Map of DunwichTransport of material eroded at Dunwich
Cliff material eroded along the Dunwich coast is transported down the coast. Material is transported within the sea by suspension (carried within the water); solution (dissolved within the water); traction (rolled along the sea bed) or saltation (bounced along the sea bed). The material is moved up and down the beach by the swash and backwash of the waves. These processes result in the large scale movement of material down the coast, called Longshore Drift. Material is carried up the beach in the swash, in the direction of the approaching wave, and is taken straight back down the beach in the backwash (under the pull of gravity). At Dunwich, material is being moved from north to south due to the prevailing north wind (see diagram below).
To test the longshore drift movement we observed and timed the movement of an orange along a 25 metre stretch of the coast. A clear link was observed between the direction of the wind and waves and the direction of the movement. On this occasion, the wind happened to be blowing from the South East, suggesting a northerly movement of material, however we know that the predominant movement is actually north-south due to the prevailing north wind.
Map of Dunwich
Longshore Drift along the Dunwich coastline
The impact of coastal erosion at Dunwich
Although today Dunwich is a small coastal hamlet with a population of about 120, it was in Roman times a flourishing port. In 1286 however severe storms resulted in heavy losses to the sea along the East coast. Similar erosion events in the following years eventually resulted in the silting up of the harbour as a spit formed. The decline of the port resulted in the decline of Dunwich itself due to trade losses and a significant reduction in population as a result. From 1600 further significant land losses occurred and many buildings were lost to the sea. All Saints church the last to be lost to the sea fell over the cliff between 1903 and 1919. The site of the former city of Dunwich is now completely under water in the North Sea (a reconstructed model can be seen in the museum).
Coastal Management at Dunwich
Very little management has been undertaken at Dunwich and the sea still eats gradually away at the cliff line. There have been some attempts to slow erosion by bulldozing shingle against the cliffs in the winter, although the cliffs are still attacked by sub-aerial weathering processes. The shingle sea wall to the North of the present day car park is also kept in place by bulldozers to protect the marshland beyond it from the sea. The natural creation of a new beach to absorb wave energy naturally is seen as the most effective solution along this unmanaged stretch of the coastline which may mean losing some of the shore.
Deposition of the Beach store transported south from Dunwich
Further south down the coast at Thorpeness, the coastline changes direction from south to south-west and the sand and shingle moving south from Dunwich has continued to be moved south. This has resulted in the development of a spit (Orford Ness Spit) which has deflected the River Alde, the mouth of which used to be at Aldeburgh. The material deposited at Aldeburgh is used by the waves as a beach store for extending the spit further south.