Physical Agents of Land Loss: Waves, Currents, & Storm Surges
Most exposed coasts lose land primarily as a result of erosion, the wearing away or removal of the land surface by mechanical, chemical, and biological agents. Landward retreat of the shore is commonly the result of this erosion. Coastal erosion is initiated by the movement of water in the form of high waves and strong currents. Breaking waves erode the coast by suspending sediment particles or dislodging rocks. Ice, chemical weathering, and mechanical abrasion also aid the erosion of some rocky headlands and sea cliffs. Beach cobbles and sand act as tools that repeatedly strike the rocks and gradually wear them down.
Powerful storms rapidly raise water levels and accelerate coastal currents causing the most rapid losses of land and perhaps most of the permanent land losses worldwide. Land loss during storms depends on many things, including distance from the storm center, storm-surge heights, wave characteristics, direction of storm movement, angle of wave approach, forward speed and duration of the storm, and tidal stage during storm landfall.
Storms that strike the coast at high tide, and especially at spring high tide, tend to cause more damage because the storm surge superimposed on the high tide causes greater flooding and overwash. In areas where the tide range is small, tidal stage is of little consequence to storm-surge heights, but it is often important where the tidal range is large. Land loss and property damage can increase when a peak storm surge coincides with high tide.
The most damaging coastal storms are either extratropical cyclones (winter storms) or tropical cyclones (hurricanes) that form around centers of low barometric pressure (Morton, 1988a). Winter storms derive their energy from the atmosphere, whereas the ocean is the principal energy source for summer hurricanes. Although the two storm systems are quite different, their influences on water bodies and nearshore environments are similar. Both generate high, steep waves and strong currents that introduce new sediment into the littoral system and redistribute pre-existing sediments over large areas of the shoreface and continental shelf. Although some coastal regions, like the middle Atlantic states, are ravaged by both winter storms and hurricanes, each region is usually affected most by one type of storm.
A combination of beach composition and storm characteristics determines how beaches are affected by storms. For example, retreat of bluffs and muddy shores occurs in an episodic, stepwise pattern without any seaward advancement between retreat events (Fig. 3). This is in contrast to sandy beaches that tend to at least partly recover after storms.
The sand beach is like a conveyor belt operating between the dunes and offshore sand bars. These two large reservoirs of sand and the beach constitute a sand-sharing system operated by the wind and waves. Storm waves erode the beach and dunes and transfer sand offshore; some of the sand is temporarily stored in offshore bars or on the shoreface and some is transported onto the shelf and deposited in relatively deep water where it is permanently lost from the littoral system. Non-storm waves tend to move the bar and shoreface sand back onto the beach where it is blown landward to form the dunes. Any interference with the onshore and offshore movement of sand, such as emplacement of coastal engineering structures, could promote land loss.