COASTLINES
Learning Objectives
The Oceans:
The oceans cover 71% of the Earth's surface. They play an important role in the distribution of heat between the poles and the equator. The oceans also act to moderate temperatures where they interact with the continents.
Water in the ocean is layered by both temperature and salinity. In general there are three temperature layers in the ocean. The surface layer is the warmest and is constantly mixed by wave action. The Thermocline is a layer of rapidly decreasing temperature with depth. Below the thermocline is the deep cold layer. Salinity varies in the ocean from place to place and is controlled by fresh water added to the oceans from rivers and precipitation and evaporation of water off of the surface.
As temperature and salinity of seawater changes density of seawater changes. Water with a higher salt content is more dense and colder water is more dense. Water that is more dense sinks. This sinking of higher density seawater helps drive ocean currents. Water circulates within and around ocean basins via currents. Surface currents are also driven by global wind patterns. Surface currents are deflected to the right in the Northern Hemisphere (to the left in the Southern Hemisphere) due to the Coriolis Effect. The Coriolis Effect is produced because the Earth is spinning on its axis. Surface currents are known as Gyres and move water around an ocean basin.
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:
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.
Sea Cliffs: on a rocky coastline the ocean erodes rock material landward forming steep cliffs.
Sea Cave: In areas of headlands or rocky coasts waves will pound rock units and erode them back landward. Oftentimes this forms a cave at the base of a sea cliff.
Wave-cut Platform: as wave action pounds a coastline the rock will be eroded back landward forming a shelf type feature along the shoreline.
Sea Arch: sea caves eroded into a headland connect forming an arch.
Sea Stack: Rock promontory left when the top of a sea arch collapses.
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.
Spit: if longshore current is slowed as it flows past an inlet sand will be deposited. This finger of sand extends out into the water and builds over time.
Baymouth Bar: a sand barrier that cuts a bay off from the ocean. Longshore current flowing past an inlet deposits sand at the mouth of the inlet and over time can cut this inlet off.
Tombolo (tied island): Offshore islands or sea stacks cause wave refraction, this causes deposition landward of the sea stack or island eventually connecting the island to the mainland.
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 (spit). 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.
Emergent and Submergent 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.
Emergent Coastline: appears to be undergoing a drop in sea level. Common features include wave cut platforms, sea caves and sea arches.
Submergent Coastline: appears to be undergoing a sea level rise. Common features include drowned river valleys (estuaries) and fiords.
Tides:
Related web sites:
http://www.sfgate.com/getoutside/1996/jun/tides.html
The gravitational attraction of the Moon causes water in the oceans to be displaced vertically. This happens twice a day on most coastlines.
The Sun has an impact on tides but it is much less of an influence than the Moon. The Sun's influence tends to enhance the Moon's effect or lessen it.
Neap Tide: when the Sun and the Moon are positioned at right angles to one another and the Sun lessens the effect of the Moon. Tidal ranges are lower: lower high tides and higher low tides.
Spring Tides: when the Sun and the Moon are aligned with one another and the Sun enhances the Moon's effect. Tidal ranges are higher: higher high tides and lower low tides.
