Waves

¥ Description

ª Wave height

ª Wavelength

ª Wave period

¥ Classification -according to wave period

ª Capillary < 0.1 sec

ª Chop 1-10 sec

ª Swell 10-30 sec

ª Seiche 10 min - 10 hrs

ª Tsunami 10 min - 60 min

ª Tide 12.4 - 24.8 hrs

Winds and Waves

¥ Most (not all) waves generated by wind

¥ Controls:

ª Velocity

ª Duration

ª Fetch

ª Original sea state - "roughness"

¥ Waves increase with wind if:

ª sufficient fetch

ª wind blows long enough

Long-Winded

¥ Waves grow gradually with wind

ª one puff won't cause waves

ª 90 km/hr wind takes 42 hrs to build waves

¥ Fully developed sea

ª waves no longer grow - energy supplied by wind is balanced by energy lost by breaking waves

¥ Significant wave height

ª Average of highest third of all waves in an area

ª important because big waves most destructive

Wave Motion

¥ Waves have two kinds of motion:

(1) motion of the wave form

ª wave crests move in one direction

(2) motion of the water

ª water moves in circular orbit

For example:

ª Wind blows out of a bay

ª Wave move out of bay - transport energy

ª Water doesn't move out of bay

Size and Distribution of Orbits

¥ Size of water orbits decrease with depth

ª no water motion at 1/2 L

ª called "wave base"

¥ Shape of orbits depend on water depth

ª Deep water waves- water depth > 1/2L

ª Shallow water waves- water depth < 1/20L

ª Intermediate water waves

Wave Speed

¥ By definition of wavelength and period, can formulate equation for speed:

C = L/T

ª Here C = celerity, meaning motion of wave, not or mass, otherwise called "speed"

¥ Speed actually controlled by water depth and wavelength - complicated formula

ª For d > L/2, C proportional to L

ª For d < L/20, C proportional to d

ª For L/20 > d > L/2, C proportional to d & L - very complicated

Wave History

¥ Wave generation:

ª Sea surface chaotic

ª Breaking waves, many different sizes

¥ Constructive interference

ª Rogue waves

¥ Destructive interference

¥ Waves outside area of generation:

ª dispersion - waves move away from generation area at celerity depending on period & wavelength

Shallow Water Waves

¥ Changes in waves:

ª Shape & Speed

ª Refraction

ª Breakers

¥ Shape and Speed:

¥ Slow when approach shore - controlled by d

¥ Wavelength decreases:

ª Leading wave slows

ª Following waves same speed

¥ The shape of wave changes:

ª Taller, irregular

¥ Period stays the same, L decrease, C decrease

¥ Refrection

ª Most waves approach coast at angle

ª Waves approach coast first, slow first

ª Net result is waves tend to "bend" parallel to coast

ª Wave orthogonals:

* Lines constructed perpendicular to wave crests

* Wave energy between orthogonals constant

* Energy concentrated at points, dispersed at embayments

¥ Breakers

ª wave steepness - H/L

ª critical value 1/7 - at this point wave collapses - i.e. breaks

¥ Three types of breakers:

ª Spilling - flat beach (mushy)

ª Plunging - steep beach - curling

ª Surging - very steep beach - reflect energy, common at seawalls & cliffs

Standing Waves

¥ Waves that do not move horizontally (like wind-waves)

ª seesaw analogy

ª Nodes - no vertical motion

ª Antinodes - maximum vertical motion

¥ The property of the waves depend on the "container"

Closed basins:

Open basins:

Internal Waves

¥ Waves can form at pycnocline

ª different in density of fluids

ª very large -

* P = minutes

* L = hundreds of meters

* H = > 100 m

¥ Observation difficult - understanding poor

ª Tides, friction, slumping, storms

Tsunamis

¥ Waves from movement of seafloor

ª earthquake, volcano, slump

¥ Typically L > 100 km

ª Shallow water waves

ª very fast 700 km/hr

ª flat - won't be felt at sea.