Waves 2 Flashcards
Sea state is the ___ at sea
degree of turbulence
What three things are needed to get BIG waves?
- Fast wind speed
- Sustained wind (blow for long time)
- Long fetch
Long fetch
large, uninterrupted distance (fetch) over which wind blows
As wind speed, blowing time, or long fetch increases, what happens to the waves?
- more energy is added
- wave height increases = bigger waves
The Beaufort scale rates ___
relates wind speeds
to observed conditions at sea & on land
- from 0 to 12
Waves develop in a ___ sea state
turbulent
Is there a maximum sea state?
Yes
- the fully developed sea
In a fully developed sea, there is a practical limit for ___
EVERY wind speed
The practical limit for a wind speed means that
extra time or distance will NOT produce larger waves
The biggest wind waves are found in ___ because ___
Southern ocean around Antarctica
- cuz nothing affects their flow = large fetch
How big are wind waves usually in the open ocean?
About 2-6m height
What happens when waves become too steep in the open ocean?
they become unstable and break or fall over
How can we calculate when a wave will break?
steepness (H/L) > 1/7 (0.14)
Wind waves may have wavelengths of up to ___ meaning they will break at max ___ m high
- L about 150 m
- break at 21m
Swells are waves that move ___ a storm region and can travel distances of ____
- away from
- thousands of km
What happens when swells from diff directions run into each other?
they interact thru constructive & destructive interference
- height of resulting wave is a sum of individual interacting waves
- result of this produces complex mixture of waves
Rogue waves
unusually large waves created by constructive interference
- monster or freak wave
- super random and fast (seconds)
- can be 3-4 times larger than other waves in the same area
How to calculate whether a wave is rogue?
H ÷ Hs > 2.2
OR
Crest height ÷ Hs > 1.25
(Hs is Height of sea state)
True or False: rogue waves are only hazardous for ships at sea
False :O
- most hazardous for ships at sea, but can also hit coast & drown ppl on shore
What place is notorious for rogue waves and why?
Cape of Good Hope in south coast of Southern Africa, becuz strong currents collide with large wind waves
(Angulhas current flow from north
Antarctic storm waves from south)
Seiche
standing wave oscillating in a closed/semi-enclosed body of water
- time btwn high and low can be hrs
The properties of a standing wave depend on ___
the properties of the basin
For seiches, a longer length of a basin means
a longer period of the standing wave
How are seiches generated?
mostly change of atmospheric pressure !
- Seismic
- fault motion or landslides into water
- ex/ lake tahoe - Meterological
- wind
- change in atmos pressure
- ex/ Great Lakes - Tsunami entering a harbor / bay
Seiches normally have what kinds of properties?
long wavelengths and periods
The period of a seiche is determined by?
the natural resonant period of the basin
The natural resonant period of a basin is determined by?
its size, depth, and contour
Resonance is the tendency of ___
a system to oscillate with greater amplitude at a given period
Resonance is longer for ___ bodies of water
larger
ex/ Coffee cup < 1s
Lake ~ mins to hrs
What happens when a force is applied at intervals that coincide with the natural resonant period of a system?
the response amplitude will be at its relative max
True or False: every object and system has a natural resonant period
true
Equation to calculate the period of a seiche
T = 2L / (√g x d)
OR
T = 0.64 x (L / √d )
L = longest length of body
d = average water depth
