planet earth coast_powerpoint_presentation

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  • 1. Coastal Systems Wave PhysicsLongshore transport, wave refraction.Coastal Features Coastal Erosion and Mitigation


  • Waves form as wind blows over the ocean surface. Waves increase insizewith the following factors:
  • - greater windvelocity
  • - greateramounts of timethat the wind blows (wind duration)
  • greaterfetch : the distance of water over which the wind blows
  • Why is the Pacific (CA, HI, Australia) better for surfing than the Atlantic?

3. In waves far out at sea, in deep water, the motion of water in the waves is circular. As waves move by, the water moves in a circle. You can observe this by watching the way an object (like the seagull in this figure) moves as the wave passes. Such waves are calledwaves of oscillation . The wave form itself moves, but the water does not have net forward motion -- instead the water travels in circles. 4. Orbitals: Water doesnt move (is not displaced)but the energy travels THROUGH the water.Water molecules circulate in orbitals as the wave passes.Note, wave length, height (H) and wave base (L/2).At wave base water is not effected by wave energy. 5.

  • We measure the sizes of waves by:
  • wave height-- the vertical distance from low point (trough) to high point (crest)
  • wavelength-- the horizontal distance from one crest to the next crest

Note in this figure the circular motion of the water, and notice that the motion becomes less with depth. At a water depth equal tohalf the wavelength , you can see that there is no more water motion.This is called WAVE BASEmovement of water particles is not felt below wave base. 6. Wherever they form out at sea, waves eventually will come into shore. When this occurs some important changes take place. At a water depth equal to half the wave length, the waves begin toslow down(orfeel bottom ) due to friction with the sea floor. (Recall from the previous slide that water motion in a wave occurs down to a depth equal to half the wavelength. Therefore waves feel friction with the bottom and slow down when the depth becomes half the wavelength.) 7. The shallower the water, the slower the waves move. This causes the waves tobunch up(wavelengths decrease) andgrow taller(wave heights increase). Eventually the waves move slower than the water orbiting within them, causing the waves to tumble toward the shore, orbreak . Breaking waves are calledwaves of translationbecause the water in them moves landward (rather than in a circle), hitting the shore with great force. 8. 9. The goal of waves is to fill in depressions and erode back headlands extending out to sea.A well worked coastline is flat (I.e. atlantic).As waves move onshore, they slow down in areas around the headlands (shallow water).By slowing down, the wave bends (refraction) and most of the wave energy is exerted on the headland. 10. Refraction erodes headlands.The rest of the wave that is still in deep water gently rolls up the beaches (depositing sand and filling them in). 11. Wave refraction! 12. wave refraction. 13. Wherever waves approach the shore at anangle , they bend in toward the shore somewhat, because the part of the wave closer to shore, in shallower water, moves slower than the part farther from shore, in deeper water. This bending of waves is calledrefraction . 14. Longshore Drift When waves strike the shore at an angle, it causes acurrent of water to flow along the shore in the direction of the waves . This is called alongshore current .The longshore current, along with the waves breaking on the beach at an angle, causelittoral drift: the movement of large amounts of sand along the shore in the direction of the waves . 15. This photograph shows the waves hitting the beach at an angle. Therefore the direction of thelongshore current/drift(the movement of water) and thelittoral drift(the movement of sand) is along the beach toward the viewer. We measure littoral drift by the volume of sand moved in cubic meters per year or cubic yards per year. 16. Depositional Beach Environments Depositional coasts include the eastern seaboard of the U.S.Here, the coasts form gentle slopes with the continental shelves.Continental shelves are wide in these areas (coincidental that they are passive continental margins?). 17. Barrier Island Cross Section 18. 19. Barrier island are places of change and motion. Islands grow and shrink depending on the amount of sediment available and global conditions like change in sea level.An island that is migrating toward the mainland is termed retrograding. In most cases an island that is retrograding undergoes a process known as "island rollover". This process commonly occurs on islands that are experiencing erosion due to a stifled sediment supply. Two major types of evidence that show an island is rolling over are washovers and exposed marsh mud on the beach. 