In summer, the waves tend to be more constructive so the main work being done is deposition. This results in material being moved up the beach gradually, so the profile is gentler than in the winter when there are a lot of steep undulations close to the tide.
Sea level change
The global water balance is a closed system (energy can go in and out, but material cannot), but sea levels can change due to two main influences:
- Availability of water
- Tectonic changes
Eustatic sea level change is due to movement of the water – E=W
Isostatic sea level change is due to movement of the land – I=L
During maximum glaciations, large volumes of water have been stored on the land as ice, meaning less water was able to get to the sea, leading to a eustatic drop in sea level. However, when ice accumulates, its weight presses down on that part of the crust, causing the land to sink – this causes isostatic sea level rise – isostatic depression.
As ice sheets melt, there is eustatic rise because the water goes back into the sea. When there is less ice on the land, reduced weight presses down so there is isostatic movement upwards – isostatic uplift/recovery.
Tectonic changes have resulted in uplift (orogeny) of new mountains. Seashells have been found high in rocks of the Alps, Andes and Himalayas, all of which are fold mountains. Tilting (epeirogeny) of the land has led to submergence of several ancient ports in the Mediterranean.
These sorts of changes have various impacts:
- Shape of coastlines and formations of new features by increased erosion and deposition
- Balance between erosion and deposition by rivers resulting in drowning of lower sections of valleys and rejuvenation of rivers
- Migration of plants, animals and humans
- Hull and London are both at risk from sea level rise
- Salt-water intrusion
- Numerous villages have been lost from low-lying east coast areas due to subsidence
Coastlines of submergence
These develop when sea levels rise and as a result, river valleys are drowned. The river floodplain vanishes beneath the rising waters. At the edge of uplands, only the lower parts off the valley are filled with sea water so higher land is left dry. This is a common feature in south-west England where sea level has risen and drowned the valleys of rivers which flow off Dartmoor and uplands of Cornwall. There is the Fowey Estuary in Cornwall and Kingsbridge estuary in south Devon.
These are drowned glacial valleys normally found on coast of Norway, southwest New Zealand, British Columbia in Canada, southern Chile, and Greenland. They have steep sides which are quite straight and narrow. They form following the retreat of a glacier after carving out a U-shaped valley. Sea water fills the valley. An example is the Sogne Fjord in Norway, which is nearly 200km long.
When the land topography runs parallel to the coastline, so valleys follow this parallel pattern as well. When sea level rises, the valleys are filled, and it appears as small islands and peninsulas which are parallel to the shore. The name is derived from the Croatian Coast in the Adriatic.