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11 Recommended Glossary Marty agencies at various levels of government, utilities, professional orga- nizations, and other entities have responsibilities and other interests in the design, construction, and safe operation of dams. Consequently, the no- menclature that has developed over time by the large number of entities and for different purposes in association with dams is not always consis- tent. Inconsistencies range from subtle to contradictory. These differences in definition have occasionally resulted in confusion even among profes- sional engineers involved in dam safety. In fact, the National Research Council's Committee on Safety of Nonfederal Dams recommended in its 1982 report* to FE MA that "FEMA, with the help of ICODS, should de- velop a glossary of common terms for use in dam safety activities." The present committee, based on its review of the various sources of nomencla- ture and the members' experience, recommends the following glossary of terms to assist FEMA in implementing this recommendation. This glossary is based principally on the International Commission on Large Dams (ICOLD) Technical Dictionary on Dams, 1978. Many of the ICOLD terms were modified or supplemented to conform more closely to American us- age. Terms, not found in the ICOLD glossary, were supplied by committee members. It is noted that all of these terms are not contained in the body of the present report. *National Research Council, Committee on Safety of Nonfederal Dams (1982) Safety of Nonfe- deral Dams: A Review of the Federal Role, National Academy Press, Washington, D.C. 309

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310 Recommended Glossary ABUTMENT That part of the valley side against which the dam is con- structed. An artificial abutment (see Block) is sometimes constructed, as a concrete gravity section, to take the thrust of an arch dam where there is no suitable natural abutment. Right and left abutments are those on respective sides of an observer when viewed looking downstream. ACTIVE STORAGE The volume of the reservoir that is available for use either for power generation, irrigation, flood control, or other purposes. Active storage excludes flood surcharge. It is the reservoir capacity less inactive and dead storages. The terms useful storage or usahie storage or working storage are sometimes used instead of active storage but are not recommended. ADIT Tunnel for exploratory or test purposes; opening in the face of a dam for access to galleries or operating chambers; or access tunnel to a tunnel for construction or maintenance purposes. AFTERBAY DAM (REREGULATING DAM) A dam constructed to reg- ulate the discharges from an upstream power plant. AMBURSEN DAM See Buttress Dam. ANCHOR BLOCK See Block. APPURTENANT STRUCTURES Refers to ancillary features of a dam, such as the outlet, spillway, powerhouse, tunnels, etc. AQUEDUCT An artificial way of conveying water, i.e., by canal, pipe, or tunnel; hence the terms connecting aqueduct and diversion aqueduct. ARCH DAM A concrete or masonry dam that is curved in plan so as to transmit the major part of the water load to the abutments. ARCH CENTERLINE The locus of all midpoints of the thickness of an arch section. ARCH ELEMENT That portion of a dam bounded by two horizontal planes spaced 1 foot apart. CANTILEVER ELEMENT That portion of an arch dam that is contained within the vertical planes normal to the extrados and spaced 1 foot apart at the axis. CONSTANT ANGLE ARCH DAM An arch dam in which the angle subtended by any horizontal section is constant throughout the whole height of the dam. CONSTANT RADIUS ARCH DAM An arch dam in which every horizon- tal segment or slice of the dam has approximately the same radius of curvature. CROWN CANTILEVER That cantilever element located at the point of maximum depth in the canyon. DOUBLE CURVATURE ARCH DAM An arch dam that is curved vertically as well as horizontally. FILLET An increase in thickness of a dam beginning near and extending

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Recommended Glossary 311 to the abutments of the arches or base of cantilevers. Usually placed at the downstream face. LENGTH OF ARCH The distance along a curve that is concentric with the extrados and passes through the midpoint of the arch thickness at the crown. LINE OF CENT=S The loci of centers for circular arcs used to describe a face of a dam or a portion thereof. ARCH BUTTRESS DAM See Buttress Dam. ARCH GRAVITY DAM See Gravity Dam. AUXILIARY SPILLWAY See Spillway. AXIS OF DAM The plane or curved surface, arbitrarily chosen by a de- signer, appearing as a line, in plan or in cross section, to which the hori- zontal dimensions of the dam can be referred. BACKWATER CURVE The longitudinal profile of the water surface in an open channel where the depth of flow has been increased by an ob- struction such as a weir or dam across the channel, or by an increase in channel roughness, by a decrease in channel width, or by a flattening of the bed slope. BAFFLE BLOCK See Block. BANK STORAGE (GROUND STORAGE) See Storage. BASE WIDTH (BASE THICKNESS) The maximum thickness or width of a dam measured horizontally between upstream and downstream faces and normal to the axis of the dam but excluding projections for outlets, etc. BATTER Angle of inclination from the vertical. BERM A horizontal step or bench in the sloping profile of an embank- ment dam. BLANKET DRAINAGE BLANKET A drainage layer placed directly over the founda- tion material. GROUT BLANKET See Consolidation Grouting. UPSTREAM BLANKET An impervious layer placed on the reservoir floor upstream of a dam. In the case of an embankment dam the blanket may be connected to the impermeable element in the dam. BLOCK BAFFLE BLOCK (IMPACT BLOCK) A block of concrete or concrete and steel constructed in a channel or stilling basin to dissipate the energy of wa- ter flowing at high velocity. CHUTE BLOCK A baffle block constructed in a spillway chute. THRUST BLOCK (ANCHOR BLOCK) A massive block of concrete built to withstand a thrust or pull from an arch dam. BULKHEAD GATE See Gate.

