Tides & Tides & Tides & Tides & Tides & Tides & Tides & Tides &

WavesWavesWavesWavesWavesWavesWavesWaves

••Very long waves that move through Very long waves that move through oceansoceans

••Response to forces exerted by moon Response to forces exerted by moon & sun& sun

••Originate in middle of oceans & Originate in middle of oceans & progress toward coastprogress toward coast

TidesTidesTidesTidesTidesTidesTidesTides

••Appear as regular Appear as regular rise and fall of the rise and fall of the sea surfacesea surface

••High tideHigh tide = = CrestCrest (highest part of (highest part of

wave) reaches a locationwave) reaches a location

••Low tideLow tide = = TroughTrough (lowest part) (lowest part)

••Tidal rangeTidal range = Difference in height = Difference in height

between high tide & low tidebetween high tide & low tide

TidesTidesTidesTidesTidesTidesTidesTides

High tideHigh tideHigh tideHigh tideHigh tideHigh tideHigh tideHigh tide Low tideLow tideLow tideLow tideLow tideLow tideLow tideLow tide

Causes of TidesCauses of TidesCauses of TidesCauses of TidesCauses of TidesCauses of TidesCauses of TidesCauses of Tides

Created by imbalance between 2 Created by imbalance between 2 forces: Gravity & inertiaforces: Gravity & inertia

1.1.GravitationalGravitational force of Moon & Sun on force of Moon & Sun on EarthEarth

•• Greater mass = gravitational force Greater mass = gravitational force ��

•• If distance If distance ��, then gravitational , then gravitational force force ����

SunSun’’s tides tide--generating force is generating force is ½½ that of that of the moon the moon

WHY?????WHY?????

Causes of TidesCauses of TidesCauses of TidesCauses of TidesCauses of TidesCauses of TidesCauses of TidesCauses of Tides

•• SunSun’’s mass is 27 million times s mass is 27 million times >>>>>>>> moonmoon’’s s

•• SunSun’’s gravitational attraction to Earth is > > s gravitational attraction to Earth is > >

moonmoon’’s (177X)s (177X)

•• Sun should have >> tideSun should have >> tide--generating force generating force

••Sun is further Sun is further

from Earth than from Earth than

moon moon →→→→→→→→

••SunSun’’s tides tide--

generating force is generating force is

59 million times < 59 million times <

moonmoon

Tidal bulges (High tides)Tidal bulges (High tides)

TideTide--generating forces produce 2 bulges:generating forces produce 2 bulges:

1.1. Away from MoonAway from Moon –– side of Earth opposite side of Earth opposite

Moon (inertia = straight line)Moon (inertia = straight line)

2.2. Toward MoonToward Moon –– side of Earth facing Moon side of Earth facing Moon

(moon(moon’’s gravity)s gravity)

••Earth rotates Earth rotates

into & out of into & out of

tidal bulges tidal bulges

→→ Creates high & Creates high &

low tideslow tides

Low tide

Low tide

•Takes Earth 50 min. to “catch up” to moon

•Earth spins once on its axis – 24 hrs.

•Moon revolves with Earth in same direction (revolution takes ~ 1 month)

•Takes Earth 50 min. to “catch up” to moon

•Earth spins once on its axis – 24 hrs.

•Moon revolves with Earth in same direction (revolution takes ~ 1 month)

The Lunar day = The Lunar day = The Lunar day = The Lunar day = The Lunar day = The Lunar day = The Lunar day = The Lunar day = 24 hrs. & 50 min.24 hrs. & 50 min.

•Earth rotates through 2 tidal “bulges” each lunar day →

•2 high & low tides every 24 hr. & 50 min.

