Limited intervention – this is where sediment and vegetation are introduced to reduce the wave energy to cause the waves to be ineffective.
Method
Figure 3
Atlántida beach, Sitges
The first data collection was on Atlantida beach, and on there I was trying to find out whether longshore drift was in action. Also to see what type of waves there were destructive or constructive. It is a shingle beach, not usually used for sunbathing. It is used for protection to break and slow down the waves, due to the importance of the land usage behind. The nightclub is used a lot for tourism and lies on the western edge of the Sitges coastline as can been seen in the map above figure3, needing to be protected to encourage tourism. Also with the railway line running immediately behind this, linking Barcelona to Sitges and another way of bringing tourism in. At the other end of Atlántida beach there is a sewage works that needs to be protected because the risk of polluting the sea and turning people away.
First of all we were in 10 sets of two evenly spread along the beach. Then we picked up a handful of pebbles and disposed of every third one, in the swash zone and also further up the beach. We discarded every third stone to make it a fair result of recordings. We recorded the size of the pebble and measured the long axes of them. We did this for 58 pebbles in the swash zone and further up the beach to get a representative sample. The reason why we did 58 pebbles was because this is was the lowest number of pebbles recorded to get a representative sample.
At the higher part of the beach we first of all measured each of the pebbles orientations, to try to get a similarity throughout the results to see if and what direction longshore drift was travelling. Then we did the long axis measurements. On the other hand in the swash zone we only measured the long axes of the pebbles and put them back due to the waves constantly disrupting the pebble orientation to get a recording of them. We recorded the data using a compass to measure the orientation and a ruler to measure the long pebble axes in cm.
Our next data was collected on beach number 4 which is shown on figure 3. We did a beach profile, when we were there we found out various different things such as beach width, beach depth and beach profile
The beach depth was the first measurement recorded and we did this at the groyne at the edge of the beach. We got a recording from both sides of the groyne to compare the difference in sediment levels. If the amount of sediment on one side of the beach is higher than the other, this shows a strong possibility of the direction that longshore drift is in affect. We used a clinometer and ranging poles to get an accurate reading.
Then was the beach width and what we did to achieve this was just simply to measure the width using a tape measure to get a correct measurement.
Lastly we managed to do a beach profile at the left, right and centre of the beach; we started at the edge of the shore and worked our way up to the promenade. We used ranging poles which were placed 10 metres apart all the way up the beach and a clinometer to test the gradient of the beach. The reason why we did this was to find out to see if there was a lot of material left on the beach and to see if beach replenishment was needed.
Also I will analyse the data and compare the location to pebble size by doing spearman’s rank correlation coefficient if there is any significant result to suggest that longshore drift is taking place.
Figure 4
Showing longshore drift
Data analysis
Spearman's Rank Correlation is a technique used to test the direction and strength of the relationship between two variables. In other words, it’s a device to show whether any one set of numbers has an effect on another set of numbers. We analysed our result by doing spearman’s rank correlation coefficient and found out that in the swash zone there was a very weak positive correlation of 0.19. In the storm beach there was also a very weak positive correlation of 0.18. From this you can see that the difference in pebble size along the beach is varied but this doesn’t show much of a correlation to suggest that the pebble size gradually gets smaller or gradually bigger along the beach. This suggests that there isn’t any longshore drift operating there. As we only did ten sites along the beach this could therefore not be statistically accurate. After consulting student t-tables I can conclude that my results are not statistically significant at any meaningful level of confidence.
Figure 5 Spearman’s Rank
In the figure6 shown below it illustrates the sizes of the pebbles in the swash zone and in the storm berm, at all 10 locations all the way down the coastline of Atlántida beach.
We were looking to see if there was any indication that longshore drift was affecting the Sitges coastline. The things that we looked out for were to see if that along the coastline to see if either side of the beach had a higher level of sediment. Also to see if there was a side of the beach that had more sediment, to see if this was a continuing trend throughout the rest of the beaches along the coast.
We can see that from this that in the middle of the coastline there was less sediment, there could be an indication that, if they have sea defences that they are not effective. The sizes of the two different types of data are separate and they storm berm pebble sizes are all bigger except for the group threes data which seems to be anonymous. This shows that the wave direction known as longshore drift could be in motion. Due to both sides of the beach having bigger size pebbles than all the rest, the erosion to cause this pebbles to get smaller is all done by the destruction that the waves cause.
