| Technical
Glossary |
Hydrology
Terms
A
Ablation :
The processes by which ice and snow dissipate owing to
melting and evaporation.
Abutment :
The part of a valley or canyon wall against which a dam
is constructed. Right and left abutments are those on
respective sides of an observer looking downstream
Abutment Seepage :
Reservoir water that moves through seams or pores in the
natural abutment material and exits as seepage.
Acre-foot :
The amount of water required to cover one acre to a depth
of one foot. An acre-foot equals 326,851 gallons, or 43,560
cubic feet.
Active Conservation Storage :
The portion of water stored in a reservoir that can be
released for all useful purposes such as municipal water
supply, power, irrigation, recreation, fish, wildlife,
etc. Conservation storage is the volume of water stored
between the inactive pool elevation and flood control
stage.
Active (Usable) Storage Capacity :
The total amount of reservoir capacity normally available
for release from a reservoir below the maximum storage
level. It is total or reservoir capacity minus inactive
storage capacity. More specifically, it is the volume
of water between the outlet works and the spillway crest.
Adirondack Type Snow Sampling Set :
A snow sampler consisting of a 5-foot fiberglass tube,
3 inches in diameter, with a serrated-edge steel cutter
at one end and a twisting handle at the other. This sampler
has a 60-inch snow depth capacity.
ADVIS :
A program which combines the Antecedent Precipitation
Index (API) method of estimating runoff with unit hydrograph
theory to estimate streamflow for a headwater basin.
Aeration Zone :
A portion of the lithosphere in which the functional interstices
of permeable rock or earth are not filled with water under
hydrostatic pressure. The interstices either are not filled
with water or are filled with water that is no held by
capillarity.
AFOS :
Automation of Field Operations and Services
Afterbay :
The tail race of a hydroelectric power plant at the outlet
of the turbines. The term may be applied to a short stretch
of stream or conduit, or to a pond or reservoir.
Agglomerate :
An Ice cover of floe formed by the freezing together of
various forms of ice.
AHOS :
Automatic Hydrologic Observing System
AHOS-S :
Automatic Hydrologic Observing System - Satellite
AHOS-T :
Automatic Hydrologic Observing System - Telephone
Airborne Snow Survey Program :
Center (NOHRSC) program that makes airborne snow water
equivalent and soil moisture measurements over large areas
of the country that are subject to severe and chronic
snowmelt flooding.
Airborne Snow Water Equivalent Measurement Theory :
A theory based on the fact that natural terrestrial gamma
radiation is emitted from the potassium, uranium, and
thorium radioisotopes in the upper eight inches of the
soil. The radiation is sensed from low flying aircraft
500 feet above the ground. Water mass in the snow cover
attenuates the terrestrial radiation signal. The difference
between airborne radiation measurements made over bare
ground and snow-covered ground can be used to calculate
a mean areal snow water equivalent value with a root mean
square error of less than a half inch.
Albedo :
The portion of incoming radiation which is reflected by
a surface.
ALERT Flood Warning System :
A cooperative, community-operated flood warning system;
the acronym stands for Automated Local Evaluation (in)
Real Time.
Alluvial :
An adjective referring to alluvium.
Alluvium :
Sediments deposited by erosional processes, usual by streams.
Anabranch :
A diverging branch of a river which re-enters the main
stream.
Anchor Ice :
Submerged Frazil ice attached or anchored to the river
bottom, irrespective of its formation.
Anchor Ice Dam :
An accumulation of anchor ice which acts as a dam and
raises the water level.
Annual Flood :
The maximum discharge peak during a given water year (October
1 - September 30).
Antecedent Precipitation Index (API) :
An index of moisture stored within a drainage basin before
a storm.
API Method :
A statistical method to estimate the amount of surface
runoff which will occur from a basin from a given rainstorm
based on the antecedent precipitation index, physical
characteristics of the basin, time of year, storm duration,
rainfall amount, and rainfall intensity.
Aquiclude :
A formation which contains water but cannot transmit it
rapidly enough to furnish a significant supply to a well
or spring.
Aquifer :
Permeable layers of underground rock, or sand that hold
or transmit groundwater below the water table that will
yield water to a well in sufficient quantities to produce
water for beneficial use.
Aquifuge :
A geologic formation which has no interconnected openings
and cannot hold or transmit water.
Arch Dam :
A concrete arch dam is used in sites where the ratio of
width between abutments to height is not great and where
the foundation at the abutments is solid rock capable
of resisting great forces. The arch provides resistance
to movement. When combined with the weight of concrete
(arch-gravity dam), both the weight and shape of the structure
provide great resistance to the pressure of water.
Area of Influence :
The area covered by the drawdown curves of a given pumping
well or combination of wells at a particular time.
Area-Capacity Curve :
A graph showing the relation between the surface area
of the water in a reservoir, the corresponding volume,
and elevation.
Area Wide Hydrologic Prediction System (AWHPS) :
A computer system which automatically ingests areal flash
flood guidance values and WSR-88D products and displays
this data and other hydrologic information on a map background.
Arid :
An adjunctive applied to regions where precipitation is
so deficient in quantity, or occurs at such times, that
agriculture is impracticable without irrigation.
Arroyo :
A water-carved channel or gully in arid country, usually
rather small with steep banks, dry most of the time, due
to infrequent rainfall and the shallowness of the cut
which does not penetrate below the level of permanent
ground water.
Artesian Well :
a well drilled into a confined aquifer with enough hydraulic
pressure for the water to flow to the surface without
pumping. Also called a flowing well.
Artificial Control :
A weir or other man-made structure which serves as the
control for a stream-gaging station.
ASAP :
AHOS SHEF Automatic Processing System
ASAPTRAN :
The software component of ASAP.
ASOS :
The ASOS program is a joint effort of the National Weather
Service (NWS), the Federal Aviation Administration (FAA),
and the Department of Defense (DOD). When installation
is completed in the mid-1990s, the ASOS systems will serve
as the nation's primary surface weather observing network.
ASOS is designed to support weather forecast activities
and aviation operations and, at the same time, support
the needs of the meteorological, hydrological, and climatological
research communities.
Automatic Surface Observing System
ASOS
ATDTDCS (Automated Tone Dial Telephone Data Collection
System) :
Data collection system where cooperative observers collect
precipitation, stage, and temperature data then transmit
the data to the NWS ATDTDCS computer through the telephone
lines. The ATDTDCS computer transmits the data to AFOS.
Attenuation :
The process where the flood crest is reduced as it progresses
downs
Automated Event-Reporting Gage:
(see Tipping Bucket Rain Gage); for river stage gages,
IFLOWS pressure transducer type gages can be programmed
to report if water surface rises or falls by a predetermined
amount.
Automated Local Evaluation in Real Time (ALERT) :
A local Flood warning system where river and rainfall
data are collected via radio signals in real-time at an
ALERT base station.
AWIPS :
Advanced Weather Interactive Processing System. A system
that will eventually replace Automation of Field Operations
and Services, AFOS.
B
B-44 Form, Cooperative Station Report :
A Weather Service form documenting station management,
exposure, topography, driving instructions, payment information,
hydrometeorlogic equipment, and observing information.
Backflow :
The backing up of water through a conduit or channel in
the direction opposite to normal flow.
