OCEAN WAVES

OBJECTIVES

• Identify the generating force of different waves

• Describe the factors that influence wind wave development

• Describe the basic properties of deep and shallow water waves

• Calculate the speed of a wave

WAVE INTRODUCTION

• Most waves generated by wind.• Other forces–seismic activity from subduction zones–Tidal forces from gravity–Impacts from space

WAVE MOTION

• Waves represent transmission of energy• In fact, in the deep ocean,

water moves in circles rather than toward shore.

WAVE ANATOMY

• The image below shows the ‘anatomy’ of a wave

WAVE ANATOMY

• CREST: peak of a wave• TROUGH: ‘bottom’ of the wave• WAVELENGTH (L): distance between two

successive crests (or troughs) in meters.• WAVE HEIGHT (H): vertical distance between

the crest and trough of a wave, in meters.• WAVE PERIOD (T): amount of time (in

seconds) that it takes for one wave to pass.

WAVE BASE

• WAVE BASE: water depth at which you can’t feel the effect of the wave anymore.

WAVE BASE CALCULATION

• Wave base = L/2

• If the wavelength of a wave is 200m, what is the wave base? 200/2 = 100m

WAVE TYPES

• Wave base distinguishes between ‘deep water waves’, and ‘shallow water waves’.

• Deep water wave travels = open ocean?• Shallow water waves = close to the coast• A tsunami is a shallow water wave everywhere

in the ocean – even traveling in the deepest parts of the

WAVES VIDEO

• http://www.montereyinstitute.org/noaa/lesson09.html

• Answer the questions on the lined page.

DEEP vs. SHALLOW WATER WAVES• Waves are classified as deep water, intermediate, or shallow water waves. • Depends on water depth and wavelength• Deep water wave

– travels in water that is deep enough that it won’t feel the effect of the bottom – in other words, it travels in water that is deeper than its base.

– If a wave travels in water that is deeper than L/2, it is a deep water wave. • Intermediate water wave

– Once the wave gets closer to shore and ‘feels’ the seabed – that is, in water depths less than L/2 – it becomes an ‘intermediate’ wave.

– The motion of the water is disrupted- rather than carving out lovely circular orbits, the friction between the water and the seabed causes the orbits to become more oval in shape.

• Shallow water wave– Encounters water is that is shallower than its wavelength divided by 20.– This is where the water molecules begin to move essentially back and forth

(↔), rather than in an orbital motion.

DEEP vs. SHALLOW WATER WAVES

DEEP vs. SHALLOW WATER WAVES

• TO SUMMARIZE AND REVIEW• A deep water wave is one that travels in water that

is ____________ (less than, greater than) L/2.• An intermediate wave travels in water that is

____________ (less than, greater than) L/2, but ____________ (less than, greater than) L/20.

• A shallow water wave is one that travels in water that is ____________ (less than, greater than) L/20.

CALCULATING WAVE SPEED

• Formula to calculate wave speed is different for deep or shallow water waves.

• The full formula to calculate the speed, or ‘celerity’ (C) of an ocean wave is: __________________C=√(gL/2π*tanh(2πd/L))

• It’s not as bad as it looks, we can simplify this if we know what type it is.

DEEP WATER WAVE SPEED

• For a deep water wave the equation becomes:(1) C=1.25√L OR(2) C= 1.56T

• So if we know that we have a deep water wave, then we can calculate speed simply by knowing the wavelength (equation 1) or the wave period (equation 2). Other formulae that are worth knowing for deep water waves:(3) L = 1.56T2 (4) T = √L ÷1.25

SHALLOW WATER WAVE SPEED

• For a shallow water wave the equation becomes: (5) C = 3.13*√d

• Other useful equations for shallow water waves:(6) L = T * 3.13√d _(7) T = L ÷ (3.13*√d)

• Note that these equations are only valid for wavelength in meters, depth in meters, wave period is seconds, and speed in meters per second.

WAVE SPEED PRACTICE

• There’s no reason for you to memorize any of these equations, but you do need to be able to use them.

• Let’s get some practice. Try the Wave Speed Worksheet.