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312 Recommended Glossary BUTTRESS DAM A dam consisting of a watertight upstream face sup- ported at intervals on the downstream side by a series of buttresses. But- tress dams can take many forms. ARCH BUTTRESS DAM (CURVED BUTTRESS DAM) curved in plan. A buttress dam that is MULTIPLE ARCH DAM A buttress dam the upstream part of which com- prises a series of arches. FLAT SLAB DAM (AMBURSEN DAM) (DECK DAM) A buttress dam in which the upstream part is a relatively thin flat slab usually made of rein- forced concrete. SOLID HEAD BUTTRESS DAM A buttress dam in which the upstream end of each buttress is enlarged to span the gap between buttresses. The terms round head, diamond head, tee head refer to the shape of the upstream enlargement. CELLULAR GRAVITY DAM See Gravity Dam. COFFERDAM A temporary structure enclosing all or part of the con- struction area so that construction can proceed in the dry. A diversion cofferdam diverts a river into a pipe, channel, or tunnel. CONCRETE LIFT In concrete work the vertical distance between suc- cessive horizontal construction joints. CONDUIT A closed channel to convey the discharge through or under a dam. CONSOLIDATION GROUTING (BLANKET GROUTING) Consoli- dating a layer of the foundation to achieve greater impermeability and/ or strength by injecting grout. CONSTRUCTION JOINT The interface between two successive plac- ings or pours of concrete where bond, not permanent, separation is intended. CONTACT GROUTING Filling with cement grout any voids existing at the contact of two zones of different materials, e.g., between a concrete tunnel lining and the surrounding rock. The grouting operation is usu- ally carried out at low pressure. CORE (IMPERVIOUS CORE) (IMPERVIOUS ZONE) A zone of mate- rial of low permeability in an embankment dam; hence the terms central core, inclined core, puddle clay core, and rolled clay core. CORE WALL A wall built of impervious material, usually of concrete or asphaltic concrete in the body of an embankment dam to prevent leak- age. See also Membrane or Diaphragm. CREST GATE See Gate. CREST LENGTH The developed length of the top of the dam. This in- cludes the length of spillway, powerhouse, navigation lock, fish pass, etc., where these structures form part of the length of the dam. If de- tached from the dam these structures should not be included.

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Recommended Glossary 313 CREST OF DAM The crown of an overflow section of the dam. In the United States, the term crest of dam is often used when top of dam is intended. To avoid confusion, the terms crest of spillway and top of dam should be used for referring to the overflow section and dam proper, respectively. CRIB DAM A gravity dam built up of boxes, cribs, crossed timbers, or gabions and filled with earth or rock. CULVERT (a) A drain or waterway structure built transversely under a road, railway, or embankment. A culvert usually comprises a pipe or a covered channel of box section. (b) A gallery or waterway constructed through any type of dam, which is normally dry but is used occasionally for discharging water; hence the terms scour culvert, drawoff culvert, and spillway culvert. CURTAIN GROUT CURTAIN (GROUT CUTOFF) A barrier produced by injecting grout into a vertical zone, usually narrow in horizontal width, in the foun- dation to reduce seepage under a dam. DRAINAGE CURTAIN See Drainage Wells. CURVED BUTTRESS DAM (ARCH BUTTRESS DAM) See Buttress Dam. CURVED GRAVITY DAM See Gravity Dam. CUTOFF An impervious construction by means of which seepage is re- duced or prevented from passing through foundation material. CUTOFF TRENCH The excavation later to be filled with impervious material so as to form the cutoff. Sometimes used incorrectly to describe the cutoff itself. CUTOFF WALL A wall of impervious material (e.g., concrete, asphal- tic concrete, steel sheet piling) built into the foundation to reduce seep- age under the dam. DAM A barrier built across a watercourse for impounding or diverting the flow of water. DEAD STORAGE The storage that lies below the invert of the lowest outlet and that, therefore, cannot be withdrawn from the reservoir. DESIGN FLOOD See Spillway Design Flood. DIAMOND HEAD BUTTRESS DAM See Buttress Dam. DIAPHRAGM See Membrane. DIKE (LEVEE) A long low embankment. The height is usually less than 4 to 5 meters and the length more than 10 or 15 times the maximum height. Usually applied to embankments or structures built to protect land from flooding. If built of concrete or masonry the structure is usu- ally referred to as a flood wall. Also used to describe embankments that block areas on the reservoir rim that are lower than the top of the main dam and that are quite long.

