NPDP Dam Dictionary

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Term AKA Definition
Top of inactive capacity The reservoir water surface elevation below which the reservoir will not be evacuated under normal conditions. The highest applicable water surface elevation described below usually determines the top of inactive capacity. (1) The lowest water surface elevation at which the planned minimum rate of release for water supply purposes can be made to canals, conduits, the river, or other downstream conveyance systems. This elevation is normally established during the planning and design phases and is the elevation at the end of extreme drawdown periods. (2) The established minimum water surface elevation for fish and wildlife purposes. (3) The established minimum water surface elevation for recreation purposes. (4) The minimum water surface elevation as set forth in compacts and/or agreements with political sudivision. (5) The minimum water surface elevation at which the powerplant is designed to operate. (6) The minimum water surface elevation to which the reservoir can be drawn down using established operating procedures without endangering the dam, appurtenant structures, or reservoir shoreline. (7) The minimum water surface elevation or top of inactive capacity established by legislative action.
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. See reservoir.
Top of surcharge capacity The maximum water surface of a reservoir.
Top width or thickness The thickness or width of a dam at the level of the top of dam (excluding corbels or parapets). In general, the term thickness is used for gravity and arch dams, and width is used for other dams.
Topography Physical shape of the ground surface.
Topsoil The topmost layer of soil, usually containing organic matter. Usually refers to soil containing humus which is capable of supporting plant growth.
Total capacity The reservoir capacity below the highest of the elevations representing either the top of exclusive flood control capacity, the top of joint use capacity, or the top of active conservation capacity. In the case of a natural lake which has been enlarged, the total capacity includes the dead capacity of the lake. Total capacity is used to express the total quantity of water which can be impounded and is exclusive of surcharge capacity.
Total dissolved solids TDS A quantitative measure of the residual mineral dissolved in water that remains after the evaporation of a solution. Usually expressed in milligrams per liter or parts per million. Total amount of dissolved material, organic and inorganic, contained in water.
Total Economic Losses Description Describe the damages that occurred; e.g., damage to homes, etc. Provide estimates of the number of items that were damaged; e.g., 10 houses.
Total Fatalities Owner Number of fatalities of owner employees.
Toxin Poisonous substance, generally from a plant or animal.
Training wall A wall built to confine or guide the flow of water.
Tranquil flow Distinguished from rapid flow by a dimensionless number called the Froude number. If the Froude number is less than one, the flow is tranquil. If the Froude number is greater than one, the flow is rapid. If the Froude number is equal to one, the flow is critical. In tranquil flow, surface waves propagate upstream as well as downstream. Control of tranquil flow depth is always downstream.
Transition zone Semipervious zone A substantial part of the cross section of an embankment dam comprising material whose grading is of intermediate size between that of an impervious zone and that of a permeable zone.
Transmission line Facility for transmitting electrical energy at high voltage from one point to another point. See distribution line.
Transpiration The process by which water in plants is transferred into water vapor in the atmosphere.
Transport capacity The capacity of a river to carry sediment in suspension or to move sediment along the riverbed. Usually expressed as mass per unit of time.
Trash rake A device that is used to remove debris which has collected on a trashrack to prevent blocking the associated intake.
Trashrack A metal or reinforced concrete structure placed at the intake of a conduit, pipe, or tunnel that prevents entrance of debris over a certain size.
Trench(es) See ditch.
Tributary River or stream flowing into a larger river or stream.
Trophic level Place of an animal in the food chain.
Tube valve A valve which is opened or closed by mechanically moving a tube upstream or downstream by an actuating screw.
Tunnel Covered portion of spillway between the gate or crest structure and the terminal structure, where open channel flow and/or pressure flow conditions may exist. Portion of an outlet works between upstream and downstream portals, excluding the gate chamber. Tunnels are generally located in the dam abutments, and are concrete lined or concrete/steel lined. An enclosed channel that is constructed by excavating through natural ground. A tunnel can convey water or house conduits or pipes.
Turbidity Measure of extent to which light passing through water is reduced due to suspended materials. Cloudiness of water, measured by how deeply light can penetrate into the water from the surface. The scattering and absorption of light that makes the water look murky. Caused by the content and shape of matter suspended in the water.
Turbulent flow Open channel flow characterized by random fluid motion. The flow is laminar or turbulent depending on the value of the Reynolds number, which is a dimensionless ratio of the inertial forces to the viscous forces. In laminar flow, viscous forces are dominant and the Reynolds number is relatively small. In turbulent flow, the inertial forces are very much greater than the viscous forces and the Reynolds number is large. Turbulent flows are predominant in nature.
Turnout A structure used to divert water from a supply channel to a smaller channel.