A
acre-foot
the quantity of water required to cover one acre of land to a depth of one foot; equivalent to 43,560 cubic feet of water or
325,851 gallons of water.
area, usable
the area under the surface of a stream available to aquatic organisms.
area, weighted usable (WUA)
the area under the surface of a stream, weighted by its suitablity, available to a life stage of an aquatic organism. (see PHABSIM)
average annual flow
the rate at which water flows through a channel, determined by averaging daily measurements of the flow during one entire
year.
avoided cost
the estimated sum of money that would have been spent on the lowest cost alternative generating plant that would be used if
the small hydro project did not exist; used in setting power prices.
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B
base flow
flow in a channel sustained by ground-water discharge in the absence of direct runoff.
base load
the minimum amount of electric power that is needed at all times during all seasons.
bed
the bottom of the stream channel; may be wet or dry.
bypass
structure used to convey water around the natural channel to the powerplant and back to the natural channel. ( see headrace, tailrace, penstock )
bypassed channel 
the length of natural river channel impacted by the removal of water for hydropower generation. The upstream boundary is
where the withdrawal occurs, typically located at a dam, and the downstream boundary is the confluence of the channel with
the powerhouse tailrace.
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C
canal
an open manmade channel used for conveying water.
cavitation
violent collapse of bubbles of water vapor that may form in the vacuum normally found at local points within the runner and
draft tube. Occurrence of cavitation depends on atmospheric pressure, hydrostatic pressure, water temperature, water
velocity, and directional changes. Cavitation breaks down the surface of metal objects, encouraging oxidation, and, in severe
cases, results in the development of a honeycomb structure in the metal.
cogeneration
the use of waste heat to drive turbine-generators for electricity generation. Also, the use of low-pressure exhaust steam from
an electric generating plant to heat an industrial process or a space.
control
a downstream channel feature--a channel constriction, a bedrock outcrop, a gravel bar, woody debris, an artificial
structure--in the channel that physically influences the upstream water-surface elevation.
cofferdam
a temporary, watertight enclosure from which the water is pumped to expose the bottom and allow construction.
crest 
the top edge of a dam , dike, spillway, or weir .
critical stream flow
the amount of water available for hydroelectric power generation during the most adverse streamflow period.
cubic feet per second (cfs)
a standard measure of the total amount of water passing by a particular location of a river, canal, pipe or tunnel during a one second interval. One cfs is equal to 7.4805 gallons per second, 28.31369 liters per second, 0.028 cubic meters per second, or 0.6463145 million gallons per day (mgd). Also called second-feet.
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D
dam
a structure to impound water or raise water elevation for storage or diversion or to create hydraulic head.
datum
a geometric plane of known or arbitrary elevation used as a point of reference to determine the elevation, or change of elevation, of another plane. (see gage datum)
dead storage
the portion of a storage basin or reservoir that cannot be use for temporary water storage.
discharge
the volume of fluid that passes a given point in a given amount of time, usually cubic feet per second.
diversion 
the act of, or structure built for, partially obstructing the flow of water in a channel in order to direct or alter the course of the water.
draft tube
The conical shaped passageway downstream of the turbine that slows the water exiting the turbine runner and allows uniform
recovery of water pressure between the runner and the tailrace .
drainage area
the area of land that drains to a specific point on a river. Also called catchment area.
drainage basin
part of the Earth's surface that is occupied by a drainage system with a common outlet for its surface runoff.
drawdown
lowering of the water level of a reservoir by removing water at a greater rate than the inflow to the reservoir.
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E
energy dissipater
a device used to reduce water pressure or velocity to a safe level.
entrainment
when organisms are pulled into the current and subsequently transported through a hydropower plant's conduits and
generation equipment.
exemption
permission to operate a hydropower facility, issued by the FERC, that waives the
requirement that a project be licensed under the Federal Power Act if it meets certain capacity, project type, land
ownership, and environmental criteria.
