The figure on the left represents a cross section through a tidal channel. The water velocity is generally at its maximum at the surface but at the seabed itself it will be near to zero. The generally accepted velocity shear relationship with depth follows a one-seventh power rule. As with wind the energy content of a stream varies with the cube of the flow velocity.
The figure on the left indicates that a seabed-mounted single rotor unit may only capture 25% of the available energy, missing out on the more energetic flows near the surface. The Triton multi-turbine system captures energy from all flow layers including those near the surface.
Some turbine systems rely on floating surface support structures, and would appear attractive in that they gather only the most energetic surface layers. However, such systems are vulnerable to storms and the resulting severe sea states, and would not be suitable to the most energetic and exposed sites.