16_coast_09_post - 16: Surface Processes 3 Tides, Waves,...

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Unformatted text preview: 16: Surface Processes 3 Tides, Waves, and Coastlines Why this is important • Most waves are caused by wind, which is driven by Earth’s external heat engine. Rocky shoreline, Oregon. Photo: Steve Terrill Barrier islands Surfers off Maui coast. Photo: R.W. Schlische Photo: NASA Tides & Waves: 1 Tides & Waves: 2 Why this is important Tides Fig. 18.15 • 60% of global population live on or near coast. Coastlines are dynamic features that represent a huge investment in property, recreation, etc. Tidal range Block diagram Size of intertidal zone depends on… Side view Tides & Waves: 3 Photo: W.K. Hamblin Tides & Waves: 4 Tides Tide height Tides Q1. Approximately how many high tides occur per day? A. 1 B. 2 C. 3 D. 4 • Tides: rise / fall of sea surface caused by gravitational pull of moon and sun and centrifugal force caused by the revolution of the Earth-moon system around its center of mass. Q2. Approximately what is the difference in time between two successive high tides? A. 6 B. 12 C. 18 D. 24 Q3. Is the time of high tide the same for each day of the month? A. Yes B. No Fig. 18.15 Q4. Is the height of high tide the same for each day of the month? A. Yes B. No Time/height of high tide Time/height of low tide Time/height of high tide Tides & Waves: 5 Hours Tides: Analogy Time/height of low tide Approx. 2 high tides per day due to Earth’s rotation beneath tidal bulges Tides & Waves: 6 Tide-generating forces Each point on Earth feels same centrifugal force due to spin of Earth-moon system around its center of mass but feels different gravitational attraction due to pull of moon (e.g., force is greatest closest to moon). Top views Tides & Waves: 7 Tide-generating force is sum of centrifugal force and gravitational attraction. On side of Earth closest to moon, tide-generating force vector is dominated by gravitational force and points toward moon. On opposite side, centrifugal force dominates, and tidegenerating force points away from moon. Thus, two tidal bulges exist. Tides & Waves: 8 Side views Tides Tidal amplitude varies greatly around the world (~1-2 m in NJ, up to 16 m in Bay of Fundy). Amplitude depends on latitude and shape of coastlines. For example, the Bay of Fundy acts like a funnel that forces water higher and higher near its narrow end during high tide. Tides & Waves: 9 Tides Low tide Top view • Spring tides: tides with higher- thannormal tidal range that are produced when the Earth, Moon, and sun are aligned. Bay of Fundy, Nova Scotia, Canada Map views High tide • Neap tides: tides with lower-thannormal tidal range that are produced when the moon is at right-angles to the Earth-sun system. Top view Fig. 18.15 Tides & Waves: 10 Review Questions Review Questions 16-1. A. True / B. False: Coastal processes are of interest to geologists, but affect relatively few people because less than 10% of the world’s population lives within 10 km of a sea coast. 16-5. Tidal range (difference in height between high and low tide) is affected by ____________. A. the slope of the coastline B. the position of the moon C. Both A and B are correct D. None of the above is correct. 16-2. A. True / B. False: When high tide occurs at any single shoreline, that means it is also high tide at all other coasts throughout the world. 16-3. The most prominent force inducing tides on Earth is the ____________. A. Sun’s gravitational pull B. Moon’s gravitational pull C. electromagnetic attraction between Earth and the Sun D. gravitational attraction between ocean waters and the continents on Earth 16-6. A. True / B. False: A very high tidal range results when the Earth, Moon, and sun are all lined up. 16-7. A. True / B. False: A very low tidal range results when the sun is at right angles to the Earth-moon system. 