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F
Federal Energy Regulatory Commission (FERC)
an agency of the Department of Energy that licenses non-federal hydropower projects and regulates interstate transfer of
electric energy. { FERC's web site }
firm capacity
capacity of a plant that can be guaranteed to meet load requirements.
fish ladder
an artificial waterway composed of a series of stepped pools allowing fish to ascend a vertical gradient, usually built at one
end of a dam.
fish screen
barrier installed to prevent fish from passing through the turbine .
flashboards
boards installed on the crest of a dam to raise the water level.
flow-duration curve 
a graphic presentation of flow values plotted in descending order of magnitude against the percentage of time that a
particular flow is equaled or exceeded. For example, the flow that equals the 90th percentile is the flow that 90 percent of all
recorded flows for the river will equal or exceed.
forebay
the body of water immediately upstream from the generating plant, from which water is fed into the generators.
Francis turbine
a reaction turbine that uses the combined action of the pressure and velocity of the water to drive generating equipment.
Water enters the unit radially and exits axially.
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G
gage datum
elevation of the zero point of the reference gage from which gage height is determined as compared to sea level.
gage height
water-surface elevation referenced to the gage datum.
gaging station 
a specific site on a stream where systematic observations of streamflow or other hydrologic data are obtained.
generator 
a machine that converts mechanical energy into electrical energy.
generating capacity
the maximum load that the system can generate under specified conditions for a given time interval without exceeding
approved limits of temperature and stress.
gigawatt (gw)
one billion watts
glory hole spillway
( see shaft spillway )
guide vanes 
moveable vanes control the amount of flow to the turbine runner . They can be used to shut off flow to the turbine , with
varying amounts of leakage, or opened to admit flow. They also impart a spin to the flowing water so that it acts more
efficiently on the runner blades.
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H
head
the difference in elevation between two water surfaces (see gross head , net head ).
head, critical
the head at which the output of a turbine operating at full capacity equals the nameplate rating of an associated generator .
head, design
the head at which the turbine is designed to give the best overall efficiency under various operating conditions.
head, gross (h)
the difference in elevation between the headwater surface above and the tailwater surface below a hydroelectric power plant,
under specified conditions.
head, net
equal to the gross head minus hydraulic losses in the waterways as the water passes from headwater to tailwater . Normally
used to express the head available to the turbine .
head, rated
the net hydraulic head at which the turbine produces the generator's rated output.
headgate
a water control structure at the entrance to a conduit leading to a powerhouse.
headrace
an open channel for conducting water into the power plant.
headwater
water upstream of a dam or powerhouse
hydraulic head
a measure of energy or pressure, expressed in terms of the vertical height of a column of water that has the same pressure
difference.
hydraulic jump
a sudden turbulent rise in water level, such as often occurs at the foot of a spillway when the velocity of rapidly flowing
water is instantaneously slowed.
hydrology
the science of the behavior of water in the atmosphere, on the earth's surface, and underground.
hydroelectric or hydropower plant
a facility which uses falling water to generate electricity with turbine-generators.
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I
impingement
when organisms are drawn against the intake trash rack or screen and held by the force of the water current.
impulse turbine
a collective term for hydraulic turbines in which the kinetic energy of water from high pressure flow jets is transferred to the
runner shaft by impact with the bowl-shaped buckets on the runner. An example of an Impulse turbine is the Pelton.
installed capacity
the total capacity of the generating units in a hydroelectric plant shown on the nameplates of the units.
instream flow
the amount of water in a stream that is required to sustain downstream uses within the channel, such as aquatic habitat, aquatic life,
recreation, or wastewater assimilation.
instream flow incremental methodology (IFIM)
a five phase management and negotiation tool used for water allocation. The five phases are problem identification, study planning, study implementation, alternatives analysis, and problem resolution. Analysis is based on stream channel characteristics, water column dynamics, the historical flow record and target species habitat requirements or management goals.
intake
a structure to divert water into a conduit leading to the power plant.
interbasin transfer
the physical transfer of water from one river basin to another.
intervention
a formal action taken by a person or group to ensure that its interests are addressed by FERC in the course of reviewing a
license or exemption application.
