07 Tides

07 Tides - Tides Overview Rhythmic rise and fall of sea...

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Unformatted text preview: Tides Overview Rhythmic rise and fall of sea level Very long and regular shallow-water waves Caused by gravitational attraction of Sun, Moon, and Earth Tides Tide Generating Forces Spring and Neap Tides Other Complicating Factors Idealized Tide Prediction Coastal Interaction with Tides Tides and Marine Life Tidal Power Centripetal force Center-seeking force Tethers Earth and Moon to each other Fig. 9.3 Tidegenerating forces Barycenter between Moon and Earth Mutual orbit due to gravity and motion Gravitational forces Every particle attracts every other particle Gravitational force proportional to product of masses Inversely proportional to square of separation distance Fig. 9.2 Tides Tide Generating Forces Spring and Neap Tides Other Complicating Factors Idealized Tide Prediction Coastal Interaction with Tides Tides and Marine Life Tidal Power Lunar Tidal Bulges Small horizontal forces push seawater into two bulges One bulge faces Moon Other bulge opposite side Earth Tidal bulges (lunar) Moon closer to Earth so lunar tideproducing force greater than that of Sun Ideal Earth covered by ocean Two tidal bulges Two high tides, 12 hours apart High tide, flood tide, seawater moves on shore Low tide, ebb tide, seawater moves offshore Tidal bulges (solar) Similar to lunar bulges but much smaller Moon closer to Earth New/full moon tidal range greatest spring tide Quarter moons tidal range least neap tide Time between spring tides about two weeks Tides Monthly tidal cycle Spring tide During new and full moons Gravitational forces added together Especially high and low tides Large daily tidal range Earth-Moon-Sun positions during the Spring tide Tides Monthly tidal cycle Neap tide First and third quarters of the Moon Gravitational forces are offset Daily tidal range is least Earth-Moon-Sun positions during the Neap tide Earth-Moon-Sun positions and spring and neap tides Tides Tide Generating Forces Spring and Neap Tides Other Complicating Factors Idealized Tide Prediction Coastal Interaction with Tides Tides and Marine Life Tidal Power Lunar Day Moon orbits Earth 24 hours 50 minutes for observer to see subsequent Moons directly overhead High tides are 12 hours and 25 minutes apart Other complicating factors: declination Angular distance Moon or Sun above or below Earth's equator Sun to Earth: 23.5o N or S of equator Moon to Earth: 28.5o N or S of equator Shifts lunar and solar bulges from equator Unequal tides Fig. 9.11 Declination and tides Unequal tides (unequal tidal ranges) Fig. 9.13 Tidal range greatest at perihelion (January) and perigee Tidal range least at aphelion (July) and apogee Perigee and apogee cycle 27.5 days Other complicating factors: elliptical orbits Tides Tide Generating Forces Spring and Neap Tides Other Complicating Factors Idealized Tide Prediction Coastal Interaction with Tides Tides and Marine Life Tidal Power Idealized tide prediction Two high tides/two low tides per lunar day Six lunar hours between high and low tides Idealized tidal bulges on Earth Real tides Earth not covered completely by ocean Continents and friction with seafloor modify tidal bulges Tides are shallow water waves with speed determined by depth of water Tidal bulges cannot form (too slow) Tidal cells rotate around amphidromic point Tidal cells in world ocean Cotidal lines Tide wave rotates once in 12 hours Counterclockwise in Northern Hemisphere Clockwise in Southern Hemisphere Tidal patterns Diurnal Semidiurnal Mixed Tides Tidal patterns Diurnal tidal pattern A single high and low tide each tidal day Occurs along the northern shore of the Gulf of Mexico One high tide and one low tide in a 24 hour day. Tides Tidal patterns Semidiurnal tidal pattern Two high and two low tides each tidal day Little difference in the high and low water heights Common along the Atlantic Coast of the U.S. Two high tides and two low tides in a 24 hour day. Tides Tidal patterns Mixed tidal pattern Two high and two low waters each day Large inequality in high water heights, low water heights, or both Prevalent along the Pacific Coast of the U.S. Uneven high and low tides in a 24 hour day. Tidal frequencies for North America Tides Tide Generating Forces Spring and Neap Tides Other Complicating Factors Idealized Tide Prediction Coastal Interaction with Tides Tides and Marine Life Tidal Power Tides in coastal waters Tide waves reflected by coast Amplification of tidal range Example, Bay of Fundy maximum tidal range 17 m (56 ft) High tide in the Bay of Fundy along the Nova Scotia coast Low tide in the Bay of Fundy along the Nova Scotia coast Tides in coastal waters Tidal bore in lowgradient rivers Coastal tidal currents Flood current Ebb current High velocity flow in restricted channels Tidal currents Horizontal flow accompanying the rise and fall of tides Types of tidal currents Flood current advances into the coastal zone Ebb current seaward moving water Sometimes tidal deltas are created by tidal currents Features associated with tidal currents Coastal tidal currents Whirlpool Rapidly spinning seawater Restricted channel connecting two basins with different tidal cycles Tides Tide Generating Forces Spring and Neap Tides Other Complicating Factors Idealized Tide Prediction Coastal Interaction with Tides Tides and Marine Life Tidal Power Tides and marine life Tide pools and life Grunion spawning Tides Tide Generating Forces Spring and Neap Tides Other Complicating Factors Idealized Tide Prediction Coastal Interaction with Tides Tides and Marine Life Tidal Power Tide-generated power Renewable resource Does not produce power on demand Possible harmful environmental effects Potential Sites for Tidal Power Fig. 9.21 End of Tides Tide-producing forces Resultant forces = differences between centripetal and gravitational forces Tide-generating forces are horizontal components Fig. 9.4 ...
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