•High tides: 12 hr. & 25 min. apart

•High & low tides: 6 hr. & 12.5 min. apartapart

Monthly Tidal CycleMonthly Tidal CycleMonthly Tidal CycleMonthly Tidal CycleMonthly Tidal CycleMonthly Tidal CycleMonthly Tidal CycleMonthly Tidal Cycle

Every ~7 days, Earth alternates between:Every ~7 days, Earth alternates between:

1.1. Spring tideSpring tide -- EarthEarth--MoonMoon--Sun in alignmentSun in alignment

••New or full New or full

moonmoon

••Solar tide Solar tide has has additiveadditiveeffect on effect on lunar tidelunar tide

→→→→→→→→ ExtraExtra--high, high, high tides & high tides & very low, low very low, low

tidestides

Monthly Tidal CycleMonthly Tidal CycleMonthly Tidal CycleMonthly Tidal CycleMonthly Tidal CycleMonthly Tidal CycleMonthly Tidal CycleMonthly Tidal Cycle

2. 2. Neap tideNeap tide -- Sun & moon are at right anglesSun & moon are at right angles

••Solar tide Solar tide

partially partially

cancelscancels

out lunar out lunar

tide tide

••Produces Produces

moderate moderate

tidestides

Monthly Tidal CycleMonthly Tidal CycleMonthly Tidal CycleMonthly Tidal CycleMonthly Tidal CycleMonthly Tidal CycleMonthly Tidal CycleMonthly Tidal Cycle

Tidal patterns in Tidal patterns in

the U.S.the U.S.

Tidal CurrentsTidal CurrentsTidal CurrentsTidal CurrentsTidal CurrentsTidal CurrentsTidal CurrentsTidal Currents•• Horizontal movement of water w/ rising & Horizontal movement of water w/ rising & falling of tidesfalling of tides

•• Flood tideFlood tide = incoming tide along coast, = incoming tide along coast, bays, & estuariesbays, & estuaries

•• Ebb currentEbb current = outgoing tide= outgoing tide

Figure 9-18

••Slack waterSlack water = =

still water still water

(lasts 1 hour (lasts 1 hour

afterafter set of set of

each high & each high &

low tide)low tide)

Importance of Understanding & Importance of Understanding & Importance of Understanding & Importance of Understanding & Importance of Understanding & Importance of Understanding & Importance of Understanding & Importance of Understanding &

Monitoring TidesMonitoring TidesMonitoring TidesMonitoring TidesMonitoring TidesMonitoring TidesMonitoring TidesMonitoring Tides

Movement of

organisms

(plankton, grunion)

Surfing

Navigation

Pollution

Nutrient mixing

WavesWavesWavesWavesWavesWavesWavesWaves

Properties of Ocean WavesProperties of Ocean Waves

Ocean wave = undulation of the sea surfaceOcean wave = undulation of the sea surface

Distance

between

2 crests

Wave PeriodWave Period –– Time (sec.) between 2 crests Time (sec.) between 2 crests

(fixed point)(fixed point)

Wave frequencyWave frequency -- The number of waves that The number of waves that

pass a given point in a given timepass a given point in a given time

= distance = distance btwnbtwn. .

trough & cresttrough & crest

What Causes Waves?What Causes Waves?What Causes Waves?What Causes Waves?

••••Gravitational attraction of sun & moon

Wind !!!!

Wind Generation of WavesWind Generation of WavesWind Generation of WavesWind Generation of Waves

•The type of wave generated by wind is determined by:

• Wind velocity (distance traveled per unit time)

• Wind duration

• Fetch (distance that wind blows over ocean surface)

Wave velocityWave velocityWave velocityWave velocityWave velocityWave velocityWave velocityWave velocity

DDDDetermined by:

LENGTH (Feet or Meters)

PERIOD (seconds)

(Distance between Crests) (Distance between Crests) (Distance between Crests) (Distance between Crests) (Time between Crests)(Time between Crests)(Time between Crests)(Time between Crests)

Wave windowsWave windows

••Swells approach our coastline from 2 Swells approach our coastline from 2

directions: directions: South or NorthwestSouth or Northwest

••Arrive via Arrive via wave windowswave windows (unobstructed (unobstructed

approach to coast)approach to coast)

•All other directions

blocked

offshore by:

•Channel islands

(Catalina,

San Clemente)

•Coastline itself (Pt.

