COASTLINES
Where water meets the land. Coastlines are very dynamic features. The primary agent of erosion, transportation and deposition is the ocean.
Water in the ocean is in motion. Water in the ocean moves due to wind blowing across its surface and differences in density from temperature and salinity. On the surface water moves in waves. Waves in the ocean transfer energy through the water but the water in a wave is actually moving in an orbital pattern with little movement of the water itself. Water moves from place to place because of currents NOT waves.
Wave:
Related sites for waves:
Crest: the high point of the wave.
Trough: the low point of the wave.
Wave height: distance from crest to trough.
Wave length: distance between troughs or crests.
Wave depth: the depth to which the wave form extends, 1/2 the wavelength.
As a wave travels across the ocean it moves closer to the coast. As it approaches the shore the bottom of the wave form "feels" bottom and starts to drag. It also is "pushed up" forming a breaking wave. As the wave approaches the shore the wave is bent or refracted as the leading edge slows in the shallows. This refraction forms a current that moves parallel to the coastline known as longshore drift or longshore current. The longshore current is an active component of coastal erosion, transportation and deposition.
Erosion
Many erosional features can be found along a coastline. In general, waves and longshore currents act to straighten a coastline, eroding along areas that jut out and depositing along areas that indent. Waves erode rock material through the hydraulic action of pounding surf. Longshore currents erode unconsolidated material in much the same manner as a stream, velocity of the current determines the amount of material carried by the current.
Transport:
The primary agent of transport along a coastline is longshore current. The longshore current carries millions of tons of sand along a beach every year. The sand is moved along in a zig-zag pattern. Wave action moves individual sand grains onto and off of the beach as the long shore current moves the sand down the beach.
Deposition:
If the longshore current slows deposition will occur. Depositional features:
Barrier islands, barrier bars: large strips of sand, offshore from and parallel to the coast.
Beach:
A strip of unconsolidated material (usually sand or gravel) bordering the coastline.
A beach is divided into distinct zones:
Backshore: area that extends from the high-tide mark to the farthest extent of water during a storm event. This area is dry except during storms.
Foreshore: area between the high-tide and low-tide mark. This area is alternately submerged and dry.
Offshore: area from low-tide mark to a depth of about 30 ft. Although this area is submerged the sediment in this area is still part of the beach and will move onto and off of the beach between winter and summer.
Summer beach: during the summer sediment is moved up onto the beach from the offshore portion. This gives the beach a flattened, more gentle profile.
Winter beach: during the winter sediment moves off of the beach into the offshore area. This gives the beach a steeper profile.
Sand is constantly moving onto and off of a beach. Example of a grain of sand: Wave action pulls the sand grain out into the surf zone. Here it is picked up by the longshore current and moved down the beach. Wave action then pushes this grain back onto the beach but farther down the beach from where it started. The sand grain has moved in a zig-zag pattern. The longshore current carries an approximate volume of sand, wave action is constantly moving individual sand grains onto and off of the beach. Unless some force changes the velocity of the longshore current there is no net erosion or deposition on the beach.
If the velocity of the longshore current decreases (as when it passes an inlet) deposition occurs. If velocity of the longshore current increases erosion occurs.
Beach Erosion:
In many areas along the East coast of the United States beach erosion is an important concern. Structures that manipulate the longshore current can deter erosion and even encourage deposition.
Strategies for living on a shrinking beach:
Groin: barrier built out from the shoreline that acts to intercept the longshore current. Deposition will occur because the current is slowed, HOWEVER erosion will increase after the current passes by the groin giving the shoreline a scalloped appearance.
Sea Wall: feature built along the shoreline to keep waves from moving up the beach. This protects structures built behind the sea wall but encourages beach erosion because it concentrates wave action at the base of the sea wall causing a scouring of the beach.
Move: traditionally structures along the shoreline were moved as ocean waves encroached. Cape Hatteras Light is threatened and there is a plan to move the lighthouse 1500 ft inland.
Changing Coastlines:
Sea level has risen and fallen many times during the geologic past. Also the land surface is uplifted and subsides. The result is a changing water level along the coastlines.
Eustatic sea level change: a sea level change that is world-wide. Caused by:
- Climate change and increase/decrease of glacial ice.
- Increase in rate of sea floor spreading increases sea level.
- Warming of the ocean water causes it to expand thereby rising.
Local changes in sea level caused by:
- Isostatic uplift in areas once covered by glaciers.
- Regional tectonic uplift.
- Land subsidence caused by increased sedimentation.
- Local land subsidence caused by withdrawal of groundwater or oil deposits.
