NPDP Dam Dictionary

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Term AKA Definition
Intake structure Concrete portion of an outlet works, including trashracks and/or fish screens, upstream from the tunnel or conduit portions. The entrance to an outlet works.
Intangible factors Factors that affect a decision, but that cannot be expressed in monetary terms. Examples include employee morale, safety, system reliability, environmental effects, and politics.
Intensity scale An arbitrary scale to describe the degree of shaking at a particular place. The scale is not based on measurement, but on assessment by an experienced observer. Several scales are utilized (the modified Mercalli scale, the MSK scale) all with grades indicated by Roman numerals from I to XII. For more information, visit the U.S. Geological Survey National Earthquake Information Center.
Interconnected system System consisting of two or more individual power systems normally operating with connecting tie lines.
Intermittent stream A stream which flows part of the time, usually after rainstorm, during wet weather, or for only part of the year. Also referred to as an ephemeral stream. Stream on or in contact with the groundwater table that flows only at certain times of the year when the groundwater table is high.
Internal drainage Movement of water down through soil to porous aquifers or to surface outlets at lower elevations. Drainage within a basin that has no outlet.
Internal erosion The formation of voids within soil or soft rock caused by the mechanical or chemical removal of material by seepage. See erosion.
Internal vibration A method of consolidating soil in which vibrators are used within a thoroughly wetted soil mass to consolidate the soil to the desired density.
Interruptible demands Demands that, by contract, can be interrupted if the supplying system lacks capacity.
Interstitial flow Portion of surface water that infiltrates the streambed and moves through pores in subsurface.
Intrusion A feature (landform, vegetation, or structure) that is generally considered out of context because of excessive contrast and disharmony with characteristic landscape.
Inundate To cover with impounded waters or floodwaters
Inundation map A map of the ground surfaces downstream of a dam showing the probable encroachment by water released because of failure of the dam or from abnormal flood flows released through a dam's spillway.
Invert The lowest point of an underground excavation or the lowest point of the interior of a circular conduit, pipe, or tunnel.
Invertebrates All animals without a vertebral column, e.g. spiders, crabs, or worms.
Ion An atom or molecule that has lost or gained one or more electron
Iowa vane A flow deflector that will divert water in a way calculated to attract fish.
Irretrievable Commitments that are lost for a period of time.
Irreversible Commitments that cannot be reversed, except perhaps in the extreme long term.
Irrigable acreage for service The acreage classified as irrigable for which works have been constructed and water is available. This figure may change from year to year, generally upward, as project works are completed and service is made available to additional acreage. Upon completion of the project, the irrigable acreage for service will equal the irrigable land as presented in the repayment contract or most recent project authorization. See irrigated acreage.
Irrigable land Arable land under a specific plan for which a water supply is or can be made available and which is provided with or planned to be provided with irrigation, drainage, flood protection, and other facilities as necessary for sustained irrigation.
Irrigated acreage The irrigable acreage actually irrigated in any one year. It will include irrigated crop land harvested, irrigated pasture, crop land planted but not harvested, and acreage in irrigated rotation used for soil-building crops.
Irrigation Act of supplying dry land with water in order to grow crops or other plants.