–Semi-diurnal tides and the lunar "tidal day". –Diurnal ... Here's what the two tidal
bulges actually look like. ... occur as the Earth rotates beneath the tidal bulges.
Fig. 8.13
Fig. 8.14 Fig. 9.13
Longshore transport 1. Wave fronts and onshore "swash" -- oblique to beach front 2. "Backswash" perpendicular to beach front
Result: longshore transport, Water and sediment move parallel to the beach front Fig. 9.13
Longshore transport (cont’d) •Erosion occurs in areas of strong wave action (headlands) •Longshore transport tends to be away from headlands and toward quieter areas •Both sediment and water converge in recessed areas •Sediment collects there (beaches) •Water turns back seaward: Rip currents
Fig. 9.14
1
BEACH PROCESSES -- Seasonal •Summer -- gentle waves, sand deposited on the beach •Winter -- storm waves, erosion and seaward transport
Fig. 9.12
BEACH PROCESSES -Longshore Transport •Beach sediments -- continuous movement due to longshore transport •Longshore transport -- depends on wave energy and barriers
Human Impact On Beach/coastal Processes (1) Dams on rivers •Reduced sediment input: coasts erode (2) "Groins" to stop loss of beach sand •Starves the next beach of its sediment supply (3) "Jetties" to protect harbors •Same result
2
Coastal Woes Intervention in response to a single coastal problem or demand of society often leads to unforeseen consequences- erosion or deposition.
TIDES
Class #29 -- The Oceans TIDES •Types of tides •Tide-raising forces –Gravitational attraction –Centrifugal forces •Equilibrium Tide Theory –Semi-diurnal tides and the lunar "tidal day" –Diurnal tides –"Spring" and "Neap" tides
TIDES- Two Bulges (Why?)
Periodic rise and fall of water level along coastlines that changes with the phases of the Moon.
Is this correct? Earth Moon
Moon’s Gravity (Size of bulge is greatly exaggerated, of course)
The moon orbits around the earth, and this makes the earth “wobble”.
Here’s what the two tidal bulges actually look like. Earth Moon
What force is this?
Moon’s Gravity
3
More precisely, the earth and moon both orbit around their center of mass.
X
X
TIDES- Two Bulges.... Why? There are two forces
Tides depend on Gravity (G) and Centrifugal force (C)
1. Gravity force from Moon 2. Centrifugal force caused by earth’s “wobble” Earth Moon
Centrifugal force stronger here
Because the earth is orbiting (slightly) there is a centrifugal force.
Moon’s Gravity stronger here
TIDES- Two Bulges....
1. Moon’s orbit (T = 29.5 days). 2. C has same magnitude and direction for all parts of the Earth 3. G and C are exactly balanced at center of the earth. 4. But... –Moon’s G stronger on the side close to Moon –C stronger away from Moon
How many high tides per day? •One would expect 2 highs and 2 lows per day. –As the earth rotates, we are close t the moon once per day and opposite the moon once per day.
Earth Moon
Centrifugal force stronger here
Moon’s Gravity stronger here
4
TIDES- Two Bulges....
Earth Moon
Centrifugal force stronger here
Moon’s Gravity stronger here
•One would expect 2 highs and 2 lows per day. •Reality: It depends on where you are on the earth. 1. Some places have Diurnal tides –1 High and 1 Low per day 2. Some places have Semi-diurnal tides –2 Highs and 2 Lows per day 3. Some places have Semi-diurnal mixed tides – 2 Highs and 2 Lows per day –Uneven: 1 major high, 1 major low, others less
Tidal period is actually 12 hours, 25 min. (or 24 hours, 50 min. for diurnal tides)
Diurnal
Cause: The tilt of the earth’s axis relative to the orbital plane of the moon
Semi- Diurnal
Semi-Diurnal Mixed
Diurnal tides and semidiurnal mixed tides . . occur because the Moon and the Sun are not directly overhead at the Equator, but at a different latitude. - "Declination" •Sun's declination -- 23.5 deg N to 23.5 deg S •Moon's declination -- 28.5 deg N to 28.5 deg S •Tidal bulges are in both the N. and S. Hemispheres •Resulting tides as Earth rotates: •Diurnal immediately beneath the bulge •Semi-diurnal near the Equator •Semi-diurnal mixed elsewhere
2-week tides ("Spring" & "Neap" tides)
•Variation in high and low tidal levels with a period of about 2 weeks •These change correlate with the phases of the moon •Higher highs and lower lows at full moon and new moon
5
Fig. 8.26
Explaining types of tides from Equilibrium Theory: Semi-diurnal tides . . occur as the Earth rotates beneath the tidal bulges. Any location on Earth should experience 2 high tides and 2 low tides per revolution (per day). Expected T = 12 hrs exactly -- the case for solar semi-diurnal tides. Lunar semi-diurnal tides: T = 12 hr 25 min. Moon rotates as Earth spins. Any location on Earth's surface must rotate a little further each day (about 50 min. more rotation) to keep up. "Lunar tidal day" = 24 hr 50 min. Period of the lunar semi-diurnal tide is one-half of that, or 12 hr 25 min.
6
Two-week variations in tidal ranges -- spring and neap tides -caused by changes in the alignment of the Earth-MoonSun (EMS) system. EMS along straight line: M&S tides reinforce each other -- SPRING TIDES EMS forms a right angle, M&S tides "cancel" -- NEAP TIDES
