BIOL 331

Lecture 4

Waves

Wave Terminology

• Crest
• Trough
• Wave length
• Wave height
• Wave Period

Types of Waves and Relative Energy

1. Ripples
2. Chop
3. Sea
4. Swell
5. Surf Beat
6. Tsunamis
7. Tides

Historical Studies on Ideal Waves

1802: Franz Gerstner

• Water particles in waves move in circles
• Surface trace of a wave is approximately a trochoid
• (like sine waves).

1825: Ernst & Wilhelm Weber

• Used a wave channel to study waves
• Filled it with variety of liquids (brandy, mercury,
• water, etc.)
• Three important discoveries:

1. Waves reflected with no energy loss
2. Suspended particles trace circles
3. Near bottom orbits are nearly flat

Fundamental Properties of Waves

I. Wave Length, Period & Velocity

• Wave length depends upon period

The following hold true for deep water waves (d/L > 1/2)

L = length in feet, T = period in seconds, g = acceleration of gravity

• Wave velocity depends on wave length

C = velocity in feet per second

• For shallow water the velocity equation must be modified

For d/L between 0.05 and 0.5:

For d/L < 0.05:

II. Wave Height

• Appears at first to be independent of wave period & length
• Holding wave length constant (e.g. 5.12 feet) and increasing wave height results in waves over 8 inches (0.75 feet) "breaking"
• Repeated experiments show that wave crest angle cannot be more than 120o
• Steepness (ratio of H/L) cannot exceed 1:7
• Waves at 1:7 move 10% faster than predicted.

III. Orbital Motion

• Movement of water molecules studied using "gunk" (xylol & butyl phthalate whitened and weighted with zinc oxide
• Surface particles make an orbit = the wave height, deeper orbits are progressively smaller
• Repeated observations show that at D = L/9 the diameter of the orbit is halved
• Deeper orbits become more and more flattened
• At bottom the particles simply move back and forth

IV. Mass Transit

• If particles are watched over time, the orbits gradually move toward direction of waves.
• Movement of particles is small, but significant given time

Waves Caused by Wind

Factors Determining Wave Development:

1. Wind velocity
2. Length of time wind blows
3. Length of fetch

Other Considerations

• Wind is never constant
• Direction of wind often shifts erratically
• Wave size and direction therefore highly variable
• Waves of 75 to over 100 feet have been documented

Characteristics of Shallow Water Waves

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