Backsight :
A rod reading taken on a point of known elevation, a benchmark
or a turning point. Backsights are added to the known
elevation to arrive at the height of the instrument. With
a known height of the instrument, the telescope can be
used to determine the elevation of other points in the
vicinity.
Backwater Curve:
The longitudinal profile of the surface of a liquid in
a non-uniform flow in an open channel, when the water
surface is not parallel to the invert owing to the depth
of water having been increased by the interposition of
an obstruction such as a dam or weir. The term is sometimes
used in a generic sense to denote all water surface profiles;
or for profiles where the water is flowing at depths greater
than the critical.
Backwater Effect:
The effect which a dam or other obstruction has in raising
the surface of the water upstream from it.
Backwater Flooding :
Upstream flooding caused by downstream conditions such
as channel restriction and/ or high flow in a downstream
confluence stream.
Bank :
The margins of a channel. Banks are called right or left
as viewed facing in the direction of the flow.
Bank Storage:
Water absorbed and stored in the void in the soil cover
in the bed and banks of a stream, lake, or reservoir,
and returned in whole or in part as the level of water
body surface falls.
Bankfull Stage/Elevation :
An established river stage/water surface elevation at
a given location along a river which is intended to represent
the maximum water level that will not overflow the river
banks or cause any significant damages from flooding.
Bankfull Stage :
An established river stage at a certain point along a
river which is intended to represent the maximum safe
water level which will not overflow the river banks or
cause any significant damage within the reach of the river.
Barrage :
Any artificial obstruction placed in water to increase
water level or divert it. Usually the idea is to control
peak flow for later release.
Base Flood:
The national standard for floodplain management is the
base, or one percent chance flood. This flood has at least
one chance in 100 of occurring in any given year. It is
also called a 100 year flood.
Base Station :
A computer which accepts radio signals from ALERT gaging
sites, decodes the data, places the data in a database,
and makes the data available to other users.
Baseflow :
Streamflow which results from precipitation that infiltrates
into the soil and eventually moves through the soil to
the stream channel. This is also referred to as ground
water flow, or dry-weather flow.
Base Width :
The time duration of a unit hydrograph.
Basin :
An area having a common outlet for its surface runoff.
Basin Boundary :
The topographic dividing line around the perimeter of
a basin, beyond which overland flow (i.e.; runoff) drains
away into another basin.
Basin Lag :
The time it takes from the centroid of rainfall for the
hydrograph to peak.
Basin Recharge :
Rainfall that adds to the residual moisture of the basin
in order to help recharge the water deficit. i.e; water
absorbed into the soil that does not take the form of
direct runoff.
Bed Load :
Sand, silt, gravel, or soil and rock detritus carried
by a stream on or immediately above its bed. The particles
of this material have a density or grain size such as
to preclude movement far above or for a long distance
out of contact with the stream bed under natural conditions
of flow.
Beginning of the Breakup :
Date of definite breaking, movement, or melting of ice
cover or significant rise of water level.
Beginning of Freezup :
Date on which ice forming a stable winter ice cover is
first observed on the water surface.
Benchmark (BM) :
A permanent point whose known elevation is tied to a national
network. These points are created to serve as a point
of reference. Benchmarks have generally been established
by the USGS, but may have been established by other Federal
or local agencies. Benchmarks can be found on USGS maps.
Black Ice :
Transparent ice formed in rivers and lakes.
Border ice :
An ice sheet in the form of a long border attached to
the bank or shore.; shore ice.
Brackish Ice :
Ice formed from Brackish water.
Braided Stream :
Characterized by successive division and rejoining of
streamflow with accompanying islands. A braided stream
is composed of anabranches.
Brash Ice :
Accumulation of floating ice made up of fragments not
more than 2 meters across; the wreckage of other forms
of ice.
Breach :
The failed opening in a dam.
Breakup :
The time when a river whose surface has been frozen from
bank to bank for a significant portion of its length begins
to change to an open water flow condition. Breakup is
signaled by the breaking of the ice and often associated
with ice jams and flooding.
Breakup date :
Date on which a body of water is first observed to be
entirely clear of ice and remains clear thereafter.
Breakup jam :
Ice jam that occurs as a result of the accumulation of
broken ice pieces.
Breakup period :
The period of disintegration of an ice cover.
Bubbler Gage :
A water stage recording device that is capable of attaching
to a LARC for data automation purposes.
Buttress Dam :
Buttress dams are comprised of reinforced masonry or stonework
built against concrete. They are usually in the form of
flat decks or multiple arches. They require about 60 percent
less concrete than gravity dams, but the increased form
work and reinforcement steel required usually offset the
savings in concrete. Many were built in the 1930's when
the ratio of labor cost to materials was comparatively
low. However, this type of construction is not competitive
with other types of dams when labor costs are high.
C
CADAS (Centralized Automated Data Acquisition System)
:
A system of two minicomputers in NWSH that interrogates
LARCs and DARDCs by telephone every 6 hours and transmits
the data to AFOS via HADS.
Calibration :
The process of using historical data to estimate parameters
in a hydrologic forecast technique such as SACSMA, routings,
and unit hydrographs.
Capillarity :
(1)The degree to which a material or object containing
minute openings or passages, when immersed in a liquid,
will draw the surface of the liquid above the hydrostatic
level. Unless otherwise defined, the liquid is generally
assumed to be water. (2)The phenomenon by which water
is held in interstices above the normal hydrostatic level,
due to attraction between water molecules.
Capillary Fringe :
The soil area just above the water table where water can
rise up slightly through the cohesive force of capillary
action. This layer ranges in depth from a couple of inches,
to a few feet, and it depends on the pore sizes of the
materials. The capillary zone is also called the capillary
zone.
Capillary Potential :
The work required to move a unit mass of water from the
reference plane to any point in the soil column.
Capillary Zone :
The soil area just above the water table where water can
rise up slightly through the cohesive force of capillary
action. This layer ranges in depth from a couple of inches,
to a few feet, and it depends on the pore sizes of the
materials. The capillary zone is also called the capillary
fringe.
Catchment Area :
An area having a common outlet for its surface runoff
(also see Drainage Area or Basin, Watershed).
Caution Stage :
Same as alert stage.
CFS (Cubic Feet per Second) :
The flow rate or discharge equal to one cubic foot (of
water, usually) per second. This rate is equivalent to
approximately 7.48 gallons per second. This is also referred
to as a second-foot.
Cfs-Day :
The volume of water discharged in twenty four hours, with
a flow of one cubic foot per second is widely used; 1
cfs-day is 24 x 60 x 60 = 86,000 cubic feet, 1.983471
acre-feet, or 646,317 gallons. The average flow in cubic
feet per second for any time period is the volume of flow
in cfs-days.
Channel (watercourse) :
An open conduit either naturally or artificially created
which periodically, or continuously contains moving water,
or forms a connecting link between two bodies of water.
River, creek, run, branch, anabranch, and tributary are
some of the terms used to describe natural channels. Natural
channels may be single or braided. Canal and floodway
are some of the terms used to describe artificial channels.
Channel Inflow :
Water, which at any instant, is flowing into the channel
system form surface flow, subsurface flow, base flow,
and rainfall that has directly fallen onto the channel.
Channel Lead :
An elongated opening in the ice cover caused by a water
current.
