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Delta |
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An alluvial sediment deposit normally formed where a river or stream enters a lake or estuary. Sediment deltas are usually triangular in plan view, narrow at the upstream end and relatively wide at the downstream end. The sediment particles deposit because the river velocity and gradient are too low to keep the particles in motion. Active deltas contain diverging multiple channels that continually deposit sediment and migrate back and forth across the delta surface. The sediment particles of the delta deposit are usually well sorted such that the coarser particles (gravel and sand) deposit first at the upstream end, while finer particles (silt and clay) deposit farther downstream. |
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Demand |
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Rate at which electric energy is used, expressed in kilowatts, whether at a given instant, or averaged over any designated period of time. Maximum water use under a specified condition. |
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Demersal |
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Fish eggs or organisms that hatch on the bottom of a lake or stream. |
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Demographics |
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Relating to the statistical study of human populations. |
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Dendritic |
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Channel pattern of streams with tributaries that branch to form a tree-like pattern. |
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Density |
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The total mass (solids plus water) per total volume. The weight of soil per unit volume, usually the weight of soil in one cubic foot (unit weight). The weight of a substance per unit of volume of the substance; for example, water has a density of 62.4 pounds per 1 cubic foot. Number per unit area of individuals of any given species at any given time (see population density). |
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Dentate |
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See baffle block. |
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Depletion |
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To permanently remove water from a system for a specific use. |
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Deposition |
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Material settling out of the water onto the streambed. Occurs when the energy of the flowing water is unable to support the load of suspended sediment. |
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Depth of cutoff |
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The vertical distance that the cutoff penetrates into the dam foundation. |
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Derrick |
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Usually a non-mobile tower equipped with a hoist, but may be used as a synonym for a crane. |
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Desiccate |
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To dry up; remove moisture from a substance. |
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Design basis earthquake |
DBE |
The earthquake which the structure is required to safely withstand with repairable damage. Those systems and components important to safety must remain functional and/or operable. For design purposes, the intended use of this earthquake loading is for economic design of structures or components whose damage or failure would not lead to catastrophic loss. For most usage in Reclamation, the DBE is defined to have a 90% probability of nonoccurrence in a 50-year-exposure period, which is equivalent to a recurrence interval of 474 years. Economic considerations for specific projects may lead to consideration of other values. |
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Design response spectra |
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Smooth, broad-banded spectra appropriate for specifying the level of seismic design force, or displacement, for earthquake-resistant design purposes. |
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Desorption |
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The release or removal of an adsorbed material from the surface of a solid adsorbent. |
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Detention dam |
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A dam built to store streamflow or surface runoff, and to control the release of such stored water. |
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Detonating cord |
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Round, flexible textile, or plastic cord with a center core of high explosive. A common use is to tie in shots using nonelectric delays (MS connectors) when the use of electric caps would be hazardous or undesirable. |
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Detonation |
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Practically instantaneous decomposition or combustion of an unstable compound, with tremendous increase in volume. |
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Detonator |
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A device to start an explosion, as a fuse or cap. |
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Detritus |
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Loose material that results directly from disintegration. |
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Dewatering |
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As opposed to unwatering, dewatering is the removal and control of groundwater from pores or other open spaces in soil or rock formations to the extent that allows construction activities to proceed as intended, including the relief of groundwater pressure. Removing water by pumping, drainage, or evaporation. The removal of groundwater and seepage from below the surface of the ground or other surfaces through the use of deep wells and wellpoints. |
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Diagnostic tool |
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Any artifact which, because of form, shape, or function, provides chronological or manufacturing information. |
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Diaphragm |
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See membrane. |
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Diaphragm-type earthfill |
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An embankment dam which is constructed mostly of pervious material and having a diaphragm of impermeable material which forms a water barrier. The diaphragm which forms the water barrier may consist of earth, Portland cement concrete, bituminous concrete, or other material, and may occupy a position within the embankment or on the upstream face. |
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Diatom |
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Single-celled or colonial algae whose cell walls are made of silica |
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Differential head |
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The condition in which the water pressure on the upstream and downstream sides of an object differ (also called unbalanced head). |
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Dike |
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A low embankment, usually constructed to close up low areas of the reservoir rim and thus limit the extent of the reservoir. Embankment for restraining a river or a stream. Usually applied to dams built to protect land from flooding. |
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Dilatancy |
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The expansion of cohesionless soils when subject to shear deformation. |
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Dilution |
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Reduction of the concentration of a substance in air or water. |
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Dip |
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The angle that a stratum or any planar feature makes with the horizontal, measured perpendicular to the strike and in the vertical plane. The slope of layers of soil or rock. |
