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Apron |
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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. |
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Arch dam |
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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. |
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Arch-buttress dam |
Curved buttress dam |
A buttress dam which is curved in plan. |
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Arch-gravity dam |
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An arch dam which is only slightly thinner than a gravity dam. |
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Armoring |
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See riprap. |
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Arroyo |
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A gully or channel cut by an intermittent stream. |
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Atmospheric pressure |
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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. |
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Auger |
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A rotating drill having a screw thread that carries cuttings away from the face. |
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Auxiliary spillway |
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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. |
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Axis |
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A straight line around which a shaft or body revolves. |
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Axis of dam |
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A vertical plane or curved surface, appearing as a line in plan or cross section, to which horizontal dimensions can be referred. |
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Axis of dam (concrete) |
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A vertical reference surface coincident with the upstream face at the top of the dam. |