Channel Routing :
The process of determining progressively timing and shape
of the flood wave at successive points along a river.
Channelization :
The modification of a natural river channel; may include
deepening, widening, or straightening.
Closed Basin :
A basin draining to some depression or pond within its
area, from which water is lost only by evaporation or
percolation. A basin without a surface outlet for precipitation
falling precipitation.
Closed Basin Lake Flooding :
Flooding that occurs on lakes with either no outlet or
a relatively small one. Seasonal increases in rainfall
cause the lake level to rise faster than it can drain.
The water may stay at flood stage for weeks, months, or
years.
CNIF :
Calibration Network Information Files
Coastal Flooding :
Flooding that occurs from storms where water is driven
onto land from an adjacent body of water. These can be
hurricanes, "nor’easters," or tropical storms,
but even a severe winter storm or thunderstorm can cause
this type of flooding.
COE :
Corps of Engineers
Columnar Ice :
Ice consisting of columnar shaped grain. The ordinary
black ice is usually columnar-grained.
Composite Hydrograph :
A stream discharge hydrograph which includes base flow,
or one which corresponds to a net rain storm of duration
longer than one unit period.
Cone of Depression :
The depression, roughly conical in shape, produced in
a water table, or other piezometric surface, by the extraction
of water from a well at a given rate. The volume of the
cone will vary with the rate of withdrawal of water. Also
called the Cone of Influence.
Cone of Influence :
The depression, roughly conical in shape, produced in
a water table, or other piezometric surface, by the extraction
of water from a well at a given rate. The volume of the
cone will vary with rate of withdrawal of water. Also
called the Cone of Depression.
Confined Ground Water :
Ground water held under an aquiclude or an aquifuge called
artesian if the pressure is positive.
Congressional Organic Act of 1890 :
The act that assigned the responsibility of river and
floor forecasting for the benefit of the general welfare
of the Nation’s people and economy to the Weather Bureau,
and subsequently the National Weather Service.
Conservation Storage :
Storage of water for later release for usual purposes
such as municipal water supply, power, or irrigation in
contrast with storage capacity used for flood control..
Consolidated Ice Cover :
Ice cover formed by the packing and freezing together
of floes, brash ice and other forms of floating ice.
Contents :
The volume of water in a reservoir. Unless otherwise indicated
reservoir content is computed on the basis of a level
pool and does not include bank storage.
Control Points: Horizontal and Vertical :
Small monuments securely embedded in the surface of the
dam. Any movement of the monument indicates a movement
in the dam itself. Movements in the dam are detected by
comparing control points location to location of fixed
monuments located off the dam using accurate survey techniques.
Cooperative Observer :
An individual (or institution) who takes precipitation
and temperature observations-and in some cases other observations
such as river stage, soil temperature, and evaporation-at
or near their home, or place of business. Many observers
transmit their reports by touch-tone telephone to an NWS
computer, and nearly all observers mail monthly reports
to the National Climatic Data Center to be archived and
published.
Conveyance Loss :
The loss of water from a conduit due to leakage, seepage,
evaporation, or evapo-transpiration.
Corn Snow Ice :
Rotten granular ice.
County Warning Forecast Area (CWFA) :
The area (aggregation of counties, or parishes and sometimes
portions of counties, or parishes) served by a WFO for
which weather forecast and warning services are provided.
Crack :
A separation formed in an ice cover of floe that does
not divide it into two or more pieces.
Creek :
A small stream of water which serves as the natural drainage
course for a drainage basin of nominal, or small size.
The term is a relative one as to size, some creeks in
the humid section would be called rivers if they occurred
in the arid portion.
Crest :
(1)The highest stage or level of a flood wave as it passes
a point. (2)The top of a dam, dike, spillway, or weir,
to which water must rise before passing over the structure.
Crest Gage :
A gage used to obtain a record of flood crests at sites
where recording gages are installed.
Crest (Top) of Dam :
The elevation of the uppermost surface of a dam excluding
any parapet walls, railings, etc.
Crest Width (Top thickness) :
The thickness or width of a dam at the level of the crest
(top) of the dam. The term "thickness" is used
for gravity and arch dams and "width" for other
types of dams.
Critical Depth :
The depth of water flowing in an open channel or conduit,
partially filled, corresponding to one of the recognized
critical velocities.
Critical Flow :
A condition of flow where the mean velocity is at one
of the critical values; ordinarily at Belanger's critical
depth and velocity. Another important usage is in reference
to the Reynolds' critical velocities which define the
point at which the flow changes from streamline or nonturbulent
to turbulent flow.
Critical Rainfall Probability (CRP) :
The Probability that the actual precipitation during a
rainfall event has exceeded or will exceed the flash flood
guidance value.
Cross-sectional area :
Area perpendicular to the direction of flow.
CRP :
Critical Rainfall Probability. The Probability that a
given rainfall will cause a river, or stream to rise above
flood stage.
Cryology :
The science of the physical aspects of snow, ice, hail,
and sleet and other forms of water produced by temperatures
below Zero degrees Celsius.
Cubic Feet Per Second :
A unit expressing rates of discharge. One cubic foot per
second is equal to the discharge through a rectangular
cross section, 1 foot wide by 1 foot deep, flowing at
an average velocity of 1 foot per second. It is also approximately
7.48 gallons per second.
Curtain Drain :
A drain constructed at the upper end of the area to be
drained, to intercept surface or ground water flowing
toward the protected area from higher ground, and carry
it away from the area. Also called an Intercepting Drain.
Current meter :
Device used to measure the water velocity or current in
a river.
Cuttoff :
from passing through a dam’s foundation material.An impervious
construction or material which reduces seepage or prevents
it.
C
CADAS (Centralized Automated Data Acquisition System)
:
A system of two minicomputers in NWSH that interrogates
LARCs and DARDCs by telephone every 6 hours and transmits
the data to AFOS via HADS.
Calibration :
The process of using historical data to estimate parameters
in a hydrologic forecast technique such as SACSMA, routings,
and unit hydrographs.
Capillarity :
(1)The degree to which a material or object containing
minute openings or passages, when immersed in a liquid,
will draw the surface of the liquid above the hydrostatic
level. Unless otherwise defined, the liquid is generally
assumed to be water. (2)The phenomenon by which water
is held in interstices above the normal hydrostatic level,
due to attraction between water molecules.
Capillary Fringe :
The soil area just above the water table where water can
rise up slightly through the cohesive force of capillary
action. This layer ranges in depth from a couple of inches,
to a few feet, and it depends on the pore sizes of the
materials. The capillary zone is also called the capillary
zone.
Capillary Potential :
The work required to move a unit mass of water from the
reference plane to any point in the soil column.
Capillary Zone :
The soil area just above the water table where water can
rise up slightly through the cohesive force of capillary
action. This layer ranges in depth from a couple of inches,
to a few feet, and it depends on the pore sizes of the
materials. The capillary zone is also called the capillary
fringe.
Catchment Area :
An area having a common outlet for its surface runoff
(also see Drainage Area or Basin, Watershed).
Caution Stage :
Same as alert stage.
CFS (Cubic Feet per Second) :
The flow rate or discharge equal to one cubic foot (of
water, usually) per second. This rate is equivalent to
approximately 7.48 gallons per second. This is also referred
to as a second-foot.
