NPDP Dam Dictionary

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Term AKA Definition
Apron A section of concrete or riprap constructed upstream or downstream from a control structure to prevent undercutting of the structure. A short ramp with a slight pitch.
Arch dam A concrete or masonry dam which is curved upstream in plan so as to transmit part of the water load to the abutments. See thin arch dam, medium-thick arch dam, thick arch dam, arch-buttress dam, arch-gravity dam, constant angle arch dam, constant radius arch dam, double curvature arch dam, and multiple arch dam.
Arch-buttress dam Curved buttress dam A buttress dam which is curved in plan.
Arch-gravity dam An arch dam which is only slightly thinner than a gravity dam.
Armoring See riprap.
Arroyo A gully or channel cut by an intermittent stream.
Atmospheric pressure Pressure of air enveloping the earth, averaged as 14.7 psi at sea level, or 29.92 inches of mercury as measured by a standard barometer.
Auger A rotating drill having a screw thread that carries cuttings away from the face.
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 A straight line around which a shaft or body revolves.
Axis of dam A vertical plane or curved surface, appearing as a line in plan or cross section, to which horizontal dimensions can be referred.
Axis of dam (concrete) A vertical reference surface coincident with the upstream face at the top of the dam.