16-4. The size of the intertidal zone is affected by ____________. A. the slope of the coastline B. the position of the moon C. Both A and B are correct D. None of the above is correct Tides & Waves: 11 Tides & Waves: 12 Waves and wind Waves and wind • Waves are a form of vibration that transfers energy from the open ocean to the coast. . Waves approaching shoreline, Oceanside, CA. Photo: J.S. Shelton Waves are produced by wind blowing over water surface. Wave height in open water depends on wind speed and fetch length (length of water acted upon by wind). Tides & Waves: 13 Q5. As the wind speed increases (for a constant fetch length), the wave height ___. A. increases B. decreases C. remains the same Q6. As the fetch length increases (for a constant wind speed), the wave height ___. A. increases B. decreases C. remains the same Tides & Waves: 14 Wave motion Wave geometry • Wave height: vertical distance between crest and trough • Wavelength: distance between two adjacent crests (or troughs) • Period: time it takes successive crests to pass a given point • Wave base: depth at which particle motion is negligible Fig. 18.18 • Circular orbits Q7. Distance from height of trough to wave base is equal to: A. amplitude B. half the wavelength C. the wavelength D. the wave height • Wave moves, but water particles move… move… Side views Tides & Waves: 15 Side views Tides & Waves: 16 Fig. 18.18 Wave erosion Breaking waves Fig. 18.19 • Below sea level --> • Surf zone --> rubbing & grinding of wave-transported particles Fig. 18.19 Block diagram Block diagram • Wave enters shallow water --> lower orbits feel bottom--> wave crests bunch together --> amplitude increases --> wave oversteepens --> collapses--> • Breaker: collapsed wave that rushes up the shoreface • Surf: turbulent water related to breakers Tides & Waves: 17 Tides & Waves: 18 Wave refraction Wave refraction Headland • Wave refraction: for waves approaching shoreline at angle, part of wave closest to shoreline feels bottom first & slows down, bending wave Block diagram Embayment Headland Embayment Fig. 18.20 Tides & Waves: 19 Q8. Wave crests are: A. closer together at embayments and farther apart at headlands B. farther apart at embayments and closer together at headlands C. the same distance apart at embayments and headlands Tides & Waves: 20 Review Questions Wave refraction 16-8. A. True / B. False: In the open ocean, the passage of a wave results in no significant forward motion of packets of water. 16-9. As waves approach shore in shallow water their speed ____________. A. increases B. decreases 16-10. If the wavelength of a wave is 20 m, the approximate depth of wave base is ___. A. 1 m B. 5 m C. 10 m D. 20 m 16-11. A. True / B. False: Breakers form in the region where wave base is located at or above the seafloor. Fig. 18.20 Block diagram • Effects on headlands: waves converge and… 16-12. Ripple marks in sediment caused by ocean waves are found ____________. A. at all depths B. only within the tidal range C. only above the wave base D. only below the wave base 16-13. A. True / B. False: Wave refraction causes wave action to be concentrated on headlands. • Geometry of coastline: irregular coastline… Tides & Waves: 21 Tides & Waves: 22 Sediment transport Sediment transport • Swash: waves travel up beach in direction of breaking, not slope of beach • Backwash: water runs back down beach parallel to slope • Longshore currents: sediment & water transport parallel to beach. currents: • Rip current: water flowing back out to sea current: • Spit: elongated ridge of sand or gravel that projects from land and ends in open water Fig. 18.20 Fig. 18.21 Block diagrams If caught in rip current, swim parallel to shore to exit current Tides & Waves: 23 • Longshore currents deposit sediment at bay mouths --> deeper water --> velocity decrease • Examples: Sandy Hook, NJ; Cape Cod, MA Tides & Waves: 24 Photo: S. Dunwell Sediment transport / landforms Sediment transport / landforms • Baymouth bar: ridge of sediment in front of a bay created when a spit extends completely across a bay • Barrier islands: long, narrow sandy islands lying offshore and parallel to the coast, separated from the mainland by a lagoon Top view Animation Barrier island, South Pea Island, North Carolina. Photo: P.L. Kresan Fig. 18.25 Tides & Waves: 25 Tides & Waves: 26 Coastal landforms Beach profile • Beach: dynamic body of sediment supplied by rivers and coastal erosion and sculpted by waves and near-shore currents • Winter beaches: in stormy weather, short wavelength waves break close to shore, carrying sediment offshore, steepening and narrowing beach • Summer beaches: in calm weather, long wavelength waves break offshore, sweeping sediment toward beach, flattening and widening beach Photo: J. Valentine Block diagram