Cfs-Day :
The volume of water discharged in twenty four hours, with
a flow of one cubic foot per second is widely used; 1
cfs-day is 24 x 60 x 60 = 86,000 cubic feet, 1.983471
acre-feet, or 646,317 gallons. The average flow in cubic
feet per second for any time period is the volume of flow
in cfs-days.
Channel (watercourse) :
An open conduit either naturally or artificially created
which periodically, or continuously contains moving water,
or forms a connecting link between two bodies of water.
River, creek, run, branch, anabranch, and tributary are
some of the terms used to describe natural channels. Natural
channels may be single or braided. Canal and floodway
are some of the terms used to describe artificial channels.
Channel Inflow :
Water, which at any instant, is flowing into the channel
system form surface flow, subsurface flow, base flow,
and rainfall that has directly fallen onto the channel.
Channel Lead :
An elongated opening in the ice cover caused by a water
current.
Channel Routing :
The process of determining progressively timing and shape
of the flood wave at successive points along a river.
Channelization :
The modification of a natural river channel; may include
deepening, widening, or straightening.
Closed Basin :
A basin draining to some depression or pond within its
area, from which water is lost only by evaporation or
percolation. A basin without a surface outlet for precipitation
falling precipitation.
Closed Basin Lake Flooding :
Flooding that occurs on lakes with either no outlet or
a relatively small one. Seasonal increases in rainfall
cause the lake level to rise faster than it can drain.
The water may stay at flood stage for weeks, months, or
years.
CNIF :
Calibration Network Information Files
Coastal Flooding :
Flooding that occurs from storms where water is driven
onto land from an adjacent body of water. These can be
hurricanes, "nor’easters," or tropical storms,
but even a severe winter storm or thunderstorm can cause
this type of flooding.
COE :
Corps of Engineers
Columnar Ice :
Ice consisting of columnar shaped grain. The ordinary
black ice is usually columnar-grained.
Composite Hydrograph :
A stream discharge hydrograph which includes base flow,
or one which corresponds to a net rain storm of duration
longer than one unit period.
Cone of Depression :
The depression, roughly conical in shape, produced in
a water table, or other piezometric surface, by the extraction
of water from a well at a given rate. The volume of the
cone will vary with the rate of withdrawal of water. Also
called the Cone of Influence.
Cone of Influence :
The depression, roughly conical in shape, produced in
a water table, or other piezometric surface, by the extraction
of water from a well at a given rate. The volume of the
cone will vary with rate of withdrawal of water. Also
called the Cone of Depression.
Confined Ground Water :
Ground water held under an aquiclude or an aquifuge called
artesian if the pressure is positive.
Congressional Organic Act of 1890 :
The act that assigned the responsibility of river and
floor forecasting for the benefit of the general welfare
of the Nation’s people and economy to the Weather Bureau,
and subsequently the National Weather Service.
Conservation Storage :
Storage of water for later release for usual purposes
such as municipal water supply, power, or irrigation in
contrast with storage capacity used for flood control..
Consolidated Ice Cover :
Ice cover formed by the packing and freezing together
of floes, brash ice and other forms of floating ice.
Contents :
The volume of water in a reservoir. Unless otherwise indicated
reservoir content is computed on the basis of a level
pool and does not include bank storage.
Control Points: Horizontal and Vertical :
Small monuments securely embedded in the surface of the
dam. Any movement of the monument indicates a movement
in the dam itself. Movements in the dam are detected by
comparing control points location to location of fixed
monuments located off the dam using accurate survey techniques.
Cooperative Observer :
An individual (or institution) who takes precipitation
and temperature observations-and in some cases other observations
such as river stage, soil temperature, and evaporation-at
or near their home, or place of business. Many observers
transmit their reports by touch-tone telephone to an NWS
computer, and nearly all observers mail monthly reports
to the National Climatic Data Center to be archived and
published.
Conveyance Loss :
The loss of water from a conduit due to leakage, seepage,
evaporation, or evapo-transpiration.
Corn Snow Ice :
Rotten granular ice.
County Warning Forecast Area (CWFA) :
The area (aggregation of counties, or parishes and sometimes
portions of counties, or parishes) served by a WFO for
which weather forecast and warning services are provided.
Crack :
A separation formed in an ice cover of floe that does
not divide it into two or more pieces.
Creek :
A small stream of water which serves as the natural drainage
course for a drainage basin of nominal, or small size.
The term is a relative one as to size, some creeks in
the humid section would be called rivers if they occurred
in the arid portion.
Crest :
(1)The highest stage or level of a flood wave as it passes
a point. (2)The top of a dam, dike, spillway, or weir,
to which water must rise before passing over the structure.
Crest Gage :
A gage used to obtain a record of flood crests at sites
where recording gages are installed.
Crest (Top) of Dam :
The elevation of the uppermost surface of a dam excluding
any parapet walls, railings, etc.
Crest Width (Top thickness) :
The thickness or width of a dam at the level of the crest
(top) of the dam. The term "thickness" is used
for gravity and arch dams and "width" for other
types of dams.
Critical Depth :
The depth of water flowing in an open channel or conduit,
partially filled, corresponding to one of the recognized
critical velocities.
Critical Flow :
A condition of flow where the mean velocity is at one
of the critical values; ordinarily at Belanger's critical
depth and velocity. Another important usage is in reference
to the Reynolds' critical velocities which define the
point at which the flow changes from streamline or nonturbulent
to turbulent flow.
Critical Rainfall Probability (CRP) :
The Probability that the actual precipitation during a
rainfall event has exceeded or will exceed the flash flood
guidance value.
Cross-sectional area :
Area perpendicular to the direction of flow.
CRP :
Critical Rainfall Probability. The Probability that a
given rainfall will cause a river, or stream to rise above
flood stage.
Cryology :
The science of the physical aspects of snow, ice, hail,
and sleet and other forms of water produced by temperatures
below Zero degrees Celsius.
Cubic Feet Per Second :
A unit expressing rates of discharge. One cubic foot per
second is equal to the discharge through a rectangular
cross section, 1 foot wide by 1 foot deep, flowing at
an average velocity of 1 foot per second. It is also approximately
7.48 gallons per second.
Curtain Drain :
A drain constructed at the upper end of the area to be
drained, to intercept surface or ground water flowing
toward the protected area from higher ground, and carry
it away from the area. Also called an Intercepting Drain.
Current meter :
Device used to measure the water velocity or current in
a river.
Cuttoff :
from passing through a dam’s foundation material.An impervious
construction or material which reduces seepage or prevents
it.
D
Daily Flood Peak:
The maximum mean daily discharge occuring in a stream
during a given flood event.
Dam :
Any artificial barrier which impounds or diverts water.
The dam is generally hydrologically significant if it
is:
1. 25 feet or more in height from the natural bed of the
stream and has a storage of at least 15 acre-feet.
2. Or has an impounding capacity of 50 acre-feet or more
and is at least six feet above the natural bed of the
stream.
DAMBRK :
The Dam Break Forecasting Model.
Dam Failure :
Catastrophic event characterized by the sudden, rapid,
and uncontrolled release of impounded water.
DAPM :
The Data Acquisition Program Manager.
DATACOL :
The Software System that supports RFC gateway functions.
DATANET :
Hydrologic Data Network Analysis Software.
