NPDP Dam Dictionary

Browse by Letter:
All  A  B  C  D  E  F  G  H  I  J  K  L  M  N  O  P  Q  R  S  T  U  V  W  X  Y  Z  


Term AKA Definition
Tunnel Covered portion of spillway between the gate or crest structure and the terminal structure, where open channel flow and/or pressure flow conditions may exist. Portion of an outlet works between upstream and downstream portals, excluding the gate chamber. Tunnels are generally located in the dam abutments, and are concrete lined or concrete/steel lined. An enclosed channel that is constructed by excavating through natural ground. A tunnel can convey water or house conduits or pipes.
Turbidity Measure of extent to which light passing through water is reduced due to suspended materials. Cloudiness of water, measured by how deeply light can penetrate into the water from the surface. The scattering and absorption of light that makes the water look murky. Caused by the content and shape of matter suspended in the water.
Turbulent flow Open channel flow characterized by random fluid motion. The flow is laminar or turbulent depending on the value of the Reynolds number, which is a dimensionless ratio of the inertial forces to the viscous forces. In laminar flow, viscous forces are dominant and the Reynolds number is relatively small. In turbulent flow, the inertial forces are very much greater than the viscous forces and the Reynolds number is large. Turbulent flows are predominant in nature.
Turnout A structure used to divert water from a supply channel to a smaller channel.
U.S. National Inventory of Dams (NID) The NID is a database maintained by the U.S. Army Corps of Engineers. It is periodically updated with data provided by the states and federal agencies. The NID includes dams currently in operation that meet the following criteria: 1. Dam Height (feet) >= 25 2. Reservoir Volume (acre feet) >= 50 3. Hazard Class: High hazard, any size
Unbalanced head See differential head.
Unclassified excavation Excavation paid for at a fixed price per yard, regardless of whether it is common or rock excavation.
Uncompacted backfill Material used in refilling an excavation without the material being compacted. See backfill.
Understory Vegetation underneath the trees.
Unified soil classification system A method of grouping and describing soils according to their engineering properties.
Uniform flow Open 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 hydrograph The direct runoff hydrograph resulting from a unit depth of excess rainfall produced by a storm of uniform intensity and specified duration.
Unit weight See density.
Unlisted items Line item used in an appraisal estimate for design changes and to estimate pay items that have little influence on the total cost. The allowance for unlisted items in appraisal estimates should be at least 10 percent of the listed items. Line item used in a feasibility estimate for quantity changes due to receiving more design data and to estimate pay items that have little influence on the total cost. The allowance for unlisted items in a feasibility estimate varies between 2 percent and 15 percent of the listed items, depending on the form of the specifications quantities.
Unsteady flow The velocity at a point varies with time.
Unsuitable material Those soils that cannot be compacted in embankment or backfill or where excavated to finished grade result in unstable material.
Unwatering As opposed to dewatering, unwatering is the interception and removal of groundwater outside of excavations and the removal of ponded or flowing surface water from within excavations. To remove or drain off water. The removal and control of ponded or flowing surface water, surface seepage, and precipitation from within and adjacent to excavations by the use of channels, ditches, and sumps.
Uplift The upward pressure in the pores of a material (interstitial pressure) on the base of a structure. An upward force on a structure caused by frost heave or windforce.
Uplift pressure See pore-water pressure.
Upstream blanket An impervious blanket placed on the reservoir floor upstream of a dam. In the case of an embankment dam, the blanket may be connected to the impermeable element.
Upstream face The inclined surface of the dam that is in contact with the reservoir. See face.
Urbanization To become urban in nature or character; residential, commercial, and industrial development.
Use value The economic benefit associated with the physical use of a resource, usually measured by the consumer surplus or net economic value associated such use. The contingent value method is one technique used to estimate use value.
User day The participation in a recreation activity at a given resource during a 24-hour period by one person.
V-notch weir A weir that is V-shaped, with its apex downward, used to accurately measure small rates of flow.
Valve A device used to control the flow in a conduit, pipe, or tunnel that permanently obstructs a portion of the waterway. See butterfly valve, gate valve, hollow- jet valve, and tube valve.
Vebe time The time for concrete to fully consolidate in a Vebe cylinder.
Velocity Rate of flow of water expressed in feet per second or miles per hour. The time rate of displacement of a fluid particle from one point to another. Velocity is a vector quantity that has magnitude and direction.
Visitor day Twelve visitor hours which may be aggregated by one or more persons in single or multiple visits.
Visitor use Visitor use of wilderness resource for inspiration, stimulation, solitude, relaxation, education, pleasure, or satisfaction.