Fig. 18.28 Tides & Waves: 27 Tides & Waves: 28 Rocky coasts Block diagram Fig. 18.29 Rocky coasts Side view Sea stacks resulting from shoreline retreat, Australia. Photo: K. Schafer Collapse of a sea arch along the coast of California. (L) 1969. (R) 1987. Photos: W.K. Hamblin Tides & Waves: 29 Tides & Waves: 30 Rocky coasts Rocky coasts • Wave-cut bench: bedrock platform cut by waves; located at mean wave base • Wave-cut notch: hollow cut into a cliff by wave action • Wave-cut cliff: cliff produced by rock falls above the notch Effect of falling sea level or rising land Block diagrams Side view Fig. 18.38a Uplifted wave-cut platform at Big Sur, California. Wave-cut notch, Oman. Photo: W.K. Hamblin Tides & Waves: 31 Fig. 18.29 Photo: W.K. Hamblin Tides & Waves: 32 Changing sea level Effect of rising sea level or sinking land Map (top view) Types of coastlines Block diagrams a. Rising sea level (or sinking land) b. Falling sea level (or uplifting land) c. Large fluvial input on a subsiding coastline Fig. 18.38 Block diagrams Fig. 18.33 Delaware & Chesapeake Bays are drowned river valleys Tides & Waves: 33 Most reefs are located within… Tides & Waves: 35 Fig. 18.23 Tides & Waves: 34 Reefs Reefs • Fringing reef: attached to or closely borders adjacent land. • Barrier reef: separated from the land by a lagoon. • Atoll: approximately circular reef enclosing a shallow lagoon; form as a result of the progressive submergence of a volcanic island in which upward reef growth keeps pace with subsidence Photo: P.L. Kresan • Organic reefs: mounds of colonies of tiny marine organisms that secrete calcium carbonate in shallow, clear water with a temperature between 18°C and 30°C. d. Rising sea level and high sand supply e. Ice age forms deep valleys, melting ice fills valleys f. High-sediment supply plus wind g. Warm, sunny conditions year round Bermuda Block diagrams Tides & Waves: 36 Fig. 18.36 Depositional landforms: reefs Depositional landforms: reefs Barrier reef surrounding volcanic island in the Pacific Ocean. Photo: J.-M. Truchet Tides & Waves: 37 Review Questions 16-14. A. True / B. False: Water associated with swash travels up the beach in the direction of wave motion whereas water associated with backwash always travels down the maximum slope of a beach. 16-15. Currents of water that travel out to sea in discrete locations are called ___. A. rip currents B. longshore drift C. beach drift D. backwash 16-16. Longshore currents flow ____________. A. directly toward the shoreline B. directly away from the shoreline C. parallel to the shoreline 16-17. A swimmer caught in a current heading out to sea should swim ____________. A. in direct opposition to the current B. in the direction of the current C. perpendicular to the current (parallel to the shoreline) 16-18. A. True / B. False: Beaches generally become narrower in the winter because short-wavelength waves break near the beach. Tides & Waves: 39 Atoll: Loughlan (Nada) Island, Solomon Sea, Pacific. Photo: S. Titley Tides & Waves: 38 Review Questions 16-19. Which term does not belong with the others? A. wave-cut bench B. spit C. baymouth bar D. barrier island 16-20. On a coastline that is trending east-west, a spit projects from the east side of a bay toward the middle of the bay. Longshore currents are flowing ___. A. from the north to the south B. from the east to the west C. from the south to the north D. from the west to the east 16-21. A wave-cut notch is located ____ and a wave-cut bench is located ___. A. above wave base; at wave base B. at wave base; below wave base C. above wave base; below wave base 16-22. A. True / B. False: Bermuda (slide 31) is located north of 30° latitude, yet still has coral reefs. This is possible because warm water from the Gulf Stream current flows near Bermuda. 16-23. Which sequence of terms describes the landforms that are associated with the progressive subsidence of a volcanic island? A. atoll-->barrier reef-->fringing reef B. barrier reef-->fringing reef-->atoll C. fringing reef-->barrier reef-->atoll D. fringing reef-->atoll-->barrier reef Tides & Waves: 40 ...
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This note was uploaded on 09/25/2011 for the course GEOLOGY 100 taught by Professor Lepre during the Fall '11 term at Rutgers.

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