Ocean101-Ch09-Part2 - What happens when waves approach...

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Unformatted text preview: What happens when waves approach shore? Deep-water orbital waves Fig. 9.16, p. 211 Wave motion is influenced by water depth and shape of the shoreline wave buildup zone surf beach zone Wave base deepwater wave Wave base shallowwater wave 1 Wind Waves "Breaking" Breaking" Waves reach surf zone Wave speed decreases Wave length decreases Wave height increases Wind Waves "Breaking" Breaking" Spilling breakers occur on a gradually sloping bottom (gentle beach slope). The crest of a spilling wave slides down the face of the wave as it breaks on shore. http://topex-www.jpl.nasa.gov/education/activities/ts2enac1.pdf Plunging breakers break violently against the shore, leaving an air-filled tube (channel) between the crest and foot of the wave. Form when waves approach a shore over a moderately steeply sloped bottom. Wave steepness 1/7, wave breaks 1/7, Surface tension no longer able to hold wave together Surging breakers surge forward, when bottom has abrupt slope. The force of breaking waves is important for shaping our coastlines. What happens when the water depth becomes less than onehalf the wave length? 1) The deep-water wave becomes a shallow-water wave. 2) The wave slows down. 3) The wave enters the zone of wave buildup. What happens when the wave slows down (in the zone of wave buildup)? Wave period remains constant = Wave length decreases Wave height increases Wave refraction can occur! 2 Wave Refraction A wave will bend and change direction when one part of it goes slower or faster than another part. Wave refraction is the bending of waves in shallow water. As waves approach shore, the part of the wave in shallower water slows more than the part in deeper water. This makes the wave bend and change direction. View from above Fig. 9.17a, p. 212 Fig. 9.17b, p. 212 Wave Refraction: The bending of waves as they change velocity Wave base 3 Bending of wave crests due to refraction as waves slow down in progressively shallower water depths Wave refraction concentrates energy at headlands, thereby causing increased erosion Wave refraction decreases energy at bays, thereby causing increased deposition Wave Refraction, Diffraction, & Reflection Wave refraction - The slowing & bending of waves in shallow water. Wave diffraction - Propagation of a wave around an obstacle. Wave reflection - Waves "bounce back" from an obstacle they encounter. Reflected waves can cause interference with oncoming waves, creating standing waves. waves Wave Diffraction Wave energy transferred around or behind barriers 4 Wave Reflection Waves bounce back from steep slopes or seawalls Reflected wave may constructively interfere with other waves "The Wedge" Wedge" Longshore Drift The transport of sediment (e.g., sand) parallel to the shoreline, mainly in the surf and swash zone. 5 Coastal Erosion Rates in the U.S. Erosion Fact: 30-50% of all the structures within 500 feet of the present Gulf shoreline will be lost due to erosion in the next 60 years U.S. Geological Survey Source: Heinz Center Report to FEMA, 2000 Q: What determines whether a beach is eroding or stable? What is the sand budget? budget? The rate at which sand is supplied (input) to the beach, compared to the rate at which it is being removed (output). A: The sand budget 6 Sand Budget Inputs Outputs What do we do to try to "prevent" coastal erosion? prevent" Structural approaches: Groins Jetties Seawalls Breakwaters Non-structural approaches: Beach nourishment Abandonment / relocation / zoning Groins = Impermeable structures perpendicular to the shoreline Purpose: = To trap sediment, thereby reducing beach erosion What happens when you build a groin? 7 Groin Erosion Up-drift Deposition Down-drift Erosion Deposition Lessens up-drift erosion BUT... BUT... Causes down-drift erosion Q: What do jetties, groins, seawalls, and breakwaters all have in common? Beach "Nourishment" = The artificial addition of sand to the beach to reduce the rate of beach erosion. A: They all cause down-drift erosion! erosion 8 Beach "Nourishment" = The artificial addition of sand to the beach to reduce the rate of beach erosion. And, it does not cause down-drift erosion. Beach "Nourishment" = The artificial addition of sand to the beach to reduce the rate of beach erosion. But, it must be periodically replenished! ($$$) Some Key Concepts * Wind waves transmit energy, not water mass, across the ocean!s surface. * Waves are classified by several characteristics. * The behavior of a wave depends on the relation between the wave!s size and the depth of water through which it is moving. * Wind waves form when energy is transferred from wind to water. * Waves can change direction by refraction and diffraction, can interfere with one another, and reflect from solid objects. Beach "Nourishment" Monmouth Beach, New Jersey 9 " 5-Minute Write " Summarize the main points of today's lecture. List 3 to 5 questions you have, based on today's lecture. What did you find most interesting about today's lecture? How was the lecture relevant to you? 10 ...
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