Glossary of Hydro PowerTerminology

Coanda Screen A downstream metal screen which prevents both fish and debris entering the penstock.
Depleted Reach The length of river or stream between the offtake point and the discharge point in which the flow is reduced by the volume of water going through the penstock, turbine and tailrace.
Dynamic Head The water pressure at the turbine inlet when water is flowing and the turbine is operating
EA Environment Agency
Fish Ladder A series of stepped pools allowing mirating fish to pass up or down a weir or impoundment.
Flow Duration Curve A graph which plots time (ususally 12 months of the year) on one axis and the water flow in a river on the other axis. The plot line shows the percentage of time that a prescribed flow is equalled or exceeded.
Flow Rate The volume of water moving down a river or stream at a given point and time measured in either litres per second (l/s) or cubic metres per second (m³/s)
Forebay Tank A tank placed between the river and the penstock to slow the water down and allow silt to settle and entrapped air to escape.
Hands Off Flow The amount of water which must remain in the river at all times. Also known as the compensation or reserved flow.
Head The vertical distance between the penstock inlet and the turbine inlet usually measured in metres
Head Efficiency The dynamic head pressure as a percentage of the static head pressure. Normally over 90%
Hydromorphology The study of the banks and bed of a river or stream. Especially related to the movement of sediments within a river bed in which fish spawn and how that might be modified by water abstraction for a hydro power scheme.
Impoundment A weir or small dam across a river or stream which holds back the water into which the penstock is connected.
Intake Screen Ususally a mesh screen at the inlet point to prevent debris enetring the penstock.
Leat An open water channel that feeds a water wheel.
Low Flows A software programme that uses historical rainfall data and topography to estimate the monthly flow in a river at a specific location. 
NRW Natural Resources Wales
Offtake The structure which extracts water from the river, screens it and then feeds it into the penstock. This may be built into a dam or similar impoundment or a "side offtake" built into the bank of the river or stream which extracts sufficient water without restricting the downstream flow.
Pelton Turbine Generally suited to locations with high heads and low flows.
Penstock The pipe, ususally MDPE between the water offtake and the turbine inlet
Runner The circular disc which is rotated by the water flow which in turn rotates the generator
Spillway Part of the offtake structure which permits a pre-determined amount of water to stay in the river at all times.
Static Head The water pressure at the turbine inlet when no water is flowing
Tailstock The pipe which returns the water to the river or stream
Turbine A machine for converting water flow under pressure into electricity
Turgo Turbine Generally suited to low heads and high flows