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WIND / SWELL WAVES

WIND / SWELL WAVES. Ocean Waves. Characteristics Wave crest - the highest portion of the wave between two successive troughs Wave trough - the lowest portion of the wave between two successive crests Wave amplitude - 1/2 of the wave height

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WIND / SWELL WAVES

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  1. WIND / SWELL WAVES

  2. Ocean Waves • Characteristics • Wave crest - the highest portion of the wave between two successive troughs • Wave trough - the lowest portion of the wave between two successive crests • Wave amplitude - 1/2 of the wave height • Wave length - the horizontal distance between two successive crests or troughs and measured in feet

  3. Ocean Waves • Characteristics (cont) • Wave period - the time interval between successive wave crests or wave troughs as they pass a fixed point and measured in seconds • Wave speed - the rate at which the wave moves through the water and measured in knots • Wave frequency - the number of waves passing a given point per unit time NOTE: Wave frequency and wave height are inversely proportional. Question: The lower the wave heights = the higher the wave frequency

  4. OCEAN WAVE WAVE MOTION 2 1 3 STILL WATER LEVEL 4 • Crest • Wave Length • Amplitude • Wave Height • Trough 5

  5. Sea Waves Sea waves (aka wind waves) • Wind generated waves are generated by the local wind field within a “fetch area” fetch area = area of constant wind direction and speed • Beaufort Scale is used to predict wind generated waves

  6. BF: 0 WIND: 0-1KT WAVES: 0FT BF: 1 WIND: 1-3KT WAVES: 0FT BF: 2 WIND: 4-5KT WAVES: 1FT BF: 3 WIND: 7-10KT WAVES: 2-3FT

  7. BF: 4 WIND: 11-16KT WAVES: 4-5FT BF: 5 WIND: 17-21KT WAVES: 6-8FT BF: 7 WIND: 28-33KT WAVES: 14-19FT BF: 6 WIND: 22-27KT WAVES: 9-13FT

  8. BF: 8 WIND: 34-40KT WAVES: 18-25FT BF: 9 WIND: 41-47KT WAVES: 23-32FT BF: 11 WIND: 56-63KT WAVES: 37-52FT BF: 10 WIND: 48-55KT WAVES: 29-41FT

  9. BF: 12 WIND: > 64KT WAVES: > 46FT The scale was created in 1806 by Sir Francis Beaufort, an Irish-born British admiral and hydrographer. The scale was made a standard for ship's log entries on Royal Navy vessels in the late 1830s and was adapted to non-naval use in the 1850s. The Beaufort scale was extended in 1946, when Forces 13 to 17 were added. However, Forces 13 to 17 were intended to apply only to special cases, such as tropical cyclones. Nowadays, the extended scale is only used in Taiwan and mainland China, which are often affected by typhoons.

  10. Swell Waves Swell waves (aka surface waves) • Waves that began as wind generated waves and propigated out of the fetch area • Can travel distances up to thousand’s of miles • Swells generally have periods of 10 seconds or greater

  11. Ocean Waves Development • Max sea height for each wind speed (scientific study) is based on wind speed, time, and length of the fetch area • Numerous variables cause lower heights - variable wind speed/direction - depth of water - ocean currents - islands • Based on wind speed, wave reaches top height - known as “Fully Arisen Sea” • North Wall is an exception • Maximum sea height can physically reach up to a height of 212 feet based on mathematics

  12. Ocean Waves Development • Growth determined by tangential stress and pressure transfer • Forces of gravity and surface tension causes wavelets or ripples with 2-3 knots of wind • Transfer of energy from wind to the sea is known as pressure transfer • The wind generated surface current will develop at 45° to the right of the predominate wind direction (due to the Ecmond Spiral)

  13. Ocean Waves Decay • Dispersion • As a swell wave leaves the fetch area it leaves at an angle to the wind direction, which causes a decrease in height • Air resistance • Gravity • Angular Spreading • The highest swells will be plus or minus 30 from the predominate wind direction • Swells lose approximately 1/3rd of their height each time they travel a distance in nautical miles equal to their length in feet

  14. BF: 9 WIND: 41-47KT WAVES: 23-32FT WW3

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