Abutment - That portion of the foundation, especially in the sides of a valley, which is in contact with a dam. Also, that portion of a dam which makes contact with and abuts against the foundation at the sides of a valley.

Active Basin - The portion of a reservoir basin above a given elevation which can be used for power generation or other beneficial purpose.

Active Capacity - The reservoir capacity normally usable for storage and regulation of reservoir inflows to meet established reservoir operating requirements. It extends from the highest of (l) the top of exclusive flood control capacity, (2) the top of joint use capacity, or (3) the top of active conservation capacity, to the top of inactive capacity. It is also the total capacity less the sum of the inactive and dead capacities.

Active Conservation Capacity - The reservoir capacity assigned to regulate reservoir inflow for irrigation, power, municipal and industrial use, fish and wildlife, navigation, recreation, water quality, and other purposes. It does not include exclusive flood control or joint use capacity. It extends from the top of the active conservation capacity to the top of the inactive capacity.

Active pool - See Active Basin

Active storage - See Active Basin

Aggregate - Natural material used in the manufacture of concrete. Also any natural material, sorted or unsorted, used in dam or other construction. Aggregate for concrete commonly is obtained from alluvial stream deposits or from rock quarries.

Appurtenant feature - Any physical feature other than the dam itself which contributes to the operation of the dam and reservoir for its intended purpose or purposes.

Aquifers - Water bearing strata of the ground

Arch Dam - A concrete or masonry dam which is curved in plan so to transmit the major part of the water load to the abutements.

Arch-Gravity Dam - An arch dam which is only slightly thinner than a gravity dam.

Artificial abutment - An abutment, usually constructed from concrete, to sustain the lateral thrusts of an arch dam. Such abutments are constructed where existing topographic or bedrock geologic conditions are not adequate for the design of the dam.

Asphaltic concrete - An impervious mixture of aggregate and bitumens used in cores or upstream surfaces of embankment dams.

Auxiliary Spillway - A spillway, usually located in a saddle or depression in the reservoir rim which leads to a natural or excavated waterway, located away from the dam which permits the planned release of excess flood flow beyond the capacity of the service spillway. A control structure is seldom furnished. The crest is set at the maximum water surface elevation for a 100-year flood or some other specific frequency flood. The auxiliary spillway thus has only infrequent use.

Axis of dam - A reference line used for control of surveying during construction of a dam. Commonly the axis defines the location of the upstream portion of the crest of a dam, whether the crest is straight or curved.

Base Widthor Thickness - The maximum thickness or width of the dam measured horizontally between upstream and downstream faces and normal to the axis or centerline crest of the dam, but excluding projections for outlets, etc. In general. the term thickness is used for gravity or arch dams, and width is used for other dams.

Bedrock - The natural, more or less undisturbed rock in the foundation of a dam.

Blanket - A thin blanket or inclined layer of material forming a part of an embankment dam.

Blanket grouting - Shallow, systematic grouting with cement-water mixtures or chemical solutions of bedrock exposed in an excavation for a dam.

Block - Many concrete dams are built in sections or blocks. A section of a concrete dam emplaced within forms or contained between upstream and downstream forms and adjacent sections of the dam is a block.

Borrow area - The source area for natural materials used in dam construction

Bucket - The curved bottom portion of a spillway. The bucket deflects upward and outward the water flowing down the inclined surface of the spillway.

Bulkhead - A structure built to resist rock pressure or to shut off water flow, as in a tunnel.

Buttress - A thin, erect, tabular concrete supporting member used in construction of slab and buttress dams. Also a projecting structure providing lateral support to a rock face or a portion of a dam.

Buttress Dam - A dam consisting of a watertight upstream part supported at intervals on the downstream side by a series of buttresses (walls normal to the axis of the dam).

Cableway - A steel cable used in placing concrete in a dam and to transport excavated materials and construction materials over and above a dam.

Chute - An inclined open trough or lined canal through which water is discharged.

Cofferdam - A temporary dam designed to contain and divert water away from the excavation for a dam or other facility during construction. In some embankment dams the cofferdam is subsequently incoporated into the main, larger structure.

Clay blanket - a thin layer of impervious clay placed upstream from an embankment dam to reduce or eliminate seepage beneath the dam.

Compacted fill - Material in an embankment dam that has been compressed by rolling or impact vibration.

Concrete piling - Pillars of conrete driven vertically downward into unconsolidated materials below an embankment dam to reduce or eliminate seepage beneath the dam.

