NPDP Dam Dictionary

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Term AKA Definition
Ichthyology The scientific study of fish.
Igneous Rock formed by molten magma.
Impeller A rotary pump member using centrifugal force to discharge a fluid into outlet passages
Impervious Not permeable; not allowing liquid to pass through.
Impervious core See core.
Impervious material Relatively waterproof soils. Clay, for example, since water will percolate through clay at about one millionth of the rate at which it will pass through gravel.
Impervious zone See core.
Impinge To collide or strike.
Impoundment Body of water created by a dam.
Improvement Structural measures for the betterment, modernization, or enhancement of an existing facility or system to improve the social, economic, and environmental benefits of the project.
In Operation Selecting this option searches all, currently operational dams, irrespective of dam size, and whether the dam is regulated or not.
Inactive capacity The reservoir capacity exclusive of and above the dead capacity from which the stored water is normally not available because of operating agreements or physical restrictions. Under abnormal conditions, such as a shortage of water or a requirement for structural repairs, water may be evacuated from this space. The inactive capacity extends from the top of inactive capacity to the top of dead capacity.
Incident Type Type of the incident.
Incident Year The year in which a dam incident occured.
Incised stream A stream that has cut its channel into the bed of the valley through degradation.
Incremental Fatalities The number of fatalities associated with dam incident; above those that may have occurred during flooding for example.
Incremental loss of life A measurement for assessing damage after a dam fails. The difference between the projected loss of life had the dam not been built and the projected loss of life with the dam in place.
Independent power producers Non-utility owned electric resources.
Indicator Organism, species, or community which indicates certain environmental conditions.
Indigenous Native to a given area.
Infiltration Flow of a liquid into a substance through pores of small openings. See permeability.
Inflow Water that flows into a body of water.
Inflow design flood IDF The flood used to design and/or modify a specific dam and its appurtenant works; particularly for sizing the spillway and outlet works, and for determining surcharge storage requirements. The flood used for design of a safe structure. It may be the probable maximum flood (PMF), but in sparsely developed areas where judgment indicates minimal property damage and no probable loss of life, the IDF may be less than the PMF.
Inflow reservoir The amount of water entering a reservoir expressed in acre-feet per day or cubic feet per second.
Inlet channel Inlet structure Concrete lined portion of spillway between approach channel and gate or crest structure.
Inlet structure See inlet channel.
Inspection Type This is a data field in the NPDP database that is used to identify what type of dam inspection was performed. Some agencies have different types of periodic dam safety inspections they perform. For example two types of periodic inspections might be reconnaisance and detailed. A reconnaisance inspection is a brief inspection to assess the general state of the dam. A detailed inspection is a comprehensive inspection to evaluate the condition of all the elements of a dam. A reconnaisance inspection is typically short in duration, whereas a detailed inspection is more lengthy.
Instream flow requirements Amount of water flowing through a defined stream channel needed to sustain instream values, e.g. flows designated for fish and wildlife.
Instrumentation Any device used to monitor the performance of the structure during its construction and throughout its useful life.
Intake Any structure through which water can be drawn into a waterway.