TIDES

Chapter 8

101

100

10-1

10-2

• A tide has a waveform.- Shallow water wave (large L compared to water depth).- Crest of wave is high tide.- Trough of wave is low tide.- Period is time between successive high (or low) tides.- Wave height of tidal wave is the range

crest

trough

L

I. TIDAL RANGES VARY OVER THE DAILY (DIURNAL) PERIOD (vary geographically):

1. Diurnala) One low one high tide each day.

b) Tidal period of 24hrs 25 min.

2. Semidiurnala) Two high and two low tides each day.

b) Tidal period of 12 hrs 25 min.

3. Mixeda) Two high and two low tides of dissimilar range.

b) Periods between 12 hrs 25 min & 24 hrs 25 mins.

0 1 2Day

-3

-2

-1

0

1

2

3T

idal

Hei

gh

t (f

eet)

0 1 2Day

-3

-2

-1

0

1

2

3

Tid

al H

eig

ht

(fee

t)

D iurnal (once a day)

Sem idiurnal (tw ice a day)

Diurnal and Semidiurnal

0 1 2 3Day

-3

-2

-1

0

1

2

3

Tid

al H

eig

ht

(fee

t)Superposition of W aves or Adding waves up

0 1 2 3Day

-3

-2

-1

0

1

2

3

Tid

al H

eig

ht

(fee

t)

0 1 2 3

Day

-3

-2

-1

0

1

2

3

Tid

al H

eig

ht

(fee

t)

D iurnal (once a day)

Sem idiurnal (tw ice a day)

Diurnal + Sem idiurnalReal tides are the resultOf constructive and destructive wave interferenceof multiple tidal components, making a “mixed tide”.

0 1 2 3Day

-3

-2

-1

0

1

2

3

Tid

al H

eig

ht

(fee

t)Superposition of W aves or Adding waves up

0 1 2 3Day

-3

-2

-1

0

1

2

3

Tid

al H

eig

ht

(fee

t)

0 1 2 3

Day

-3

-2

-1

0

1

2

3

Tid

al H

eig

ht

(fee

t)

D iurnal (once a day)

Sem idiurnal (tw ice a day)

Diurnal + Sem idiurnal

1. Spring tidesa) The low and high tides each month with

highest range.

b) Coincides with full and new moons.

2. Neap tidesa) The low and high tides each month with

lowest range.

b) Coincides with first and last quarter moons.

TIDAL RANGES ALSO VARY OVER THE MONTLY PERIOD, WITH CYCLE OF MOON

Examples of Tidal Records

Two Models of Tides

1. Equilibrium modela) (over)-simplified representation.

2. Dynamic modela) Adds more realistic complexity.

How to account for the temporal and geographicvariations of the tides???

• Assumptions:– Earth’s surface is covered by seawater

of infinite depth.

– Wave is progressive.

– Wave is in equilibrium with tide generating forces (gravitational and centrifugal).

Equilibrium Model of Tides

TIDES RESULT FROM TWO FORCES:

1. Gravitational force of moon & sun

2. Centrifugal force

Gravitational effect

Centrifugal effect

Combined gravitation & centrifugal.

Equilibrium Tides- Declination

Spring and Neap Tides

Spring and Neap Tides

So, to summarize…

The equilibrium model of the tides helps explain:

1. Geographic variation of tidal periods (diurnal, semidiurnal, mixed).a. Moon’s declination causes latitudinal variation in sizes of

‘bulges’ seen at a point on the rotating earth.

2. Temporal variation of tidal periods (spring, neap).a) Moon’s phases and alignment with moon & sun.

• Add more complexity to the simple equilibrium model. To make more realistic we must consider:

-shape of ocean basins, land masses

-coriolis deflection & geostrophic flow

The Dynamic Model of Tides

Rotary Tidal Flow in NH

P1

P2

P1 > P2

Remember: P = grh

N

W

E

S

Tidal wave crest

Rotary Tidal Flow in NH

Remember: P = grh

N

PG

CD

W

E

S

Rotary Tidal Flow in NH

P2

Remember: P = grh

N

CDW

E

S

PGCD

Rotary Tidal Flow in NH

P2

Remember: P = grh

N

W

E

S

P2

CD

X

PG

Rotary Tidal Flow in NH

Remember: P = grh

N

W

E

SPG

X

PGCD

Rotary Tidal Motion

Rotary motion of tide is counter-clockwise in NH and clockwise in SH.

Amphidromic System

Some other terminology…

•Tidal resonance• period of main tidal component equals

natural period of basin.

•Tidal Bore• “wall” of water that surges upriver with

advancing tide

•Tidal currents• Flood current in advancing tide• Ebb currents in retreating tide

Electricity can be generated from tidal currents if the tidal range is greater than 5 m in a large bay connected to the ocean by a narrow opening.

• A dam is constructed across the opening and water is allowed to flow into and out of the bay when sufficient hydraulic head exist to drive turbines and generate power.

Power from Tides8-5