Control Tower - A tower commonly constructed a short distance upstream from a dam and within the reservoir to control flow of water from the reservoir into the conduits and tunnels.

Control works - Facilities such as valves and gates designed to control flow from the reservoir through, voer, or around a dam.

Construction joint - A joint between adjacent blocks of concrete. Also, a joint, usually nearly horizontal, between a leyer of concrete and the next one placed over it during construction.

Core - The central portion or zone of an embankment dam consisting of impervious material.

Core trench - The trench excavated below the general level of the base of an embankment dam and filled with the impervious material used to construct the core.

Crest - The top of a dam.

Curtain - A zone of foundation grouting or piling parallel to a dam axis designed to prevent or diminish seepage beneath the dam.

Curtain grouting - Grouting of foundation materials to produce a barrier to seepage beneath a dam.

Curved Gravity Dam - A gravity dam which is curved in plan.

Cut-off - A fabricated structure or a grout curtain placed to intercept seepage flow beneath a dam.

Dam - A barrier, either natural or artificially constructed, that impounds or diverts the flow of water, especially in a water course. Also, the body of water confined by a dam.

Dead Capacity - The reservoir capacity from which stored water cannot be evacuated by gravity.

Dead storage - Water in the lower elevations of a reservoir that is unavailable for use or diversion.

Design basis earthquake (DBE) - The earthquake which the structure is required to safely withstand with repairable damage. Those systems and components important to safety must remain functional and/or operable. For design purposes, the intended use of this earthquake loading is for economic design of structures or components whose damage or failure would not lead to catastrophic loss. For most usage in Reclamation, the DBE is defined to have a 90% probability of nonoccurrence in a 50-year-exposure period, which is equivalent to a recurrence interval of 474 years. Economic considerations for specific projects may lead to consideration of other values.

Dike - A long, low embankment. The height is usually less than four to five meters and the lenghth more than ten or fifteen times the maximum height.

Diversion Capacity - The flow which can be passed through the canal headworks at a dam under normal head.

Double Curvature Arch Dam - An arch dam which is curved in plan and elevation, with undercutting of the heel and a downstream overhang near the crest of downstream cantilever.

Drain - A facility for collecting and diverting water that seeps through a dam or through the foundation of a dam.

Drainage holes - Drilled holes designed to intercept seepage water within or beneath a dam.

Drainage prism - A geometrically shaped zone of permeable materials installed in or below an embankment dam to intercept seepage.

Drawdown - Reduction of the water level of a reservoir.

Dumped fill - Material that is placed in an embankment dam without special additional treatment, such as rolling.

Earth fill - Material consisting of earth excavated from a nearby borrow area used in the construction of an embankment dam. The term is imprecisely defined but is generally applied to materials containing abundant soil and clayey substances with or without rocky components.

Embankment - A raised structure built up from unconsolidated materials.

Emergency Spillway - A spillway which provides for additional safety should emergencies not contemplated by normal design assumptions be encountered, i.e., inoperable outlet works, spillway gates, or spillway structure problems. The crest is usually set at maximum water surface.

Exclusive Flood Control Capacity - The reservoir capacity assigned to the sole purpose of regulating flood inflows to reduce possible damage downstream. In some instances, the top of exclusive flood control capacity is above the maximum controllable water surface elevation.

Fill - The natural material used to construct an embankment dam.

Filter zone - A porous zone in or below a dam designed to intercept and divert seepage water.

Fish ladder - A structure built at the side or up the face of a dam to enable migration of fish upstream and downstream.

Flash board - A wood plank or a steel member place at the top of a spillway to increase the storage capacity of a reservoir.

Flat Slab or Slab and Buttress - A buttress dam with buttresses which support the flat slab of reinforced concrete which forms the upstream face.

Foundation - The surface and the natural material beneath it on which a dam and appurtenant features rest.

Foundation cut-off - An excavated trench beneath or adjacent to a dam filled with impermeable material or a grout curtain designed to prevent seepage in the foundation beneath a dam.

Freeboard - That portion of a dam maximum water level in a reservoir.

Gallery - A long, narrow passage inside of a dam used for inspection, grouting, or spillway.

Gate - A movable facility for controlling flow of water over a dam through a spillway.

Gravity Dam - A dam constructed of concrete and/or masonry which relies on its mass for stability.

Gravity tunnel - A tunnel through with water flows without restraint under the force of gravity.

Grout - A mixture of water and cement or a chemical solution that is forced by pumping into foundation rocks or joints in a dam to prevent seepage and to increase strength.