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K
Kaplan turbine
a hydraulic turbine using a propeller-type runner and wicket gates in which the pitch of the propeller blades and wicket gates
is adjustable under load to achieve optimum performance in the generation of hydroelectric power.
kilowatt (kW)
one thousand watts (W) or 1.34 horsepower (hp), a measure of electric power or generating capacity.
kilowatt-hour (kWh)
one thousand watt-hours (Wh). The amount of electrical energy produced or consumed by one kW unit for one hour; the
basic unit of electric energy and the unit by which electric energy is sold.
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L
license
the basic form of FERC authorization to construct a new hydroelectric project or
continue operating an existing hydroelectric project for a specific period of time.
load
the amount of power required at a given point on an electric system.
log boom
a device used to prevent large objects floating on the water surface from entering an area like an intake or spillway.
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M
macrohabitat
habitat conditions in a reach of river controlling longitudinal distribution of aquatic organisms, e.g. channel characteristics, streamflow, water quality, temperature.
manifold
a section of steel pipeline that diverts flow from a single penstock into several smaller penstocks that feed multiple
turbine-generator units.
market value
the monetary value of power at the load center as measured by the cost of producing and delivering equivalent alternative
power to the market.
megawatt (MW)
one million watts; a measure of electric power or generating capacity.
microhabitat
small area habitat of a river that controls specific locations or home ranges of aquatic organisms, e.g. hydraulic features, structural features.
million gallons per day (mgd)
a standard measure of the total amount of water passing by a particular location of a river, canal, pipe or tunnel during a 24-hour interval. One mgd is equal to 1.5472323 cubic feet per second (cfs), or 43.782837 liters per second
morning glory spillway
( see shaft spillway )
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N
nameplate capability
the nominal rated capacity of a generator or other similar apparatus. The term gives an indication of the approximate
generating capacity of the unit; however, in many cases, the unit is capable of generating substantially more than the
nameplate capacity on a continuous basis.
nameplate rating
the full load continuous rating of a generator or other piece of electrical equipment under specified conditions, as designated
by the manufacturer and written on the nameplate.
notice of intent
formal notice that a competing application for a license or preliminary permit will be filed with the FERC .
notice period
a specified period of time during which the public must be notified of an application pending before FERC . Interventions and
Notices of Intent must be filed during the Notice Period.
nozzle
a control valve that directs flow onto the runner of a Pelton-type impulse turbine .
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O
operating range
the range of hydraulic head and flow existing during operation of a hydroelectric plant, within which the turbine can be
operated without undue vibration or cavitation drainage.
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P
peak load
the greatest of all load demands on an interconnected electric transmission network occurring in a specified period of time.
peak load plant
a power plant that is operated to provide energy only during maximum demand, peak load, periods. Also called a peaking
plant.
peaking 
as distinguished from run-of-river, a type of hydropower project that uses its reservoir as a storage facility, releasing water
to generate power only when power is needed, or more profitable.
peaking capability
the maximum peak load that can be supplied by a generating unit, station, or system in a stated time period. For a
hydroelectric project, the peaking capability equals the maximum plant capability under favorable pool and flow conditions.
peaking capacity
that part of a system's generating capacity that is operated during the hours of highest power demand.