Conception)

Wave DiffractionWave DiffractionWave DiffractionWave DiffractionWave DiffractionWave DiffractionWave DiffractionWave Diffraction

Bending of waves around an obstacleBending of waves around an obstacle

Causes the Causes the ““scallopscallop”” effect along our coasteffect along our coast

Wave Refraction & Wave DiffractionWave Refraction & Wave DiffractionWave Refraction & Wave DiffractionWave Refraction & Wave DiffractionWave Refraction & Wave DiffractionWave Refraction & Wave DiffractionWave Refraction & Wave DiffractionWave Refraction & Wave Diffraction

Bending of waves Bending of waves

around an obstacle. around an obstacle.

This is what causes This is what causes

the the ““scallopscallop”” effect effect

along our coast.along our coast.

DiffractionDiffractionRefractionRefraction

Bending of waves as they Bending of waves as they

approach a shore at an angle. approach a shore at an angle.

As drag along the bottom and As drag along the bottom and

differential speed along the differential speed along the

crest, waves refract It is this crest, waves refract It is this

action that allows surfers to action that allows surfers to

surf.surf.

Wave ReflectionWave ReflectionWave ReflectionWave ReflectionWave ReflectionWave ReflectionWave ReflectionWave Reflection

Wave Reflection occurs when incoming Wave Reflection occurs when incoming

waves do not break, expending energy, waves do not break, expending energy,

but are reflected back only to interfere but are reflected back only to interfere

with incoming waves. This occurs at the with incoming waves. This occurs at the

surf spot known as the surf spot known as the ““WedgeWedge”” in in

Newport, See the diagram below.Newport, See the diagram below.

The WedgeThe Wedge

Shore breakers (surf) are collapsing waves. Shore breakers (surf) are collapsing waves.

Breaking is determined by wave steepness. When the wave Breaking is determined by wave steepness. When the wave

height/wave length ratio is about 1/7, height/wave length ratio is about 1/7,

waves begin to break. waves begin to break.

BreakersBreakersBreakersBreakersBreakersBreakersBreakersBreakers

TsunamisTsunamisTsunamisTsunamisTsunamisTsunamisTsunamisTsunamis

Tsunamis are incorrectly called Tsunamis are incorrectly called ““tidal wavestidal waves”” because because

they have nothing to do with tides. They are caused by they have nothing to do with tides. They are caused by

movement of the sea floor (earthquakes, landslides). As movement of the sea floor (earthquakes, landslides). As

they move out from their source, the waves formed end up they move out from their source, the waves formed end up

having very small heights (1 having very small heights (1 -- 2 ft.), very long wave lengths 2 ft.), very long wave lengths

(100+ miles long) and they move at high speed (hundreds of (100+ miles long) and they move at high speed (hundreds of

miles per hour). When they approach shallow water, the miles per hour). When they approach shallow water, the

water water ““piles uppiles up”” and a very high wave may form, and when and a very high wave may form, and when

it breaks, thousands of people living near the shore could it breaks, thousands of people living near the shore could

be killed.be killed.

Where do the tides start?Where do the tides start?

••MMMMMMMMiddles of the worldiddles of the world’’s oceanss oceans

••Similar to how water in a bowl moves Similar to how water in a bowl moves

(spin bowl in a circle) (spin bowl in a circle)

••““NodeNode”” (Fig. 1) never rises or falls (point (Fig. 1) never rises or falls (point

around which water moves)around which water moves)

Fig. 1Fig. 1

The Bay of Fundy: Site of the worldThe Bay of Fundy: Site of the worldThe Bay of Fundy: Site of the worldThe Bay of Fundy: Site of the worldThe Bay of Fundy: Site of the worldThe Bay of Fundy: Site of the worldThe Bay of Fundy: Site of the worldThe Bay of Fundy: Site of the world’’’’’’’’s largest s largest s largest s largest s largest s largest s largest s largest

tidal rangetidal rangetidal rangetidal rangetidal rangetidal rangetidal rangetidal rangeTidal RangeTidal Range

Tidal Range is the difference between high tide and low Tidal Range is the difference between high tide and low tide. The Bay of Fundy in Canada has the greatest tidal tide. The Bay of Fundy in Canada has the greatest tidal range on the planet.range on the planet.

