Tidal streams are powerful, naturally occurring currents found in specific coastal environments where the ocean’s water is channeled. They represent the horizontal flow of water created by the regular rise and fall of the sea surface, known as the tide. These flows are distinct from continuous, deep-ocean currents driven by temperature and salinity differences. Tidal streams are cyclical, wholly predictable, and their energy is recognized as a reliable source of renewable energy.
Defining a Tidal Stream
A tidal stream is a fast-moving, oscillating current caused by the horizontal movement of water in response to the vertical change of the tide. It is often referred to as a tidal current and is defined by its location in a constrained geographic area. These areas include narrow straits, the entrances to bays, or estuaries where the flow is amplified.
The stream’s velocity and direction are governed entirely by the tidal cycle, making their timing and strength exceptionally predictable, unlike weather-driven ocean currents. Tidal streams are categorized as reversing currents because they flow strongly in one direction before slowing, stopping, and then reversing. This cyclical nature is the core identity of the phenomenon.
The Mechanics of Tidal Stream Formation
The formation of a tidal stream begins with the gravitational forces exerted by the Moon and Sun, which create bulges of water on the Earth’s surface. As the Earth rotates, these bulges move across the ocean basins, causing a large-scale, horizontal movement of water known as the general tidal flow.
A true tidal stream achieves high velocity when this massive flow of water is forced through a constricted area, such as a narrow channel or strait. This process is analogous to the Venturi effect, where a fluid’s speed increases as it passes through a choke point. The funneling of the water between islands, headlands, or over shallow banks dramatically increases the current’s kinetic energy.
The geographical features of the seabed and coastline act as an accelerator, converting the large volume of the tidal flow into a high-speed stream. In locations with high tidal ranges and significant constrictions, the speed of the current can easily exceed several meters per second. The resulting force is significantly stronger than general tidal currents in the open ocean.
The Cycle of Tidal Stream Behavior
A tidal stream operates on a repeating cycle characterized by three distinct phases: the flood stream, the ebb stream, and slack water. The flood stream is the period when the current flows toward the land, causing the water level to rise in bays and estuaries. This phase is driven by the incoming high tide.
Conversely, the ebb stream is the phase when the current reverses and flows away from the shore, resulting in the falling water level. The strongest velocities for both the flood and ebb streams occur around the midway point between high and low tide. This peak velocity does not occur at the absolute peak of the tide’s height.
Separating these two powerful flows is the period of slack water, a brief window when the current velocity drops to near zero before changing direction. Slack water typically coincides with the moments of high and low tide, lasting only for a few minutes to an hour. Following this momentary pause, the current begins to build speed again, restarting the predictable cycle.
Harnessing Tidal Streams for Energy
The predictability and high energy density of tidal streams make them an attractive source for generating renewable electricity. The technology used is the Tidal Stream Generator (TSG), which is an underwater turbine similar in concept to a wind turbine. These devices are anchored to the seabed in areas known for consistently strong currents.
Water is approximately 800 times denser than air, allowing a tidal turbine to generate the same power as a wind turbine at significantly lower flow speeds. A tidal stream current of only 5.6 miles per hour (2.5 meters per second) can produce substantial power. The turbines capture the kinetic energy as the water flows across their blades, turning a rotor that powers an internal generator.
A primary advantage of TSGs is the precise predictability of the power output, as the tides are governed by known astronomical cycles. This allows utility companies to integrate tidal energy into the power grid with high reliability, unlike intermittent sources like solar or wind power. Projects like the MeyGen tidal stream array in Scotland demonstrate the viability of this technology, tapping into currents to deliver grid-scale electricity.