U.S. Geological Survey (USGS)(USGS)The USGS serves the Nation by providing reliable scientific information to describe and understand the earth; minimize loss of life and property from…
Ultimate bearing capacityThe load per unit of area required to produce failure by rupture of a supporting soil or rock mass.
Unbundling (functional unbundling)Electric service is traditionally provided on a bundled basis, meaning that generation, transmission, and distribution services are provided as a single…
UncertaintyDescribes situations where potential outcomes cannot be estimated based on historical events.
Unclassified excavationExcavation paid for at a fixed price per yard, regardless of whether it is common or rock excavation.
Uncompacted backfillMaterial used in refilling an excavation without the material being compacted. See backfill.
UnconformityA break or gap in the geologic record where rock layers were eroded or never deposited.
UndercurrentA current below the upper currents or surface of a fluid body.
UnderflowThe movement of groundwater through the soil or a subsurface stratum, or under a structure; specifically, the water flowing beneath the bed of a stream…
UnderstoryThe trees and other woody species growing under a relatively continuous cover of branches and foliage formed by the overstory trees.
UndertakingAny project, activity, or program that could change the character or use of historic properties.
Undisturbed sampleA soil sample that has been obtained by methods in which every precaution has been taken to minimize disturbance to the sample.
Uniform flowOpen channel flow where the depth and discharge remain constant with respect to space. Also, the velocity at a given depth is the same everywhere.
Unit HydrographThe direct runoff hydrograph resulting from a unit depth of excess rainfall produced by a storm of uniform intensity and specified duration.
Unit Hydrograph (or Unitgraph)The discharge hydrograph from one inch of surface runoff distributed uniformly over the entire basin for a given time period.
Unit Hydrograph DurationThe time over which one inch of surface runoff is distributed for unit hydrograph theory.
Unit Hydrograph TheoryUnit Hydrograph Theory states that surface runoff hydrographs for storm events of the same duration will have the same shape, and the ordinates of the…
Unit weightWeight per unit volume. See density, dry unit weight, maximum unit weight, saturated unit weight, unit weight of water, wet unit weight.
United StatesIncludes the several States, the District of Columbia, the Commonwealth of Puerto Rico, the Virgin Islands of the United States, American Samoa, Guam, and…
Universal Type Weighting and Recording GageA gage which collects precipitation and then converts the weight onto an inked pen movement which traces on graph paper fixed to a clock driven drum.
Unlisted itemsLine item used in an appraisal estimate for design changes and to estimate pay items that have little influence on the total cost.
Unload-reload cycleA loading sequence resulting in rebound and recompression.
Unpaid Federal investmentA year-end amount owned to the Treasury which remains to be repaid and is synonymous to "balance to be repaid."
Unsaturated zoneThe subsurface zone, usually starting at the land surface and ending at the water table, that includes both water and air in spaces between rocks.
Unsteady flowThe velocity at a point varies with time.
Unsuitable materialThose soils that cannot be compacted in embankment or backfill or where excavated to finished grade result in unstable material.
UnwateringAs opposed to dewatering, unwatering is the interception and removal of ground water outside of excavations and the removal of ponded or flowing surface…
UpliftCharges from an RTO/ISO collected outside of the market-clearing commodity price; these charges can include payments to reliability must run (RMR) units…
UprateAn increase of greater than 15 percent in powerplant unit output at an existing facility through modifications or replacement of equipment.
Upstream BlanketAn impervious blanket placed on the reservoir floor and abutments upstream of a dam.
Upstream faceThe inclined surface of the dam that is in contact with the reservoir. See face.
Upstream SlopeThe part of the dam which is in contact with the reservoir water. On earthen dams, this slope must be protected from the erosive action of waves by rock…
Urban areaPredominantly cities, towns or developed areas where the population is significant.
Urban Flash Flood GuidanceA specific type of flash flood guidance which estimates the average amount of rain needed over an urban area during a specified period of time to initiate…
Urban FloodingFlooding of streets, underpasses, low lying areas, or storm drains. This type of flooding is mainly an inconvenience and is generally not life threatening.
UrbanizationTo become urban in nature or character; residential, commercial, and industrial development.
Usable storage capacityThe quantity of ground water of acceptable quality that can be economically withdrawn from storage.
Use valueThe economic benefit associated with the physical use of a resource, usually measured by the consumer surplus or net economic value associated with such…
User dayThe participation in a recreation activity at a given resource during a 24-hour period by one person.
