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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.