CHAPTER 14 shorelines - 2009 Allan Ludman and Stephen...

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1 CHAPTER 14 SHORELINE LANDSCAPES PURPOSE To become familiar with processes of shoreline erosion and deposition To become familiar with different types of shoreline landforms and landscapes To interpret active shoreline processes and identify possible hazards MATERIALS NEEDED Colored pencils Magnifying glass or hand lens to help read close-spaced contour lines 14.1 INTRODUCTION Europeans coming to North America first settled the Pacific, Atlantic, Gulf of Mexico, and Great Lakes shores. These areas are still home to much of the continent’s population and millions more vacation at their beaches but shorelines are the most dynamic places on Earth, capable of changing in hours or days. Shores are unique places where the rapidly moving atmosphere and hydrosphere interact with the more stable geosphere. Results of these interactions depend on factors involving all parts of the Earth System. 14.2 FACTORS CONTROLLING SHORELINE EROSION AND DEPOSITION Factors shaping shoreline landscapes include rock or sediment type, wind direction and strength, interaction with mass wasting, wind, stream, and other erosional agents, and tectonic activity. A few basic principles explain how these interact to produce shoreline erosion and deposition: • Waves are the dominant erosional and depositional agent at shorelines. • Waves are generated by the interaction of wind and the surfaces of oceans or large lakes. • The kinetic energy of waves causes erosion and redeposition of unconsolidated material. • Wind also moves sediment by itself, forming coastal sand dunes. • Shoreline currents redistribute sediment to produce barrier islands, spits, and other landforms. • Like streams and glaciers, waves use loose sediment to abrade the bases of solid bedrock cliffs. When support at the base of cliffs is undermined, the cliffs collapse by rockfall and slump. Waves erode the rubble, exposing the new cliff to wave erosion and the cycle repeats. In this way, shoreline cliffs gradually retreat inland. © 2009 Allan Ludman and Stephen Marshak W.W. Norton & Company
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2 • Shorelines are in a constant state of conflict between the destructional processes of wave and wind and constructional shoreline deposition, lava flows, reef building, etc. 14.3 EVOLUTION OF COASTAL LANDFORMS The evolution of coastal landforms depends on the complex combination of factors listed above and on long-term climate change. The following sections and Exercise 14.1 help understand the role each of these factors plays. 14.3.1 Waves : Waves form by friction generated when wind blows across the surface of an ocean or lake. The symmetrical shape of waves offshore shows how the kinetic energy of the wind is transferred to the water. Offshore, water molecules move in circular paths, each molecule passing some energy on to those it contacts (Figure 14.1). Loss of energy in these contacts limits the depth of wave action to approximately ½ of the wavelength, a depth referred to as the wave base .
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