Day-Second Feet :
Often abbreviated as DSF. See Second-Day Feet.
DCP (Data Collection Platform) :
An electronic device that connects to a river or rainfall
gage that records data from the gage and at pre-determined
times transmits that data through a satellite to a remote
computer.
DDS :
Data Distribution System.
Dead Storage:
The volume in a reservoir below the lowest controllable
level.
Deep Percolation Loss:
Water that percolates downward through the soil beyond
the reach of plant roots.
Deep Seepage:
Infiltration which reaches the water table.
Deep well:
A well whosepumping head is too great to permit use of
a suction pump.
Deformed Ice :
A general term for ice which has been squeezed together
and forced upwards and downwards in places. Subdivisions
are rated ice, ridge ice, hummocked ice, and other similar
deformations.
Degradation :
The geologic process by means of which various parts of
the surface of the earth are worn down and carried away
and their general level lowered, by the action of wind
and water.
Delta :
An alluvial deposit, often in the shape of the Greek letter
"delta", which is formed where a stream drops
its debris load on entering a body of quieter water.
Dendrites :
Thin branch-like growth of ice on the water surface.
Dendritic :
The form of the drainage pattern of a stream and it's
tributaries when it follows a treelike shape, with the
main trunk, branches, and twigs corresponding to the main
stream, tributaries, and subtributaries, respectively,
of the stream.
Density Current:
A flow of water maintained by gravity through a large
body of water, such as a reservoir or lake, and retaining
its unmixed identity because of a difference in density.
Density of Snow:
The ratio, expressed as a percentage, of the volume which
a given quantity of snow would occupy if it were reduced
to water, to the volume of the snow. When a snow sampler
is used, it is the ratio expressed as percentage of the
scale reading on the sampler to the length of the snow
core or sample.
Depletion Curve:
That part of the hydrograph extending from the point of
termination of the Recession Curve to the subsequent rise
or alternation of inflow due to additional water becomming
available for stream flow.
Depression Storage:
The volume of water contained in natural depressions in
the land surface, such as puddles.
Depth of Runoff:
The total runoff from a drainage basin, divided by its
area. For convenience in comparing runoff with precipitation,
the term is usually expressed in inches of depth during
a given period of time over the drainage area or acre-feet
per square mile.
Design Criteria :
The hypothetical flood used in the sizing of the dam and
the associated structures to prevent dam failure by overtopping,
especially for the spillway and outlet works.
Detention Basins :
Structures which are built upstream from a populated area
so that precipitation flows do not flood and cause the
loss of life or property. They are normally dry, but are
designed to detain surface water temporarily during, and
immediately after a runoff event. Their primary function
is to attenuate the storm flows by releasing flows at
a lower flow rate. There are no gates or valves allowed
on the outlet so that water can never be stored on a long-term
basis. Typical detention times in such a basin would be
on the order of 24 to 72 hours although some are as long
as 5 to 10 days.
Detention Storage:
The volume of water, other than depression storage, existing
on the land surface as flowing water which has not yet
reached the channel.
Detritus :
(1) the heavier mineral debris moved by natural watercourses,
usually in bed-load form. (2) the sand, grit, and other
coarse material removed by differential sedimentation
in a relatively short period of detention.
Diffuse Ice :
Poorly defined ice edge limiting an area of dispersed
ice; usually on the leeward side of an area of floating
ice.
Direct Flood Damage:
The damage done to property, structures, goods, etc.,
by a flood as measured by the cost of replacement and
repairs.
Direct Runoff:
The runoff entering stream channels promptly after rainfall
or snowmelt. Superposed on base runoff, it forms the bulk
of the hydrograph of a flood.
Discharge :
The rate at which water passes a given point. Discharge
is expressed in a volume per time with units of L3/T.
Discharge is often used interchangeably with streamflow.
Discharge Curve:
A curve that expresses the relation between the discharge
of a stream or open conduit at a given location and the
stage or elevation of the liquid surface at or near that
location. Also called Rating Curve and Discharge Rating
Curve.
Discharge Table:
(1) A table showing the relation between two mutually
dependant quantities or variable over a given range of
magnitude. (2) A table showing the relation between the
gage height and the discharge of a stream or conduit at
a given gaging station. Also called a Rating Table.
Distribution (Hydro)Graph:
A unit hydrograph of direct runoff modified to show the
proportions of the volume of runoff that occur during
successive equal units of time.
Diversion :
The taking of water from a stream or other body of water
into a canal, pipe, or other conduit.
Divide :
The high ground that forms the boundary of a watershed.
A divide is also called a ridge.
DNR :
Department of Natural Resources.
DOH :
Development and Operations Hydrologist.
Domestic Consumption:
The quantity, or quantity per capita, of water consumed
in a municipality or district for domestic useses or purposes
during a given period, generally one day. It is usually
taken to include all uses included withinthe term Municipal
Use of Water and quantity wasted, lost, or otherwise unaccounted
for.
Domestic Use of water:
The use of water primarily for household purposes, the
watering of livestock, the irrigation of gardens, lawns,
shrubbery, etc., surrounding a house or domicile.
Downstream Slope :
The slope or face of the dam away from the reservoir water.
This slope requires some kind of protection (e.g.; grass)
from the erosive effects of rain and surface flow.
Drainage Area :
An area having a common outlet for its surface runoff
(also see Watershed and Catchment Area).
Drainage Basin:
A part of the surface of the earth that is occupied by
a drainage system, which consists of a surface stream
or a body of impounded surface water together with all
tributary surface streams and bodies of impounded surface
water..
Drainage Density:
The relative density of natural drainage channels in a
given area. It is usually expressed in terms of miles
of natural drainage or stream channel per square mile
of area, and obtained by dividing the total length of
stream channels in the area in miles by the area in square
miles.
Drainage Divide:
The boundary line, along a topographic ridge or along
a subsurface formation, separating two adjacent drainage
basins.
Drains (Relief Wells) :
A vertical well or borehole, usually downstream of impervious
cores, grout curtains or cutoffs, designed to collect
and direct seepage through or under a dam to reduce uplift
pressure under or within a dam. A line of such wells forms
a "drainage curtain".
Drawdown :
The lowering of the surface elevation of a body of water,
the water surface of a well, the water table, or the piezometric
surface adjacent to the well, resulting from the withdrawl
of water therefrom..
Dredging :
The scooping, or suction of underwater material from a
harbor, or waterway. Dredging is one form of channel modification.
It is often too expensive to be practical because the
dredged material must be disposed of somewhere and the
stream will usually fill back up with sediment in a few
years. Dredging is usually undertaken only on large rivers
to maintain a navigation channel.
Drifting Ice :
Pieces of floating ice moving under the action of wind
and/ or currents.
Drought :
A period of abnormally dry weather sufficiently prolonged
from the lack of precipitation to cause a serious hydrologic
imbalance.
Drought Index :
Computed value which is related to some of the cumulative
effects of a prolonged and abnormal moisture deficiency.
(An index of hydrological drought corresponding to levels
below the mean in streams, lakes, and reservoirs.)
Dry Crack :
Crack visible at the surface but not going right through
the ice cover, and therefore it is dry.
Dry Floodproofing :
A dry floodproofed building is sealed against floodwaters.
All areas below the flood protection level are made watertight.
