around 10 months before mine and it looks very different from my profile. As we took all our results in the space of 2 days our results were as fair as we could get them to be.
Place
I also feel that we should have taken results from a range of sites along the bay, as I do not feel that the two ends were enough. If information was taken from 5 different places along the bay, a much better idea would have been given about how the shape and size of the stones change as they move along the bay. This would give us an idea of where longshore drift and attrition was happening the most and fastest.
If information were recorded from other bays along the coastline a better idea would have been given about the occurrence of longshore drift along the entire Exmoor coastline. This would have tied in nicely with the possible implications from defenses that had taken place in Minehead. If we were to carry this out I would have been able to see how much longshore drift occurred with the sea defenses in place. This would have most definitely altered my solution to the problem. Unfortunately we were unable to investigate other bays due to our limited amount of time in than area.
Explanation of sea defences
Sea walls, groynes and offshore breakwaters are often popular methods of defence. This is not because they are less expensive, in contrary they are more efficient. They all work by absorbing most of the wave's energy and this prevents excess erosion.
An offshore breakwater is a large concrete structure that spans the area you are trying to protect, but about 80 meters away from the actual coastline, therefore absorbing the wave's energy and not allowing them to reach the beach and cause any damage. However if large wave's were on attractive feature of the beach, it stands chance of losing some appeal. On the other hand, a nice peaceful sea will be there instead. Perfect for young children and adults who wish to relax. Neither of these is the case in Porlock bay as it is primarily not a tourist beach and all people are interested ii, it is also the protection of their land rather then a calm sea or where wave's are big enough to surf on.
Sea walls are expensive concrete structures that will span the area you are trying to protect and therefore eliminate any further erosion as well as practically using up the beach itself. This should not be too much of a problem as Porlock bay is far from a narrow beach and land behind the wall will then become available to the farmers and residents of the area.
Groynes are the long wooden structures that protrude out of the coastline. They are slow and if not stopped by L.S.D. stopping the beach material from being moved along the coastline. This method would not be as effective as other methods as almost all the sediments on the bay are large rocks that would quickly damage the weak wooden structures. Apart from that, the cause of the problem in Porlock, there is Groynes being built at Minehead, so building them or any other seas defences at Porlock would just cause even more problems further down the coast, and almost defeating the object of coastal defences.
Gabions stones in the wire baskets and the concrete revetments are less popular methods. As they are unsightly and will probably put people off and not come to the affected beach. They are large wire baskets in which large angular rocks are placed inside to take the blow of the sea. These are probably the easiest to be installed but don't allow much access to the sea itself as they must span the whole area for the operation to work. Access to the sea is not necessary for Porlock as there are very few users of seas in the bay area.
Beach nourishment is only useful in the short term, as it is not ma permanent solution and will have to be replaced every few years or so. Once you add more sand, the wind or sea will take it away again. This could prove problematic due to the area access difficulties. Nourishment involves large trucks carrying sand to form unnatural sources or from beaches with excess, will have to have access to the beach to deliver their payload. As Porlock is predominantly stones as opposed to sand, stones will have to be imported instead. This could prove even more problematic as stones will be even harder to transport than sand would be. Road access to the area will prove most difficult as the land behind the bay and closest to the main road is shared by two separate owners, one of which allows no access across his land and the fact that there are no real roads across either owners land will add even more problems.
Access may add difficulties to all of these methods, as all require the delivery of some sort of materials most of which will need to be delivered by a lorry or some other large vehicle.
However, if any action is taken to defend the coastline, if you hinder in nature's course of beach erosion, there will always be after effects resulting from unnatural intrusion. This proves that any sea defence could prove a problem especially with the land owned by Nation Trust.
Nevertheless if you could prove to the owners of the land to the extent of which erosion is taken place and show them how much LSD is affecting the area, action may well become a better option.
Whilst staying in the area I decided to investigate the effects of LSD on the coastline and show how it affects the coastline. If LSD has affected the area there would be more material at one end of bay as well smaller smother stones at the same end. This is because they would be eroded down whilst traveling from one end to the other.