NPDP Dam Dictionary

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Term AKA Definition
Abrasion Wearing away of surfaces by friction.
Absorbed water Water held mechanically in a soil mass and having physical properties not substantially different from ordinary water at the same temperature and pressure.
Abutment That part of the valley wall against which the dam is constructed. The part of a dam that contacts the riverbank. A structure that supports the ends of a dam or bridge. An artificial abutment is sometimes constructed, as a concrete gravity section, to take the thrust of an arch dam where there is no suitable natural abutment. Action or place of abutting; the part of a structure that is the terminal point or receives thrust or pressure. Defined in terms of left and right as looking away from the reservoir, looking downstream (i.e., left abutment, right abutment).
Accelerogram The record from an accelerometer showing acceleration as a function of time.
Access shaft Concrete portion of an outlet works between the shaft house and the gate chamber. The access shaft provides vertical access to the gates.
Accretion Process of growth whereby material is added to the outside of nonliving matter. The gradual increase in flow of a stream due to seepage.
Acre Unit for measuring land, equal to 43,560 sq. ft., 4840 sq. yds., or 160 sq. rds.
Acre-foot A term used in measuring the volume of water, or amount of water, needed to cover 1 acre (43,560 square feet) 1 foot deep (325,851 gallons).
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 the top of exclusive flood control capacity, the top of joint use capacity, or 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 Active storage 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 (or dead capacity where there is no inactive capacity).
Adhesion Shearing resistance between soil and another material under zero externally applied pressure.
Adit A nearly horizontal underground excavation in an abutment having an opening in only one end. An opening in the face of a dam for access to galleries or operating chambers.
Aerate To impregnate with gas, usually air.
Afterbay (tailrace) The body of water immediately downstream from a powerplant or pumping plant.
Afterbay dam Reregulating dam A dam located downstream from a large hydroelectric powerplant used to regulate discharges.
Aggregate Crushed rock or gravel screened to sizes for use in road surfaces, concrete, or bituminous mixes.
Air release valve A valve, usually manually operated, which is used to release air from a pipe or fitting.
Air slaking The process of breaking up or sloughing when an indurated soil is exposed to air.
Air-void ratio The ratio of the volume of airspace to the total volume of voids in a soil mass.
Airspace ratio Ratio of volume of water that can be drained from a saturated soil or rock under the action of force of gravity to total volume of voids.
Alkali A soluble salt obtained from the ashes of plants. A substance having marked basic properties.
Alkali-aggregate reaction (AAR) A deterioration of concrete by which the alkali in the cement paste in the concrete reacts chemically with the silica or carbonate present in some aggregates. In the presence of free moisture, the gel (product of the reaction) will expand and manifest into cracking and differential movement in structures as well as other deleterious effects such as reduction in freeze-thaw durability and compressive and tensile strength. Three forms of alkali-aggregate reaction have been identified, see alkali-silica reaction, the slow/late-expanding type of reaction referred to as alkali-silicate reaction, and the alkali- carbonate reaction.
Alkali-carbonate reaction (ACR) Reaction of alkalis which occurs between certain argillaceous dolomitic limestones and the alkaline pore solution in the concrete and causes expansion and extensive cracking. Expansive dolomite limestones are characterized by a matrix of fine calcite and clay minerals with scattered dolomite rhobohedra. This reaction usually occurs early and structures may show cracking within 5 years after construction. See alkali-aggregate reaction.
Alkali-silica reaction (ASR) Reaction of alkalis with aggregate with various forms of poorly crystalline reactive silica: opal, chert, flint and chalcedony and also tridymite, crystoblite and volcanic glasses. Aggregate containing such materials (e.g., some cherty gravels) may cause deterioration of concrete when present in amounts of 1% to 5%. Concrete made of these aggregates is characterized by the early onset of a relatively rapid expansion. Cracking of structures is often observed within 10 years of construction. See alkali-aggregate reaction.
Alkali-silicate/silica reaction (ASSR) Reaction of alkalis with strained quartz is thought to be one reactive component of aggregates causing this reaction. A wide variety of quartz-bearing rocks have been found to be reactive including graywackes, argillites, quartzwackes, quartzarenites, quartzites, hornfels, quartz biotite, gneiss, granite, phyllite, arkose and sandstone. This type of reaction is characterized by a delayed onset of expansion and cracking may not become evident for up to 20 years after construction. See alkali-aggregate reaction.
Alkaline Having a pH of 7.0 or above.
Ampere A unit of electric current or rate of flow of electrons. One volt across 1 ohm of resistance causes a current flow of 1 ampere.
Anchor block See thrust block.
Approach channel The channel upstream from that portion of the spillway having a concrete lining or concrete structure. Channel upstream from intake structure of an outlet works. Channel is generally unlined, excavated in rock or soil, with or without riprap, soil cement or other types of erosion protection.
Appurtentant structures Outlet works, spillways, bridges, drain systems, tunnels, towers, etc.