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314 Recommended Glossary In the Mississippi River basin, where the old French word levee has sur- vived, this now applies to flood protecting embankments whose height can average up to 10 to 15 meters. DIVERSION CHANNEL, CANAL, OR TUNNEL A waterway used to divert water from its natural course. The term is generally applied to a temporary arrangement, e.g., to bypass water round a dam site during construction. Channel is normally used instead of canal when the water- way is short. Occasionally the term is applied to a permanent arrange- ment (diversion canal, diversion tunnel, diversion aqueducts). DOLOSSE A precast concrete shape, named after the knucklebone of a sheep. Dolosses are placed randomly in an interlocking pattern and are extremely effective in dissipating the energy of waves or flowing water. DRAINAGE AREA The area that drains naturally to a particular point on a river. DRAINAGE LAYER OR BLANKET A layer of pervious material in a dam to relieve pore pressures or to facilitate drainage of the fill. DRAINAGE WELLS (RELIEF WELLS) Vertical wells or boreholes usually downstream of impervious cores, grout curtains, or cutoffs, de- signed to collect and control seepage through or under a dam so as to reduce uplift pressures under or within a dam. A line of such wells forms a drainage curtain. DRAWDOWN The resultant lowering of water surface level due to re- lease of water from the reservoir. EARTH DAM OR EARTHFILL DAM See Embankment Dam. EMBANKMENT Fill material, usually earth or rock, placed with slop- ing sides. EMBANKMENT DAM (FILL DAM) Any dam constructed of excavated natural materials or of industrial waste materials. EARTH DAM (EARTHFILL DAM) An embankment dam in which more than 50 % of the total volume is formed of compacted fine-grained material obtained from a borrow area. HOMOGENEOUS EARTHFILL DAM An embankment dam constructed of similar earth material throughout, except for possible inclusion of in- ternal drains or drainage blankets. Used to differentiate from a zoned earthfill dam. HYDRAULIC FILL DAM An embankment dam constructed of materials, often dredged, that are conveyed and placed by suspension in flowing water. ROCKFILL DAM An embankment dam in which more than 50 % of the total volume comprises compacted or dumped pervious natural or crushed rock. ROLLED FILL DAM An embankment dam of earth or rock in which the

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Recommended Glossary 315 material is placed in layers and compacted by using rollers or rolling equipment. ZONED EMBANKMENT DAM An embankment dam the thickness of which is composed of zones of selected materials having different degrees of porosity, permeability, and density. 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. EMERGENCY GATE A standby or reserve gate used only when the nor- mal means of water control is not available. EMERGENCY SPILLWAY See Spillway. ENERGY DISSIPATOR Any device constructed in a waterway to reduce or destroy the energy of fast-flowing water. ENERGY DISSIPATING VALVE A generic term used to describe those regulating valves that are designed to dissipate as much energy as possi- ble through the valve. Included are Howell-Bunger valves and Hollow Jet valves. EPICENTER That point on the earth's surface that is directly above the focus of an earthquake. EXTRADOS The curved upstream surface of an arch clam. FACE With reference to a structure, the external surface that limits the structure, e.g., the face of a wall or dam. FACING With reference to a wall or concrete dam, a coating of a differ- ent material, masonry or brick, for architectural or protection purposes, e.g., stonework facing, brickwork facing. With reference to an embank- ment dam, an impervious coating or face on the upstream slope of the dam. FAILURE An incident resulting in the uncontrolled release of water from an operating dam. FETCH The straight line distance between a dam and the farthest reser- voir shore. The fetch is one of the factors used in calculating wave heights in a reservoir. FILTER (FILTER ZONE) A band or zone of granular material that is incorporated in a dam and is graded (either naturally or by selection) so as to allow seepage to flow across or down the filter without causing the migration of material from zones adjacent to the filter. FINGER DRAINS A series of parallel drains of narrow width (instead of a continuous drainage blanket) draining to the downstream toe of the embankment dam. FIXED CREST WEIR See Weir. FLASHBOARDS Lengths of timber, concrete, or steel placed on the crest of a spillway to raise the retention water level but that may be quickly

