Shoreline erosion and migration
Shoreline erosion is the temporary or permanent loss of sand from the beach or dune systems. Although sea level rise is not thought to cause significant beach erosion in itself, higher water levels in combination with wind-driven waves can cause severe erosion.
Storms are major causes of shoreline erosion, though storm-induced erosion is rather temporary or local in nature. Man-made structures like groins, jetties, and sea walls also can contribute to erosion by altering the movement of sediment along the coast.
Sea level rise is linked to shoreline migration, which refers to the gradual movement of shorelines and dune features in a landward direction, though not necessarily with any decrease in size or perceptible erosion. The illustration at right shows the typical evolution of a marsh as the sea level rises, and how hard structural stabilization of coastal property (via bulkheads or sea walls) could lead to complete wetland loss. These hard structures also deflect wave energy downward, often leading to more severe erosion at the site. Some states have banned hard-structural stabilization projects along the coastline because of their associated erosion problems.
Shorelines are one of the most dynamic areas of the ocean environment, and scientists have long known that sea level has been higher and lower in times past. By taking core samples and conducting sediment analysis, for example, geologists can determine which areas were above or below sea level at different points in the earth’s geologic history.
Visual evidence of past shoreline migration can be seen by the untrained eye as well.
In the photograph at right, exposed roots and tree stumps along the shoreline are remnants of Delaware forests that grew several hundred years ago when the sea level was lower and the shoreline extended further out.
Sea level rise also has the potential to exacerbate the damaging effects of coastal storms by increasing the severity of flooding in coastal communities. The graph below, based on tidal gauge data at Breakwater Harbor in Lewes, Delaware, shows the trend in sea level rise for this area during the 20th century. While the average daily tidal range at Breakwater Harbor is over four feet, there is a measurable rise in sea level of about one foot per 100 years. This sea level rise itself will not cause a rapid flooding event because it occurs gradually over a long period of time. However, one foot of sea level rise, when combined with a storm causing storm surge, will measurably increase flood risk and flood damage.