Pelton turbine
an impulse type hydraulic turbine normally used for high head hydroelectric power plants. The turbine works by the impact
of high velocity jets of water, directed by nozzles, onto a series of buckets fixed around the edge of the runner .
penstock 
pipe or other enclosed, pressurized conduit for the specific use of conveying water to a power plant from a dam or diversion
above the plant.
physical habitat simulation (PHABSIM)
computated relationship between stream flow and either physical habitat for an aquatic organism or a recreational activity. The relationship is based on channel characteristics (substrate, cover), stream dynamics (depth, velocity) and habitat suitability criteria for a recreational activity or various life stages (adult, spawning, fry, juvenile) of a target species. Habitat is presented as weighted usable area.
pipeline
a pressurized conduit for the specific use of conveying fluid from one location to another.
pondage
water stored behind a dam that is used for daily or weekly regulation of the flow of a river. Generally applies to reservoir
storage at run-of-river hydropower plants
powerhouse
a structure that houses the turbines , generators, and associated control equipment.
preliminary permit
a permit granted by FERC for a particular project site, giving the holder priority
status for filing an application for a license or exemption. A preliminary permit may be granted for a term of up to 36
months, and is not renewable.
pressure shaft
a vertical or inclined conduit excavated in rock and capable of carrying water under pressure.
propeller turbine
an axial flow reaction turbine , i.e., the flow moves parallel with the axis of the turbine shaft. The turbine runner is similar to
a ship's propeller and the turbine is used for low and ultra low head hydro projects. If the angle of the blades to the flow can
be adjusted, the turbine is called a Kaplan turbine.
pumped-storage hydroelectric plant
a hydroelectric plant that generates energy for periods of peak demand or emergency demand by using water pumped during
off-peak periods from a lower-elevation reservoir to a higher-elevation reservoir.
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Q
Q
hydrological abbreviation for discharge .
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R
rated output
the power output at which a turbine or generator is rated; this normally corresponds to the output at the selected design
point of head and flow.
reaction turbine
a generic term for bladed turbines that operate full of water and in which water enters the turbine runner under pressure and
interacts with the runner in such a way that the hydraulic energy is converted to kinetic energy in the turbine shaft.
Reaction turbines include the Francis fixed-blade propeller and the Kaplan
.
recurrence interval
the average time, usually expressed in years, between occurrences of hydrologic events of a specified type (such as
exceedance of a specified high flow or non-exceedance of a specified low flow). The term does not imply a regular cyclic
occurrence. The actual times between occurrences vary randomly, with most of the times being less than the average and a
few being substantially greater than the average. For example, the 100-year flood is the flow rate that is exceeded by the
annual maximum peak flow at intervals whose average length is 100 years (that is, once in 100 years, on average); almost
two-thirds of all exceedances of the 100-year flood occur less than 100 years after the previous exceedance, half occur less
than 70 years after the previous exceedance, and about one-eighth occur more than 200 years after the previous exceedance.
Similarly, the 7-day 10-year low flow (7Q10 ) is the flow rate below which the annual minimum 7-day-mean flow dips at
intervals whose average length is 10 years (that is, once in 10 years, on average); almost two-thirds of the non-exceedances
of the 7Q10 occur less than 10 years after the previous non-exceedance, half occur less than 7 years after, and about
one-eighth occur more than 20 years after the previous non-exceedance. The recurrence interval for annual events is the
reciprocal of the annual probability of occurrence. Thus, the 100-year flood has a 1-percent chance of being exceeded by the
maximum peak flow in any year, and there is a 10-percent chance in any year that the annual minimum 7-day-mean flow will
be less than the 7Q10.
relicensing
the administrative proceeding in which the FERC , in consultation with other
federal and state agencies, decides whether and on what terms to issue a new license for an existing hydroelectric project at
the expiration of the preceding license.
reregulating reservoir
an impoundment, located downstream from a hydroelectric peaking plant, that has sufficient pondage to store the widely
fluctuating discharges from the peaking plant and release them downstream in a relatively uniform manner.
reservoir
a body of water, either natural or artificial, that is used to manipulate flow or store water for future use.
retention time
time, usually in days, required for a volume of water equal to the reservoir capacity to move through the reservoir and be
discharged downstream.
retrofitting
furnishing a power plant with new parts or equipment not purchased or available at the time of construction.
