![]() ![]() Much of the east coast of the united states has a moderately wide and shallow shelf that is able to dissipate about 20 percent of the energy of most tsunamis by friction along the wave base. such is the case for the part of the northeast seas of China (Yellow sea, Bohai, and related areas) where the water depth is quite shallow for many hundreds of miles, making the northeast coast of China much less susceptible to tsunamis than the southeastern coast, where the shelf area is deeper and narrower. In most settings the amount of dissipation of energy by friction is minor (less than 3 percent), but in some cases where the continental shelf areas are very wide and narrow, the dissipation may be significant enough to reduce the tsunami threat to the region dramatically. The friction at the base of the wave dissipates or takes some of the energy away from the tsunami. Seafloor topography very close to the shore can modify this refraction, and either focus the energy into specific locations, or disperse it across the shoreline. ![]() This effect bends tsunamis, like other waves, so that they hit most shoreline areas nearly head-on. This refraction occurs because the part of the wave that encounters shallow water first will be slowed down by the increased friction, whereas the other part of the wave still in deepwater will continue to move faster, until it catches up with the rest of the wave by being in the same water depth, then moves at the same rate. One of the main effects of the friction at the base of the tsunami as it enters shallow water is that the wave fronts tend to be strongly refracted, or bent, so that they approach land at less than 10° no matter what the original angle of approach to the shore was. This causes the wave height or amplitude to increase dramatically, sometimes 10 to 150 feet (3-45 m) above the normal stillwater line for tsunamis. When waves encounter shallow water, the friction of the seafloor along the base of the wave becomes greater than when the waves were traveling in deep water, causing them to slow down dramatically, and the waves effectively pile up on themselves as successive waves move into shore. In some cases many of the crests will merge and the troughs will disappear during this process, producing huge solitary waves, whose height from base to top is entirely above sea level. ![]() When this occurs the wave must become taller and narrower to accommodate the waves moving into the same space from behind thus as the tsunami moves from deep water into shallow waters, it becomes taller (larger amplitude), has a shorter distance between crests ( shorter wavelength), and moves slower (velocity). This slowing of the wave speed as it begins to encounter shallow water causes the waves at the back of the train to move faster than those in the front. The wave speeds slow down as the tsunamis encounter shallow water, typically in the range of 60-180 miles per hour (100-300 km/hr) across the continental shelves, and about 22 miles per hour (36 km/hr) at the shore. Normal ocean waves travel at less than 55 miles per hour (90 km/hr), whereas many tsunamis travel at 375 to 600 miles per hour (800 to 900 km/hr), faster than most commercial airliners. Since the longer the wavelength the faster the wave in deep open water, tsunamis travel extremely fast across the ocean. Waves across a boundary 44.Waves with long wavelengths travel faster than waves with short wavelengths. GCSE Keywords: water waves, shallow, deep, wave speed, wavelength, direction The deeper the water, the faster the waves travel, and so waves will refract (change direction) when they enter deeper or shallower water at an angle. The depth of water affects the speed of these waves directly without having anything to do with the density of the water. So a Rayleigh wave is a mixture of a longitudinal and a transverse wave! If you look at a particle closely, you'll see that it oscillates both perpendicular and parallel to the direction of the wave motion. For GCSE purposes we treat them as being transverse waves, but they're actually more complex than that - here's an animation: Water waves are actually called 'Rayleigh waves'. However, water waves are a bit different because these waves are mechanical waves - it's the oscillations of actual water molecules which cause the wave to move. If the refractive index of the material is higher than the refractive index of air (which has the value of 1.0), then light will travel slower in the material. Light waves will speed up or slow down when they enter or exit a material of a different optical density, which is the refractive index of the material. ![]() Water waves moving from shallow to deeper water ![]()
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