Grout blanket - A grouted zone in the shallow portion of a foundation which has been treated to improve its strength and reduce its permeability.

Grout cap - A cap, usually consisting of concrete, through which grouting operations of foundations are performed.

Grout curtain - A zone in bedrock beneath a dam and parallel to its length that has been injected with grout to stop or reduce seepage beneath a dam.

Grout trench - A trench excavated to enable construction of a grout cap.

Grout veil - The same as a grout curtain.

Grouting - The operation whereby grout is injected under pressure into openings in a dam or in its foundations.

Gut - A term used for the cableway above a dam used for transportation of construction materials.

Head - The hydrostatic pressure generated by water in a reservoir.

Headrace - The flow of water in the direction of a controlling valve or, more specially, through a conduit or tunnel towards a power generating unit.

Headrace conduit - A conduit that conducts water under a head to a valve or into a power generating unit.

Headrace tunnel - A pressure tunnel which conducts water from the reservoir to control work and ultimately into a power generating unit.

Heel - The upstream contact of a dam with its foundation.

Hydraulic fill - Fill pumped or directed by channel flow into and embankment dam during construction.

Hydraulic Height - Height to which the water rises behind the dam, and is the difference between the lowest point in the original streambed at the axis or the centerline crest of the dam and the maximum controllable water surface.

Impervious blanket - A thin layer of impervious material placed within and embankment dam or on the floor of a valley upstream from the dam to reduce or eliminate seepage through or beneath the dam.

Impervious core - A core in a zoned embankment dam consisting of impervious material.

Impervious material - Material, usually rich in clay and/or silt size particles that resists penetration by water.

Inactive basin - That portion of the bottom of a reservoir that contains water that can not be put to beneficial use for drained from the reservoir.

Inactive Capacity - The reservoir capacity exclusive of and above the dead storage from which the stored water is normally not available because of operating agreements or physical restrictions. Under abnormal conditions, such as a shortage of water or a requirement for structural repairs, water may be evacuated from this space.

Inactive storage - The storage in an inactive basin.

Infiltrate - If the gruound surface layer is porous and has minute passages available for the passage of water droplets, the water is then said to infiltrate the subsurface soil.

Inflow Design Flood (IDF) - The flood used to design and/or modify a specific dam and its appurtenant works; particularly for sizing the spillway and outlet works, and for determining surcharge storage requirements. The IDF is equated to less than the Probable Maximum Flood.

Inspection gallery - A gallery within a dam which enables examination of the performance of the dam with time and reservoir filling.

Instrumentation - Devices installed on and within a dam to monitor cyclic or progressive changes during and after construction of the dam.

Intake - The entrance to any water transporting facility such as a conduit or a tunnel.

Intake structure - The structure built at the intake.

Joint Use Capacity - The reservoir capacity assigned to flood control purposes during certain periods of the year and to conservation purposes during other periods of the year.

Left abutment - That portion of the dam that makes contact with it foundation on the left side of a valley as you from upstream.

Left or Right Designation - The designation of left or right is made with the observer looking downstream.

Length of Dam - The distance, measured along the axis of the dam at the level of the top of the main body of the dam or the roadway surface on the crest, from abutment contact to abutment contact.

Mass Concrete - Any large volume of concrete cast-in-place, generally as a monolithic structure. Dimensions of the structure are of such magnitude that measures must be taken to cope with the generation of heat and the resulting volume changes and cracking.

Massive Head Buttress - A buttress dam in which the buttress is greatly enlarged on the upstream side to span the gap between buttresses.

Maximum Controllable Water Surface - The highest reservoir water surface elevation at which gravity flows from the reservoir can be completely shut off.

Maximum credible earthquake (MCE) - The largest earthquake that a fault or other seismic source could produce under the current tectonic setting. The seismic evaluation criteria determines which faults or seismic sources are assigned an MCE.

Maximum Water Surface - The highest acceptable water surface elevation with all factors affecting the safety of the structure considered. It is the highest water surface elevation resulting from a computed routing of the inflow design flood through the reservoir under established operating criteria. This surface elevation is also the top of the surcharge capacity.

Maximum design earthquake (MDE) - The earthquake selected for design or evaluation of the structure. This earthquake would generate the most critical ground motions for evaluation of the seismic performance of the structure among those loadings to which the structure might be exposed. For example, if a site were assigned MCEs for two separate sources, the MCE which would be expected to generate the most severe ground motions would be the maximum design earthquake. The response of the structure to specific ground motion parameters (frequency, duration, etc.) needs to be considered in specifying this event. In certain cases, more than one maximum design earthquake may be specified to reflect the differing response of various components of the structure to earthquake loading.

