CHAPTER 9 Tides http://www.bbso.njit.edu/espr/sci_images/composite_earth1_red.gif

Overview Rhythmic rise and fall of sea level Very long and regular shallow-water waves Caused by gravitational attraction of Sun, Moon, and Earth http://soconnell.web.wesleyan.edu/ees106/lecture_notes/lecture-tides/HTML%20Presentation%20folder/img064.gif

Tide-generating forces http://www.lhup.edu/~dsimanek/scenario/img008.gif Tide-generating forces Barycenter between Moon and Earth – center of gravity of “Earth-Moon System” Mutual orbit due to gravity and motion Fig. 9.1

Gravitational forces Every particle attracts every other particle Gravitational force proportional to product of masses, toward center of gravity of Moon Inversely proportional to square of separation distance – decrease the farther away from Moon Fig. 9.2

Centripetal force Center-seeking force – force equal at all points Tethers Earth and Moon to each other Fig. 9.3

Tide-producing forces Resultant forces = differences between centripetal and gravitational forces (blue arrows) Near moon, gravitational forces are greater Away from moon, centripetal forces are greater Tide-generating forces are horizontal components resulting in “bulges” Fig. 9.4

Blue arrows are formed from the tip of the red arrows (centripedal forces) to the tip of the black arrows (gravitational forces). Fig. 9.4

Tidal bulges (lunar) Small horizontal forces push seawater into two bulges on opposite sides of Earth One bulge faces Moon Other bulge opposite side Earth Fig. 9.6

http://www.lhup.edu/~dsimanek/scenario/img008.gif Tidal bulges (lunar) Moon closer to Earth so lunar tide-producing 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

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 Fig. 9.7

Tidal bulges (solar) Similar to lunar bulges but much smaller because Moon is much closer to Earth New/full moon tidal range greatest – spring tide lunar and solar tides in line – constructive interference Time between spring tides about two weeks Quarter moons tidal range least – neap tide lunar and solar tides at 90O – destructive interference

Earth-Moon-Sun positions and spring and neap tides Fig. 9.9

https://www.youtube.com/watch?v=l37ofe9haMU

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 at given latitude Fig. 9.11 http://www.nos.noaa.gov/education/kits/tides/media/

Declination and tides Unequal tides (unequal tidal ranges at given latitude) Fig. 9.13

Other complicating factors: elliptical orbits Tidal range greatest at perihelion (January) and perigee Tidal range least at aphelion (July) and apogee Moon’s perigee and apogee cycle 27.5 days Omit Fig. 9.12

Idealized tide prediction Two high tides/two low tides per lunar day Six lunar hours between high and low tides http://www.aztecsailing.co.uk/theory/Ch2-Fig-2-tidal-cycle.gif

Real tides affected by many factors http://www.princeton.edu/~pccm/outreach/scsp/water_on_earth/tides/inquiry 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 “slosh” around amphidromic point http://ffden-2.phys.uaf.edu/645fall2003_web.dir

Tidal cells in world ocean Cotidal lines Tide wave rotates once in 12 hours Counterclockwise in Northern Hemisphere Omit

Omit Fig. 9.14

Tidal patterns Semidiurnal Two high tides/two low tides per day Tidal range about same Diurnal One high tide/one low tide per day Rarest Mixed Tidal range different Most common http://www.fhwa.dot.gov/engineering/hydraulics/images/h25_b6.jpg

Global distribution of tides www.nos.noaa.gov/.../ tides/media/

Tides in coastal waters Standing waves Tide waves reflected by coast Amplification of tidal range Example, Bay of Fundy maximum tidal range 17 m (56 ft) Video http://www.ludssurfingpage.bravepages.com/wswave4.jpg http://academics.sru.edu/GGE/pictures/Newfoundland/Images

Bay of Fundy http://academics.sru.edu/GGE/pictures/Newfoundland/Images http://www.eicc.bio.usyd.edu.au/Newslette www.sfu.ca/ ~dutting/photos.htm http://user.fundy.net/nature/hope.jpg

Tides in coastal waters Tidal bore in low-gradient rivers Video Fig. 9A

Coastal tidal currents Reversing current Flood current or tide – approaching high tide (crest moves into harbor) Ebb current or tide – approaching low tide (trough moves into harbor) Slack tide – period of little current at high or low tide High velocity flow in restricted channels Fig. 9.18

Peak flood and ebb currents in middle of tidal cycle ~1/2 tidal flow moves thru within middle two hours of cycles Example for semidiurnal tide - “rule of 12ths” 1 – 1/12 of volume  slack tide 2 – 2/12 of volume 3 – 3/12 of volume  fastest current 4 – 3/12 of volume  fastest current 5 – 2/12 of volume 6 – 1/12 of volume  slack tide http://www.hka.org.uk/images/rule12.gif

Tidal range is not a good predictor of tidal currents; altered by: Shape and volume of basin Restriction of flow at basin mouth Winds Currents stronger at channel center than at edges http://www.skypic.com/florida

Coastal tidal currents Whirlpool Rapidly spinning seawater Restricted channel connecting two basins with different tidal cycles Fig. 9.19

Tides and marine life Tide pools and life Grunion spawning Fig. 9C http://www.stripersonline.com/surftalk/attachment.php?attachmentid=30481&stc=1&d=1169261518 Tides and marine life Tide pools and life Grunion spawning Fig. 9C

Tide-generated power Renewable resource – mostly hypothetical at this point Does not produce power on demand Possible harmful environmental effects http://www.bluenergy.com/technology.html

End of CHAPTER 9 Tides Fig. 9.21