revetment
a facing of masonry or concrete, used to protect an embankment from erosion or slumping.
riffle
shallow rapids in an open stream where the water surface is broken into waves by obstructions wholly or partly submerged.
riparian rights
the rights of a land owner to the water on or bordering his property, including the right to prevent diversion or misuse of
upstream water.
riprap
large stones or concrete placed for the purpose of protecting a slope from erosion due to flowing water.
river mile
the distance of a point on a river measured in miles from the river's mouth along the low-water channel.
rule curve
guides used in water reservoir operation. They graphically show desired water levels and certain operating rights,
entitlements, obligations, and limitations for a reservoir through the year.
run-of-river
a type of hydroelectric project in which the amount of electricity generated is controlled mainly by the volume of water
flowing in the stream above the project, i.e., outflow equals inflow. Any project which cannot store significant quantities of
water at or above the site must be operated as a run-of-river facility. Some run-of-river plants have limited storage capacity
(pondage), which allows for streamflow regulation on a daily or weekly basis.
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S
safe yield
the maximum dependable quantity of water that can be withdrawn continuously from a water source--surface or ground--for a period of years without depleting the source beyond its ability to be replenished naturally. A reservoir safe yield estimate takes into consideration the physical size of the reservoir, the size of its watershed, the maximum volume of water that can be expected to flow into the system under conditions similar to the worst drought on record, and the duration of the worst drought on record. 20-year and 50-year safe yields are typical, and are based on 20- or 50-year worst drought flows, respectively.
scroll case
a spiral casing, made of steel or concrete, that carries water around the periphery of a turbine . As the water spirals in and is
discharged through the wicket gates to the runner , the diameter of the scroll case decreases to keep the flow velocity
uniform.
seasonal storage
storage of water in a reservoir during that portion of the year when a surplus occurs.
settling basin
a chamber designed to remove sediment from water by providing still conditions that allow sediment to fall to the floor of
the chamber. They are used in cases where sediment would otherwise block waterways or damage the turbine .
seven-day ten-year low flow (7Q10)
the minimum flow averaged over 7 consecutive days that is expected to occur on average, once in any 10-year period. The
7Q10 has a 10-percent chance of occurring in any given year. ( see recurrence interval )
shaft spillway
a vertical shaft having a funnel-shaped entrance and ending in an outlet tunnel that provides an outflow from a reservoir.
Also known as a morning glory or glory hole spillway.
spillway
an outlet from a reservoir or section of a dam designed to release surplus water that is not discharged through a turbine or
other outlet works.
stage
the elevation, or vertical distance, of the water surface above a datum.
stage-discharge relation
the relation between the water-surface elevation, termed stage (gage height), and the volume of water flowing in a channel
per unit time.
stilling basin
the area on the downstream side of a spillway where water velocity is reduced to prevent erosion damage to hydraulic
structures or the natural riverbed and banks.
storage project
a reservoir of sufficient size to permit carry over from the high flow season to the low flow season and, thus,
develop a firm flow that is substantially greater than the minimum natural flow. A storage project may have its own power
plant or may be used only for increasing generation at a downstream plant.
storage reservoir
the available volume behind a dam used to store water.
stream flow
the amount of water passing a given point in a river in a given period of time, usually expressed in cubic feet per second
(cfs), or million gallons per day (mgd).
surface area
area encompassed by the boundary of a lake or impoundment, as shown on a map or photograph, at a specific water
elevation.
surge tank 
a hydraulic structure designed to control pressure and flow fluctuations in a penstock or tunnel. It functions as a standpipe
and a quick-acting reservoir that temporarily stores or releases water to the penstock. (see water hammer)
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T
tailrace 
the channel which directs water flow away from the powerhouse after passing through the generating turbines . Also called
an afterbay.
tailwater
water downstream of a powerhouse.
thalweg
the longitudinal line connecting points of minimum bed elevation along the stream course.
