NPDP Dam Dictionary

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Term AKA Definition
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.