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105_16-shorelines_part_1 - Waves Beaches and Coasts Waves...

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Unformatted text preview: Waves, Beaches and Coasts Waves – Wind driven – Energy transfers from wind to the water surface – Size and speed of wave controlled by Wind Wind speed Length Length of time wind blows Distance Distance that the wind blows over the water (fetch) Chapter 16 Waves – Wave height – distance between crest and trough – Wavelength – distance between two crests (or troughs) Wave Motion Motion of a WAVE is not the same as the motion of the WATER through which the wave travels. The wave progresses from one place to another, while the water moves in a circular path called an orbit. orbit Wave Wave motion Wave motion Waves slow down, their wavelengths decrease and their heights increase near shore when their lowest level of orbital motion (~ ½ wavelength) is equal to the water depth. 1 Wave motion Waves become steeper and steeper until the crest of the wave topples forward, and the wave becomes turbulent and foamy. (breaks) NearNear-shore circulation Wave Wave refraction – Bending of waves as they approach the shore – Waves generally approach shoreline at an angle, and then refract to become nearly parallel to the shoreline. NearNear-shore circulation Wave Wave refraction NearNear-shore circulation Longshore Longshore Currents – Waves hit shoreline at a slight angle – Water is pushed up and along the beach up along – A current develops which moves parallel to the shoreline = longshore current NearNear-shore circulation Longshore Longshore currents NearNear-shore circulation Rip Rip Currents – Narrow currents that flow straight out to sea – Travel at the water surface, and die out with depth – Can be spotted by a discoloration of the water where current it carrying sediment seaward – If you are being carried out to sea in a rip current, swim parallel to shore to get out of the narrow current 2 NearNear-shore circulation Rip Rip currents Shoreline deposition Beaches Beaches – pocket beaches – continuous beaches (strands) Beaches Parts Parts of a Beach Beaches Parts Parts of a Beach – Foreshore: water during high tide, none during low tide exposed to wave action – Backshore: usually dry – Berm: sediment deposited by waves. Flat or slightly sloping landward – Beach face: area exposed to wave action – Marine terrace: broad, gently sloping platform exposed at low tide 3 Beaches Shoreline deposition Beach Beach sands are generally composed of quartz, but locally other material may be dominant – shell fragments (Florida) – black basalt fragments (Hawaii) Shoreline deposition Spits: Spits: fingerlike continuation of beach Baymouth Baymouth bar: spit that closes off bay Black Sand Beach, Hawaii Spit 4 Shoreline deposition Tombolos: Tombolos: bar of sediment connecting beach with island Martha’s Vineyard, Massachusetts Shoreline deposition Barrier Barrier islands – Long narrow islands of sand lying a short distance offshore from the mainland – Ridges of sand that separate ocean from lagoon or bay – Grow parallel coast by beach drift and longshore drift – Constantly changing – Line the East Coast for 1300 km and the Gulf Coast for 1300 km, string of 295 islands Barrier Islands 5 Origin of Barrier Islands ….from spits Shoreline erosion Coastal Coastal Erosion – 14,000 waves strike coasts every day ….from drowned beach ridges Shoreline Erosion Wave Wave action forces water into cracks Slope Slope of sea floor – Gradual slopes dissipate energy Erosional coastal landforms Headlands: noseHeadlands: nose-shaped protruding ridges – Waves refract and erode headland WaveWave-cut bench, near San Francisco 6 ...
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