Walls are coated with waterproofing compounds or plastic
sheeting. Openings like doors windows, sewer lines and
vents are closed, wether permanently, with removable shields,
or with sandbags. The flood protection level should be
no more than 2 or 3 feet above the top of the foundation
because the buildings walls and floors cannot withstand
the pressure of deeper water.
Dry Weather Flow :
Streamflow which results from precipitation that infiltrates
into the soil and eventually moves through the soil to
the stream channel. This is also referred to as baseflow,
or ground water flow.
Duration Curve:
A cumulative frequency curve that shows the percent of
time during which specified units of items (e.g. discharge,
head, power,etc.) were equaled or exceeded in a given
period. It is the integral of the frequency diagram.
Duration of Ice Cover :
The time from freeze-up to break-up of an ice cover.
Dynamic Ice :
Pressure due to a moving ice cover or drifting ice. Pressure
occuring at movement of first contact termed Ice Impact
Pressure
Dynamic Wave Routing Model (DWOPER) :
A computerized hydraulic routing program whose algorithms
incorporate the complete one-dimensional equations of
unsteady flow originally developed by Barre’ De Saint-Venant
in 1871.
E
E-3, Flood Stage Report :
A form that a Service Hydrologist/ Hydrology Focal Point
completes to document the dates in which forecast points
are above flood stage, as well as the crest dates and
stages. Discussion of the flood event must also be included
in the E-5, Monthly Report of River and Flood conditions.
An E-3 report is sent to Regional Headquarters, the appropriate
RFC, as well as the Office of Hydrology (OH).
E-5, Monthly Report of River and Flood conditions :
A monthly narrative report covering flooding which occurred
over the past month. Flood stage, flood crest and dates
in which flooding occurred is covered within this report
for each data point which was in flood. If the flooding
involved a forecast point, an E-3 must be filled out as
well. If no flooding has occurred within the past month,
a climatic summary of the past month can be included as
well as other interesting non-flood events, such as water
supply, ice jams and the occurrence of drought. An E-5
report is sent to Regional Headquarters, the appropriate
RFC, as well as the Office of Hydrology (OH).
E-7, Flood Damage Report :
A report to be completed anytime there is reported flood
damage or loss of life as a direct result of flooding.
An E-7 report is sent to Regional Headquarters, as well
as the Office of Hydrology (OH).
E-19, Report on River Gage Station :
A report to be completed every 5 years providing a complete
history of a river station and all gages that have been
used for public forecasts since the establishment of the
station.
E-19a, Abridged Report on River Gage Station :
An abridged version of an E-19, an E-19a updates the E-19
as additional information, or changes occur at the station
during the intervening five year period. An E-19a is to
be completed anytime a significant change occurs at a
forecast point. An E-19a is also used to take the place
of an E-19 in documenting any gage history, or information
of any non-forecast point (i.e; data point).
Earthen (or Earthfill) Dam:
An embankment dam in which more than 50% of the total
volume is formed of compacted fine-grained material. A
homogeneous earthen dam is constructed of similar earthen
material throughout. These are the most common type of
dam because their construction involves using materials
in the natural state, requiring little processing.
Effective Porosity:
The ratio, usually expressed as a percentage, of the volume
of water or other liquid which a given saturated volume
of rock or soil will yield under any specified hydraulic
condition, to the given volume of soil or rock.
Effective Precipitation (Rainfall):
(1) That part of the precipitation that produces runoff.
(2) A weighted average of current and antecedent precipitation
that is "effective" in correlating with runoff.
(3) That part of the precipitation falling on an irrigated
area that is effective in meeting the consumptive use
requirements.
Effluent Seepage:
Diffuse discharge of ground water to the ground surface.
Effluent Stream :
Any watercourse in which all, or a portion of the water
volume came from the Phreatic zone, or zone of saturation
by way of groundwater flow, or baseflow.
EIF :
Enhanced IFLOWS Format.
Embankment:
Fill material, usually earth or rock, placed with sloping
sides and usually with length greater than height. All
dams are types of embankments.
Emergency Action Plan :
A predetermined plan of action to be taken to reduce the
potential for property damage and loss of life in an area
affected by a dam break or excessive spillway.
Emergency Services :
Services provided in order to minimize the impact of a
flood that is already happening. These measures are the
responsibility of city, or county emergency management
staff and the owners or operators of major, or critical
facilities. Some examples of emergency services are flood
warning and evacuation, flood response, and post flood
activities.
Energy Dissipator :
A structure which slows fast-moving spillway flows in
order to prevent erosion of the stream channel.
Engineer's Level :
A telescope which is attached to a spirit-tube level,
all revolving around a vertical axis and is mounted on
a tripod. An Engineer's Level is used for determining
the difference in elevation between two points. The telescope
on the level has a vertical cross hair and a horizontal
cross hair. Once the instrument is leveled, the sighting
through the horizontal cross hair represent a horizontal
plane of equal elevation.
Ensemble Hydrologic Forecasting :
A process whereby a continuous hydrologic model is successively
executed several times for the same forecast period by
use of varied data input scenarios, or a perturbation
of a key variable state for each model run. A common method
employed to obtain a varied data input scenario is to
use the historical meteorological record, with the assumption
that several years of observed data covering the time
period beginning on the current date and extending through
the forecast period comprises a reasonable estimate of
the possible range of future conditions.
Equilibrium Drawdown:
The ultimate, constant drawdown for a steady rate of pumped
discharge.
Equilibrium Surface Discharge:
The steady rate of surface discharge which results from
a long-continued, steady rate of net rainfall, with discharge
rate equal to net rainfall rate.
Equilibrium Time:
The time when flow conditions become substantially equal
to those corresponding to equilibrium discharge or equilibrium
drawdown..
Equi-Potential Line:
A line, in a field of flow, such that the total head is
the same for all points on th line, and therefore the
direction of flow is perpendicular to the line at all
points.
Erosion :
Wearing away of the lands by running water, glaciers,winds,
and waves, can be subdivided into three process: Corrasion,
Corrosion, and Transportation. Weathering, although sometimes
included here, is a distant process which does not imply
removal of any material.
ESP :
Extended Streamflow Prediction.
ESPINIT :
ESP Initialization Program
Esturine waters :
Deepwater tidal habitats and tidal wetlands that are usually
enclosed by land but have access to the ocean and are
at least occasionally diluted by freshwater runoff from
the land (such as bays, mouths of rivers, salt marshes,
lagoons).
Esturine Zone :
The area near the coastline that consists of esturaries
and coastal saltwater wetlands.
Estuary :
The thin zone along a coastline where freshwater systems
and rivers meet and mix with a salty ocean (such as a
bay, mouth of a river, salt marsh, lagoon).
Evaporimeter :
An instrument which measures the evaporation rate of water
into the atmosphere.
Evaporation :
Process by which liquid water is converted into water
vapor.
Evaporation Pan:
A pan used to hold water during observations for the determination
of the quantity of evaporation at a given location. Such
pans are of varying sizes and shapes, the most commonly
used being circular or square.
Evaporation Rate:
The quantity of water, expressed in terms of depth of
liquid water, which is evaporated from a given surface
per unit of time. It is usually expressed in inches depth,
per day, month, or year.
Evapotranspiration :
Combination of evaporation from free water surfaces and
transpiration of water from plant surfaces to the atmosphere.
