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The magnitude of the UK tidal current resource has been examined by a number of different reports, each attempting to define a figure which is representative of the commercially exploitable energy in the flow. Recent work suggests that the exploitable resource is of the order of one fifth the size of the raw flux passing through a channel. It is not possible to develop an indicative value of the exploitable energy simply from knowledge of the harmonic constants or local tide tables, and over-optimistic resource forecasting has resulted from attempts to do so. Tidal currents are bounded, finite systems with no capacity for energy replenishment from other sources, whereas the atmospheric energy source is, from a wind turbine standpoint, almost infinite, being easily replenished from the upper atmosphere or neighbouring weather systems. The principle sources of tidal flow data to date have been navigational charts and Admiralty tidal stream atlases, but these values are based on surface or near surface measurements of the flow velocities, which can be used to develop a baseline model of the energy, but do not necessarily represent the flow conditions as seen by a turbine fixed in position close to the sea bed. The bathymetry and topography of the geological structures that enclose the tidal current site will heavily influence the nature of the flow, both in terms of velocity and in the generation of metre length scale vortices. Additionally, since tidal currents are driven largely by head difference across a locally narrow channel, meteorological events such as barometric pressure, wind strength and direction, and the situation of a site relative to a large fetch will affect the tidal current flow velocities.