20. Barrier Island systems are not STABLE!!!But in fact are dynamic and motile.During storms, overwash deposits form.Waves essentially push beach material behind the barrier island system moving it TOWARDS the coast.Problems arise when beach front properties try to stabilize the barrier island. 21. 22. This figure shows how Hog Island, Virginia, has changed over the past 150 years, as a result of hurricanes and wave erosion. The dashed lines show the former shorelines of the island. Entire towns, such as the once-thriving community of Broadwater, have disappeared into the ocean as the barrier has shifted toward the mainland. 23. Barrier Island Systems run parallel to the mainland.Sediments that supply the barrier island system arise from deposits from streams that discharge into the ocean.Outer Banks, NC (left), Sea Bright, NJ (right).In many cases, barrier islands are thin (~3 miles wide, if that).Standing on a sea wall in seabright, you can see the bay water. Its about 4 city blocks wide!!! 24. Long Beach Island, NJ riddled with beautiful beach homes.Note that from bay (left) to ocean (right) is just a few city blocks.The only way to get onto LBI is via only a few bridges.Evacuations can be hectic. 25. Littoral drift builds up very large features over time. In this photograph of San Diego, you can see the largesand spit(about 10 miles long) called Coronado Island, that encloses San Diego Bay. The sand spit formed by northward (to the left in the picture) littoral drift of sand from the Tijuana River in Mexico to the south (out of view to the right). San Diego Bay forms an excellent natural harbor as a result, and is the main reason why this area is a major U.S. Navy base.SAND SPIT(Coronado Island) 26. Oftentimes, because of a long shore transport current, barrier islands may have hook like extensions.These spits are deposits of sand.Sandy Hook is connected to the mainland by a thin strip of land at its southern end.You can see where the older tips of sandy hook were once. 27. Human structures may interfere with the movement sand by littoral drift . In this figure, the rock jetties built to create an open inlet block the movement of sand moving by littoral drift. The sand builds up on one side of the jetties, and gets eroded on the other side because no sand comes along to replace the sand that moves away.The result: severe shore erosion on the downstream side of the jetties. 28. The effect of jetties on littoral drift is dramatically illustrated in this figure. In the 1930s jetties were built at Ocean City, Maryland, to maintain a channel connection to the ocean. The direction of littoral drift here is from north to south (top to bottom in the figure). By stopping the sand from coming south, the jetties deprived Assateague Island of the sand it would normally have received. As a result, the island has eroded and shifted landward by more than half a kilometer in places. (The red line shows the former location of the island.) 29. Groins are built specifically to trap sand being carried by longshore drift and build up the beach updrift of the structure.Unfortunately these structures starve downdrift beaches of sand, and erosion on those beaches is accelerated.The only way to save downdrift beaches is to construct more groins, and therefore groins tend to multiply into groin fields. 30. This photograph shows how littoral drift was interrupted by the rock groins that stick out into the water. Sand accumulates on one side and erodes on the other. What is the direction of littoral drift here? Shore erosion in this area of coastal New Jersey has caused damage to roads and private property. 31. Erosional Coastal Features Common to areas that are tectonically active (I.e. pacific coast), erosional coasts do not have wide continental shelves offshore.Tectonic uplift and downdrop of coastal areas can result in stranded marine terrace structures or drowned stream valley coastlines (such as San Francisco).Erosional features are evident in these types of coasts. 32. Some shorelines, particularly on the western coast of the U.S., are dominated bysea cliffsrather than beaches. The tops of these cliffs form prime real estate. But sea cliffs are subject to erosion by bothwave undercuttingandmass wasting . How long is that nice ocean view shown here going to last ?! 33. Cliffs usually meet the sea in these areas and as such, are subject to wave erosion.Recall the goal of waves is to erode back headlands and fill in depressions.Tectonic movements of coastal areas however do not allow for wide sandy beaches to develop. 34. Erosion of headland areas form sea arches as seen below.The high energy waves that strike headlands erode a tunnel.Over time, the tunnel ceiling will become weak enough and fall. 35. Once the ceiling falls, you are left with a projection of land off shore (originally