ERTH 2403 - Introduction To Oceanography Lecture 8 - Tides Tides
ERTH 2403 - Introduction To Oceanography Lecture 8 - Tides Tides
ERTH 2403 - Introduction To Oceanography Lecture 8 - Tides Tides
Lecture 8 Tides
Tides
Periodic-short term changes in the height of ocean surface at particular
places
L = circumference of Earth
H = 2m to rarely higher
D = < L shallow water wave
Disturbing Forces
1. Gravitational attraction of Moon & Sun & Earth
2. Rotation of the Earth on its axis
Generating Force
Tide generating force is inversely proportional to cube of the distance
between their centers
Distance is more important than mass
Sun 27 million times the moon (mass) but is 387 further
Tidal Theories
1. Newtons Equilibrium Model
a. Ideal
b. Water-Covered Earth
c. Spins frictionless under ocean
2. Laplaces Dynamic Theory
a. Real friction, continents
Tidal Patterns
1. Diurnal
a. One high tide and one low tide
2. Semidiurnal
a. Two high and two lows of equal value
3. Mixed
a. Semidiurnal unequal
4. Coriolis Effect
a. Deflects tide wave crest
b. Limited by edge of basin
c. Rotate about no-tide points
d. Amphidromic Point
i. Node: A no tide point in the ocean around which the tide crest
rotates through one tidal cycle counter clockwise in the Northern
Hemisphere
Tidal Datum
Mixed Tides
o MLLW mean low low water
Diurnal and Semidiurnal
o MLW mean low water
Tidal Range
o Difference between high and low
Tidal Currents
o In Basins
o Flood currents
o Slack water
o Ebb current
Tidal Bore
o Forced wave
Tidal Friction
o Slow Earths rotation
o Devonian day = 22 hours ~ 430m.y.a
Predicting Tides
o Complex
o Local
o Navigational info
o Very accurate
Power Generator
o Rance River France
Classification of Coast
1. Erosional Coasts
a. Net wearing down by ocean
b. By river/land source processes
2. Deposition Coast
a. Net sediments build-up by ocean
b. River/biological/land source processes
Erosional Coastal Features
Coasts are shaped by
o Ocean erosion
Rocky
Irregular
Sea cliffs, sea stacks, blow holes arches, wave-cut platforms
High Energy
Headland erosion : Shore Straightening
Shore Straightening
Wave refract
o Feel bottom and slow
o Bend +- parallel to shore
o High energy
o Headland erosion
Erosional Coastal Features
Land Erosion
o Drowned rivers
Rivers cut when sea level low ( ice age) drown as sea level rose
o Fjords
Glaciated valleys when sea level low
Water-filled as sea level rose
Volcanism
o Volcanic action hot lavas cutting into coast
Tectonics
o Faulting can uplift or down drop a coastline
Beaches
Accumulation of sediments
~30% U.S Coasts
More on east coasts (passive margin)
Dynamic states sediments in motion
Beach Profile
1. Backshore:
a. Above high tide
2. Foreshore & Nearshore
a. Intertidal: highest water
b. Berm: high water mark
c. tide line
d. Berm crest: highest point
e. Beach scrap: high water cutting into the berm
f. Swash zone: flat wet surface water moves back and forth and
sediments move back and forth
g. Surf zone breakers
3. Offshore
a. Below low tide
b. Long shore trough
c. Longshore bars
d. Longshore transport
4. Rip Currents
a. Water piles up with longshore currents
b. Periodically
i. Deeper area
ii. Water breaks through current
iii. Moves out to sea
iv. Rapid
v. Muddy
vi. Dangerous
Barrier Islands
Coastal dunes during last ice age = low sea level
Lagoons formed as sea level rose
~295 islands off east coast
~2500km shore
Migrate landward (wave erosion) ~ 2m/yr
Deltas
Land built
Passive margins
Broad continental shelves
Low ocean energy
Low tides
Low currents
Low waves
No large deltas on Atlantic coast (high energy)
Balance between river and ocean processes
River dominated
o Birds foot delta Mississippi
Wave dominated
o Small, smooth
o Single channel
o Uncommon
o San Francisco
Tide Dominated
o More or less parallel channels
o Ganges Brahmaputra (Bay of Bengal)
Coasts Dominated by biological activity
Coral Reef (tropical)
o Fringing reef
o Barriers
o Atolls
Mangroves
o Dominated in tropics
Salt water marshes
o Dominate outside of tropics
Estuary
A body of water partially surrounded by land where fresh water from a river
mixes with ocean water
Creating an area of remarkable biological productivity
Four types based on Origin
o Drowned Rivers
o Fjords
o Bar-built
o Tectonic faulting
Estuary Circulation
1. Salt wedge estuary
a. Deep
b. Strong river large tide
c. Sharp halocline
d. Hudson River
2. Well mixed estuary
a. Shallow, broad
b. Low river volume
c. Strong tide
d. Internal transfer of salt water
e. Vertical mixing
f. Coriolis Effect
g. Columbia River
3. Partially mixed estuary
a. Deeper, more or less broad
b. Coriolis
c. Strong River and Tide
d. Turbulent upward mixing of water
e. Chesapeake Bay
4. Fjord Estuary
a. Deep
b. Narrow
c. Still obstruction of outflow
d. Strong River
e. Stagnant
5. Reversed Estuary
a. Arid coast
b. Periodic drying of river
c. Tide upstream
d. Evaporation result in high salinity upstream
e.