In various parts of this beach there was no sea defences and that is taking its toll as closely behind this beach is a nightclub, but, more importantly a railway line connecting Sitges to Barcelona so it is crucial to keep that protected. Sitges relies a lot on tourism and with limited beach access; the local economy will suffer from lack of beach, with restricted access to the sea side resort as the railway line will no longer be safe enough to keep it in use safely.
Figure 5
Pebble sizes
In the graph above labelled figure 5 if you section it into the first three sites, then the next four and then the final three sites, if you do this you can create an average of the size of pebbles on the central part of the beach the west of the beach and the east of the beach.
Next we moved further down the coastline to do more analysis. At Atlántida beach we also had a separate beach in each of our groups which had groynes to separate them. From the results we collected, we split each of the 5 beaches up into three parts the west, centre and east, and identified which side of the beach has the most sediment there. We compared it to the rest of the groups data to see if the was any correlation to show which direction longshore drift was travelling in if any. As shown in figure 6
Figure6
Graphs showing beach profiles
Evidence from the graphs above shows us that the centre of the beach tends to have the lowest amount of sediment and also be the lowest point. The east side is predominantly the highest throughout all the graphs except for beach 8 which is the opposite and the west side has the higher amount of sediment.
The results from our separate beaches are shown below in figure6
Figure 7
Sediment levels either side of the 5 groynes
From this you are able to see that there is a trend, most of the sediment is on the west of each beach. This suggests that longshore drift is happening and that it is coming in from the east and travelling towards the west. Except one anomaly which is beach number 5, where the majority of the sediment is on the opposite side. Beach number 4 was the most successfully defended, because it had by far the most amount of sediment left and was worth protecting for economical gain. Also other reasons why this stood out as being the best is due to the groyne on the eastern side being a lot longer than the others and being at a different angle, almost parallel to the direction of where the longshore drift was coming from. The sea defences that are already in place are the groynes and are affective. As they are keeping each beach divided and prevents all the sediment ending up all down the western side of the coast. The groynes are slowing down the process of longshore drift and if there were more groynes along the coast the effect of longshore drift would be minimal. The only concern is that is it worth having the groynes in parts of the coast due to lack of sediment remaining to protect. Below shows the widths of each beach and the amount of sediment that remains there in figure8.
Figure 8
Beach width
The graph shows us that beach 4 has by far the most sediment with 73.5 metres of sand. The defences are very effective as the sand is not being moved. As the direction of the waves to the direction of the groyne are parallel so longshore drift cannot take great affect. It is more important to protect a beach that is popular with tourists and with enough sediment to attract tourists and sunbathers but if not it not as essential to protect it.
Figure9
The effects of sub-aerial weathering
Here shown above in figure8 is a section of the Atlántida beach, in the car park next to the nightclub, in front of the railway line which was not only being eroded by the waves but also sub-aerial weathering. This is playing a major part in this. Sub-aerial weathering is where destruction is caused to the land by natural weather such as rain. Where the bottom part is getting eroded, this has created a wave cut. As the size of the cut increases the weight of the top of the cliff becomes far greater and will eventually topple due to the surface water. This will continue to happen but as soon as the initial edge of the cliff has fallen it provides temporary protection for the cliff behind. Then the whole process starts over again. This is not being managed well enough as the car park is slowly eroding and some measures that could be put in place are that sea defences, as this will prevent the cut forming and maintain the strength of the cliff. This problem needs to start to be addressed before it causes damage to the local economy.
Conclusion
The hypothesis ‘the coastline of the resort of Sitges is being effectively managed’ cannot be answered fully on the whole of the coastline. As this investigation has shown parts of the coastline are being effectively managed and others are clearly not. The level of sediment to protect is a factor on the effectiveness of the defences in place. Some of the beaches are classed as beaches but you can barely fit anyone on it, so they still need protecting i.e. the promenade. Other beaches had a far greater amount of sediment so the need for the protection of it is vital for tourism especially; as Sitges’ main income is from tourism. There is a strong indication that longshore drift is happening and going from an east to a west direction. Some of the defences in place may need to be adapted to prevent sediment being removed and transported. So the main parts that need to be protected are being protected but could improve to prevent increased lack of sediment.
References
