NPDP Dam Dictionary

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Term AKA Definition
Discharge Volume of water that passes a given point within a given period of time. See flow.
Discharge capacity The maximum amount of water that can safely released from a given waterway.
Dissolved oxygen DO Amount of free oxygen found in water; perhaps the most commonly employed measurement of water quality. Low DO levels adversely affect fish and other aquatic life. The ideal dissolved oxygen for fish life is between 7 and 9 mg/L; most fish cannot survive when the DO level falls below 3 mg/L.
Distribution line Low voltage electric power line, usually 69 kilovolts or less.
District An entity that has a contract with the Reclamation for the delivery of irrigation water. Such entities include, but are not limited to: canal companies; conservancy districts, ditch companies, irrigation and drainage districts, irrigation companies, irrigation districts, reclamation districts, service districts, storage districts, water districts, and water users associations.
Ditch Generally, a long narrow excavation.
Diversion A process which, having return flow and consumptive use elements, turns water from a given path. Removal of water from its natural channel for human use.
Diversion capacity The flow which can be passed through the canal headworks at a dam under normal head.
Diversion channel Canal; 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 around a damsite during construction. Channel is normally used instead of canal when the waterway is short. Occasionally the term is applied to a permanent arrangement (diversion canal, diversion tunnel, diversion aqueducts).
Diversion inlet A conduit or tunnel upstream from an intake structure. Diversion inlet may be integral with the outlet works or be part of a separate conveyance structure that will only be used during construction.
Double curvature arch dam An arch dam which is curved in plan and elevation, with undercutting of the heel and in most instances, a downstream overhang near the crest. An arch dam which is curved vertically as well as horizontally.
Downstream face The inclined surface of a dam away from the reservoir. See face.
Drainage blanket A layer of pervious material placed directly over the foundation material to facilitate drainage of the foundation and/or embankment.
Drainage layer A layer of pervious material in an earthfill dam to relieve pore pressures or to facilitate drainage of the fill.
Drainage well Relief well Vertical wells or boreholes downstream of, or in downstream shoulder of, an embankment dam to collect and control seepage through or under the dam and so reduce water pressure. A line of such wells forms a drainage curtain.
Draw A small valley or gully.
Drawdown Lowering of a reservoir's water level; process of depleting a reservoir or groundwater storage. Vertical distance the free water surface elevation is lowered or the reduction of the pressure head due to the removal of free water.
Dredge To dig under water. A machine that digs under water.
Drift Food organisms, including algae, plankton, and even larval fish dislodged and moved by river current. A small, nearly horizontal tunnel.
Drop structure A structure that conveys water to a lower elevation and dissipates the excess energy resulting from the drop.
Drum gate A movable crest gate in the form of a sector of a cylinder hinged at the centerline. The arc face effects a seal with edge of a recess and the gate is operated by a reservoir pressure. Design permits overtopping.
Dry well A deep hole, covered, and usually lined or filled with rocks, that holds drainage water until it soaks into the ground.
Dumped A method of compacting soil by dumping the soil into place with no compactive effort.
Dynamic compaction A method of compacting soil by dropping a heavy weight onto loose soil.
Earth dam Earthfill dam An embankment dam in which more than 50 percent of the total volume is formed of compacted fine-grained material.
Earth lining Compacted layer of earth on surface of canal or other excavation.
Earthquake See design basis earthquake (DBE), maximum credible earthquake (MCE), maximum design earthquake (MDE), operating basis earthquake (OBE), and random earthquake.
Earthwork Any one or combination of the operations involved in altering or movement of earth.
Ecology Branch of biological science which deals with relationships between living organisms and their environment.
Ecosystem Complex system composed of a community of animals and plants as well as the chemical and physical environment.