thalweg depth
the vertical distance of the lowest point of a channel section to the water surface.
toe
the downstream, lower edge or edges of a dam structure.
trash rack 
a screen or rack of parallel bars installed in the headrace to collect debris and prevent damage to the turbine .
trash rake
a mechanism used to clean the trash rack.
turbine 
a rotary machine in which the pressure or kinetic energy of water is converted to mechanical energy that, in the case of a
hydroelectric plant, is then converted to electrical energy by a generator .
turbine-generator
a unit consisting of a turbine driving an electric generator .
turbine runner 
the part of a turbine , consisting of curved vanes, blades, or buckets on a wheel or hub, that is turned by the pressure of high
velocity water, thereby transforming falling water energy into rotating mechanical energy.
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U
usable storage
that portion of the gross storage in a reservoir that may be used for a designated purpose.
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V
valve
a closure device for controlling the flow of water.
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W
water hammer
dramatic pressure fluctuations within a contained, pumped liquid when it is started or stopped suddenly.
water year
twelve-month period from October 1 through September 30. The water year is designated by the calendar year in which it
ends and which includes 9 of the 12 months. Thus, the year ending September 30, 2001, is called the "2001 water year."
water room
lower bay of the powerhouse with submerged turbines in an open-flume setting, i.e. water enters the water room from the headrace, passes through the guide vanes, falls past the runners into the draft tube and out the tailrace.
weir 
low elevation dam used to impound water for gaging, reregulation or withdrawal.
wetted perimeter
the distance along the bottom and sides of a channel cross section in contact with the water.
wheeling
transportation of electricity by a utility over its lines for another party. It includes the receipt from and delivery to another
system of like amounts but may not necessarily be the same energy.
wicket gates
a series of overlapping, movable guide vanes around the circumference of a turbine that control the flow of water through a
turbine and, consequently, controlling the output of the hydroelectric unit. (See guide vanes)
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Y
yield
the amount of water that can be supplied from a reservoir, or other source, in a specified time period.
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References
Bovee, K.D. and R. Milhous. Hydraulic simulation in instream flow studies: theory and techniques. Instream flow information paper number 5. 130 pp., U.S. Fish and Wildlife Service Report Number FWS/OBS-78/33, 1978.
Christensen, J.P., C.H. Cunningham, T.S. Eanes, and A.R. Engebretsen. Small hydropower development: the process, pitfalls, and experience. Volume IV: Guide for developers. 195 pp., U.S. Department of Energy Report Number DOE/ID/12254-1. 1985.
Echeverria, J.D., P. Barrow, and R. Roos-Collins. Rivers at risk: the concerned citizen's guide to hydropower. 217 pp., Island Press, Washington, D.C., 1989.
Milhous, R.T., M.A. Updike, and D.M. Schneider. Physical habitat simulation system reference manual - version II. Instream flow information paper number 26. U.S. Fish and Wildlife Service, Biological Report 89(16), 1989.
Ragland, B.C., D.A. Walters, G.D. Cartano, and J.E. Taylor. Water Resources Data, North Carolina, Water Year 1999, Volume 1A. Surface-water data. 574 pp., U.S. Geological Survey Report Number USGS-WDR-NC-99-1A, 2000.
Rochester, Jr., H., T. Lloyd, and M. Farr. Physical impacts of small-scale hydroelectric facilities and their effects on fish and wildlife. 191 pp., U.S. Fish and Wildlife Service Report Number FWS/OBS-84/19, 1984.
Stalnaker, C., B.L. Lamb, J. Henriksen, K. Bovee, J. Bartholow. The Instream Flow Incremental Methodology: A primer for IFIM. 99pp., National Ecology Research Center, Internal Publication. National Biological Survey. Fort Collins, Colorado, 1994.
Wolfe, M.E. A landowner's guide to western water rights. 175 pp., Roberts Rinehart Publishers, Boulder, Colorado. 1996.
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