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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. |
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Backfill |
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Material used in refilling excavation, or the process of such refilling. |
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Backfill concrete |
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Concrete used in refilling excavation in lieu of earth material. |
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Backwater |
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A small, generally shallow body of water attached to the main canal, with little or no current of its own. Stagnant water in a small stream or inlet. Water moved backward or held back by a dam, tide, etc. |
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Baffle block |
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One of a series of upright obstructions designed to dissipate energy as in the case of a stilling basin or drop structure (also referred to as dentate). |
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Balanced head condition |
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The condition in which the water pressure on the upstream and downstream sides of an object are equal (such as a emergency or regulating gate). |
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Ball-milling |
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The repeated churning action of cobbles, gravel, and sand caused by the force of water in a stilling basin or other structure by which severe concrete abrasion can occur. |
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Bank storage |
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Water that has inflitrated from a reservoir into the surrounding land where it remains in storage until water level in the reservoir is lowered. |
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Barrage |
Gate-structure dam |
A barrier built across a river, comprising a series of gates which when fully open allow the flood to pass without appreciably increasing the flood level upstream of the barrage. |
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Base flow |
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Groundwater inflow to the river. Portion of stream discharge that is derived from natural storage. |
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Base width or thickness |
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The maximum thickness or width of the dam measured horizontally between upstream and downstream faces and normal to the axis or centerline crest of the dam, but excluding projections for outlets, etc. In general, the term thickness is used for gravity or arch dams, and width is used for other dams. |
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Baseload plant |
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Powerplant normally operated to carry baseload; consequently, it operates essentially at a constant load. |
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Baseloading |
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Running water through a powerplant at a roughly steady rate, thereby producing power at a steady rate. |
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Batter |
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Inclination from the vertical. A pile driven at an angle to widen the area of support and to resist thrust. |
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Beaching |
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The action of water waves by which beach materials settle into the water because of removal of finer materials. |
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Bed material |
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Unconsolidated material of which a streambed is composed. |
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Bedding |
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Ground, or layer of such, for support purposes on which pipe is laid. Soil placed beneath and beside a pipe to support the load on the pipe. |
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Bedding plane |
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A separation or weakness between two layers of rock, caused by changes during the building up of the rock-forming material. |
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Bedload |
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Coarse sediments carried along near the bottom of a river. |
