changing with the tides

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Engineers have known for at least a century that tides can change locally. In 1899, builders predicted that tides would increase in the river Ems upstream from a weir they planned to construct. (After the weir was built, the tides did increase about as much as they were expecting.)

Intertidal Zone - National Geographic Society Intertidal Zone - National Geographic Society

Tidal energy is a renewable resource that many engineers and consumers hope will be developed on a large scale. Now, small programs in Northern Ireland, South Korea, and the U.S. state of Maine are experimenting with harnessing the power of tides. Forces that contribute to tides are called tidal constituents. The Earth’s rotation is a tidal constituent. The major tidal constituent is the moon’s gravitational pull on the Earth. The closer objects are, the greater the gravitational force is between them. Although the sun and moon both exert gravitational force on the Earth, the moon’s pull is stronger because the moon is much closer to the Earth than the sun is. As Earth rotates, the moon's gravity pulls on different parts of our planet. Even though the moon only has about 1/100th the mass of Earth, since it's so close to us, it has enough gravity to move things around. The moon's gravity even pulls on the land, but not enough for anyone to tell (unless they use special, really precise instruments). While a tidal bore is a tidal wave, a tsunami is not. Tsunami is taken from the Japanese words for “harbor wave.” Tsunamis are caused not by tides, but by underwater earthquakes and volcanoes. Tsunamis are associated with tides because their reach surpasses the tidal range of an area. If sea level were to rise between 0.5 and 2.1 meters (1.6 and 6.9 feet) in the delta, then cities in its upstream reaches would see tidal increases between 0.1 and 0.5 meters (0.3 and 1.6 feet), the scientists reported in February in Geophysical Research Letters. Add those numbers together and it looks as if water levels could go up between 0.6 and 2.6 meters (2 and 8.5 feet).When the sun, moon and Earth are all lined up, the sun’s tidal force works with the moon’s tidal force. The combined pull can cause the highest and lowest tides, called spring tides. Spring tides happen whenever there is a new moon or a full moon and have nothing to do with the season of spring. (The term comes from the German word springen, which means “to jump.”)

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Rather, people are causing these changing tides. Dredging river channels like the Ems or filling in coastal wetlands can trigger shifts. The nature of those shifts is complicated: In some locations the tidal range grows more dramatic, whereas in others it shrinks. Either way, the shifting tides have big implications for hundreds of millions of coastal residents. The vertical difference between high and low tide is called the tidal range. Each month, the range changes in a regular pattern as a result of the sun’s gravitational force on the Earth. Although the sun is almost 390 times farther away from the Earth than is the moon, its high mass still affects the tides. I feel my body and mind are disconnected, except when my worries wage against me and I stay up all night concerned over past transgressions and burden with future bad decisions that have not happened yet. But those changes also changed the rhythm in which tides ebbed and flowed into the river from the sea. Those shifting tides stirred up sediment from the river bottom and muddied its waters. Over the last 120 years the tidal range — the distance between high and low tide — has quintupled in the Ems estuary. The intertidal zone can be further divided into three zones: high tide, middle tide, and low tide. The high tide zone is only submerged at high tide and is hotter and drier as a result. The middle tide zone is submerged and exposed for equal amounts of time. The low tide zone is only exposed during low tide and has the greatest biodiversity of the three zones because it provides more favorable conditions for those organisms that cannot tolerate air exposure for long.Several cities in the delta, including Guangzhou and Shenzhen, are among the world’s most vulnerable to rising water levels, says De Dominicis. Knowing how high the water levels could rise can help planners build better barriers and other coastal defenses.