Waves in the Sea

1. Waves in the Sea • An ocean wave is a rhythmic rise and fall of the water’s surface. • Most commonly produced by wind. • Also by undersea earthquakes and the effects of the moon in producing tides. • The height of a wind formed wave depends on: • The length of time that the wind blows. • Fetch: the length of open water over which the wind blows • Choppy seas form from gusts that change direction. • Swells are regular intervals of smooth waves.

2. Features of Water Waves • Wave height is the difference between its high point (crest) and its low point (trough). • Wavelength is the distance from one crest to the next. • On average the wavelength is 20-30 times its height.

3. The period of a wave is the time it takes one wavelength to pass a given point. • Most ocean waves have a period that ranges from 2s to 10s. • Speed=wavelength/period • Find the speed of a wave 30m long with a period of 5 sec. • 6 m/s

4. Tsunamis • Waves with periods ranging from 5-60 minutes are called long waves. • Tsunamis are caused by underwater earthquakes. • A tsunami may have a wavelength of 150 km and a period of 12 minutes, giving it a speed of 12.5 km/min, or 748 km/h. • Such speed can cause immense damage. http://videos.howstuffworks.com/science-channel/7401-tsunami-video.htm

5. Water is not carried along with the motion of a wave. • Each water particle moves in place in a circular motion. • Water particles bump into the next one and passes the energy along, passing the energy through the water. • Wave motion also takes place below the surface, moving water molecules in smaller and smaller circles.

6. Most waves approach a shoreline at an angle. • The waves tend to bend as they hit the shoreline, called refraction. • The end of the wave closest to shore scrapes the bottom first and slows down, • The end in the deep water continues at normal speed and the wave approaches nearly parallel to the shore. • This reduces erosion in shallow bay. Headlands erode faster than bays because the wave energy gets concentrated at headlands

7. Breakers • Waves approach the shoreline smoothly until they reach water so shallow that they touch the bottom. • Water depth is usually about half the wavelength • The lower part of the wave slows down • The upper part of the wave moves ahead, until there is not enough water to support the wave • The crest falls over and breaks into surf. • The line of breakers is where the crests break • Surf is a powerful agent of erosion.

8. Shoreline Currents • Waves may come from any direction, most strike the shore at an angle. • The motion of water up the beach is swash, it moves sand up the beach at an angle. • Most water runs back down the beach in a backwash current, which drags sand straight back to the sea. • A very strong backwash is an undertow. • A longshore current forms beyond the breakers that runs almost parallel to shore. • Rip currents are strong surface currents that flows away from the beach.