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316 Recommended Glossary removed in the event of a flood either by a tripping device or by deliber- ately designed failure of the flashboards or their supports. FLAT SLAB DAM See Buttress Dam. FLIP BUCKET (SKIJUMP SPILLWAY) The downstream end of a spill- way shaped such that water flowing at high velocity is deflected upward in a trajectory away from the end of the spillway. FLOOD PLAIN An area adjoining a body of water or natural stream that has been or may be covered by flood water. FLOOD PLAIN MANAGEMENT A management program to reduce the consequences of flooding, either by natural runoff or by dam failure, to properties in a flood plain, both existing and future. FLOOD ROUTING The determination of the attenuating effect of stor- age on a flood passing through a valley, channel, or reservoir. FLOOD STORAGE See Storage. FLOOD SURCHARGE The volume or space in a reservoir between the controlled retention water level and the maximum water level. Flood surcharge cannot be retained in the reservoir but will flow over the spill- way until the controlled retention water level is reached. (The term wet freeboard for describing the depth of flood surcharge is not recom- mended, see Freeboard.) FLOOD WALL A concrete wall constructed adjacent to a stream for the purpose of preventing flooding of property on the landside of the wall; normally constructed in lieu of or to supplement a levee where the land required for levee construction is more expensive or not available. FOCUS (HYPOCENTER) The point within the earth that is the center of an earthquake and the origin of its elastic waves. FOUNDATION OF DAM The natural material on which the dam struc- ture is placed. FREEBOARD The vertical distance between a stated water level and the top of a dam. NET FREEBOARD, DRY FREEBOARD, FLOOD ~EBOA~, or ~SID- UAL ~BOA~ iS the vertical distance between the estimated maximum water level and the top of a dam. GROSS ~EBOA~ or TOTAL ~EBOA~ iS the vertical distance between the maximum planned controlled reten- tion water level and the top of a dam. (That part of the GROSS FREEBOARD attributable to the depth of flood surcharge is sometimes referred to as the wEr FREEBOARD, but this term is not recommended as it is preferable that freeboard be stated with reference to the top of a dam.) FUSE PLUG SPILLWAY See Spillway. GABION A prefabricated basket of rock within a wire cage that is free draining and capable of being stacked. GABION DAM Name given to a crib dam when built of gabions.

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Recommended Glossary 317 GALLERY (a) A passageway within the body of a dam or abutment; hence the terms grouting gallery, inspection gallery, and drainage gat- lery. (b) A long and rather narrow hall; hence the following terms for a power plant: valve gallery, transformer gallery, and busbar gallery. GATE In general, a device in which a leaf or member is moved across the waterway from an external position to control or stop the flow. BASCULE GATE See Flap Gate. BULKHEAD GATE A gate used either for temporary closure of a channel or conduit before dewatering it for inspection or maintenance or for closure against flowing water when the head difference is small, e. g., for diversion tunnel closure. Although a bulkhead gate is usually opened and closed under nearly balanced pressures, it nevertheless may be capable of withstanding a high differential head when in the closed position. CREST GATE (SPILLWAY GATE) A gate on the crest of a spillway to control overflow or reservoir water level. DRUM GATE A type of spillway gate consisting of a long hollow drum. The drum may be held in its raised position by the water pressure in a flotation chamber beneath the drum. EMERGENCY GATE A standby or reserve gate used only when the normal means of water control is not available. FIXED WHEEL GATE (FIXED ROLLER GATE) (FIXED AXLE GATE) A gate having wheels or rollers mounted on the end posts of the gate. The wheels bear against rails fixed in side grooves or gate guides. FLAP GATE A gate hinged along one edge, usually either the top or bot- tom edge. Examples of bottom-hinged flap gates are tilting gates and fish belly gates so called from their shape in cross section. FLOOD GATE A gate to control flood release from a reservoir. GUARD GATE (GUARD VALVE) G ate or valve that operates fully open or closed. May function as a secondary device for shutting off the flow of water in case the primary closure device becomes inoperable. Usually operated under balanced pressure no-flow conditions, except for clo- . sure in emergencies. OUTLET GATE A gate controlling the outflow of water from a reservoir. RADIAL GATE (TAINTER GATE) A gate with a curved upstream plate and radial arms hinged to piers or other supporting structure. REGULATING GATE (REGULATING VALVE) A gate or valve that operates un- der full pressure and flow conditions to throttle and vary the rate of discharge. ROLLER DRUM GATE A crest gate for dam spillways comprising a long horizontal cylinder spanning between piers. The cylinder is fitted

