NPDP Dam Dictionary

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Term AKA Definition
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.