•• Tidal energy is focused by shape and shallowness of bayTidal energy is focused by shape and shallowness of bay

•• Maximum spring tidal range in Minas Basin 53 feetMaximum spring tidal range in Minas Basin 53 feet

•• Nova Scotia bends when the tide comes in! Nova Scotia bends when the tide comes in!

As 14 billion tons (14 cubic kilometers) of As 14 billion tons (14 cubic kilometers) of

sea water flow into Minas Basin twice daily, sea water flow into Minas Basin twice daily,

the Nova Scotia countryside actually tilts the Nova Scotia countryside actually tilts

slightly under the immense load! slightly under the immense load!

Figure 9-17

Bay of FundyBay of Fundy

tidal boretidal bore

Tidal BoresTidal BoresTidal BoresTidal BoresTidal BoresTidal BoresTidal BoresTidal Bores

The wave on the incoming tide is known as a Tidal The wave on the incoming tide is known as a Tidal

Bore. Tidal bores occur in about 100 rivers throughout Bore. Tidal bores occur in about 100 rivers throughout

the world. Three of the better known bores are the the world. Three of the better known bores are the

Qiantang Bore in China. This bore reaches heights of 15 Qiantang Bore in China. This bore reaches heights of 15

ft. and travels 15 mph. The Pororoca, in the Amazon ft. and travels 15 mph. The Pororoca, in the Amazon

River, forms waves 12 ft. high and can reach speeds of River, forms waves 12 ft. high and can reach speeds of

20 mph. The Severn Bore in England is a popular one to 20 mph. The Severn Bore in England is a popular one to

surf as shown below.surf as shown below.

Qiantang BoreQiantang Bore Surfing the Severn BoreSurfing the Severn Bore

Tides in the Tides in the Tides in the Tides in the Tides in the Tides in the Tides in the Tides in the

oceanoceanoceanoceanoceanoceanoceanocean•• Cotidal map shows tides rotate around Cotidal map shows tides rotate around amphidromic points.amphidromic points.

There are 140 amphidromic points in the There are 140 amphidromic points in the worldworld’’s oceans.s oceans.

•• More realistic pattern of tides in the oceanMore realistic pattern of tides in the ocean

Figure 9-14

Amphidromic PointsAmphidromic Points

2.2.2.2.2.2.2.2. CentripetalCentripetalCentripetalCentripetalCentripetalCentripetalCentripetalCentripetal

ForceForceForceForceForceForceForceForce

•Center-seeking

•Required to

keep bodies in

nearly circular

orbits

Spring tideSpring tide

Neap tideNeap tide

As the storm subsides, the waves move out of the As the storm subsides, the waves move out of the

storm zone and carry a lot of energy. Swells, storm zone and carry a lot of energy. Swells, -- more more

regular waves, beyond area of generation, form and the regular waves, beyond area of generation, form and the

progressive wave move across the ocean.progressive wave move across the ocean.

These waves may travel hundreds of miles across the These waves may travel hundreds of miles across the

ocean,only to break on some distant shore.ocean,only to break on some distant shore.

••MoonMoon’’s Force is same s Force is same

everywhere on Eartheverywhere on Earth

•• Force is directed Force is directed

perpendicular to Earthperpendicular to Earth’’s s

center (everywhere)center (everywhere)

MoonMoonMoonMoonMoonMoonMoonMoon’’’’’’’’ s Gravitational forces on Earths Gravitational forces on Earths Gravitational forces on Earths Gravitational forces on Earths Gravitational forces on Earths Gravitational forces on Earths Gravitational forces on Earths Gravitational forces on Earth

•• Force Force ↓↓↓↓↓↓↓↓ as as distance distance ↑↑↑↑↑↑↑↑

•• Force directed Force directed toward Moontoward Moon’’s s center of masscenter of mass

Centrifugal forces on EarthCentrifugal forces on Earth