Coastal ProcessesChapter 13Waves, Beaches And Coastal Erosion: Rivers of SandWave Processes and Shore ErosionWaves form as wind blows over the ocean surface. Waves increase in size with the following factors:greater wind velocitygreater amount of time the wind blowsgreater fetchWave height and wavelength increase with increasing energy.wave height -- the vertical distance from low point (trough) to high point (crest)wavelength -- the horizontal distance from one crest to the next crestIn deep water the wave form itself moves, but the water does not have net forward motion -- instead the water travels in circlular orbits. The circular motion of the water fades downward to a depth equal to ~half the wavelength, called wave base. At a water depth equal to or less than half the wave length, the waves begin to ‘feel bottom’. The crest of the wave moves forward as the base drags on the bottom. This causes the waves to bunch up (wavelengths decrease) and grow taller (wave heights increase). Eventually the waves oversteepen, causing them to tumble toward the shore, or break, and the water moves landwardBeachesBeaches are accumulations of sand or gravel supplied by Sea Cliff Erosion and by River Transport of Sediment to the Coast. The sand and gravel supplied to the coast is then moved by wave action.Wave Refraction and Cliffed CoastsWave refraction is the bending of waves to conform to the shape of the coastline.Wave refraction concentrates wave energy on headlands and deposits the products of that erosion in bays, ultimately straightening the coastline.Longshore DriftThe gradual migration of sand along the shoreline.Sand grains are pushed up onto the beach in the direction of wave travel. Gravity pulls it back directly down the slope of the beach.We measure longshore drift by the volume of sand moved in cubic meters per year.Storm Response Loss of Sand from the BeachLarger waves and storm surge erode sand from the shallow portion of the beach and transport much of it just offshore. Much of the eroded material comes from the surface of the beach, eroding it to a flatter profile.Barrier IslandsBarrier islands are long, narrow, offshore deposits of sand that parallel the coastline, separated from the main land by a shallow sound, bay or lagoon. They are the products of dynamic coastline processes: erosion, deposition, longshore drift, and wind transport.Barrier islands are often found in chains along the coast line and are separated from each other by narrow tidal inlets.A baymouth barrier is a barrier beach that is attached at both ends to the mainland and backed by a bay or lagoon.A spit is a protrusion or linear beach extending from the mainland.Most barrier islands in North America are along the Atlantic Coast.Barrier islands are places of change and motion.Longshore drift moves sand from one beach to another.Islands grow and shrink depending on the amount of sediment available (dams, sand mining, hardening).Sea level rise causes them to erode and to shift toward the mainland over time. 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". Two forms of evidence that show an island is rolling over are washovers and exposed marsh mud and oyster shells on the beach.Barrier islands maintain their equilibrium profile by migrating shoreward with rising sea level. Because many barrier islands are now covered with buildings, the island cannot migrate but is progressively eroded.BreakwatersBreakwaters, built offshore and parallel to the shore, have a similar effect, causing deposition in the protected area behind the breakwater and erosion on the downdrift side.Coastal ProcessesChapter 13Waves, Beaches And Coastal Erosion: Rivers of SandWave Processes and Shore ErosionWaves form as wind blows over the ocean surface. Waves increase in size with the followingfactors: • greater wind velocity• greater amount of time the wind blows• greater fetchWave height and wavelength increase with increasing energy.• wave height -- the vertical distance from low point (trough) to high point (crest)• wavelength -- the horizontal distance from one crest to the next crestIn deep water the wave form itself moves, but the water does not have net forward motion -- instead the water travels in circlular orbits. The circular motion of the water fades downward to a depth equal to ~half the wavelength, called wave base. At a water depth equal to or less than half the wave length, the waves begin to ‘feel bottom’. The crest of the wave moves forward as the base drags on the bottom. This causes the waves to bunch up (wavelengths decrease) and grow taller (wave heights increase). Eventually the waves oversteepen, causing them to tumble toward the shore, or break, and the water moves landwardBeachesBeaches are accumulations of sand or gravel supplied by Sea Cliff Erosion and by River Transport of Sediment to the Coast. The sand and gravel supplied to the coast is then moved by wave action.Wave Refraction and Cliffed CoastsWave refraction is the bending of waves to conform to the shape of the coastline.Wave refraction concentrates wave energy on headlands and deposits the products of that erosionin bays, ultimately straightening the coastline. Longshore DriftThe gradual migration of sand along the shoreline.Sand grains are pushed up onto the beach in the direction of wave travel. Gravity pulls it back directly down the slope of the beach.We measure longshore drift by the volume of sand moved in cubic meters per year.Storm ResponseLoss of Sand from the BeachLarger waves and storm surge erode sand from the shallow portion of the beach and transport much of it just offshore. Much of the eroded material comes from the surface of the beach, eroding it to a flatter profile.Barrier IslandsBarrier islands are long, narrow, offshore deposits of sand that parallel the coastline, separated from the main land by a shallow sound, bay or lagoon. They are the products of dynamic coastline processes: erosion, deposition, longshore drift, and wind transport.- Barrier islands are often found in chains along the coast line and are separated from each other by narrow tidal inlets.- A baymouth barrier is a barrier beach that is attached at both ends to the mainland and backed by a bay or lagoon. - A spit is a protrusion or linear beach extending from the mainland.- Most barrier islands
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