Medium-thick Arch - An arch dam with a base thickness to structural height ratio between 0.2 and 0.3.

Muck - A common expression or unconsolidated, usually wet and muddy natural material.

Multiple Arch Dam - A buttress dam, the upstream part of which comprises a series of arches.

Natural abutment - An abutment in natural foundation materials. Contrasted with an artificial abutment which is constructed from concrete at the site.

Natural frequency (f) - The natural frequencies of a structure are the frequencies of free vibration. Free vibration is vibration that occurs in the absence of forced vibration. In a structure undergoing vibration, a mode of vibration is a characteristic pattern (shape) assumed by the structure in which the motion of every particle is simple harmonic motion with the same frequency. The fundamental mode of vibration of a structure is the mode having the lowest natural frequency.

Natural period of vibration (T) - The period of vibration of a a structure is the time required for one cycle of the simple harmonic motion in one of these characteristic patterns (shapes). T = 1/f.

Normal Water Surface - The elevation at the top of the active conservation capacity. The maximum elevation to which the reservoir may rise under normal operating conditions exclusive of flood control storage. (The term is no longer used by the Service but is offered because of its prior usage.)

Observation well - An excavated well or vertical borehole used to observe changes in seepage flow through or beneath dams during filling and drawdown.

Operating basis earthquake (OBE) - The earthquake that the structure must safely withstand with no damage. All systems and components necessary to the uninterrupted functioning of the project are designed to remain operable during the ground motions associated with the OBE. This includes the dam, appurtenant structures, electrical and mechanical equipment, relays, spillway gates, and valves. For most usage in the Bureau of Reclamation, the OBE is specified to have a 90% probability of nonoccurrence in a 25-year-exposure period. This is equivalent to a recurrence interval of 237 years. Economic considerations for specific projects may lead to consideration of other values.

Outlet - Any facility, such as the exit of a tunnel, from which water issues by controlled flow.

Outlet structure - A fabricated structure at the conflict of a canal, conduit, or tunnel.

Overflow section - That portion of a dam, usually occupied by a spillway, over which water above the spillway elevation flows.

Outlet Works - A combination of structures and equipment required for the safe operation and control of water released from the reservoir to serve various purposes, i.e., regulate stream flow and quality; release floodwater; and provide irrigation, municipal, and/or industrial water. Included in the outlet works are the intake structure, conduit, control house-gates, regulating gate or valve, gate chamber, and stilling basin.

Parapet - Usually construed to be a low protective wall along the crest of a dam.

Pendulum Shaft - A narrow vertical opening in a dam used for surveying control during structures and subsequently for occupation of deflections of the dam under load.

Penstock - A conduit, commonly steel pipe, leading from the reservoir to a power generating plant downstream from the reservoir.

Pervious Material - Material through which water flows with relative ease. Contrasted with impervious material.

Phreatic Surface - As the groundwater percolates down the acquifer becomes saturated. The surface of saturation os referred to as the groundwater table or the phreatic surface. It falls during dry spells and rises in rainy spells.

Piling - Elongate, post like steel or concrete members or steel sheets driven into a dam foundation to reduce or eliminate seepage.

Power Intake - The intake to a conduit or tunnel which leads to a power generating unit.

Power Plant - The facility constructed at or near the downstream face of a dam to generate hydroelectric power.

Pressure Tunnel - A tunnel which transmits water under moderate to high pressure.

Prestressing - Strengths of rocks in foundation and elements within concrete dams are increased by installation of steel rocks or steel cables which are injected to tensioning. The procedure that is followed is called prestressing.

Purge tunnel - A tunnel that is used to much more frequently used tunnels of obstructions or deposits of sediment.

Relief Well - An excavated well below a dam to collect seepage water in the foundation.

Reservoir - In the present context a reservoir is a basin, usually artificially created, that impounds and stores water.

Right abutment - The abutment to the right as observed from a point upstream from a dam.

Rock blanket - A layer of rocks placed on the face of a dam to prevent wave erosion of deeper materials.

Rock bolt - A threaded steel rod placed in a drilled hole and tensioned to increase strength of rock masses.

Rock fill - Rock aggregate placed in an embankment dam.

Rolled fill - Fill, usually rich in clayey or silty components, that is compacted by rolling, especially with sheep's foot rollers or vibratory compactors.

