Tidal currents occur when ocean levels rise and fall, causing water to flow in and out of bays and rivers. Tides are caused by the gravitational pull of the sun and moon, which is caused by the mass of the planets. As the moon and Earth orbit the sun, their gravity pulls the water from the oceans toward the planets. Because the sun and moon are in different places every day, there are usually two high and low tides per day.
The height of the rising water will cause a flow into bays, rivers, or other lower areas as the tide rises, causing a flood tide. Slack tide occurs when the tide effect reaches its maximum height and there is no water movement in either direction. The tide will then begin to fall, or ebb, and the tidal currents will flow outward toward the ocean until the next slack tide, when the tide will fall again.
These tidal changes occur every six hours, moving from flood to slack to ebb, but they can vary depending on the sun and moon’s position. The gravitational forces are stronger when the sun and moon align on the same or opposite sides of the Earth. A spring tide is defined as a high tide that is higher than at other times. Because spring tides occur at the same time as a full or new moon, they can be predicted by looking at the moon at night.
The sun and moon occasionally produce a neap tide, which is a lower tide than usual. When the moon is in the first or third quarter phase, and the moon and sun are at an angle to each other, the gravitational effect is reduced. Between the new and full moons, these moon phases occur.
In the past, sailing ships needed high ebb tides to leave ports of call and allow the ship to be carried out to sea by the tidal currents. The moon phases were used by navigators to predict the best times to arrive and depart from ports. This was especially important in tide-affected rivers or bays. Before the invention of the steam engine, sailing ships lacked engines and were unable to navigate narrow rivers or in the absence of wind. They relied solely on the tides to get into and out of ports of call.
The interaction of the sun and moon complicates tidal current calculations. Furthermore, due to the shape of the river or bay inlet and other factors, the amount of tidal flow can vary greatly from place to place. Various government and private organizations publish tidal tables for a variety of locations around the world, providing mariners with data that can help them avoid groundings or low-water ship damage.
There was a growing interest in using tidal currents to generate electricity in the early twentieth century. In some parts of the world, tidal flow rates are extremely high, and installing generators beneath the water could generate electricity except during slack tides. The benefits of using tidal currents include not having to build dams or disrupt ship and wildlife movement. Research into commercial systems that could generate enough electricity to make them useful continued well into the twenty-first century.