11.8: Chapter Summary

Processes That Form and Modify the Coastline.

Coasts can be erosional or depositional. Primary coasts are formed by volcanoes, earthquake movements, landslides, deposition of river-borne sediment, and sea-level change. Sea level has risen about 130 m in the past 19,000 years and continues to rise. Secondary coasts are formed by erosion or deposition by waves, tidal and ocean currents, and the formation of coral reefs.

Beaches.

The littoral zone extends between the seaward boundary of land vegetation and the depth where wave action does not disturb the sediments (about 10 to 20 m). Beaches are part of the littoral zone and consist of a back-shore, foreshore, and offshore. The backshore is normally dry at high tide and may be characterized by berms created by storm waves. The foreshore is the gently sloping area between the high- and low-tide lines, where scarps may form. The offshore extends seaward of the low-tide line and may have a longshore bar near its seaward boundary.

Beaches are a mixture of grains of varied composition supplied by erosion or by longshore drift. Longshore drift, caused by the incomplete refraction of waves, transports sand downcoast of the direction from which the waves approach. Sand is moved along the coast until it is transported down a submarine canyon. Some beaches may be in danger of losing sand because the supply of river-borne sand to the longshore drift system has been reduced by dams and coastal structures designed to reduce wave erosion. 

Particles that make up a beach are sorted by waves. Small particles are removed to deep water. Large particles are not moved by longshore drift and are present only if they are formed locally by the erosion of cliffs next to the beach. Grain size is larger on beaches with greater wave action. Quiet beaches are composed of fine-grained sand, whereas those with greater wave action are composed of coarse-grained sand or gravel.

Large winter waves move sand from the beach offshore to build a longshore bar. In summer, gentler waves return sand to the beach. In winter, a beach is narrower with a flat foreshore and steep backshore. In summer, the beach is broader, the foreshore is steeper and a summer berm or berms are formed. Beach slope increases as grain size of the beach materials increases.

Barrier Islands and Lagoons.

Barrier islands protect the coast from wave erosion and enclose lagoons that are nursery areas for many marine species. They are formed and maintained by sand that is moved by longshore drift. Barrier island–lagoon systems are retreating landward as sea level rises. Development on barrier islands presents problems because of this landward migration. Dredging of the lagoons and beach replenishment are necessary to reverse migration that would destroy developed structures on the islands, but these measures cannot be successful indefinitely. Developments on barrier islands repeatedly sustain expensive damage when major storms occur.

Beaches and Human Structures.

As beaches migrate inland or lose sand because river sources are reduced, seawalls and groins are built to retain the sand or protect coastal structures. Seawalls temporarily protect coastal structures from waves, but eventually they are undermined and destroyed as the shoreline recedes. Seawalls also reduce erosion that would otherwise supply sand to the longshore drift system, and consequently, they reduce beaches and longshore bars that would absorb wave energy. Groins are built across the littoral zone to retain sand on beaches by blocking longshore drift. Sand builds up on the upcurrent side of the groin and is lost from the downcurrent side. Groins are often only temporarily effective and can cause a loss of sand on down-current beaches that leads to the building of more groins. Jetties or breakwaters protect harbors from wave action but interfere with longshore drift and often cause sediment to accumulate in the harbor, making continuous dredging necessary.

Coral Reefs and Atolls.

Reef-building corals grow only in warm water shallow enough that their zooxanthellae can photosynthesize. They grow best in waters with lower turbidity and little salinity variation. Fringing coral reefs grow in shallow water offshore where erosion and freshwater inputs are low. Barrier reefs and atolls form where coral growth is faster some distance offshore than immediately along the shore, and where sea level rises or the coast subsides. Most atolls are formed from barrier reefs as volcanic islands sink isostatically below the sea surface.

Wetlands.

Tidal wetlands are flat, muddy areas covered by water part of the time. They retain detritus from river flow and vegetation that grows in them. The detritus provides food, and the sea grasses or mangroves and shallow water provide shelter for juveniles of many marine species. Large areas of coastal wetlands have been filled for development and agriculture.

Deltas.

Deltas are formed as sediments accumulate at the mouth of a river. They are flat areas maintained at an elevation slightly above sea level by periodic river floods that bring sediment, detritus, and nutrient-containing particles to be deposited on the flooded delta. They are good agricultural land because of these periodic inputs, but they are often flooded. Although levees have been built to protect many deltas from flooding, these structures cut off the supply of new sediment, so the land slowly subsides, is eroded away, and loses its nutrients and detritus.