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APPENDIX F
Glossary
Several terms used in this report are peculiar to or most often used in rather
specialized technical disciplines. Others are used in special, restricted senses.
Some such terms are defined when first used; others are not. For convenience
of the reader a number of those technical terms are defined here.
DAM A barrier built across a watercourse for impounding or diverting the
flow of water.
DRAINAGE AREA The area that drains naturally to a particular point on a
river.
EMERGENCY ACTION PLAN A predetermined plan of action to be taken
to reduce the potential for property damage and loss of lives in an area
affected by a dam break.
EPICENTER That point on the earth's surface that is directly above the
focus of an earthquake.
FAILURE An incident resulting in the uncontrolled release of water from a
dam.
FLOOD PLAIN An area adjoining a body of water or natural stream that
has been or may be covered by floodwater.
FLOOD ROUTING The determination of the modifying or attenuating
effect of passage of a flood through a valley, channel, or reservoir.
FOCUS (HYPOCENTER) The point within the earth that is the center of an
earthquake and the origin of its elastic waves.
FREEBOARD The vertical distance between a stated water level and the
top of a dam.
255
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~ 256
Appendix F
HAZARD A source of danger. In other words, something that has the po-
tential for creating adverse consequences.
HYDROGRAPH A graphical representation of discharge, stage, or other
hydraulic property with respect to time for a particular point on a stream.
(At times the term is applied to the phenomenon the graphical representa-
tion describes; hence, a flood hydrograph is the passage of a flood dis-
charge past the observation point. ~
INTENSITY SCALE An arbitrary scale to describe the degree of shaking at a
particular place. The scale is not baser] on measurement but on a descrip-
tive scale by an experienced observer. Several scales are used (e.g., the
Modified Mercalli scale, the MSK scale), all with grades indicated by
Roman numerals from I to XII.
LOADING CONDITIONS Events to which the dam is exposed, e. g., earth-
quake, flood, gravity loading.
MAGNITUDE (see also RICHTER SCALE) A rating of the size of an earth-
quake by numerical values, such as MS.6, M8.2, etc. The magnitude
number is calculated by means of the logarithm of the amplitude of matri-
ces recorded by a standard seismograph at a known distance from the
origin of the earthquake. Each higher whole number expresses an amount
of energy released that is approximately 60 times larger than that ex-
pressed by the preceding whole number, for example an Me earthquake
will release about 60 times the energy of an MS earthquake.
MAXIMUM CREDIBLE EARTHQUAKE (MCE) The severest earthquake
that is believed to be possible at the site on the basis of geologic and
seismological evidence. It is determined by regional and local studies that
include a complete review of all historic earthquake data of events suffi-
ciently nearby to influence the project, all faults in the area, and attenua-
tions from causative faults to the site.
ONE-HUNDRED-YEAR (100-YEAR) EXCEEDANCE INTERVAL FLOOD
The flood magnitude expected to be equaled or exceeded on the average of
once in 100 years. It may also be expressed as an exceedance frequency
with a 1 percent chance of being exceeded in any given year.
OPERATING BASIS EARTHQUAKE More moderate than the MCE and
may be selected on a probabilistic basis from regional and local geology
and seismology studies as being likely to occur during the life of the proj-
ect. Generally, it is at least as large as earthquakes that have occurred in
the seismotectonic province in which the site is located.
PROBABILITY The likelihood of an event's occurring.
PROBABLE MAXIMUM FLOOD (PMF) The flood that may be expected
from the most severe combination of critical meteorologic and hydrologic
conditions that are reasonably possible in the region. This term as used in
official documents of the Corps of Engineers identifies estimates of hypo-
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Appendix F
257
thetical floor! characteristics (peak discharge, volume, and hydrograph
shape) that are considered to be the most severe "reasonably possible" at a
particular location, based on relatively comprehensive hydrometeorolo-
gical analyses of critical runoff-producing precipitation (and snowmelt, if
pertinent) ant] hydrologic factors favorable for maximum floor] runoff.
(ONE-HALF PMF That flood with a peak flow equal to one-half of the
peak flow of a probable maximum flood.)
PROBABLE MAXIMUM PRECIPITATION (PMP) Theoretically, the greatest
depth of precipitation for a given duration that is physically possible over
a given size storm area at a particular geographical location at a certain
time of the year. (This definition is a 1982 revision and results from mutual
agreement among the National Weather Service, the U.S. Army Corps of
Engineers, and the Bureau of Reciamation.)
PSEUDO-ADIABATIC A term applier! to a vertical temperature variation
in the atmosphere in which the temperature at any elevation is that which
would be attained if a unit mass of air at ground] surface were carried aloft
to that elevation and allowed to expanc] to ambient pressure without loss
of heat.
RESERVOIR ROUTING The computation by which the interrelated effects
of the inflow hydrography, reservoir storage and discharge from the reser-
voir are evaluated.
RICHTER SCALE A scale proposed by C. F. Richter to describe the magni-
tude of and earthquake by measurements made in well-defined conditions
and with a given type of seismograph. The zero of the scale is fixed arbi-
trarily to fit the smallest recorded earthquakes. The largest recorded
earthquake magnitudes are near 8.7 and are the result of observations and
not an arbitrary upper limit like that of the intensity scale.
RISK The likelihood of adverse consequences.
RISK ASSESSMENT As applied to clam safety, the process of identifying the
likelihood and consequences of dam failure to provide the basis for in-
formed decisions on a course of action.
RISK COST (EXPECTED COST OF FAILURE)
the monetary consequences of failure.
SAFETY EVALUATION EARTHQUAKE (SEE) The earthquake, expressed
in terms of magnitude anal closest distance from the dam site, or in terms of
the characteristics of the time history of free-field ground motions, for
which the safety of the dam and critical structures associated with the
dam is to be evaluated. In many cases this earthquake will be the maxi-
mum credible earthquake to which the dam will be exposed. However, in
other cases, where the possible sources of ground motion are not readily
apparent, it may be a motion with prescribed characteristics selected on
the basis of a probabilistic assessment of the ground motions that may
The product of the risk and
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258 Appendix F
occur in the vicinity of the dam. It should be demonstrated that the dam
can withstand this level of earthquake shaking without release of water
from the reservoir.
SAFETY EVALUATION FLOOD (SEF) The largest reasonable hypothetical
water inflow for which the safety of a dam and appurtenant structures is
to be evaluated; it should be demonstrated that this flood level can be
accommodates] through storage, spillway releases, releases through other
outlet works or limited and acceptable overtopping without causing fail-
ures of the structure or release of impounded water.
SEISMIC INTENSITY See intensity scale.
SPILLWAY A structure over or through which flood flows are clischarged. If
the flow is controlled by gates, it is considered a controlled spillway; if the
elevation of the spillway crest is the only control, it is considered an
uncontrolled spillway.
SPILLWAY DESIGN FLOOD (SDF) The largest flood that a given project is
designed to pass safely.
STORAGE The retention of water or delay of runoff either by planned
operation, as in a reservoir, or by temporary filling of overflow areas, as in
the progression of a flood crest through a natural stream channel.
STORAGE RESERVOIR A reservoir that is operated with changing water
level for the purpose of storing and releasing water.
UNIT HYDROGRAPH The hydrograph of flow from a watershed produced
by a unit volume of runoff generated during a specific period of rainfall
excess in the watershed. In the United States a unit hydrograph normally
represents 1 inch of runoff volume. Hence, a 6-hour unit hyclrograph
represents runoff from the watershed resulting from rainfall excess (rain-
fall minus infiltration and other losses) of 1 inch over a 6-hour period.
Representative terms from entire chapter:
intensity scale
