Tsunami waves exhibit a phenomenon called diffraction when they enter bays through a narrow entrance. Areas that have narrow and steep continental shelves offshore are prone to the most severe tsunamis, since these areas do not have the ability to reduce the wave energy by friction.
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. If you run that calculation on water than is 4000 meters deep, you get a speed of over 200 m/s, which is like 450 mph.Waves with long wavelengths travel faster than waves with short wavelengths. Note that tsunamis are a special case of a very long wavelength wave, up to 200 km, so the shallow-water calculation applies even though they are in the open ocean. Waves break when the ratio of wave height to water depth is about 3:4. This is what cases the waves to break as they approach the shore. So, as water gets shallower, the waves travel more slowly. In a shallow-water situation where the depth of the water is small compared to the wavelength, the velocity is the square root of (gravity times water depth), with units again in meters / second. (The units are m/s.) So, long waves move faster. The speed of water waves depends on a few factors some of them are inherent to the planet, including gravity and surface tension, whereas others are dependent on the depth of the water, and the wavelength of the waves (which is in turn determined by things like depth and wind speed.)Ī deep-water wave moves at about g/2pi or 1.56 times the wave period in seconds.
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