eInvestigations Manual 7a.pdf - Ocean Studies...

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2/28/2019 Ocean Studies eInvestigations Manual 2018-19 1/7 Investigation 7A COASTAL PROCESSES Objectives The transition zone between the ocean (or large lake) and land is a dynamic environment. Landforms of the coastal zone are largely the products of ocean waves and currents acting on the shore. Depending on many factors including the geology of the coastline, the character of the waves arriving at the shore, tidal effects, and long-term changes in sea level, either erosion or deposition can dominate along a particular coastal segment. Variations in coastline configuration and bottom topography cause wave refraction (bending of waves) that favors erosion in some areas and deposition of sediment in other areas. In addition to wind-driven waves, ocean coastlines are vulnerable to the effects of tides and, in some areas, tsunamis. Tidal effects are especially significant where the coastline has narrow openings, including estuaries, bays, and straits. Tsunamis are relatively rare—but potentially devastating—sea waves triggered by earthquakes, volcanic eruptions, or submarine landslides. Primarily occurring in the Pacific and Indian Ocean basins, they are capable of producing run-up heights of tens of meters of ocean water as the tsunami arrives, considerably modifying the coastal zone. Tsunamis have struck with little or no warning causing great widespread death and destruction, as demonstrated by the occurrences of the 26 December 2004 Indian Ocean and the 11 March 2011 Japanese Great Tōhoku tsunamis. After completing this investigation, you should be able to: Describe ways in which wind-driven, tidal, and tsunami waves impart energy in shaping shoreline features. Demonstrate how wave refraction can concentrate or disperse wave energy. Describe the physical mechanism responsible for longshore currents and littoral drift. Wave Refraction As waves approach the coastline, they encounter increasingly shallow water. When waves enter water shallower than half their wavelength, they begin to interact with the ocean floor. The period of these shallow-water waves remains unchanged so that as the wave slows, its wavelength shortens and its wave height increases. The wave steepens and eventually becomes unstable and breaks. As a wave breaks, its potential energy converts to kinetic energy, which modifies the coastline through weathering and erosion. Among the landforms along rocky shorelines that owe their origin to such wave action are wave-cut cliffs, benches, notches, sea caves, arches, and stacks. Breaking waves approaching the shore at an oblique angle give rise to sediment-transporting currents parallel to the shoreline. (These sediments are delivered to the shore by rivers and streams, wave erosion or slumping of seaside cliffs, or wind.) A thin sheet of water follows a diagonal path up the sloping beach face and then is pulled by gravity directly down slope. This zig-zag motion rolls sediment

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