Planet earth coast_lecture_outline

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  • 1. Coastal SystemsWave PhysicsLongshore transport, wave refraction.Coastal FeaturesCoastal Erosion and MitigationWAVE PROCESSESWaves form as wind blows over the ocean surface. Waves increase in size with the followingfactors: - greater wind velocity - greater amounts of time that the wind blows (wind duration) - greater fetch: the distance of water over which the wind blowsIn waves far out at sea, in deep water, the motion of water in the waves is circular. Aswaves move by, the water moves in a circle. You can observe this by watching the way anobject (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 forwardmotion -- instead the water travels in circles.Orbitals: Water doesn’t move (is not displaced) but the energy travels THROUGH thewater. 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.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 crestWherever they form out at sea, waves eventually will come into shore. When this occurssome important changes take place. At a water depth equal to half the wave length, thewaves begin to slow down (or feel bottom) due to friction with the sea floor. (water motionin a wave occurs down to a depth equal to half the wavelength. Therefore waves feelfriction with the bottom and slow down when the depth becomes half the wavelength.)The shallower the water, the slower the waves move. This causes the waves to bunch up(wavelengths decrease) and grow taller (wave heights increase). Eventually the waves moveslower than the water orbiting within them, causing the waves to tumble toward the shore,or break. Breaking waves are called waves of translation because the water in them moveslandward (rather than in a circle), hitting the shore with great force.Wave Refraction
  • 2. The goal of waves is to fill in depressions and erode back headlands extending out to sea. Awell worked coastline is flat (I.e. atlantic). As waves move onshore, they slow down in areasaround the headlands (shallow water). By slowing down, the wave bends (refraction) andmost of the wave energy is exerted on the headland.Refraction erodes headlands. The rest of the wave that is still in ‘deep’ water gently rollsup the beaches (depositing sand and filling them in).Wherever waves approach the shore at an angle, they bend in toward the shore somewhat,because the part of the wave closer to shore, in shallower water, moves slower than thepart farther from shore, in deeper water. This bending of waves is called refraction.Longshore DriftWhen waves strike the shore at an angle, it causes a current of water to flow along theshore in the direction of the waves. This is called a longshore 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 ofthe waves.Depositional Beach EnvironmentsDepositional coasts include the eastern seaboard of the U.S. Here, the coasts form gentleslopes with the continental shelves. Continental shelves are wide in these areas(coincidental that they are passive continental margins?).Barrier Island Cross SectionBarrier island are places of change and motion. Islands grow and shrink depending on theamount 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 anisland that is retrograding undergoes a process known as "island rollover". This processcommonly 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 exposedmarsh mud on the beach.Barrier Island systems are not STABLE!!! But in fact are dynamic and motile. Duringstorms, overwash deposits form. Waves essentially push beach material behind the barrierisland system moving it TOWARDS the coast. Problems arise when beach front propertiestry to stabilize the barrier island.Barrier Island Systems run parallel to the mainland. Sediments that supply the barrierisland system arise from deposits from streams that discharge into the ocean. In manycases, 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!!!Oftentimes, because of a long shore transport current, barrier islands may have hook likeextensions. These ‘spits’ are deposits of sand. Sandy Hook is connected to the mainland bya thin strip of land at its southern end. You can see where the older ‘tips’ of sandy hook
  • 3. were once.Human structures may interfere with the movement sand by littoral drift. The rockjetties 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 nosand comes along to replace the sand that moves away. The result: severe shore erosion onthe “downstream” side of the jetties.Groins are built specifically to trap sand being carried by longshore drift and build up thebeach updrift of the structure. Unfortunately these structures starve downdrift beachesof sand, and erosion on those beaches is accelerated. The only way to save downdriftbeaches is to construct more groins, and therefore groins tend to multiply into groin fields.Erosional Coastal FeaturesCommon to areas that are tectonically active (I.e. pacific coast), erosional coasts do nothave wide continental shelves offshore. Tectonic uplift and downdrop of coastal areas canresult in stranded marine terrace structures or drowned stream valley coastlines (such asSan Francisco). Erosional features are evident in these types of coasts.Some shorelines, particularly on the western coast of the U.S., are dominated by sea cliffsrather than beaches. The tops of these cliffs form prime real estate. But sea cliffs aresubject to erosion by both wave undercutting and mass wasting.Cliffs usually meet the sea in these areas and as such, are subject to wave erosion. Recallthe goal of waves is to erode back headlands and fill in depressions. Tectonic movements ofcoastal areas however do not allow for wide sandy beaches to develop.Erosion of headland areas form sea arches. The high energy waves that strike headlandserode a tunnel. Over time, the tunnel ceiling will become weak enough and fall.Once the ceiling falls, you are left with a projection of land off shore (originally part of thesea arch). These sea stacks are bombarded with waves and eventually will erode to sealevel.Tombolos are extensions of sand, deposited from long shore transport, between a sea stackand the mainland.Protecting the Coasts: the beaches are moving!Mitigation of Shore ErosionHuman activities contribute to shore erosion in the following ways.1. River dams trap sand that would otherwise be carried down rivers to the shoreline. Thiscuts off one of the main supplies of new sand to beaches.2. Jetties and groins stick out from the shoreline and block sand moving by littoral drift.The result is shore erosion on the down-drift side of the structures.3. Seawalls or revetments cause breaking waves to bounce hard back toward the sea,carrying away sand and eroding the beach.Mitigation of shore erosion may be accomplished by:- beach replenishment: adding sand to eroded beaches; generally an expensive and short-term solution, since the sand often erodes away within a few years
  • 4. - setting structures (houses, roads) far back from the edges of eroding sea cliffs - designing groins and jetties so that they allow sand carried by littoral drift to pass through - Installation of sand dunes (with vegetation), breakwaters.Sand dunes are a natural mechanism built up by winds, that buffers the impact of stormwaves. During storms, dunes may be eroded away for the most part, but the back duneareas remain protected. Typically grasses grow on dunes and the root systems help tostabilize the sediments preventing wind erosion. This is why you are supposed to KEEP OFFTHE DUNES!To minimize the movement of sediments due to long shore transport, groins (a bunch of rockmaterial piles) are extended out perpendicular from the coastline. The idea is that they willtrap sediments from long shore transport, build up over time and sediments will pour over tothe adjacent beach upstream of the current, widening the beaches. This has yet to happen.In fact, LST continues whether groins are placed or not, so usually sediments build up onone side and erosion continues on the other. Sediment supply is essentially cut off toupdrift sides of the groin (here erosion predominates). The right image shows a groin field;once one is constructed, then others must be to avoid sediment loss.Jetties function in a similar way as groins, however they usually extend further out to seaand are constructed along inlets. LST (littoral drift) can fill in inlets, preventing ships totravel.Another way to prevent sediment erosion is to place RIP RAP along the beach. Largeobjects, remember, require more energy to transport. It makes for a lovely scenery no?Typically a last resort is to erect a seawall (rocks and cement) or bulkhead (metal, see nextslide). Basically, seawalls increase erosion rates because waves crash on them with highenergy and retreat with high energy, taking sediments (whatever little beach is present)with it. They are erected ONLY to protect the homes behind them. Beach loss isinevitable.Or millions of taxpayer dollars can be used to pump sediment from offshore, onshore andpushed around with a bulldozer to widen the beach (REPLENISHMENT).After all is tried, the beaches will move. These houses were bought out by the government.We can not make the MOST DYNAMIC NATURAL ENVIRONMENT ON EARTH, stable(although we try). Did I mention beaches generate the most tourism revenue than ANYOTHER natural environment?Lots of money and lots of politics!