Excess Rain:
Effective rainfall in excess of infiltration capacity.
Exclusive Flood Control Storage Capacity :
The space in a reservoir reserved for the sole purpose
of regulating flood inflows to abate flood damage.
F
Face :
The external surface of a structure, such as the surface
of a dam.
FCEXEC :
A component of the NWSRFS FCST Program.
FCST :
NWSRFS Forecast Program to produce operational forecasts
Federal Emergency Management Agency (FEMA) :
An agency of the federal government having responsibilities
in hazzard mitigation; FEMA also administers the National
Flood Insurance Program.
Federal Snow Sampler:
A snow sampler consisting of five or more sections of
sampling tubes, one which has a steel cutter on the end.
The combined snowpack measuring depth is 150 inches. This
instrument was formerly the Mount Rose Type Snow Sampling
Set.
Fetch :
The effective distance whcih waves have travelsed in open
water, from their point of origin to the point where they
break. (2) The distance of the water or the homogenous
type surface over which the wind blows without appreciable
change in direction.
Field (Moisture) Capacity :
The amount of water held in soil against the pull of gravity
Field Moisture Deficiency:
The quantity of water, which would be required to restore
the soil moisture to field moisture capacity.
Fill Dam :
Any dam constructed of excavated natural materials or
of industrial wastes.
Firn (Snow):
Old snow on top of glaciers, granular and compact and
not yet converted into ice. It is a transitional stage
between snow and ice. Also called Neve.
Firn Line:
The highest level to which the fresh snow on a glacier's
surface retreats during the melting season. The line separating
the accumulation area from the ablation area.
Fischer & Porter Punched Tape Recorder Gage:
A precipitation gage which converts weight into a code
disk position. The code disk position is then punched
on paper tape in a binary decimal format suitable for
automatic machine processing.
Flash Flood :
A flood which follows within a few hours (usually less
than 6 hours) of heavy or excessive rainfall, dam or levee
failure, or the sudden release of water impounded by an
ice jam.
Flash Flood Guidance (FFG) :
An internal product produced by the RFC's containing rainfall
threshold values which must be exceeded in order to produce
a flash flood.
Flash Flood Statement (FFS) :
A statement by the NWS which provides follow-up information
on flash flood watches and warnings.
Flash Flood Table :
A table of pre-computed forecast crest stage values for
small streams for a variety of antecedent moisture conditions
and rain amounts. Soil moisture conditions are often represented
by flash flood guidance values. In lieu of crest stages,
categorical representations of flooding, e.g., minor,
moderate, etc. may be used on the tables.
Flash Flood Warning (FFW) :
A warning by the NWS issued to warn of flash flooding
that is imminent or occurring.
Flash Flood Watch (FFA) :
A statement by the NWS that alerts communities to the
possibility of flash flooding in specified areas.
Flashboards :
A length of timber, concrete, or steel placed on the crest
of a spillway to raise the retention water level but which
may be quickly removed in the event of a flood by a tripping
device, or by deliberately designed failure of the flashboard
or its supports.
Float Recording Precipitation gage :
A rain gage where the rise of a float within the instrument
with increasing rainfall is recorded. Some of these gages
must be emptied manually, while others employ a self-starting
siphon to empty old rainfall amounts.
Floc :
A cluster of frazil particles
Floe :
An accumulation of frazil flocs (also known as a "pan")
or a single piece of broken ice.
Flood :
The inundation of a normally dry area caused by high flow,
or overflow of water in an established watercourse, such
as a river, stream, or drainage ditch ; or ponding of
water at or near the point where the rain fell. This is
a duration type event with a slower onset than flash flooding,
normally greater than 6 hours.
Flood Control Storage:
Storage of water in reservoirs to abate flood damage.
Flood Crest :
The Maximum height of a flood wave as it pases a location.
Flood Frequency Curve:
(1) A graph showing the number of times per year on the
average, plotted as abscissa, that floods of magnitude,
indicated by the ordinate, are equaled or exceeded. (2)
A similar graph but with recurrence intervals of floods
plotted as abscissa.
Flood Loss Reduction Measures :
The strategy for reducing flood losses. There are four
basic strategies. They are prevention, property protection,
emergency services, and structural projects. Each strategy
incorporates different measures that are appropriate for
different conditions. In many communities, a different
person may be responsible for each strategy.
Flood of Record :
The highest observed river stage or discharge at a given
location during the period of record keeping. (Not necessarily
the highest known stage.)
Flood Plain :
The portion of a river valley that has been inundated
by the river during historic floods.
Flood Plain Information Studies :
Reports usually prepared by the U.S. Army Corps of Engineers
(USACE) following a survey of a flood-impacted community.
Flood Potential Outlook (ESF on AFOS) (FPO for Acronym)
:
An NWS outlook that is issued to alert the public of potentially
heavy rainfall that could send area rivers and streams
into flood or aggravate an existing flood.
Flood Prevention :
Measures that are taken in order to keep flood problems
from getting worse. Planning, land acquisition, river
channel maintenance, wetlands protection, and other regulations
all help modify development on floodplains and watersheds
to reduce their susceptibility to flood damage. Preventive
measures are usually administered by the building, zoning,
planning and/ or code enforcement offices of the local
government.
Flood Problems :
Problems and damages that occur during a flood as a result
of human development and actions. Flood problems are a
result from: 1) Inappropriate development in the floodplain
(e.g., building too low, too close to the channel, or
blocking flood flows); 2) Development in the watershed
that increases flood flows and creates a larger floodplain,
or; 3) A combination of the previous two.
Flood Profile:
A graph of elevation of the water surface of a river in
flood, plotted as ordinate, against distance, measured
in the downstream direction, plotted as abscissa. A flood
profile may be drawn to show elevation at a given time,
crests during a particular flood, or to show stages of
concordant flows.
Flood Routing :
Process of determining progressively the timing, shape,
and amplitude of a flood wave as it moves downstream to
successive points along the river.
Flood Stage :
A gage height at which a watercouse overtops its banks
and begins to cause damage to any portion of the defined
reach. Flood stage is usually higher than or equal to
bankful stage.
Flood Statement (FLS) :
A statement issued by the NWS to inform the public of
flooding along major streams in which there is not a serious
threat to life or property. It may also follow a flood
warning to give later information.
Flood Warning (FLW) :
A release by the NWS to inform the public of flooding
along larger streams in which there is a serious threat
to life or property. A flood warning will usually contain
river stage (level) forecasts.
Flood Wave :
A rise in streamflow to a crest and its subsequent recession
caused by precipitation, snowmelt, dam failure, or reservoir
releases.
Flooded Ice :
Ice which has been flooded by melt water or river water
and is heavily loaded by water and wet snow.
Floodproofing :
The process of protecting a building from flood damage
on site. Floodproofing can be divided into wet and dry
floodproofing. In areas subject to slow-moving, shallow
flooding, buildings can be elevated, or barriers can be
constructed to block the water’s approach to the building.
These techniques have the advantage of being less disruptive
to the neighborhood. It must be noted that during a flood,
a floodproofed building may be isolated and without utilities
and therefore unusable, even though it has not been damaged.
Floodwall :
A long, narrow concrete, or masonry embankment usually
built to protect land from flooding. If built of earth
the structure is usually referred to as a levee. Floodwalls
and levees confine streamflow within a specified area
to prevent flooding. The term "dike" is used
to describe an embankment that blocks an area on a reservoir
or lake rim that is lower than the top of the dam.