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318 Recommended Glossary with a toothed rim at each end and rotates as it is moved up and down on inclined racks fixed to the piers. ROLLER GATE (STONEY GATE) A gate for large openings that bears on an intermediate train of rollers in each gate guide. SKIMMER GATE A gate at the dam crest whose prime purpose is to control the release of debris and logs with a limited amount of water. It is usually a flap or Bascule gate. SLIDE GATE (SLUICE GATE) A gate that can be opened or closed by sliding in supporting guides. GEOPIIYSICAL METEIODS Methods of studying soil and rock proper- ties and geologic structure without taking samples. GRAVITY DAM A dam constructed of concrete and/or masonry that re- lies on its weight for stability. ARCH GRAVITY DAM An arch dam where part of the water thrust is trans- mitted to the abutments by horizontal thrust and part to the founda- tion by cantilever action. CURVED GRAVITY DAM A gravity dam that is curved in plan. HOLLOW GRAVITY DAM (CELLULAR GRAVITY DAM) A dam that has the out- ward appearance of a gravity dam but that is of hollow construction. GROIN That area along the contact (or intersection) of the face of a dam with the abutments. GROSS STORAGE (RESERVOIR CAPACITY) (GROSS CAPACITY OF RESERVOIR) The gross capacity of a reservoir from the river bed up to maximum controlled retention water level. It includes active, inac- tive, and dead storage. GROUT BLANKET See Blanket. GROUT CAP A concrete pad or wall constructed to facilitate subsequent pressure grouting of the grout curtain beneath the grout cap. GROUT CURTAIN See Curtain. HAZARD A source of danger. In other words, something that has the potential for creating adverse consequences. HEADRACE A free-flow tunnel or open channel that conveys water to the upper end of a penstock; hence the terms headrace tunnel and head- race canal. HEADWATER LEVEL The level of the water in the reservoir or in the headrace at the nearest free surface to the turbine. HEEL OF DAM The junction of the upstream face of a gravity or arch dam with the foundation surface. In the case of an embankment dam the junction is referred to as the upstream toe of the dam. HEIGHT ABOVE GROUND LEVEL The maximum height from natu- ral ground surface to the top of a dam.

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Recommended Glossary 319 HEIGHT ABOVE LOWEST FOUNDATION The maximum height from the lowest point of the general foundation to the top of a dam. HYDRAULIC HEIGHT Height to which the water rises behind a dam and the difference between the lowest point in the original streambed at the axis of the dam and the maximum controllable water surface. 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 representation describes; hence a flood hydrograph is the passage of a flood discharge past the observation point.) IMPERVIOUS CORE See Core. INACTIVE STORAGE The storage volume of a reservoir measured be- tween the invert level of the lowest outlet and minimum operating level. INCLINOMETER (INCLOMETER) An instrument, usually comprising a metal or plastic tube inserted in a drill hole, and a sensitized monitor either lowered into the tube or fixed within the tube. This measures at different points the tube's inclination to the vertical. By integration, the lateral position at different levels of the tube may be found relative to a point, usually the top or bottom of the tube, assumed to be fixed. The system may be adapted to measure settlement. INTAKE Any structure in a reservoir, dam, or river through which wa- ter can be drawn into an aqueduct. INTENSITY SCALE An arbitrary scale to describe the degree of shaking at a particular place. The scale is not based on measurement but on a descriptive 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. INTERNAL EROSION See Piping. INTRADOS The curved downstream surface of an arch dam. INUNDATION MAP A map delineating the area that would be inun- dated in the event of a dam failure. LEAKAGE Uncontrolled loss of water by flow through a hole or crack. LENGTH OF RESERVOIR The distance along the thalweg of the valley forming the reservoir from the dam to the farthest point where the prin- cipal river or a tributary enters the reservoir. LEVEE See Dike. LINING With reference to a canal, tunnel, shaft, or reservoir, a coating of asphaltic concrete, reinforced or unreinforced concrete, shotcrete, rubber or plastic to provide watertightness, prevent erosion, reduce fric- tion, or support the periphery of the structure. May also refer to the lin- ing, such as steel or concrete, of an outlet pipe or conduit.

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320 Recommended Glossary LIVE-STORAGE The sum of active and inactive storage volumes. When there is no inactive storage, e.g., in some irrigation reservoirs, live stor- age and active storage describe the same storage that is generally termed live storage. LOADING CONDITIONS Events to which the dam is exposed, e.g., earthquake, flood, gravity loading. LOWEST POINT OF FOUNDATION The lowest point of the dam foundation excluding cutoff trenches less than 10 meters wide and iso- lated pockets of excavation. LOW-LEVEL OUTLET (BOTTOM OUTLET) An opening at a low level from the reservoir generally used for emptying or for scouring sedi- ment and sometimes also for irrigation releases. MAGNITUDE (`see also RICHTER SCALE) A rating of a given earth- quake independent of the place of observation.. It is calculated from measurements on seismographs and is properly expressed in ordinary numbers and decimals based on a logarithmic scale. Each higher num- ber expresses an amount of earthquake energy that is 10 times greater than expressed by the preceding lower number, e.g., a magnitude 6 earthquake will have 10 times more energy than a magnitude 5. MASONRY DAM Any dam constructed mainly of stone, brick, or con- crete blocks that may or may not be joined with mortar. A dam having only a masonry facing should not be referred to as a masonry dam. MAXIMUM CREDIBLE EARTHQUAKE (MCE) The severest earth- quake 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 sufficiently nearby to influence the project, all faults in the area, and attenuations from causative faults to the site. MAXIMUM CROSS SECTION OF DAM Cross section of a dam at the point where the height of the dam is a maximum. MAXIMUM WATER LEVEL The maximum water level, including the flood surcharge the dam is designed to withstand. MEMBRANE (DIAPHRAGM) A sheet or thin zone or facing made of a flexible impervious material such as asphaltic concrete, plastic concrete, steel, wood, copper, plastic, etc. A cutoff wall, or core wall, if thin and flexible is sometimes referred to as a diaphragm wall or diaphragm. MINIMUM OPERATING LEVEL The lowest level to which the reser- voir is drawn down under normal operating conditions. The lower limit of active storage. MORNING GLORY SPILLWAY See Spillway. MULTIPLE ARCH DAM See Buttress Dam. NAPPE The overfalling stream from a weir or spillway.

