Wave Period
We used this information along with the swash time to confirm that there were destructive waves. The swash time is the difference in time between the wave breaking, and the swash reaching its highest point up the beach. The swash is the part of the wave that washes up the beach, whereas the backwash is the opposite, the wave going back down the beach. If there is a small swash then there is very little material being deposited on the beach, and it would suggest a strong backwash that removes material from the beach.
Swash Time
We found that the average swash time was 3.586 seconds (fig 3.31). I can use this information to check my earlier calculation that there were destructive waves: if the number created by dividing the swash time by the wave period is less than 1, then the waves are destructive.
3.586 ÷ 4.808 = 0.745
This number is less than 1 so it confirms my earlier calculation that these are destructive waves.
Long Shore Drift
It is possible to tell from geographical sources which way the prevailing long shore drift direction is. If we look at Aldeburgh further south (fig 3.6), we can see that a spit has formed, south of the town. However, on the day that we visited Dunwich, we found that the long shore drift was taking the beach material north.
Instead of taking measurements in those untypical conditions, we were given measurements from the previous year when the drift was heading southwards.
The measurements were carried out by: throwing five wine corks into the sea, and observing them for ten minutes.
Long shore Drift Measurements
There are some quite considerable differences in the distances travelled, but these could have been caused by a number of reasons. The ones with smaller distances could have just been bobbing around behind the point where the waves crests were breaking, and the ones with larger distances could have been caught in breaking waves, and had the full force of the swash acting upon them. The corks washing up onto the beach, and not being brought back down by the swash could also have caused the smaller measurements.
Even though there were some different measurements, the results clearly show that there is a strong north-south long shore drift.
Beach Profile
The first set of measurements that we took was of the beach. We looked at what the shape of the beach is, and the change in size of the material on the sections of the beach.
To get the measurements of the beach, we put two ranging poles two metres apart from each other and then used a gun clinometer to measure the average angle of that part of the beach (Fig 3.3). The ranging poles are striped red and white sticks, while the gun clinometer is a device, which measures the angle between two points. We started measuring at the bottom of the scree slope, and worked our way down to within two metres of the sea. We also measured the rocks and pebbles in the areas between the ranging poles. We measured the beach angle in degrees and the pebble sizes in millimetres (Figs 3.7 + 3.8).
Beach Results
Graph
Geology
At Dunwich, and along most of the Suffolk coast, the cliffs are made from unconsolidated fluvioglacial material. This is the sediment that was left behind when the glaciers retreated after the ice age. We can see this from the field sketch (Fig 2.5) where a layer of the cliff is made from pebbles. Because this layer of rock is not very compacted, it is more easily eroded than the rest of the cliff. Because this material is so soft, it slumps easily. Slumping is caused by rain saturating the cliff, making it heavier and less stable, which in turn causes a section of the cliff to collapse onto the beach.
In this question, I have evaluated all the evidence for erosion in question one, and I have explained why the erosion occurs. There are many factors to the erosion at Dunwich and I have shown how and why they erode the Suffolk Coastline.
