Lesson 9: Waves Physical Oceanography

You may be familiar with a type of water movement in the ocean: waves This wave is breaking over the bow of a NOAA ship Today we will learn about the physical forces that cause waves to form and to break Photo: http://www.noaanews.noaa.gov/stories2009/20091116_database.html; Accessed: November 2010 Photo: NOAA

Understanding wave physics is important for human life (and not just for surfing) 1. A wave is the transmission of energy through matter – in this case through water Two important types of waves are deepwater and shallow-water waves Do you know what a tsunami is? Teacher’s note: Lead a brief discussion of student definitions and impressions of tsunamis. You may want to encourage students to think about whether a tsunami would be classified as a shallow-water or deep-water wave. This question is addressed further in the PPT.

A wave is transmission of energy through matter When energy moves through matter, like water, matter moves and returns to its initial position The energy is transmitted to its adjacent surroundings – in this case, adjacent water particles Transmission of energy through the water moves the water particles in a circle motion known as orbital motion Wave Orbital motion

Some important features of a wave Crest is the highest point of the wave H:L is the ratio of wave height to wavelength Period (T) refers to the time it takes for the same spot on two consecutive waves to pass the same point Speed = L/T Wavelength is the distance between identical points on two waves, from crest to crest Wave height is the distance from trough to crest Teacher’s Note: This slide provides definitions for the important terms on the diagram as you click ahead on the slide show. Photo: http://www.onr.navy.mil/Focus/ocean/motion/waves1.htm; Accessed: November 2010 Trough is the lowest point of the wave Photo: Navy

There is more than one type of wave Note: D = water depth; L = wavelength Deepwater waves occur when water depth is greater than 1/2 wavelength (D >1/2 L) Shallow-water waves occur when water depth is less than 1/2 wavelength (D < 1/2 L) Shallow and deepwater waves can occur at the same time Teacher’s Notes: You may want to explain and demonstrate the ‘internal wave’ here, explained in the Teacher’s Guide. An example in which shallow and deepwater waves can exist together would be tides and capillary wave. Capillary waves are very small waves. (Think of wind blowing across still water and the small ripples that are created -Surface tension is the primary restoring force for these small capillary waves.) Tides have a wavelength up to 17,000 km, and capillary wind waves have a wavelength less than 2 cm. Using the average depth of the ocean which is 4.3 km/14,000 ft (Source: http://oceanservice.noaa.gov/facts/oceandepth.html Accessed, April 2011) and the formulas above, for tides D < L/20 meaning 4.3 km < (14,000/20 km) is true making tides shallow water waves. For capillary waves, D > ½ L meaning 4.3 km > (.00002 km/2) is true making capillary waves deepwater waves. Wind can create capillary waves at the same time waves produced from tidal forces are passing through, thus these shallow and deepwater waves can exist together.

What is a tsunami? A typical tsunami wavelength is several hundred miles long, as an example let’s say wavelength is 400 miles The deepest part of the ocean is 7 miles deep Based on this information, do you think a tsunami is a shallow or deep water wave?

What is a tsunami? L = 400 miles, D = 7 miles D < 1/2 L Tsunamis are shallow water waves! The ocean would have be deeper than 200 miles on average for a tsunami to be a deep water wave! A tsunami is a shallow-water wave triggered by displacement of a large amount of water Teacher’s Note: 1. Common triggers of tsunamis include earthquakes and underwater landslides associated with smaller earthquakes.

How does a wave break? The top part of the wave The height of the moves faster than the bottom The height of the wave increases As a deepwater wave moves toward shore, its wavelength decreases H:L ratio> 1:7 When the wave meets the ocean floor, the bottom of the wave slows Teacher’s Note: This slide walks the students through the mechanics of the wave break as you advance the slide show. As a wave moves towards the shore, the wavelength decreases and the height increases. When the bottom of the wave strikes the shore, it causes the bottom part of the wave to move more slowly than the top part. As the wave becomes much taller than it is wide with the crest of the wave moving faster than the bottom, the wave becomes unstable and the crest eventually crashes over. You should stress to your students that the height to wavelength ratio at which the wave breaks is greater than 1:7 (this number typically comes up at the NOSB). The faster-moving top of the wave crashes over causing the wave to break. This happens as the wave depth gets closer to the wave height. (H:L ratio is greater than 1:7)

Why is it important to know so many details about waves? Waves can be very destructive A tsunami is one type of destructive, shallow-water wave Studying the causes and forces underlying waves can help scientists predict the timing and magnitude of these events and potentially save lives!

Can you outrun a tsunami? In today’s activity you will use wave information to calculate the speed of impending tsunamis About how fast do you think tsunamis move? Let’s find out…