Saddle Dike - A small dam built in a topographic low in the periphery of a reservoir basin.

Service Spillway - A structure located on or adjacent to a storage or detention dam over or through which surplus or floodwaters which cannot be contained in the allotted storage space are passed, and at diversion dams to bypass flows exceeding those which are turned into the diversion system. Included as part of the spillway would be the intake and/or control structure, discharge channel, terminal structure, and entrance and outlet channels.

Sheet Pilling - Plates of steel driven into the foundation of a dam to reduce or eliminate seepage beneath a dam.

Sluiced fill - Fill usually clayey, placed in an embankment dam by running water.

Spillway - The structure on or at the side of a dam that contains and guides the flow of the excess water supplied to a reservoir. Spillways inside the reservoir are called glory holes and consist of a vertical shaft a tunnel which exits below the dam.

Spoil area - An area used to dispose of materials that are unwanted or surplus in dam construction.

Stilling basin - A basin downstream from a dam that receives the discharge from tunnels or conduits or overflow from a spillway.

Structural damage - Damage resulting from failure of a dam or its appurtenant features.

Structural Height - Distance between the lowest point in the excavated foundation (excluding narrow fault zones) and the top of dam. The structural height of a concrete dam is the vertical distance between the top of the dam and lowest point of the excavated foundation area, excluding narrow fault zones. The structural height of an embankment [earth or rockfill) dam is the vertical distance between the top of the embankment and the lowest point in the excavated foundation area, including the main cutoff trench, if any, but excluding small trenches or narrow backfilled areas. The top elevation does not include the camber, crown, or roadway surfacing.

Surcharge Capacity - The reservoir capacity provided for use in passing the inflow design flood through the reservoir. It is the reservoir capacity between the maximum water surface elevation and the highest of the following elevations (1) top of exclusive flood control capacity, (2) top of joint use capacity, or (3) top of active conservation capacity.

Surge tank or shaft - A vertical shaft above a pressure tunnel that provides equal pressures at the tunnel level in response to sudden pressure changes caused by increasing or decreasing the flow of water.

Tail water - The water issuing downstream from tunnels, conduits, or spillways.

Tail race - The movement of water below a valve or after it has passed through a power generating plant.

Thin Arch - An arch dam with a base thickness to structural height ratio of 0.2 or less.

Thick Arch - An arch dam with a base thickness to structural height ratio of 0.3 or greater.

Thrust block - That part of the foundation of and arch dam against which horizontal thrust is exerted by the dam as the reservoir behind it is filled.

Toe - The downstream contact of a dam with its foundation.

Top of Active Conservation Capacity - The reservoir water surface elevation at the top of the capacity allocated to the storage of water for conservation purposes only.

Top of Exclusive Flood Control Capacity - The reservoir water surface elevation at the top of the reservoir capacity allocated to exclusive use for regulation of flood inflows to reduce damage downstream.

Top of Inactive Capacity - The reservoir water surface elevation below which the reservoir will not be evacuated under normal conditions.

Top of Joint Use Capacity - The reservoir water surface elevation at the top of the reservoir capacity allocated to joint use, i.e., flood control and conservation purposes.

Total Capacity - The reservoir capacity below the highest of the elevations representing (l) the top of exclusive flood control capacity, (2) the top of joint use capacity, or (3) the top of active conservation capacity. Total capacity is used to express the total quantity of water which can be impounded and is exclusive of surcharge capacity. Live Capacity. That part of the total reservoir capacity which can be withdrawn by gravity. This capacity is equal to the total capacity less the dead capacity.

Tower - A vertical structure upstream from a dam designed to control flow of reservoir water through the dam into power generating facilities.

Trash rack - The screening facility built at the intake end of conduits or tunnels to prevent entrance of debris.

Transpiration - Evaporation of water from the surfaces of green plants, largely through the stomata, pore openings to the intercelluar spaces in the leaves.

Valve chamber - A chamber within a dam containing valves to control the flow of water from a reservoir.

Valve vault - An opening excavated in bedrock at the side of a dam and containing valves or control of flow from the reservoir.

Volume of Dam - The total space occupied by the materials forming the dam stucture computed between abutments and from top to bottom of dam.

Water stop - A membrane placed in joints in concrete dams to prevent seepage of water.

Weir - A channel of known cross section which enables measurement of the volume of flow of water after calibration. The top of a spillway set into a concrete dam is also sometimes designated as a weir.

Zoned dam - An embankment dam in which materials of different properties are placed systematically in various portions of the dam.