1	Waves Generation And Movement
2	Objectives Examine the life cycle of a wave Focus on wind wave generation Focus on Swell Real-world sea state with waves and swell Describing sea conditions in observations and forecasts
3	Terminology
4	It all begins with wind on water
5	Wave Generation The wind “drags” the wave with it With increased wind speed: Increased wave height Increased wave speed
6	Wave Generation With a constant wind, the wave will grow until it keeps up with the wind.
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10	Wind Waves Factors in wind wave development Wind speed Wind duration Fetch length
11	Wind Speed The higher the wind, the bigger the wave Rough relationship: Wave height proportional to square of wind speed But...growth limited by duration and fetch Wave speed cannot be faster than wind speed (wave “outruns” the wind)
12	Fetch: area of similar wind speed and direction
13	Duration: how long it blows Length of time wind blows over a given fetch area Longer the wind blows over the same area, the bigger the waves get
14	Logically: A strong wind that blows over a large area for a long time produces the biggest waves.
15	Wave growth limitations Wind Speed is the ultimate limiting factor Waves can only grow to a certain height for a given wind speed = fully developed Wind waves are almost always fully developed for low wind speeds Wind waves are almost never fully developed for high wind speeds Why?
16	Examples of growth limitations Fetch Limited Offshore winds Small, but strong wind (Thunderstorms) Any fetch limited by the coastline (bay, etc) Duration Limited Fast moving storms Short lived wind (Pre-Frontal wind) Long lived wind, moving away from fetch area (e.g. a hurricane moving perpendicular to fetch region)
17	Wind change effects (assuming fully developed waves) Wind increases wave growth must occur Wind decreases a second group of smaller waves will develop original waves become swell
18	Wind decreases instead of stopping completely
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20	Complete definition of wind wave: Waves that are being acted upon by wind that is sufficiently strong, and in the correct direction, to cause the wave to grow in size, or maintain their maximum size for that wind. “wind speed is equal to or faster than the wave speed”
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22	Swell Simple definition: If it’s not wind wave, it’s swell Waves that have left their generation area, or fetch Waves that are traveling faster than the wind, or in a different direction
23	As a swell travels, its: Height decreases Period, wave length increase Crest length increases Furthermore, These changes occur more significantly early in a swell’s life cycle.
24	Gravity
25	Different Swell Heights, Same Swell Energy!
26	Consider these different reports from a buoy 600 miles offshore: 10 feet by 6 seconds 7 feet by 18 seconds
27	Swell Speed As a swell flattens, it speeds up. Therefore: Wave speed increases as it is being generated by wind, and then when the wind stops (or moderates), the wave becomes a swell and speeds up as it flattens.
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29	Estimating Equations - Deep Water
30	Estimating Equations
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32	Estimating Equations - Summary For Deep Water
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34	Mixed seas
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37	Describing sea state
38	Important parameters: Wave height Wave length Wave direction
39	Height: Significant Wave height
40	Height: Combined Seas
41	Length: Period instead of wavelength Wavelength is intuitive, but difficult to measure Period is the time for one wavelength to pass a point – easy to measure L = ~ 5 T*T
42	Length: Period Best case is to know the period of each wave group present Dominant period is that of wave group with the highest energy. Note: Low, long-wavelength swell can have have more energy than high, steep wind waves.
43	Direction: Energy vs. Direction
44	Important assessments: Wave steepness Degree of confusion in sea surface
45	What is a “steep” wave? Subjective. 9' by 9 second is definitely steep. The “square” rule is popular, but: 9x9 implies 9' x 400' (pretty steep) 20x20 implies 20' x 2000‘ (not steep)
46	Confusion in sea surface: Wave Spectrum - Energy vs. Period
47	Describing Sea State
48	Describing Sea State The NEW Way… 4 parameters: Swell direction, Swell height, Swell period. Wind wave height. Wind wave direction is assumed to be equal to wind direction Wind wave period is assumed to correspond to steep wave Pioneered by NWS Eureka
49	What’s the Difference? Consider: - Strong 25-30kt winds generating a 6 ft wind wave. - Meanwhile, 5 ft swell at 15 seconds is coming from the W. Using 2 parameters: Combined sea height = Sqrt (55 + 66) = 7.8 ft Read as “Seas 8 ft at 15 seconds” Doesn’t sound so bad…a nice hilly ride!
50	What’s the Difference? Consider: - Strong 25-30kt winds generating a 6 ft wind wave. - Meanwhile, 5 ft swell at 15 seconds is coming from the W. Now using 4 parameters: Read as “Wind waves 6 ft. West swell 5 ft at 15 seconds.” Paints a much different picture. Wind waves are likely going to be less than 7 seconds in period. Therefore, the waves will be quite steep!
51	What’s the Difference? The “moral of the story”: The more descriptors used to describe sea condition, the better. Using only two parameters can be disastrously misleading. Indicating wind wave height, swell height, as well as period and direction of swell is much more describes the sea condition much more accurately. NWS and the research community are working on deploying more “wave spectrum” buoys.
52	"What other data from the..." What other data from the buoy can you use to determine if a wind wave exists that is not showing up?
53	The end