NPDP Dam Dictionary

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Term AKA Definition
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.
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.