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Recommended Glossary 321 NONOVERFLOW DAM (NONSPILL DAM) A dam or section of dam that is not designed to be overtopped. NORMAL WATER LEVEL For a reservoir with a fixed overflow sill it is the lowest crest level of that sill. For a reservoir the outflow from which is controlled wholly or partly by movable gates, syphons or by other means, it is the maximum level at the dam to which water may rise under normal operating conditions, exclusive of any provision for flood surcharge. OGEE SPILLWAY See Spillway. OPERATING BASIS EARTHQUAKE More moderate than the MCE and may be selected on a probabilistic basis from regional and local geol- ogy and seismology studies as being likely to occur during the life of the project. Generally, it is at least as large as earthquakes that have oc- curred in the seismotectonic province in which the site is located. ONE-HUNDRED YEAR (100-YEAR) EXCEEDANCE INTERVAL The flood magnitude expected to be equalled or exceeded on the average of once in 100 years. It may also be expressed as an exceedance frequency with a 1% chance of being exceeded in any given year. OUTLET An opening through which water can be freely discharged for a particular purpose from a reservoir. All earth materials that naturally overlie rock. OVERFLOW DAM (OVERTOPPABLE DAM) A dam designed to be overtopped. PARAPET WALL A solid wall built along the top of a dam for ornament for the safety of vehicles and pedestrians, or to prevent overtopping. PEAK FLOW The maximum instantaneous discharge that occurs during a flood. It is coincident with the peak of a flood hydrograph. PENSTOCK A pipeline or pressure shaft leading from the headrace or reservoir to the turbines. PERVIOUS ZONE A part of the cross section of an embankment dam comprising material of high permeability. PHREATIC SURFACE The free surface of groundwater at atmospheric pressure. PIEZOMETER An instrument for measuring pore water pressure within soil, rock, or concrete. PIPING The progressive development of internal erosion by seepage, ap- pearing downstream as a hole or seam discharging water that contains soil particles. PLUNGE BASIN (PLUNGE POOL) A natural or sometimes artificially created pool that dissipates the energy of free-falling water. The basin is located at a safe distance downstream of the structure from which water is being released. OVERBURDEN .,

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322 Recommended Glossary PORE PRESSURE The interstitial pressure of water within a mass of soil, rock, or concrete. POWER TUNNEL A tunnel carrying water to a hydropower plant. PRECAST DAM A dam constructed mainly of large precast concrete blocks or sections. PRESSURE CELL An instrument for measuring pressure within a mass of soil, rock, or concrete or at an interface between one and the other. PRESSURE RELIEF PIPES Pipes used to relieve uplift or pore water pressure in a dam foundation or in the dam structure. PRESTRESSED DAM A dam the stability of which depends in part on the tension in steel wires, cables, or rods that pass through the dam and that are anchored into the foundation rock. PROBABILITY The likelihood of an event occurring. PROBABLE MAXIMUM FLOOD (PMF) The flood that may be ex- pected from the most severe combination of critical meteorologic and hydrologic conditions that are possible in the region. 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) The maximum amount and duration of precipitation that can be expected to occur on a drainage basin. PUMPED STORAGE RESERVOIR A reservoir filled entirely or mainly with water pumped from outside its natural drainage area. RANDOM FILL Earth or rockfill the grading of which is not specified and that is placed without treatment just as it comes from the excavation. REGULATING DAM A dam impounding a reservoir from which water is released to regulate the flow in a river. RELIEF WELLS See Drainage Wells. REREGULATING DAM See Afterbay Dam. RESERVOIR (MAN-MADE LAKE) which water can be stored. An artificial lake, basin, or tank in RESERVOIR AREA The surface area of a reservoir when filled to con- trolled retention water level. RESERVOIR ROUTING The computation by which the interrelated ef- fects of the inflow hydrograph, reservoir storage, and discharge from the reservoir are evaluated. RESERVOIR SURFACE The surface of a reservoir at any level. RICHTER SCALE A scale proposed by C. F. Richter to describe the magnitude of an earthquake by measurements made in well-defined conditions and with a given type of seismograph. The zero of the scale is fixed arbitrarily to fit the smallest recorded earthquakes. The largest re- corded earthquake magnitudes are near 8.7 and are the result of observa-

