Barrier islands are constantly changing. They are influenced by the following conditions:
- Waves - Waves continually deposit and remove sediments from the ocean side of the island.
- Currents - Longshore currents that are caused by waves hitting the island at an angle can move the sand from one end of the island to another. For example, the offshore currents along the east coast of the United States tend to remove sand from the northern ends of barrier islands and deposit it at the southern ends.
- Tides - The tides move sediments into the salt marshes and eventually fill them in. Thus, the sound sides of barrier islands tend to build up as the ocean sides erode.
- Winds - Winds blow sediments from the beaches to help form dunes and into the marshes, which contributes to their build-up.
- Sea level changes - Rising sea levels tend to push barrier islands toward the mainland.
- Storms - Hurricanes and other storms have the most dramatic effects on barrier islands by creating overwash areas and eroding beaches as well as other portions of barrier islands.
The impact of storms on barrier islands depend upon qualities of the storm (storm surge, waves) and upon the elevation of the barrier island at landfall. To quantify the impact of storm damage, the U.S. Geological Survey (USGS) has devised a "hazard scale" as follows:
- Impact 1 - Wave erosion is confined to beach area. The eroded sands will be replenished in a few weeks to months and no significant change occurs in the system.
- Impact 2 - Waves erode the dune and cause the dune to retreat. This is a semi-permanent or permanent change to the system.
- Impact 3 - Wave action exceeds the dune's elevation, destroys the dune and pushes sediment from the dune landward (approximately 300 yards/100 m), thereby creating overwash. This change in the system pushes the barrier island landward.
- Impact 4 - The storm surge completely covers the barrier island, destroys the dune system and pushes sediments landward (approximately 0.6 miles/1 km). This is a permanent change to the barrier island or portions of it.
For more details, see USGS: Mapping Coastal Change Hazards.