Floodway :
(1) A part of the flood plain, otherwise leveed, reserved
for emergency diversion of water during floods. A part
of the flood plain which, to facilitate the passage of
floodwater, is kept clear of encumbrances.(2) The channel
of a river or stream and those parts of the flood plains
adjoining the channel, which are reasonably required to
carry and discharge the floodwater or floodflow of any
river or stream.
Flow Duration Curve:
A cumulative frequency curve that shows the percentage
of time that specified discharges are equaled or exceeded..
Flowing Well :
a well drilled into a confined aquifer with enough hydraulic
pressure for the water to flow to the surface without
pumping. Also called an Artesian well.
Forebay :
The water behind (upstream) of the dam.
Forecast Crest :
The highest elevation of river level, or stage, expected
during a specified storm event.
Forecast Point :
A location that represents an area (reach of a river),
where a forecast is made available to the public. Each
NWS river forecast point has an associated E-19a, Abridged
Report on River Gage Station, and E-19, Report on River
Gage Station.
Foresight :
A sighting on a point of unknown elevation from an instrument
of known elevation. To determine the elevation of the
point in question, the foresight is subtracted from the
height of the instrument.
Fountainhead:
The upper end of a confined-aquifer conduit, where it
initersects the land surfcae.
Fracture :
Any break or rupture formed in an ice cover or floe due
to deformation.
Fracture Zone :
An area which has a great number of fractures.
Fracturing :
Deformation process whereby ice is permanently deformed,
and fracture occurs.
Frazil Ice :
Fine spicules, plates, or discoids of ice suspended in
water. In rivers and lakes, frazil is formed in supercooled,
turbulent water.
Frazil Slush :
An agglomerate of loosely packed frazil which floats or
accumulates under the ice cover.
Free Ground Water:
Unconfined ground water whose upper boundary is a free
water table.
Freeboard:
The vertical distance between the normal maximum level
of the water surface in a channel, resrvoir, tank, canal,
etc., and the top of the sides of a levee, dam, etc.,
which is provided so that waves and other movements of
the liquid will not overtop the confining structure.
Freezeup date :
Date on which the water body was first observed to be
completely frozen over.
Freezup jam :
Ice jam formed as frazil ice accumulates and thickens.
French Drain:
An underground passageway for water through the interstices
among stones placed loosely in a trench.
Frequency Curve:
A curve that expresses the relation between the frequency
distribution plot, with the magnitude of the variables
as abscissas and the number of occurrences of each magnitude
in a given period as ordinates. The theoretical frequency
curve is a derivative of the probability curve.
Friction Head:
The decrease in total head caused by friction.
G
Gage :
(1) A device for indicating the magnitude or position
of a thing in specific units, when such magnitude or position
undergoes change, for example: The elevation of a water
surface, the velocity of flowing water, the pressure of
water, the amount or intensity of precipitation, the depth
of snowfall, etc. (2) The act or operation of registering
or measuring the magnitude or position of a thing when
these characteristics are undergoing change. (3) The operation,
including both field and office work, of measuring the
discharge of a stream of water in a waterway.
Gage Datum :
The arbitrary zero datum elevation which all stage measurements
are made from.
Gage Height :
The water-surface elevation referred to some arbitrary
gage datum. Gage height is often used interchangeably
with the more general term stage, although gage height
is more appropriate when used with a reading on a gage.
Gage Zero :
The elevation of zero stage. (Same as gage datum.)
Gaging Station :
A particular site on a watercourse where systematic observations
of stage/ and or flow are measured.
Gallery :
A passageway within the body of a dam or abutment.
Gate :
A device in which a leaf or member is moved across the
waterway from an external position to control or stop
flow. There are many different kinds of gates used on
a dam. Some include:
Bulkhead, Crest (or Spillway), Emergency, Fixed Wheel,
Flap, Flood, Guard, Outlet, Radial, Regulating, and Slide
Gates.
Geohydrology:
That branch of hydrology relating to subsurface, or subterranean
waters.
Geophysics:
The study of the physical characteristics and properties
of the earth; including geodesy, seismology, meteorology,
oceanography, atmospheric electricity, terrestrial magnetism,
and tidal phenomena.
Glacier:
Bodies of land ice that consist of recrystallized snow
accumulated on the surface of the ground, and that move
slowly downslope.
Glacier Dammed Lake:
The lake formed when a glacier flows across the mouth
of an adjoining valley and forms an ice dam.
GOES (Geostationary Orbiting Environmental Satellite):
Satellite orbiting at 22,000 miles above the earth surface
that remains above the same location on the earth. DCPs
transmit river and rainfall data to the GOES for relay
to a ground-receive site in Wallops Island, VA.
GOES DCS (Data Collection System):
A data collection system under NESDIS which is comprised
of the DCPs, and the NESDIS Command and Data Acquisition
(CDA) System components. This satellite-based system collects
a variety of environmental data from locations in the
Western Hemisphere. The system is a data relay network
for more than 10,000 DCPs which transmits data to one
of two GOES satellites (East and West). These data are
relayed to the NESDIS CDA ground station located at Wallops
Island, VA. The data are then relayed over to Silver Springs,
MD, where the data is then distributed to the appropriate
recipients.
Gravity Dam :
A concrete structure proportioned so that its own weight
provides the major resistance to the forces exerted on
it.
Ground receive sites:
A satellite dish and associated computer which receives
signals from the GOES satellite, decodes the information,
and transmits it to a another site for further processing.
The GOES satellite ground-receive site is located at Wallops
Island, VA; and the information is relayed to a mainframe
computer at NWSH for processing.
Ground Water:
Water within the earth that supplies wells and springs;
water in the zone of saturation where all openings in
rocks and soil are filled, the upper surface of which
forms the water table. Also termed Phreatic water.
Grounded ice:
Ice that has run aground or is contact with the ground
underneath it.
Ground Water Divide:
A line on a water table where on either side of which
the water table slopes downward. It is analogous to a
drainage divide between two drainage basins on a land
surface.
Ground Water Hydrology:
The branch of hydrology that specializes in ground water;
its occurrence and movements; its replenishment and depletion;
the properties of rocks that control ground water movement
and storage; and the methods of investigation and utilization
of ground water
Ground Water Flow:
Streamflow which results from precipitation that infiltrates
into the soil and eventually moves through the soil to
the stream channel. This is also referred to as baseflow,
or dry-weather flow.
Ground Water Mining:
Pumping ground water from a basin where the safe yield
is very small, thereby extracting ground water which had
accumulated over a long period of time.
Ground Water Outflow:
That part of the discharge from a drainage basin that
occurs through the ground water. The term "underflow"
is often used to describe the ground water outflow that
takes place in valley alluvium (instead of the surface
channel) and thus is not measure at a gaging station.
Ground Water Overdraft:
Pumpage of ground water in excess of safe yield.
Ground Water Runoff:
That part of the runoff which has passed into the ground,
has become ground water, and has been discharged into
a stream channel as spring, or seepage water.
Grout Curtain :
A barrier produced by injecting grout into a vertical
zone, usually narrow (horizontally), and in the foundation
to reduce seepage under a dam.
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