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Recommended Glossary 323 tions and not an arbitrary upper limit like that of the intensity scale (see Table 5-4~. RIPRAP A layer of large uncoursed stones, broken rock, or precast blocks placed in random fashion on the upstream slope of an embankment dam, on a reservoir shore, or on the sides of a channel as a protection against wave and ice action. Verv large rioran sometimes is referred to as armoring. RISK The likelihood of adverse consequences. RISK ASSESSMENT As applied to dam safety, the process of identifying the likelihood and consequences of dam failure to provide the basis for informed decisions on a course of action. RISK COST (EXPECTED COST OF FAILURE) The product of the risk and the monetary consequences of failure. ROCK ANCHOR A steel rod or cable that is placed in a hole bored into rock and held in position by grout or a steel wedge. Usually the rock anchor is more than 6 meters long and is finally prestressed. ROCK BOLT A steel rod usually less than 6 meters long that is placed in a hole drilled into rock and held in position by grout or a steel wedge. ROCKFILL DAM See Embankment Dam. ROLLCRETE A no-slump concrete that can be hauled in dump trucks, spread with a bulldozer or grader, and compacted with a vibratory roller. ROLLED FILL DAM See Embankment Dam. ROUND HEAD BUTTRESS DAM See Buttress Dam. RUBBLE DAM A masonry dam in which the stones are unshaped or uncoursed. SADDLE DAM A subsidiary dam of any type constructed across a saddle or low point on the perimeter of a reservoir. SADDLE SPILLWAY See Spillway. SEEPAGE The interstitial movement of water that may take place through a dam, its foundation, or its abutments. SEEPAGE COLLAR A projecting collar usually of concrete built around the outside of a pipe, tunnel, or conduit, under an embankment dam, to lengthen the seepage path along the outer surface of the conduit. SEISMIC INTENSITY See Intensity Scale. SEMIPERVIOUS ZONE See Transition Zone. SHAFT SPILLWAY See Spillway. SHARP-CRESTED WEIR See Weir. SHELL (SHOULDER) The upstream and downstream parts of the cross section of a zoned embankment dam on each side of the core or core wall; hence the expressions upstream shoulder and downstream shoulder. SIDE CHANNEL SPILLWAY See Spillway. , ~ ~ ~

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324 Recommended Glossary SILL (a) A submerged structure across a river to control the water level upstream. (b) The crest of a spillway. (c) The horizontal gate seating, made of wood, stone, concrete or metal at the invert of any opening or gap in a structure. Hence the expressions gate sill and stoplog siZI. SLOPE (a) Side of a hill or a mountain. (b) The inclined face of a cutting or canal or embankment. (c) Inclination from the horizontal. In the United States, measured as the ratio of the number of units of the horizontal distance to the number of corresponding units of the vertical distance. Used in English for any inclination. Expressed in percent when the slope is gentle; in this case also termed gradient. SLOPE PROTECTION The protection of a slope against wave action or erosion. SLUICEWAY SLURRY TRENCH A narrow excavation whose sides are supported by a slurry made of mud, clay, or cement and mud filling the excavation. Sometimes used to describe the cutoff itself. SOIL-CEMENT A well-compacted mixture of soil, Portland cement, and water that produces a hard pavement with more or less permanent cohesion. Used for road building and slope protection. SOLID HEAD BUTTRESS DAM See Buttress Dam. SPILLWAY A structure over or through which flood flows are dis- charged. 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. AUXILIARY SPILLWAY (EMERGENCY SPILLWAY) A secondary spillway de- signed to operate only during exceptionally large floods. FUSE PLUG SPILLWAY A form of auxiliary or emergency spillway com- prising a low embankment or a natural saddle designed to be over- topped and erroded away during a very rare and exceptionally large flood. See Low-Level Outlet. MORNING GLORY SPILLWAY OGEE SPILLWAY (OGEE SECTION) See Shaft Spillway. An overflow weir in which in cross sec- tion the crest, downstream slope, and bucket have an S or ogee form of curve. The shape is intended to match the underside of the nappe at its upper extremities. PRIMARY SPILLWAY (PRINCIPAL SPILLWAY) The principal or first-used spillway during flood flows. SADDLE SPILLWAY A spillway constructed at a low saddle on the perime- ter of a reservoir. SERVICE SPILLWAY A principal spillway used to regulate reservoir re- leases additional to or in lieu of the outlet.