User feeA fee which is collected only from those persons who use a particular service, as opposed to one collected from the public in general.
V-notch weirA weir that is V-shaped, with its apex downward, used to accurately measure small rates of flow.
Vadose ZoneThe locus of points just above the water table where soil pores may either contain air or water. This is also called the zone of aeration.
ValveA device used to control the flow in a conduit, pipe, or tunnel that permanently obstructs a portion of the waterway.
Vane shear testAn in-place shear test in which a rod with thin radial vanes at the end is forced into the soil and the resistance to rotation of the rod is determined.
Variable costsInput costs that change as the nature of the production activity of its circumstances change; for example, as production levels vary.
VarvedA sedimentary bed or lamination that is deposited within one year's time.
Vebe timeThe time for concrete to fully consolidate in a Vebe cylinder.
Vegetative controlsNonpoint source pollution control practices that involve plants (vegetative cover) to reduce erosion and minimize the loss of pollutants.
Vein(vayn) a vessel through which blood passes from various organs or parts back to the heart.
VelocityThe rate and direction of change of an object's position. The speed and direction at which something is moving.
Velocity ZonesAreas within the floodplain subject to potential high damage from waves. These sometimes appear on flood insurance rate maps.
VenturiA pressure jet that draws in and mixes air.
VernierA device permitting finer measurement or control than standard markings or adjustments.
Vertical curveThe meeting of different gradients in a road or pipe.
Vertical lift gateAll rectangular gates set in vertical guides within which the gate moves vertically in its own plane. The hoist is usually on a runway overhead.
Vesicular (vesicles)Containing many small cavities formed by the expansion of a gas bubble or steam when the rock solidifies.
Vibrating screenA screen which is vibrated to separate and move pieces resting on it.
Virgin compressionCompression corresponding to stresses greater than the preconsolidation stress.
Virgin compression curveThe portion of the compression curve corresponding to virgin compression.
ViscosityThe resistance of a fluid to flow. A liquid with a high viscosity rating will resist flow more readily than will a liquid with a low viscosity.
Visitor dayTwelve visitor hours which may be aggregated by one or more persons in single or multiple visits.
Visitor useVisitor use of recreation and wilderness resource for inspiration, stimulation, solitude, relaxation, education, pleasure, or satisfaction.
VoidInvalid; a void agreement is one for which there is no remedy.
Void ratioThe ratio of the volume of void space to the volume of solid particles in a given soil mass.
Volcanic rockRock that forms from the solidification of molten rock or magma at the earth's surface (extrusive igneous rock).
Volt (V)The unit of measurement of electromotive force. It is equivalent to the force required to produce a current of 1 ampere through a resistance of 1 ohm.
Volt-ampere (VA)A unit of apparent power in an ac circuit containing reactance. It is equal to the potential in volts multiplied by the current in amperes, without taking…
Voltage (E)Electrical pressure, i.e. the force which causes current to flow through an electrical conductor.
Voltage reductionAny intentional reduction of system voltage by 3 percent or greater for reasons of maintaining the continuity of service of the bulk electric power supply…
Volume of ConcreteThe total space occupied by concrete forming the dam structure computed between abutments and from the top to the bottom of the dam.
Volume of damThe total space occupied by the materials forming the dam structure computed between abutments and from the top to the bottom of the dam.
Volumetric shrinkageThe decrease in volume, expressed as a percentage of the soil mass when dried, of a soil mass when the moisture content is reduced from a given percentage…
VortexA revolving mass of water (whirlpool) in which the streamlines are concentric circles and in which the total head is the same.
Wall frictionFrictional resistance mobilized between a wall and the soil or rock in contact with the wall.
Warm-water fisheryGenerally, water or water system that has an environment suitable for species of fish other than salmonids.
WarningThe fourth of five Early Warning System components consisting of the processes (including the media) and equipment necessary to make the public aware of…
Warning stageThe depth of water in a river at which the National Weather Service (NWS) reviews basin conditions for potential flooding.
Warning time (WT)The amount of time between detection of failure or incipient failure and arrival of dam failure flood.
Wash loadThat part of the total sediment discharge which is composed of particle sizes finer than those found in appreciable quantities in the bed material, and is…
WasteDigging, hauling and dumping of valueless material to get it out of the way; or the valueless material itself.
WastebankA bank made of excess or unstable material excavated from a construction site.