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Recommended Glossary SHAFT SPILLWAY (MORNING GLORY SPILLWAY) 325 A vertical or inclined shaft into which flood water spills and then is conducted through, under, or around a dam by means of a conduit or tunnel. If the upper part of the shaft is splayed out and terminates in a circular horizontal weir, it is termed a belimouth or morning glory spillway. SIDE CHANNEL SPILLWAY A spillway the crest of which is roughly paral- lel to the channel immediately downstream of the spillway. SIPHON SPILLWAY A spillway with one or more siphons built at crest level. This type of spillway is sometimes used for providing automatic surface-level regulation within narrow limits or when considerable discharge capacity is necessary within a short period of time. SPILLWAY CIIANNEL (SPILLWAY TUNNEL) A channel or tunnel conveying water from the spillway to the river downstream. SPILLWAY CHUTE A sloping spillway channel. SPILLWAY DESIGN FLOOD (SDF) The largest flood that a given proj- ect is designed to pass safely. The reservoir inflow-discharge hydrograph used to estimate the spillway discharge capacity requirements and corre- sponding maximum surcharge elevation in the reservoir. STILLING BASIN A basin constructed so as to dissipate the energy of fast-flowing water, e. g., from a spillway or bottom outlet, and to protect the river bed from erosion. STOPLOGS Large logs or timbers or steel beams placed on top of each other with their ends held in guides on each side of a channel or conduit so as to provide a cheaper or more easily handled means of temporary closure than a bulkhead gate. 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 wa- ter level for the purpose of storing and releasing water. STRUCTURAL HEIGHT The distance between the lowest point in the excavated foundation (excluding narrow fault zones) and the top of the dam. SUBMERGED WEIR See Weir. SURCHARGE See Flood Surcharge. TAILRACE The tunnel, channel, or conduit that conveys the discharge from the turbine to the river; hence the terms tailrace tunnel and tailrace canal. TAILWATER LEVEL The level of water in the tailrace at the nearest free surface to the turbine or in the discharge channel immediately downstream of the dam.

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326 Recommended Glossary - THALWEG The line connecting the deepest or lowest points along the stream valley. Sometimes referred to as "the third of the stream." THRUST BLOCK See Block. TOE OF DAM The junction of the downstream face of a dam with the ground surface. Also referred to as downstream toe. For an embankment dam the junction of the upstream face with ground surface is called the upstream toe. TOE WEIGHT Additional material placed at the toe of an embankment dam to increase its stability. TOP OF DAM The elevation of the uppermost surface of a dam, usually a road or walkway, excluding any parapet wall, railings, etc. TOP THICKNESS (TOP WIDTH) The thickness or width of a dam at the level of the top of the dam. In general, the term thickness is used for gravity and arch dams and width is used for other dams. TRAINING WALL A wall built to confine or guide the flow of water. TRANSITION ZONE (SEMIPERVIOUS ZONE) A part of the cross sec- tion of a zoned embankment dam comprising material whose grading is of intermediate size between that of an impervious zone and that of a permeable zone. TRASH RACK A screen comprising metal or reinforced concrete bars lo- cated in the waterway at an intake so as to prevent the ingress of floating or submerged debris. TRIBAR A precast concrete shape consisting essentially of three cylinders connected by beams. They are placed in interlocking patterns and are effective for high-energy dissipation. TUNNEL A long underground excavation usually having a uniform cross section; hence the terms headrace tunnel, pressure tunnel, collecting tunnel, diversion tunnel, power tunnel, tailrace tunnel, navigation tun- nel, access tunnel, scour tunnel, drawoff tunnel, and spillway tunnel. UNDERSEEPAGE The interstitial movement of water through a foun- dation. UPLIFT (a) The upward water pressure in the pores of a material (inter- stitial pressure) or on the base of a structure. (b) An upward force on a structure caused by frost heave or wind force. UPSTREAM BLANKET An impervious layer placed on the reservoir floor upstream of a dam. In the case of an embankment dam the blanket may be connected to the impermeable element in the dam. VALVE In general, a device fitted to a pipeline or orifice in which the closure member is either rotated or moved transversely or longitudinally in the waterway so as to control or stop the flow. GUARD VALVE See Guard Gate. REGULATING VALVE See Regulating Gate and Energy-Dissipating Valve.

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Recommended Glossary VOLUME OF DAM 327 The total space occupied by the materials forming the dam structure computed between abutments and from the top to the bottom of a dam. No deduction is made for small openings such as gal- leries, edits, tunnels, and operating chambers within the dam structure. Portions of power houses, locks, spillway, etc., may be included only if they are necessary for the structural stability of the dam. WATERSHED DIVIDE The divide or boundary between catchment areas (or drainage areas). WATERSTOP A strip of metal, rubber, or other material used to prevent leakage through joints between adjacent sections of concrete. WAVE WALL A solid wall built along the upstream side at the top of a dam and designed to reflect waves. WEIGHTING OF A SLOPE (WEIGHTING BERM) Additional mate- rial placed on the slope of an embankment. WEIR A low dam or wall built across a stream to raise the upstream wa- ter level. Termed fixed-crest weir when uncontrolled. A structure built across a stream or channel for the purpose of measuring flow. Sometimes described as measuring weir or gauging weir. Types of weir include broad-crested weir, sharp-crested weir, drowned weir, and submerged weir. ZONED EARTHFILL See Embankment Dam.

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